Authors on Lung Function

CPET Test Interpretation, Part 4: Interpretation and Summary

Wed, 06 Nov 2019 00:00:00 +0000

After having gone through the descriptive checklists for ventilatory, gas exchange and circulatory limitations the reason(s) for a patient’s exercise limitation, if any, should be reasonably clear. However, one of the first questions that should be asked when reading an exercise test is what was the purpose of the test?

Maximum safe exercise capacity for Pulmonary Rehab?
Rule in/rule out exercise-induced bronchospasm?
Pre-operative assessment?
Dyspnea of uncertain etiology?
What is the primary limitation to exercise (pulmonary or cardiac)?
Is deconditioning suspected?

The interpretation and summary should address these concerns.

2019 ATS/ERS Spirometry Standards

Wed, 30 Oct 2019 00:00:00 +0000

The 2019 ATS/ERS Spirometry Standards were recently released. The standards are open-access and can be downloaded without charge from the October 15 th issue of the American Journal of Respiratory and Critical Care Medicine. Supplements are available from the same web page.

The 2019 Spirometry Standards have been extensively re-organized with numerous updates. Notably, a number of sections that were previously discussed in the 2005 General Considerations for Lung Function Testing have been updated and included in the 2019 Spirometry Standards. Also notably, a number of stand-alone spirometry tests, including the Flow-Volume Loop, PEF and MVV are not included in the 2019 Standards.

CPET Test Interpretation, Part 3: Circulation

Wed, 23 Oct 2019 00:00:00 +0000

I would like to re-emphasize the importance of the descriptive part of CPET interpretation. At the very least consider it to be a checklist that should always be reviewed even when you think you know what the final interpretation is going to be.

After gas exchange, the next step in the flow of gases is circulation. The descriptive elements for assessing circulation are:

What was the maximum heart rate?

CPET Test Interpretation, Part 2: Gas Exchange

Wed, 16 Oct 2019 00:00:00 +0000

I would like to re-iterate the importance of the descriptive part of CPET interpretation. At the very least consider it to be a checklist that should always be reviewed even when you think you know what the final interpretation is going to be.

After ventilation, the next step in the flow of gases is gas exchange. The descriptive elements for assessing gas exchange are:

What was the maximum oxygen consumption (VO2)?

CPET Test Interpretation, Part 1: Ventilatory response

Wed, 09 Oct 2019 00:00:00 +0000

I’ve always found interpreting CPET tests to be one of the more interesting (and enjoyable) things I’ve done. Interpreting a CPET test is both more difficult and easier than interpreting regular PFTs. More difficult because there are a lot more parameters involved and easier because determining test adequacy and the primary cause(s) of an exercise limitation tends to be clearer.

I’ve found that you have to go back to basic physiology whenever you interpret CPETs and that always boils down to the flow of oxygen and carbon dioxide.

Sharing opinions with Paul Enright

Sun, 28 Apr 2019 00:00:00 +0000

Dr. Paul Enright is a well-known name in the field of Pulmonary Function testing. He is the lead author or co-author of over a hundred articles and has served on many of the ATS/ERS standards committees.

Introduction:

We both retired in southern Arizona and live a couple of towns apart from each other. We have corresponded for a while but met face-to-face only recently. We both drive small red vehicles, Richard a Ford Transit Van and Paul a Prius Compact. We both love to visit National Parks; Richard’s favorite is Canyonlands while Paul’s favorite is Jasper, with many large wild animals. This posting is based on a set of suggestions by Paul.

N2 washout is affected by N2 excretion and other factors

Tue, 26 Mar 2019 00:00:00 +0000

The Lung Clearance Index (LCI) was first described in 1952 by Margaret Becklake, and is defined as the number of lung volume turnovers required to reduce the concentration of a tracer gas by a factor of 40. LCI is calculated as the cumulative exhaled volume (CEV) during the washout divided by the functional residual capacity (FRC).

Clinically LCI has been used most often in individuals with cystic fibrosis and this is because the LCI has been repeatably shown to be sensitive to changes in airway status that are not reflected in the FEV1. LCI has shown similar results in patients with primary ciliary dyskinesia. As expected LCI has also been tested on patients with COPD, bronchiectasis and asthma although these patients tend to show a better correlation between FEV1 and LCI.

FEV1 and VC should be measured separately

Fri, 22 Mar 2019 00:00:00 +0000

The FEV1 and VC both provide quite different information about a patient’s lungs. Unfortunately, spirometry as it is currently practiced is optimized towards generating an accurate FEV1 more than an accurate VC. This is partly due to limitations in the maneuver itself and partly due to the lack of accurate end-of-test criteria for an adequate VC. In one sense this is okay since more than one person that I’ve known and respected has said that “it’s all about the FEV1”.

I’M BACK, SORT-OF

Wed, 20 Mar 2019 00:00:00 +0000

The chemo did a real number on me. I was included in a study of a new drug and it literally almost killed me. Twice. For this reason I have left the study and have been put on a gentler and more normal regime of chemo. I have lost a lot of stamina (and strength and weight) but have recovered somewhat. I have returned to catch up on comments as best I can and apologize for not responding for so long.

Goodbye

Sun, 02 Dec 2018 00:00:00 +0000

This is probably the last post I will be able to write. I was diagnosed less than two months ago with a very nasty cancer with a poor prognosis. I thought I could power through it but a serious infection with sepsis and a week and a half stay in the hospital has convinced me that it’s time to quit and focus on other things.

Sucks, but that’s life.

There are many pulmonary function topics I would have liked to discuss but time has run out. I will leave you to ponder the two biggest elephants in the room; that of height and that of ethnicity. The relationship between height and FVC, TLC, etc. is inexact and yet nobody seems to think about any alternate anthropomorphic measurements. Sitting height is only marginally better but it is better. Is anybody using it? No. C’mon people, it’s way past time that we found better anthropomorphic correlations for FVC, FEV1, TLC and DLCO.

A modest proposal for a clinical spirometry grading system

Sun, 16 Sep 2018 00:00:00 +0000

A while back I reviewed the spirometry grading system that was included in the 2017 ATS reporting standards . My feeling was, and continues to be, that its usefulness is very limited because it’s mostly a reproducibility grading system that relies on a few easy-to-measure parameters. This doesn’t mean that a grading system can’t be helpful, just that it needs to be focused differently.

In a clinical PFT lab many patients have difficulty performing adequate and reproducible spirometry, but that doesn’t mean the results aren’t clinically useful. Moreover, suboptimal quality results may be the very best the patient is ever able to produce. So what’s more important in a grading system than reproducibility is the ability to assess the clinical utility of a reported spirometry effort.

Infection Control

Sun, 09 Sep 2018 00:00:00 +0000

The issue of infection control has been a topic of a couple of discussions I’ve had lately. In particular, it was reported to me that a PFT lab had come under fire from a Joint Commission inspector who did not believe that filter mouthpieces were adequate and that “patient valves and circuits need to be sterilized between each patient”.

Unfortunately with all the other things we have to worry about it’s all too easy to become blasé about infection control. This despite the fact that every hospital I’ve visited in the last dozen or so years has posted numerous signs about hand washing and the safe disposal of contaminated supplies. But maybe it’s because we’re inundated with reminders that we’ve developed a blind spot about it.

Is gas trapping more common than we think it is?

Sun, 17 Jun 2018 00:00:00 +0000

Over the last couple of years I’ve run across a number of test systems that do not include tidal loops along with the maximal flow-volume loop. I’ve wondered why this was done and because of this I’ve thought a lot about tidal flow-volume loops and what additional information, if any, they add to spirometry interpretation.

One of my thoughts has been about the relationship between obesity and the IC and ERV. FVC and TLC are often reasonably preserved even with relatively severe obesity. FRC, on the other hand, is often noticeably affected with even minor changes in BMI (and interestingly this applies to reduced as well as elevated BMI’s). When FRC decreases because of obesity the IC usually increases and the ERV decreases and for this reason the IC/ERV ratio has been suggested as a way to monitor changes in FRC without having to actually measure lung volumes.

Telling the right story

Tue, 08 May 2018 00:00:00 +0000

The 2005 ATS/ERS spirometry standard make it permissible and even recommends that the FVC and FEV1 be selected from different efforts. I disagree somewhat with their criteria for selecting the FEV1 but overall reporting composite results makes a lot of sense. In an ideal world we’d always get the best FVC and FEV1 in a single effort but what we more often get is a good FEV1 with a poor FVC or a poor FEV1 with a good FVC. So, it best serves the clinical needs of the patient to report the best elements from multiple spirometry efforts.

I’ve got the old back-extrapolation blues

Wed, 21 Mar 2018 00:00:00 +0000

A couple days ago I pulled my copy of the Intermountain Thoracic Society manual on pulmonary function testing off the bookshelf and thumbed through it a bit. It was first published in 1975 and was the first major attempt towards standardizing the performance and interpretation of PFTs.

My first thought was that we’ve come a long way since then. Most importantly our understanding of what spirometry can (and cannot) tell us has improved dramatically.

A spirometry quality grading system. Or is it?

Sun, 25 Feb 2018 00:00:00 +0000

A set of guidelines for grading spirometry quality was included with the recently published ATS recommendations for a standardized pulmonary function report. These guideline are similar to others published previously so they weren’t a great surprise but as much as I may respect the authors of the standard my first thought was “when was the last time any of these people performed routine spirometry?” The authors acknowledge that the source for these guidelines is epidemiological and if I was conducting a research study that required spirometry these guidelines would be useful towards knowing which results to keep and which to toss but for routine clinical spirometry, they’re pretty useless.

3-Equation DLCO

Sun, 18 Feb 2018 00:00:00 +0000

One of the limitations of the single-breath DLCO is that the equation used to calculate results implicitly assumes that the entire breath-holding period occurs at TLC. Mathematically, what happens to the diffusion of carbon monoxide (CO) during inspiration and expiration is not a consideration:

The different approaches towards measuring breath-holding time (BHT) make allowances for inspiration and expiration to one extent or another but realistically they should be considered fudge factors.

VA, two ways

Mon, 22 Jan 2018 00:00:00 +0000

One of the recommendations in the 2017 ERS/ATS DLCO standards was that VA should be calculated using a mass balance equation. I’ve discussed this approach previously , but basically the volume of the exhaled tracer gas is accumulated over the entire exhalation and the amount of tracer gas presumed to remain in the lung is used to calculate VA. The conceptual problem with this for DLCO measurements is that VA is calculated using the entire exhalation but CO uptake is based solely on the CO concentration in the alveolar sample. Since VA calculated using mass balance tends to be larger than VA calculated traditionally in subjects with ventilation inhomogeneities this mean that DLCO calculated with a mass balance VA is also going to be proportionally larger as well.

What’s normal about airway resistance?

Sun, 07 Jan 2018 00:00:00 +0000

The question that was actually posed to me a month or so ago was “when is RAW abnormal?” I didn’t have a good answer at the time since airway resistance (RAW) tests are not performed by my lab. The pulmonary physicians I work with don’t think that RAW is a clinically useful measurement and for a variety of reasons I don’t disagree with this. Nevertheless, RAW testing is routinely performed in many labs around the world so I thought it would be interesting to spend some time researching this.

Thinking about the past

Sun, 31 Dec 2017 00:00:00 +0000

This is the time of the year when it’s traditional to review the past. That’s what “Auld lang syne”, the song most associated with New Year’s celebrations, is all about. I too have been thinking about the past but it’s not been about absent friends, it’s been about trend reports and assessing trends.

In the May 2017 issue of Chest, Quanjer et al reported their study on the post-bronchodilator response in FEV1. I’ve discussed this previously and they noted that the current ATS/ERS standard for a significant post-bronchodilator change of ≥12% and ≥200 ml penalized the short and the elderly. Their finding was that a significant change was better assessed by the absolute change in percent predicted (i.e. 8%) rather than a relative change.

2017 ATS PFT Reporting Standardization

Sun, 10 Dec 2017 00:00:00 +0000

The ATS has released its first standard for reporting pulmonary function results. This report is in the December 1, 2017 issue of the American Journal of Respiratory and Critical Care Medicine. At the present time however, despite its importance it is not an open access article and you must either be a member of the ATS or pay a fee ($25) in order to access it. Hopefully, it will soon be included with the other open access ATS/ERS standards .

Making Assumptions about TGV and FRC

Fri, 01 Dec 2017 00:00:00 +0000

When lung volumes are measured in a plethysmograph the actual measurement is called the Thoracic Gas Volume (TGV). This is the volume of air in the lung at the time the shutter closes and the subject performs a panting maneuver . Ideally, the TGV measurement should be made at end-exhalation and should be approximately equal to the Functional Residual Capacity (FRC). For any number of reasons in both manual and automated systems this doesn’t happen and the point at which the TGV is measured is either above or below the FRC.

The effect of errors in Inspiratory Volume on DLCO.

Sun, 19 Nov 2017 00:00:00 +0000

Yesterday while reviewing reports I ran across an interesting error in the Inspiratory Volume (VI) from a DLCO test. I’ve probably seen this before but this time I realized what effect it could have on DLCO. Specifically, what I saw was that at the start of the DLCO test the subject had not finished exhaling and although the technician had started the test, the subject continued to exhale.

What makes this interesting is that the software used the subject’s volume at the start of the test as the initial volume. This means that the software measured the VI from the initial volume to the end of inspiration, not from the point at which the subject stopped exhaling to the end of inspiration. This also means that the VI was underestimated by 0.20 L and this affects both VA and the calculated DLCO.

DLCO, de-constructed

Wed, 15 Nov 2017 00:00:00 +0000

My wife watches the Food Network a lot and I occasionally watch it with her but I can only take so much of it before I go off and read or work on one of my projects. I’ve noticed however in the various cooking contests that sometimes a chef will deconstruct a familiar recipe. This more or less means they break the recipe down into its components and present them as separate pieces or perhaps by putting what goes inside on the outside instead.

COHb and Pulse Oximetry

Tue, 07 Nov 2017 00:00:00 +0000

I was reviewing a report recently that included the results for walking oximetry. These showed that the individual has a resting SaO2 of 97% and desaturated significantly to 86% after walking a couple hundred yards. This was curious since a DLCO had also been performed and the results for that test were 94% of predicted. It’s unusual for somebody with a normal DLCO to have that low of an SaO2 but I have seen it before in individuals who were unable to ventilate adequately because of a paralyzed diaphragm. I’ve also seen it happen sometimes when somebody has a peripheral vascular disease like Reynaud’s that produces a poor quality oximeter signal. Buried in the technician’s notes however, was an additional piece of information that called into question both the resting and the exercise SaO2 readings. Specifically, the notes mentioned that an ABG had been performed and that the subject’s COHb was 9%.

Why the FEV1/FVC ratio LLN as a percent of the predicted FEV1/FVC ratio is important

Sun, 08 Oct 2017 00:00:00 +0000

My medical director and I had a discussion today about where the cutoff for a normal FEV1/FVC ratio would be for a 93 year old patient of his. Part of the problem is that there are almost no reference equations for patients this age and the best you can usually do is to extrapolate. Another part is that anybody in their 90’s is a survivor and must have had good lung function throughout their life to reach that age, which means that they aren’t average so it’s not clear how well extrapolation actually works in this population. The final part is that the guidelines for PFT interpretation that are used by my lab were put into place about 40 years ago and reflect the thoughts at that time. I updated part of the guidelines with the 2005 ATS/ERS interpretation algorithm about 10 years ago, but the thresholds for normalcy (as well as the reference equations we use) still haven’t changed all that much. I’ve brought this issue up a number of times over the years (usually every time I get a new medical director) but haven’t gotten a consensus from the pulmonary physicians on either the need for change or for what threshold values should be used.

What’s normal about the GLI DLCO reference values?

Tue, 03 Oct 2017 00:00:00 +0000

The Global Lung Initiative (GLI) has been working for several years to develop a universal reference equation for DLCO. Although this endeavor is not necessarily complete, an article describing the GLI DLCO reference equation for Caucasians was published in the September issue of the European Respiratory Journal as an open access article and can be downloaded by anyone. The Global Lung Initiative in general and the authors of the article more particularly are to be commended for this monumental work and for the insight it brings to understanding the normal distribution of DLCO.

What’s wrong with an elevated DLCO?

Sun, 17 Sep 2017 00:00:00 +0000

Well, not necessarily anything, although as usual that depends on the circumstances. Recently I was contacted by an individual who was concerned that their DLCO had decreased from 120% of predicted to 99% of predicted. They also mentioned that their DLCO results have normally ranged from 117% to 140% of predicted over the last 9 months.

More interestingly however, they said that

“the technician told me before I even took the test that anything over 100% for DLCO is essentially a testing error.”

Should biological quality control be replaced?

Sun, 03 Sep 2017 00:00:00 +0000

I’ve been thinking about quality control and quality improvement lately. Mostly this has been about how to go about determining whether the lab has a quality problem with testing and what statistics should be used for this purpose but I was reminded recently about an issue concerning biological quality control that came up a couple months ago on the AARC diagnostics forum. Specifically, one of the participants noted that some of their technicians had refused to perform biological QC on the basis that it violated their HIPAA rights to the privacy of their medical information. Further discussion noted that this was actually a correct interpretation of the HIPAA regulations and that no PFT lab can “force” its technicians to perform biological QC.

Getting more out of the LCI with Scond and Sacin

Sun, 27 Aug 2017 00:00:00 +0000

The Lung Clearance Index (LCI) is a relatively simple test that provides a measure of ventilation inhomogeneity within the lung. This can be clinically useful information since several studies have shown that increases in LCI often precede decreases in FEV1 in cystic fibrosis and post-lung transplant. LCI results are only a general index into ventilation inhomegeneity however, and other than showing its presence, does not give any further information about its cause or location.

Oscillometry

Sun, 13 Aug 2017 00:00:00 +0000

A month or two ago in the AARC Diagnostics forum several members noted that their labs had acquired Impulse Oscillometry systems a number of years ago but that their physicians had since stopped ordering oscillometry tests, mostly because nobody understood what it was measuring and didn’t know how to interpret the results. There are a number of reasons why this is probably not an uncommon scenario and why, despite being first described in 1956, oscillometry is not used more widely.

Eye was fooled

Sat, 29 Jul 2017 00:00:00 +0000

A couple of days ago I was reviewing (triaging, actually) the spirometry portion of a full panel of PFTs performed with pretty terrible test quality and was trying to decide if the technician responsible for performing the tests had made the right selections from the patient’s test results. I noticed that the FEV1 that had been selected was actually the lowest FEV1 from the all the spirometry efforts the patient made, and was trying to decide whether this was really the correct choice. We use peak flow to help determine which FEV1 to select and that particular spirometry effort appeared to have the highest and sharpest peak flow by a large margin:

PFTs on YouTube

Sun, 23 Jul 2017 00:00:00 +0000

A friend recently sent me the links to several YouTube videos on pulmonary function testing. I’ve spent some time off and on over the last year looking at YouTube videos and in particular I’ve been looking for ones that can be used as part of technician education. Maybe I’ve set the bar too high but all too often I’ve been disappointed and frustrated with what I’ve found. One reason for this is that many videos are aimed at other audiences than technicians (i.e. medical students, physicians, patients). Another reason is that too often only simple concepts are presented, often in rote fashion and often without good visual explanations (c’mon, these are videos after all, not podcasts). A final reason is that sometimes they’re outdated, misleading or just plain wrong.

Z Score to remember is -1.645

Sun, 16 Jul 2017 00:00:00 +0000

The use of Z scores to report PFT results, both clinically and for research is occurring more and more frequently. Both the Z score and the Lower Limit of Normal (LLN) come from the same roots and in that sense can be said to be saying much the same thing. The difference between the two however, is in the emphasis each places on how results are analyzed. The LLN primarily emphasizes only whether a result is normal or abnormal. The Z score is instead a description of how far a result is from the mean value and therefore emphasizes the probability that a result is normal or abnormal.

Contraindications

Sun, 09 Jul 2017 00:00:00 +0000

A couple weeks ago I was asked whether it was safe for a patient with an abdominal aortic aneurysm (AAA) to have pulmonary function testing. My first thought was that it was probably unsafe but after a moment or two of thought I realized that I hadn’t reviewed the subject for a long time. When I checked the 2005 ATS/ERS general testing guidelines (there are no contraindications in the 2005 spirometry guidelines) I found that AAA wasn’t mentioned at all. In fact, the only absolute contraindication mentioned was that patients with a recent myocardial infarction (<1 month) should not be tested. Some relative contraindications were mentioned:

Assessing post-BD improvement in FEV1 and FVC as a percent of the predicted

Sun, 02 Jul 2017 00:00:00 +0000

The 2005 ATS/ERS standards for assessing post-bronchodilator changes in FVC and FEV1 have been criticized numerous times. A recent article in the May issue of Chest (Quanjer et al) has taken it to task on two specific points:

the change in FVC and FEV1 has to be at least 200 ml
the change is assessed based on the percent change (≥12%) from the baseline value

The article points out that the 200 ml minimum change requires a proportionally larger change for a positive bronchodilator response in the short and the elderly. Additionally, by basing the post-BD change on the baseline value it lowers the threshold (in terms of an absolute change) for a positive bronchodilator response as airway obstruction become more severe. As a way of mitigating these problems the article recommends looking at the post-bronchodilator change as a percent of predicted rather than as a percent of baseline.

What do you do when the predicted is zero?

Tue, 27 Jun 2017 00:00:00 +0000

A very strange spirometry report came across my desk a couple of days ago.

	Observed:	Predicted:	%Predicted:
FVC:	3.07	0	29767
FEV1:	2.15	0	37586
FEV1/FVC:	70	71	101%

My first thought was that some of the demographics information had been entered incorrectly but when I checked the patient’s age, height, gender and race all were present, all were reasonably within the normal range for human beings in general and more importantly, all agreed with what was in the hospital’s database for the patient. I tried changing the patient’s height, age, race and gender to see if it would make a difference and although this made small changes in the percent predicted when I did this the predicteds were still zero.

The effects of anemia on exercise

Mon, 22 May 2017 00:00:00 +0000

Last week I was reviewing the exercise test results from a patient that appeared to have a relatively straightforward cardiovascular limitation when I noticed the patient also had severe anemia (Hgb = 7.1). Once that fact came up it was no longer clear the patient actually had a cardiac limitation at all.

First the results:

	Rest:	%Predicted:	AT:	%Predicted:	Max:	%Predicted:
VO2 (LPM):	0.33	13%	0.73	28%	1.45	56%
VO2 (ml/kg/min):	5.0		11.0		21.6
VCO2 (LPM)	0.26		0.63		1.81
RER:	0.73		0.83		1.24
SaO2:	98%		97%		97%

PetCO2:	35.2		38.6		31.8
Ve/VO2:	34		26		43
Ve/VCO2:	47		31		35

Ve (LPM):	11.6	8%	19.2	13%	62.9	44%
Vt (L):	0.78		1.29		2.19
RR:	15		15		29

HR (BPM):	61	35%	92	52%	152	85%
BP (mmHg):	92/62				102/64
O2 Pulse (ml/beat):	5.8	39%	8.2	55%	9.8	66%

Other values calculated from the data include:

Is there such a thing as a normal decrease when the FEV1 isn’t normal?

Sun, 14 May 2017 00:00:00 +0000

I’ve mentioned before that my lab’s database goes back to 1990, so we now have 27 years of test results available for trending. At least a couple times a week we have a patient who was last seen 10 or even 20 years ago. When I review their results I try to see if there has been any significant change from their last tests. Since the last tests are often quite some time in the past the changes in an absolute sense are often noticeably large. The question then becomes whether or not these changes are normal.

What does an inverse I:E Ratio during exercise mean?

Sun, 07 May 2017 00:00:00 +0000

Inspiration and expiration usually take different lengths of time, with inspiration almost always being shorter than exhalation. This is due to both to the physiology of breathing and to the pathophysiology of disease processes. During incremental exercise testing there are usually patterns to the way that inspiratory and expiratory times change and these are occasionally diagnostic.

When I started in this field the relationship between inspiratory and expiratory time was usually expressed as the I:E ratio, which was most often written as something like 1:1.2. One of my medical directors pointed out to me that when talking about I:E ratio it was difficult to determine what you meant if you said it was increasing or decreasing. For this reason I started reporting the I:E ratio as the E/I ratio so that instead of 1:1.2 it’s just 1.2.

2017 ERS Methacholine Challenge Standards

Fri, 05 May 2017 00:00:00 +0000

After a couple years of waiting for the new methacholine standards to be released “any day now”, they were finally published in this month’s issue of the European Respiratory Journal. The standard is an open access article and can be downloaded by anyone.

The length of time taken to develop the standard was acknowledged and although active ATS participation was withdrawn because the original timeline was not met, for the most part the original ATS participants continued with the task group and the standard has been officially endorsed by the ATS.

Measuring respiratory resistance with the APD

Sun, 16 Apr 2017 00:00:00 +0000

Inspiratory and expiratory flow rates are a function of driving pressure (i.e. the pressure difference between the alveoli and the atmosphere) and airway resistance. For this reason it would seem that airway resistance should be one of the most commonly performed pulmonary function tests but instead it is the outcome of airway resistance and driving pressure, i.e. the expiratory and inspiratory flow rates that are measured almost exclusively. One reason for this is that resistance measurements requires relatively expensive equipment such as a body plethysmograph or an impulse oscillometer as well as a fair amount of technical expertise.

Social Security Disability Evaluations

Sun, 09 Apr 2017 00:00:00 +0000

I was contacted recently by an individual with some questions about the pulmonary function testing needed for a Social Security Disability evaluation. With a small amount of research I was able to answer their questions but this brought up an interesting point and that is that despite the number of patients we see every year my lab only rarely performs any pulmonary function testing for disability evaluations. The reason I know this is because the Social Security Administration (SSA) has very specific requirements for the content and form of pulmonary function reports and we are very rarely asked for these reports.

CPT Codes

Sun, 02 Apr 2017 00:00:00 +0000

About a month or so ago I was corresponding with the manager of a small PFT lab and in response to one of their questions I had mentioned that there were no CPT codes for MIP/MEP. They responded with “what’s a CPT code?” so I guess this means that CPT codes aren’t as well known as I thought they were.

CPT stands for Current Procedural Terminology and is managed by the American Medical Association. CPT codes are a relatively universal way to classify and describe all medical tests and procedures. They are also used by all insurance companies for medical billing so one downside to this is if there isn’t a CPT code for a test or a procedure, you can’t bill for it. CPT codes also include conditions that limit performing (or at least billing for) some tests in various combinations and to some extent this drives the way PFT tests are ordered and performed.

Another post-BD FVC conundrum

Sun, 26 Mar 2017 00:00:00 +0000

Okay, this may be wrong but at the moment I’m can’t seem to find a reason why it should be. A report like this came across my desk a couple of days ago.

	Observed:	%Predicted:	Post-BD:	%Predicted:	%Change:
FVC:	4.59	94%	4.87	100%	+6%
FEV1:	3.38	89%	3.58	94%	+6%
FEV1/FVC:	73.6	95%	73.5	95%	0

Not particularly unusual and it would usually be interpreted as being within normal limits without a significant post-BD change. If you calculate the FEV1/VC ratio using the pre-BD FEV1 and the post-BD FVC however, it’s 89% of predicted and this indicates mild airway obstruction. But you’re not supposed to use the post-BD FVC this way, are you?

When is it an expiratory plateau?

Tue, 21 Mar 2017 00:00:00 +0000

Over the last couple of weeks I’ve had an unusual number of patients with expiratory plateaus on their flow-volume loops. Expiratory plateaus are usually considered to be a sign of an intrathoracic central or upper airway obstruction and several of these patients had a diagnosis of tracheomalacia but many of them didn’t. Expiratory (and inspiratory) plateaus are mentioned in the ATS/ERS standards for interpretation but since there isn’t a specific definition (other than “plateau”), an expiratory plateau is a “know it when you see it” sort of thing.

Normal or obstruction?

Sun, 12 Mar 2017 00:00:00 +0000

I had finished reviewing a pre- and post-BD spirometry report yesterday and was about to toss it on my out pile when I noticed something a bit odd about the post-BD results. I pulled it back and spent some time trying to decide if the interpretation needed to be changed but after a lot of internal debate I finally let it go as it was. I’ve continued to think about it however, and although I’m not sure that was the right decision I still haven’t come up with a clear answer.

An unusual error in helium dilution lung volumes

Wed, 01 Mar 2017 00:00:00 +0000

Recently I was reviewing a report that included helium dilution lung volumes. What caught my eye was that the TLC and the FRC didn’t particularly fit in with the results from the other tests the patient had performed.

Test:	Observed:	%Predicted:
FVC:	2.83	114%

TLC:	3.03	71%
FRC:	0.88	39%
RV:	0.09	5%
SVC:	2.93	118%

VA:	3.64	88%

When compared to the FVC and the VA (from the DLCO test) the lung volumes are significantly lower. In particular the FRC and RV are markedly reduced. This is somewhat unusual for helium dilution lung volume since most errors usually cause FRC, RV and TLC to be over-estimated instead of being under-estimated. When I checked the other reports for the day I found that two other patients that had had their lung volumes measured on the same test system also had a TLC, FRC and RV that were noticeably reduced. Obviously we had some kind of equipment problem with that test system but it took a bit of sleuthing before I found out what had happened.

2017 ERS DLNO standards

Sun, 19 Feb 2017 00:00:00 +0000

The European Respiratory Society has just published the first standards for DLNO testing. This is a signal that DLNO is moving from a research setting into routine clinical testing. Although it is unlikely that most PFT labs will immediately jump into DLNO testing, the standard is still interesting because of an extensive discussion of DLNO, DLCO, DMCO and Vc measurements and physiology. The DLNO standards (and their supplementary material) are open-access and can be downloaded from the European Respiratory Journal .

Some DLCO errors the 2017 standards will probably fix

Sun, 12 Feb 2017 00:00:00 +0000

Last week I ran across a couple errors in some DLCO tests that I don’t remember seeing before, or at least not as distinctly as they appeared this time. If I hadn’t been looking carefully I could have missed them but both sets of errors will be a lot more evident when the 2017 ERS/ATS DLCO standards are implemented.

The first error has to do with gas analyzer offsets. What alerted me was a set of irreproducible DLCO results.

What’s the frequency, plethysmograph?

Sun, 05 Feb 2017 00:00:00 +0000

Once again we’ve had some staff turnover. Rightly or wrongly, the pattern we follow in staffing the lab is to hire people with a science degree and then train them ourselves. Our hires are usually interested in a career in medicine but often haven’t decided what specifically interests them. We look for individuals with people skills on top of their education and ask for a minimum of a year’s commitment with the requirement that they get their CPFT certification by the end of the year. Sometimes our staff only stays a year, sometimes a couple years, and most of the time when they leave they go back to college for a more advanced degree and become nurses or physician assistants and occasionally even physicians (a couple of our pulmonary fellows were former PFT lab alumni).

Which time is it?

Thu, 26 Jan 2017 00:00:00 +0000

The ATS/ERS standard for spirometry recommends reporting the highest FEV1 and the highest FVC even when they come from different tests. Our lab software allows us to do this, but only with some annoying limitations. One of the bigger limitations has to do with how expiratory time is reported. In particular, expiratory time is lumped in with a number of other values like Peak Flow (PEF) and FEF25-75. As importantly, the flow-volume loop and volume-time curve can only come from a single effort.

Calculating VA the mass balance way

Sun, 15 Jan 2017 00:00:00 +0000

One of the more significant changes that appeared in the 2017 ERS/ATS DLCO standards was the requirement that rapid-response gas analyzer (RGA) systems calculate VA using a mass balance approach. This is actually more straightforward than it sounds but it does raise several issues that weren’t fully addressed in the 2017 standards.

Up until this time VA has been calculated from the inspired volume and by the amount of dilution of the tracer gas in the exhaled alveolar sample. Specifically:

ERS/ATS 2017 DLCO standards

Sat, 07 Jan 2017 00:00:00 +0000

The new ERS/ATS standards for DLCO testing were published in the January issue of the European Respiratory Journal. The article was published as open access and can be downloaded from the ERJ website.

The biggest difference between the new standards and those from 2005 is that they are now primarily oriented towards Rapid-response Gas Analyzers (RGA). The authors explicitly state that the new standards do not make older systems that use discrete alveolar sampling and slower gas analyzers obsolete, but many of the new suggestions and requirements for labs and manufacturers require systems with a RGA.

New Year’s Resolutions for a better PFT lab

Sat, 31 Dec 2016 00:00:00 +0000

It’s a tradition to come up with New Year’s resolution in order to improve ourselves. How about some resolutions to improve our labs?

1. Review and update the procedure manual

When was the last time you reviewed your procedure manual? Procedure manuals should be reviewed by the lab manager and medical director annually. It’s time to re-read the ATS/ERS guidelines and then review and update your procedure manual. Both your old staff and your new staff need to know what to do and how to do it. Your procedure manual is also going to be the first thing that anybody looks at if your lab is ever inspected.

Is there airway obstruction when the FEV1 is normal?

Thu, 29 Dec 2016 00:00:00 +0000

I’ve been reviewing the literature on PFT interpretation lately and in doing so I ran across one of the issues that’s bothered me for a while. Specifically, my lab has been tasked with following the 2005 ATS/ERS guidelines for interpretation and using this algorithm these results:

	Observed:	%Predicted:	LLN:	Predicted:
FVC:	2.83	120%	1.76	2.36
FEV1:	1.77	100%	1.26	1.76
FEV1/FVC:	63	84%	65	75

would be read as mild airway obstruction.

Fick Cardiac Output

Sun, 18 Dec 2016 00:00:00 +0000

We’ve all run across the Fick equation for cardiac output at one time or another. There are very limited circumstances when we’d ever get to use it but at the same time it’s one of those simple but incredibly profound equations that’s also a foundation of pulmonary physiology.

The Fick equation is:

where:

VO2 = oxygen uptake

CvO2 = mixed venous oxygen content

CaO2 = arterial oxygen content

And what it describes is:

N2 washout troubleshooting

Sun, 11 Dec 2016 00:00:00 +0000

I was recently contacted by the manager of a lab that was having problems with their N2 washout lung volumes. Specifically, their N2 washout lung volumes (FRC in particular) were coming out low and everyone being tested on the system looked like they had restriction. The system has been checked by the manufacturer’s service techs several times and they’d replaced the tubing, the O2 tank and a number of parts. Service first asked them to wait between tests and then not to bother. Most lately they’ve been asked to calibrate the system before each test. Despite all this, their system continues to under-estimate lung volumes.

Estimated Lung Age (ELA)

Sun, 27 Nov 2016 00:00:00 +0000

Cigarette smoking raises the probability that an individual will get lung cancer, chronic bronchitis and/or emphysema (among many other things). Nicotine is addictive and smokers often need significant motivation in order to quit. Lung age is a tool that was designed to give smokers an additional incentive to do this. The concept is fairly simple and that is by reformulating an FEV1 reference equation it is possible to take an individual’s actual FEV1 and estimate the age of their lungs (ELA). Because cigarette smoking can cause airway obstruction it tends to mimic premature lung aging which means that when a smoker’s FEV1 is used to calculate an ELA it can be significantly greater than their real or chronological lung age (CLA).

Flow-volume loops are timeless

Sun, 20 Nov 2016 00:00:00 +0000

Recently I’ve been trying to help somebody whose spirometry results changed drastically depending on where their tests were performed. When their spirometry was performed on an office spirometer their FVC was less than 60% of predicted and when they were performed in a PFT lab on a multi-purpose test system their FVC was closer to 90% of predicted. Part of the reason for this was that different predicted equations are being used in each location but even so there was about a 1.5 liter difference in FVC.

IC, ERV and the FVC

Sun, 13 Nov 2016 00:00:00 +0000

While reviewing reports today I ran across a couple of lung volume tests from different patients where the SVC was over a liter less than the FVC. Suboptimal SVC measurement can affect both the TLC and the RV and in one case the TLC was slightly below normal (78% of predicted) and in the other the TLC was within normal limits but the RV was over 150% of predicted. Both patients had had lung volume measurements previously and the current TLC was significantly different than it had been before.

The dual tracer gas single-breath washout (DTG-SBW) and ventilation inhomogeneity

Sun, 30 Oct 2016 00:00:00 +0000

I’ve been interested in ventilation inhomogeneity for a while and as ways to measure it I have looked at VA/TLC ratios , the Lung Clearance Index (LCI) and the phase III slope of the single-breath N2 washout (SIIIN2) . All of these tests are able to provide some information about ventilation inhomogeneity but each has their own limitations and just as importantly although their results have a relatively clear relationship with ventilation inhomogeneity it’s not quite as clear what exactly it is they are measuring. A friend recently pointed me to an on-line article in Chest that discusses the dual-tracer single-breath washout test in patients with COPD. The apparent advantage of this test is that it is able to provide information about the site of the ventilation inhomogeneity. Although dual tracer gases have been used to study airway function for over 50 years the limitation of this technique has been the need to use a mass spectrometer. Some recent advances in technology have made it possible for this type of testing to be performed with a significantly less expensive gas analyzer and this has revived an interest in the dual-tracer gas single-breath washout (DTG-SBW).

Why DIY CPET reports?

Sun, 23 Oct 2016 00:00:00 +0000

When I first started performing CPETs in the 1970’s a patient’s exhaled gas was collected at intervals during the test in Douglas bags and I had a worksheet that I’d use to record the patient’s respiratory rate, heart rate and SaO2. After the test was over I’d analyze the gas concentrations with a mass spectrometer and the gas volumes with a 300 liter Tissot spirometer and then use the results from these to hand calculate VO2, VCO2, Rq, tidal volume and minute volume. These results were then passed on to the lab’s medical director who’d use them when dictating a report.

The FVC/DLCO ratio. Will the real percent predicted please stand up?

Sun, 09 Oct 2016 00:00:00 +0000

Recently a reader asked me a question about the FVC/DLCO ratio. To be honest I’d never heard of this ratio before which got me intrigued so I spent some time researching it. What I found leaves me concerned that a lack of understanding about reference equations may invalidate several dozen interrelated studies published over the last two decades.

Strictly speaking the FVC/DLCO ratio is the %predicted FVC/%predicted DLCO ratio (which is usually abbreviated FVC%/DLCO%) and it appears to be used exclusively by specialists involved in the treatment of systemic sclerosis and related disorders. Specifically, the ratio is used to determine whether or not a patient has pulmonary hypertension. The basic idea is that (quoting from a letter to the editor):

When no change is a change, or is it?

Sun, 02 Oct 2016 00:00:00 +0000

I was reviewing a spirometry report last week and when I went to compare the results with the patient’s last visit I noticed that the FVC and FEV1 hadn’t changed significantly. However, the previous results were from 2009 and when the percent predicted is considered there may have been a significant improvement.

2009	Observed:	%Predicted:
FVC:	2.58	87%
FEV1:	1.60	72%
FEV1/FVC:	62	82%

2016	Observed:	%Predicted:
FVC:	2.82	104%
FEV1:	1.65	82%
FEV1/FVC:	59	79%

The answer to whether or not there was an improvement would appear to depend on what changes you’d normally expect to see in the FVC and FEV1 over a time span of 7 years. The FVC and FEV1 normally peaks around age 20 to 25 and then declines thereafter.

Top 10 spirometry errors and mistakes

Sun, 25 Sep 2016 00:00:00 +0000

A couple of days ago my medical director and I had a short discussion about teaching pulmonary fellows to read PFTs and agreed that in order to be good at interpreting PFTs it isn’t the basic algorithms that are hard, it’s gaining an understanding of test quality and testing problems. My medical director then suggested this topic. At first I wasn’t sure I could find 10 errors but after spending a couple hours digging through my teaching files I managed to come up with just a few more than that. So strictly speaking it’s not a top 10 list but I kept the title because I liked it.

Which DLCO should be reported?

Sun, 18 Sep 2016 00:00:00 +0000

I like to think my lab is better than most but every so often something comes along that makes me realize I’m probably only fooling my self.

Earlier this week I was reviewing the DLCO test data for a patient with interstitial lung disease. At first glance the spirometry and DLCO results pretty much matched the diagnosis and I had already seen they weren’t significantly different from the last visit. The technician had written “fair DLCO reproducibility” which was reason enough to review the test data but I’ve actually been making a point of taking a careful look at all DLCO tests, not just the questionable ones, for the last couple of weeks. I took one look at the test data, put my head in my hands, and counted to ten before continuing.

When is a change in FVC significant?

Sun, 11 Sep 2016 00:00:00 +0000

Most of the COPD patients that are seen in my lab tend to have little change in their FEV1 from visit to visit but their FVC often changes significantly. A change in FVC is usually related to how long a patient is able to exhale and this in turn is usually related to how well they are feeling at the time. This would seem to imply that a significant change in FVC, particularly for a patient with COPD, is, if not clinically significant, at least clinically important even when the FEV1 hasn’t changed.

Six-Minute Walk with Helium-Oxygen

Wed, 07 Sep 2016 00:00:00 +0000

We recently performed a 6-minute walk test with helium-oxygen (heliox) for a patient of one of the physicians that specializes in airway stenting. His reasons for the test weren’t particularly clear (and he hasn’t bothered to try to clarify them with me) but most probably it has to do with differentiating between central and peripheral airway obstruction. Interestingly, he predicted the patient would have a significant improvement in 6-minute walk distance and instead there was little difference between the heliox 6MWT and one performed with 3 LPM supplemental O2.

Ventilatory response to hypoxia and hyperoxia

Sun, 28 Aug 2016 00:00:00 +0000

While reading a recently published article I found they had performed response to hypoxia and hyperoxia testing as part of the study. At one time or another in the past I’ve read about response to hypoxia testing but I’d never heard about hyperoxia testing before. I had some difficulty understanding their interpretation of the study’s results and for this reason I’ve spent some time reading up on the subject. I’m not sure this helped because there appears to be a lack of consensus not in only how to perform these tests but also in how they are interpreted, except perhaps in the most simplistic sense. Hypoxia and hyperoxia testing has been performed primarily to gain a deeper understanding of the way in which the peripheral (carotid) and central chemoreceptors function. There are a variety of sensor-feedback network models and results are often presented in terms of one model or another and this makes comparing results from different studies difficult. Interpretation and comparison is further complicated by the fact that results depend not only on the length of time that hypoxia or hyperoxia is maintained but whether the subject was exposed to hypoxia, hyperoxia or hypercapnia previously.

When hypoventilation is the primary CPET limitation

Sun, 21 Aug 2016 00:00:00 +0000

Hypoventilation is defined as ventilation below that which is needed to maintain adequate gas exchange. It can be a feature in lung diseases as diverse as chronic bronchitis and pulmonary fibrosis but determining whether it is present of not is often complicated by defects in gas exchange. When desaturation occurs during a CPET (i.e. a significant decrease in SaO2 below 95%) this is a strong indication that the primary exercise limitation is pulmonary in nature and from that point the maximum minute ventilation and the Ve-VCO2 slope can show whether the limitation is ventilatory or instead due to a gas exchange defect. But in this circumstance what what does it mean when both the maximum minute ventilation and Ve-VCO2 slope are normal?

Asleep at the wheel

Wed, 10 Aug 2016 00:00:00 +0000

During this last week I was contacted by two different individuals who were asking for help in understanding their PFT results. In both cases they had a markedly elevated TLC and the interpretation included the notation that they had gas trapping and hyperinflation. Even though the amount of information they provided was minimal I am extremely skeptical that the TLC measurements were correct.

Gas trapping usually only occurs with severe airway obstruction. Hyperinflation, which at minimum consists of a chronically elevated FRC and RV, usually only occurs after prolonged gas trapping. An elevated TLC usually occurs only with prolonged hyperinflation and given the improvements in the care and treatment of COPD I’ve seen over the last several decades, has become relatively uncommon.

Does the FEV1/SVC ratio over-diagnose airway obstruction?

Sun, 17 Jul 2016 00:00:00 +0000

A low FEV1/VC ratio is the primary indication for airway obstruction.

From ATS/ERS Interpretive Strategies for Lung Function tests, page 956.

The ATS/ERS statement on interpretation says

“The VC, FEV1, FEV1/VC ratio and TLC are the basic parameters used to properly interpret lung function (fig. 2). Although FVC is often used in place of VC, it is preferable to use the largest available VC, whether obtained on inspiration (IVC), slow expiration (SVC) or forced expiration (i.e. FVC).”

When flow-volume loops get kinky

Sun, 10 Jul 2016 00:00:00 +0000

One of the more recognizable flow-volume loop contours is the one associated with severe airway obstruction. Specifically, this type of loop shows an abrupt decrease in flow rate following the peak flow with a more gradual decrease in flow rates during the remainder of the exhalation.

This abrupt decrease in flow rates was first described on a volume-time curve and the inflection point was called a “kink” but this point also corresponds with the inflection point on the flow-volume loop. This feature has also been called a “notch” or a “spike” but a number of researchers have called this the Airway Collapse pattern (AC) and it is more formally defined as a sharp decrease in flow rate from peak flow to a discontinuity point at less than 50% of the peak flow and occurring within the first 25% of the exhaled vital capacity.

What’s a normal Flow-volume Loop?

Mon, 04 Jul 2016 00:00:00 +0000

Dozens of articles have been written about the correlation between different abnormal flow-volume loop contours and pulmonary disorders. In contrast very little has ever been written about what constitutes a normal flow-volume loop and what this looks like has been primarily anecdotal.

Interestingly, the ATS/ERS standard for spirometry includes an example of a “normal” flow-volume loop but its source and what makes it normal is not explained.

From the ATS/ERS standard on spirometry, page 327.

Selecting the best FEV1. What role should PEF play?

Sun, 26 Jun 2016 00:00:00 +0000

Recently my lab has had some turnover with a couple of older staff leaving and new staff coming on board. While reviewing reports I’ve found a number of instances where the incorrect FVC and FEV1 were reported. Taking these as “teachable moments” I’ve been annoying the staff with emails whenever I find something notably wrong. I had thought that our rules for selecting the best FVC and FEV1 were fairly straightforward but given the number of corrections I’ve made lately it seemed like it would be a good idea to revisit our policy on this subject.

A change that probably isn’t a change

Sun, 19 Jun 2016 00:00:00 +0000

Recently a report came across my desk from a patient being seen in the Tracheomalacia Clinic. The clinic is jointly operated by Cardio-Thoracic Surgery and Interventional Pulmonology and among other things they stent airways. The patient had been stented several months ago and this was a follow-up visit. Given this I expected to see an improvement in spirometry, which had happened (not a given, BTW, some people’s airways do not tolerate stenting), but what I didn’t expect to see was a significant improvement in lung volumes and DLCO.

COPD and the FEV1/FVC ratio. GOLD or LLN?

Sun, 12 Jun 2016 00:00:00 +0000

Everyone uses the FEV1/FVC ratio as the primary factor in determining the presence or absence of airway obstruction but there are differences of opinion about what value of FEV1/FVC should be used for this purpose. Currently there are two main schools of thought; those that advocate the use the GOLD fixed 70% ratio and those that instead advocate the use the lower limit of normal (LLN) for the FEV1/FVC ratio.

A glitch in time

Mon, 06 Jun 2016 00:00:00 +0000

This relatively odd DLCO testing error came across my desk today. Although it’s fairly unusual it brings up some interesting points about how the Breath-Holding Time (BHT) is determined and what effect it has on DLCO.

Specifically, at the beginning of the DLCO test the patient took a partial breath in, then exhaled, then took a complete breath in. The patient performed the DLCO test three times and did exactly the same thing each time despite being coached by the technician to only take a single breath in. I’m sure this says something about human nature but I’m not exactly sure what.

Re-branding, re-imagining and re-defining ourselves

Sun, 29 May 2016 00:00:00 +0000

This idea originates, as far as I know, from Michael Sims, president and CEO of NspireHealth. I got it second hand and suspect that it is a small part of a larger presentation but this one point is worth discussing by itself. Specifically, we call the places we work “Pulmonary Function Laboratories” and this is at best an outdated and somewhat obscure term that doesn’t do much to make it clear what we do.

Exercise and the IC, EELV and Vt/IC ratio

Sun, 22 May 2016 00:00:00 +0000

Determining whether a subject has a ventilatory limitation to exercise used to be fairly simple since it was based solely on the maximum minute ventilation (Ve) as a percent of predicted. There has been some mild controversy about how the predicted maximum ventilation is derived (FEV1 x 35, FEV1 x 40 or measured MVV) but these don’t affect the overall approach. Several decades ago however, it was realized that subjects with COPD tended to hyperinflate when their ventilation increased and that this hyperinflation could act to limit their maximum ventilation at levels below that predicted by minute ventilation alone.

Assessing changes in DLCO

Sun, 15 May 2016 00:00:00 +0000

We have a number of patients who have spirometry and DLCO testing performed at regular intervals. I’ve noticed that every so often DLCO results change significantly without a change in spirometry (or lung volumes) or there’s a modest change in spirometry and a marked change in DLCO. I’ve been concerned that this may be a symptom of problems with our DLCO (CO/CH4) gas analyzers and at least once recently this kind of discrepancy did lead to having an analyzer being serviced. Realistically though, the gas analyzers are routinely passing their calibrations and when I look at the trends in calibration there hasn’t been any systematic drift. This doesn’t rule out intermittent problems however, so in order to find out whether these changes in DLCO are “real” or an artifact of our testing systems I decided to see if taking a closer look at the results would help resolve this.

It doesn’t make any sense

Sun, 08 May 2016 00:00:00 +0000

For a variety of reasons my wife recently had a full panel of PFTs (spiro+BD, lung volumes, DLCO) at a different hospital than the one I work at. I went with her and was pleased to see the technician perform the tests pleasantly, competently and thoroughly. I was able to glance at the results as the testing proceeded so I had a fairly good idea what the overall picture looked like by the time she was done.

Ventilatory Challenge Testing

Wed, 04 May 2016 00:00:00 +0000

Airway hyper-responsiveness is a primary feature of asthma. There are a number of bronchial challenge tests designed to evoke and measure this factor, the most common of which require the inhalation of one or another bronchoconstrictive agent such as methacholine, histamine, mannitol or hypertonic saline.

An elevated ventilation can cause many asthmatics to bronchoconstrict and this is often the cause of Exercise-Induced Bronchospasm (EIB). There are two competing theories as to why this happens. A number of researchers have suggested that the mechanism is a drying of the airway mucosa which changes the osmolarity of the respiratory tract fluid which in turn causes some cells to releases mediators that cause bronchoconstriction. Other researchers assert that it is the cooling of the airways during hyperventilation and an increased blood flow and edema during subsequent re-warming that causes the bronchoconstriction. There is evidence to support both interpretations and it is likely that both mechanisms coexist, with one or the other being more predominant in any given individual.

Airway obstruction and the FVC

Sun, 17 Apr 2016 00:00:00 +0000

Spirometry is the most commonly performed (and mis-performed) pulmonary function test around the world. The apparent simplicity of spirometry is misleading since there are numerous subtleties that have a significant effect on the results.

I suspect that when the FVC is thought about it is most often considered to be an index towards the total capacity of the lung. That’s certainly true in it’s own way, but the FVC is actually a critically important factor when determining airway obstruction. I’ve had a number of reports across my desk lately where the patient had a reasonably large change in FVC when compared to their last visit but little change in FEV1, and this has made a difference in how the results are interpreted. For example:

LLN versus 80%

Sun, 10 Apr 2016 00:00:00 +0000

Recently a rather eminent reader commented on an older blog entry. He finished his comment with a paragraph on another topic, however. Specifically:

By the way, it is also high time that we scuttle the habit of expressing a measurement as percent of predicted. As Sobol wrote [5]: “It implies that all functions in pulmonary physiology have a variance around the predicted, which is a fixed per cent of predicted. Nowhere else in medicine is such a naive view taken of the limit of normal.”

State Licensure requirements for PFT Testing

Sun, 03 Apr 2016 00:00:00 +0000

There has been a fair amount of confusion about PFT lab staff licensure requirements. This information is not available on the AARC website, nor on any of the AARC state society websites. A month or so ago I reached out to all of the AARC state societies but received responses from only a handful of them. I was recently able to complete this research however, by visiting the websites of the remaining state licensing boards and state legislatures.

Short efforts, gas trapping and leaks

Sun, 20 Mar 2016 00:00:00 +0000

Outside the pulmonary lab there is this notion that spirometry is supposed to be so easy that anyone can do it. You just tell the patient to take a deep breath in and blow out fast and to keep blowing until they’re empty. What’s so hard about that?

Sheesh. GIGO. I keep finding ways in which the patient, their physiology and our equipment can conspire in ways to produce errors that even experienced technicians can miss. I’ve been paying a lot of attention to flow-volume loops lately and maybe it’s for this reason that I’ve seen a steady stream of spirometry tests that have something wrong with the FVC volume.

FVC measurements that are mostly gone but not completely forgotten

Sun, 13 Mar 2016 00:00:00 +0000

When Tiffeneau described the FEV1 in 1947 and Gaensler the timed FVC and FEV1/FVC ratio in 1950 this opened up an entirely new territory for pulmonary investigators to explore. Numerous new measurements were rapidly mapped out with often conflicting titles. The situation became confusing enough that the British Thoracic Society met in 1956 specifically to standardize terminology. At this time only the volume-time curve was available for measurement purposes (usually a pen trace on kymograph paper) and this fact helped determine what measurements it was possible for researchers to make.

An FVC is not an SVC

Sun, 06 Mar 2016 00:00:00 +0000

I’ve discussed the issue of inserting a predicted FVC into the predicted lung volumes several times now. At the risk of beating this issue to death I’d like to put to rest the notion that an FVC and an SVC are the same thing.

A Forced Vital Capacity (FVC) maneuver is designed to measure the maximum expiratory flow rates, in particular the expired volume in 1 second (FEV1). It has long been recognized that the effort involved in the FVC maneuver can cause early airway closure, even in individuals with normal lungs, and that for this reason the vital capacity can be underestimated due to gas trapping. This effect is usually magnified with increasing age and in individuals with obstructive lung disease.

The ratio-nal approach to predicted TLC

Sun, 28 Feb 2016 00:00:00 +0000

I’ve been reading Miller et al’s Laboratory evaluation of Pulmonary Function which was published in 1987. That was an interesting time since PFT equipment manufacturers had mostly transitioned to computerized systems but there were still a lot of manual systems in the field. For this reason the book’s instructions are still oriented mostly around manual pulmonary function testing and there are numerous warnings about double-checking the results from automated systems.

Underutilized spirometry, missed opportunities

Sun, 21 Feb 2016 00:00:00 +0000

A friend is taking her father to a PFT lab (2500 miles away from where I am the moment so I couldn’t go along with them) because he has been short of breath for a couple of years, but oddly enough, only when lying on his side. I expect that despite these rather specific symptoms he will only get routine spirometry. I don’t necessarily fault the PFT Lab he’s going to for this, partly because physician orders often don’t include specifics, partly because they may not have the facilities to perform supine or lateral spirometry, and partly because its not clear lateral spirometry would show anything.

Turbine spirometers

Tue, 16 Feb 2016 00:00:00 +0000

Turbine spirometers have been around in one form or another for well over a hundred years. The accuracy of the early versions of this type of spirometer was poor, partly because of the turbine designs weren’t terribly efficient and partly because these devices were mechanical in nature and the gear trains or other mechanical linkages added a lot of friction and resistance.

From: Nouveaux éléments de pathologie générale, de séméiologie et de diagnostic. by Eugène Bouchut, 1875, page 865. ]

Graphical Analysis of Flow-Volume Loops

Thu, 11 Feb 2016 00:00:00 +0000

I’ve been thinking a bit about the shape of flow-volume loops lately. In part this has been about ways to accurately describe them in reports; in part speculation about the information that may be embedded in them that isn’t in any of the routinely reported spirometry values; and in part about how the human eye perceives and categorizes them in a way that is difficult to simplify and put into a computer algorithm. A couple days ago I found a recent article where a geometrical analysis was applied to flow-volume loops in individuals with COPD and this got me curious about what other graphical techniques have been used to analyze flow-volume loops.

The importance of an earnest SVC

Thu, 04 Feb 2016 00:00:00 +0000

A report came across my desk today and at first glance it looked fairly straightforward. There was a mildly reduced TLC and FVC, and although the SVC was slightly lower than the FVC it looked like this patient had mild restriction.

	Observed:	%Predicted:	Predicted:
FVC:	1.73	68%	2.56
FEV1:	1.23	65%	1.89
FEV1/FVC:	71	97%	73

TLC:	3.58	73%	4.89
FRC:	2.07	75%	2.78
RV:	1.94	83%	2.33
RV/TLC:	54	114%	48
SVC:	1.69	66%	2.56
IC:	1.51	72%	2.11
ERV:	0.13	30%	0.45

In addition, the flow-volume loop looked fairly typical for restriction, with a normal peak flow and a reduced volume.

Shunt fraction

Wed, 27 Jan 2016 00:00:00 +0000

I was reading an article recently that made an off-hand reference to the 100% oxygen shunt fraction test. Results from the test were included in the data analysis but the equations the researchers used were not presented nor were they referenced, nor was the procedure described. This is probably because the shunt fraction test and its equations are very much old-school pulmonary physiology but even if the subject is probably covered at one time or another in physiology classes I suspect that some of the issues involved in the calculation are not as well understood as they should be.

6MWT re-visited, now with the MCID!

Wed, 20 Jan 2016 00:00:00 +0000

I often find topics for this blog in a sideways fashion. Recently while searching for something else I ran across an article about the minimum clinically important difference (MCID) of the Residual Volume (RV) in patients with emphysema. I’ve come across the MCID concept before but I had never really followed up on it. This time I started researching MCID and immediately ran across a number of articles about the MCID of the 6-minute walk test (6MWT). This got me to review the articles I have on hand and I found that since I last wrote about the 6MWT I’ve accumulated quite a few new (or at least new to me) reference equations as well as a number of articles about performance issues. Given all this how could I not re-visit the 6MWT?

Hidden FIVC and FVC. When all the data is relevent.

Sun, 03 Jan 2016 00:00:00 +0000

For the first dozen or so year that I worked in a pulmonary function lab it was with counter-weighted, volume-displacement water-seal spirometers more or less like this:

Patients would do a series of tests and I’d end up with a bunch of pen traces on kymograph paper that I’d have to measure with a ruler and use a desktop calculator (it was about a foot square, weighed a couple of pounds and had a nixie tube digital display) to create a hand-written report. I’m not going to suggest that these spirometers were in any way better than what we’re using now but I have to say that I would have seen the following problems more or less immediately.

RVD’s and OVD’s can’t mix without the FEV1/FVC ratio

Wed, 30 Dec 2015 00:00:00 +0000

The patients whose reports I review have always been very accommodating. An issue of one kind or another catches my attention and before I know it I find several more reports that are similarly involved. Thanks to our patients I’ve had a number of reports come across my desk recently that showed a combination of restrictive and obstructive defects. This particular one may not be the best possible example but it seems to illustrate several points fairly well.

Hepatopulmonary syndrome

Sun, 27 Dec 2015 00:00:00 +0000

My hospital has an active liver transplantation program and all transplant candidates get a full panel of PFTs in my lab. The number of liver transplant candidates we get varies from week to week but probably averages between 150 and 200 a year. As with any population a certain number of them have COPD or other lung diseases but there are some that have normal spirometry and lung volumes but a reduced DLCO. This latter group of patients likely have hepatopulmonary syndrome (HPS).

The clue was in the O2

Sun, 13 Dec 2015 00:00:00 +0000

One of the overlooked parts of teaching pulmonary function interpretation is developing an appreciation for the number and variety of errors that the equipment, patients and technicians can produce and how they affect the reported test results. I routinely run across a couple dozen errors each week while reviewing reports. Most are minor and do not significantly affect the reported results. Many are mundane because they appear so often and a few are interesting because they point out a particular limitation in the equipment, software or testing standards. I’ve kept a file of the more iconic examples of testing errors for years and a while ago a pulmonary staff physician and I used to hold weekly sessions for fellows and residents where we’d present a number of “zingers” to see if they could figure them out. Unfortunately that physician has moved on to a different institution and I’m no longer as available as I used to be so these sessions are no longer held but I think that they or something like them should be held in all teaching hospitals.

Goiter, upper airway obstruction and the flow-volume loop

Sun, 06 Dec 2015 00:00:00 +0000

The thyroid gland is located across the front of the upper airway a short distance below the larynx. An enlarged thyroid gland is known as a goiter. The most common worldwide cause of goiter is an iodine deficiency. This is much less common in the western nations where factors such as Hashimoto’s thyroiditis, Graves’ disease, multi-nodular thyroid disease, thyroid cancer, pregnancy and the side effects of some medications are the its primary causes. Common respiratory complaints associated with goiter include cough, hoarseness, shortness of breath and stridor.

The 8 Percent Solution

Thu, 03 Dec 2015 00:00:00 +0000

The current ATS/ERS standards for a positive bronchodilator response are an increase in FEV1 or FVC of ≥ 12% and ≥ 200 ml. These standards are largely based on the ability to detect a change that is far enough above the normal variability in FEV1 and FVC to be considered significant. One problem with this is that the amount of variability that is considered to be “normal” is overly influenced by a relatively small number of subjects that have a high degree of variability.

When does it end?

Sun, 11 Oct 2015 00:00:00 +0000

I was reviewing spirometry reports and noticed several patients in a row with an early termination of exhalation that was not reflected in the reported expiratory time. This is far from the first time I’ve noticed this but seeing several examples more or less at the same time got me curious about how the software was determining expiratory time.

The ATS/ERS standards are somewhat mute on how the total expiratory time of a spirometry effort is determined. The closest the standard comes to addressing this is the end-of-test (EOT) criteria:

Post-BD FVC. It’s about time.

Sun, 27 Sep 2015 00:00:00 +0000

When assessing the response to bronchodilators the change in FEV1 is used far more frequently than any other spirometry result. Other values such as inspiratory capacity (IC) and peak inspiratory flow (PIF) have also been proposed as indicators, but the ATS/ERS standards includes changes in FVC as well as changes in FEV1 and this is often overlooked. Specifically they:

“…recommend using the per cent change from baseline and absolute changes in FEV1 and/or FVC in an individual subject to identify a positive bronchodilator response. Values >12% and 200 mL compared with baseline during a single testing session suggest a ‘‘significant’’ bronchodilatation.”

Helium overshoot, revisited

Sun, 20 Sep 2015 00:00:00 +0000

A while back one of our technicians brought a helium dilution FRC graph to my attention and wanted to know if it showed a system leak. At that time my response was that it definitely wasn’t a leak (leaks don’t show increases in helium) and was probably due to too much oxygen being added to the system at the beginning of the test.

A couple of days ago a technician brought a similar graph to me and again I was asked why it looked unusual. I’ve had time to think about this issue since the last time and I’ve come up with an alternate explanation that I think fits the facts a bit better.

Adjusting DLCO for hemoglobin

Mon, 14 Sep 2015 00:00:00 +0000

My hospital’s Oncology division treats a number of patients with lymphoma and leukemia. It also has an active bone-marrow transplant program and for all of these patients diffusing capacity measurements are a critical part of assessing treatment progress. Since these patients are also frequently anemic, correcting DLCO results for hemoglobin is also critical.

For a factor that has as much importance for the interpretation of DLCO results as it does the effect of hemoglobin on DLCO has actually been studied a relatively small number of times. Part of the reason for this is the problem of finding an acceptable model. A reduced or elevated hemoglobin is a consequence of many diseases and conditions. When studying patients longitudinally it is often difficult to separate the changes in DLCO that occur from the disease process and those that occur from changes in hemoglobin. For this reason changes in hemoglobin pre- and post-treatment in anemia and polycythemia have been studied most frequently.

A real fixer-upper

Sun, 06 Sep 2015 00:00:00 +0000

I was reviewing reports today when I ran across one with some glaring errors. There were several things that immediately told me that the reported plethysmographic lung volumes were way off; the VA from the DLCO was almost a liter and a half larger than the TLC and the SVC was only about half the volume of the FVC.

When I took a look at the raw test data I saw at least part of the reason why the technician had selected these results to be reported and that was because the SVC quality from most of the efforts was poor. They mostly looked like this:

Anatomic dead space

Sun, 30 Aug 2015 00:00:00 +0000

I’d spent some time researching single-breath tests a while back and of course ran across the Fowler method for measuring anatomic dead space. It’s a relatively simple test but assessing its results as well as the results of alternate dead space measurement techniques turns out to be more complicated than I had remembered.

The official definition of anatomic dead space is that it is that part of the inhaled volume that remains in the airways at the end of inhalation and does not participate in gas exchange. An accurate estimate of this volume is important because respiratory dead space (Vd/Vt, discussed previously ) is composed of both anatomical and physiological dead space. The physiological component of the respiratory dead space cannot be determined without knowing the anatomical dead space.

Proposal to improve the readability of flow-volume loops

Sun, 23 Aug 2015 00:00:00 +0000

I’ve been planning on putting together a tutorial on characterizing and interpreting the contours of flow-volume loops so I’ve been accumulating flow-volume loops that are examples of different conditions. Lately I was reviewing some of them and noticed that when I tried to compare loops from different individuals with similar baseline conditions that the different sizes of the flow-volume made this difficult. For example, these two loops are both from individuals with normal spirometry.

DLNO isn’t the same as DMCO but sometimes it’s useful to pretend it (almost) is

Sun, 16 Aug 2015 00:00:00 +0000

Oxygen transport between the lungs and the body depends on numerous complex factors. Ventilation and the alveolar-capillary surface area are of course important but a critical component is hemoglobin. Oxygen is poorly soluble in water (which is what blood is mostly made of) and the transportation of oxygen throughout the body would not happen without hemoglobin’s ability to absorb and release oxygen on demand. Although it is possible to measure the diffusing capacity of oxygen (DLO2) the process is technically difficult and not at all suited to routine clinical testing.

Alveolar O2 and Altitude

Sun, 09 Aug 2015 00:00:00 +0000

Recently I was trying to explain the effect of altitude on blood oxygenation to somebody with IPF. They had observed that their oxygen saturation was fairly normal at sea level but that they needed to use their supplemental O2 when they went up to 2000 feet and didn’t understand why such a low altitude made that much of a difference. I don’t think I did very well with the explanation since at the time I was limited to only text and I much prefer pulling out diagrams and waving my hands in the air.

What’s in a name?

Sun, 02 Aug 2015 00:00:00 +0000

My lab is in the final stages of a software update that will allow for electronic signing of our reports. This has been a long and slow process partly because the release date of the software got pushed back several times but mostly because a wide variety of different hospital departments and sub-departments have had to be involved.

In all the years that I’ve had computers in the pulmonary function lab I’ve never gone through a software update that was either as easy as expected or occurred within the original schedule. This includes the time when all we had was a single IBM PC/AT with a 40 megabyte hard drive, no network and the only people that cared we were going through an update was ourselves. Since we now have a dozen networked PCs located in two different building on-campus as well as three off-site locations using an IS-managed SQL server and HL7 interface I didn’t have any expectations for a speedy update and so far I have not been disappointed.

Ultrasonic spirometers

Mon, 27 Jul 2015 00:00:00 +0000

There are at least a half dozen companies that use an ultrasonic flowmeter in their spirometer. The first patent for an ultrasonic flowmeter was made in the 1970’s but it wasn’t until the 1990’s that the first ultrasonic spirometers came to market. The basic idea is fairly simple and that is to measure the transit time of ultrasonic pulses through flowing gas. Pulses that travel in the same direction the gas is flowing will take less time to travel a given distance, while pulses traveling against the direction of gas flow take a longer time.

Temperature (in)correction

Sun, 19 Jul 2015 00:00:00 +0000

Sooner or later we all get lucky and find ourselves able to replace older equipment. When you have equipment that’s so old it can’t be repaired (either because the manufacturer no longer supports it or because the manufacturer no longer exists), you’d think this would be a no-brainer but money is always in short supply. I’ve often had to try to keep equipment running long past its expected life time and was only allowed to replace it when it finally broke beyond all hope of repair.

Filter FUD

Sun, 12 Jul 2015 00:00:00 +0000

A lab manager recently emailed me and asked my opinion about whether it was okay to use generic mouthpiece filters on their test systems. They had asked the same question of their equipment manufacturer and received the following statement (parts of which have been redacted by me):

“The [model number] PFT system was designed/tested/certified using the [manufacturer’s] filter. While other “off-label” filters may fit our devices, they have never been tested or approved for use by [the manufacturer]. The precision and accuracy of our devices could be compromised by using different type filters. It is our recommendation that you continue to use the [manufacturer’s] approved filters with your PFT equipment.”

Hyperventilation Syndrome

Wed, 08 Jul 2015 00:00:00 +0000

While reviewing a CPET I noticed the patient had a low PETCO2 throughout exercise and an elevated Ve-VCO2 slope. In addition the patient’s minute ventilation was on the high side (75% of predicted) at peak exercise. This is something you might expect to see in association with pulmonary vascular disease but the subject had a normal DLCO; normal spirometry; their oxygen saturation was normal at all times; and they had a normal maximum VO2 and a normal VO2 at anaerobic threshold. Since there didn’t seem to be any clinical reason for the low PETCO2 I had to wonder whether it was due to hyperventilation syndrome (HVS).

Have you checked the math on your reports lately?

Sun, 28 Jun 2015 00:00:00 +0000

Once again my lab was questioned by a research study’s primary investigator and study coordinator about why our lung volume results came out significantly lower than another lab’s. In order to be part of this study a subject has to have an RV that is greater than 150% of predicted. The RV we had obtained on a subject referred to the study was over a liter less than the results they had brought with them from another lab and for this reason the patient no longer qualified.

Using DL/VA (no, no, no, it’s really KCO!) to assess PFT results

Sun, 21 Jun 2015 00:00:00 +0000

Last week I was discussing the use of DL/VA to differentiate between the different causes of gas exchange defects with a physician. DL/VA is DLCO divided by the alveolar volume (VA). It is an often misunderstood value and the most frequent misconception is that it is a way to determine the amount of diffusing capacity per unit of lung volume (and therefore a way to “adjust” DLCO for lung volume). This is not the case because dividing DLCO by VA actually cancels VA out of the DLCO calculation and for this reason it is actually an index of the rate at which carbon monoxide disappears during breath-holding.

Selecting the best FEF25-75. Or not.

Sun, 14 Jun 2015 00:00:00 +0000

Oddly enough, I recently got a couple emails on the same day about the FEF25-75 and ended up corresponding for a while with the authors. FEF25-75 is a subject that somehow manages to keep resurrecting itself no matter how many stakes have gotten hammered into its heart. My opinion (expressed previously ), and those of many others, is that the measurement of FEF25-75 is overly affected by FVC volume and expiratory time; that its reproducibility is poor; that its “normal” range is too broad to be meaningful; and that the FEF25-75 is usually only abnormal when the FEV1 is also below the LLN. Despite all this the FEF25-75 still continues to be used by many clinicians and researchers.

Hyperoxic CPETs

Tue, 09 Jun 2015 00:00:00 +0000

In patients with lung disease the use of supplemental oxygen during exercise increases oxygen consumption, endurance time and maximum workload, and decreases the sensation of dyspnea without increasing minute ventilation and maximum heart rate. My lab is occasionally asked to perform a CPET with an elevated FIO2 (hyperoxic CPET). We are capable of doing this but I’ve always had reservations, partly about the logistics involved in performing the CPET but more importantly with the interpretation of the results.

Opto-Electronic Plethysmography (OEP)

Tue, 02 Jun 2015 00:00:00 +0000

Although we routinely use mouthpieces, noseclips and occasionally masks for our testing, all of these alter respiration in one way or another. Opto-electronic plethysmography (OEP) is a completely non-invasive technique for measuring chest wall volume that also allows for regional differences in expansion and contraction of the thorax to be detected.

The basic idea is simple and is the same as is used in cinematic motion-capture systems. Small (6-10 mm) reflective hemispheres are attached to a subject’s torso with double sided tape. A set of 4, 6 or 8 high-speed (60-120 frames/sec) CCD cameras are then used to monitor both the overall and the relative motion of the hemispheres while the subject breathes. The accuracy of these measurements is claimed to be on the order of 0.2 mm.

Treadmill protocols

Sun, 24 May 2015 00:00:00 +0000

Since I started performing exercise tests I’ve used both treadmills and bicycle ergometers. There are a several reasons that make ergometers somewhat better for exercise testing than treadmills. Most importantly the reduced noise and physical motion makes it easier to get blood pressure measurements and better quality ECG’s. In addition the workload can be set fairly precisely and they are safer for patients. Treadmills do have some advantages however, since patients are usually able to achieve a higher maximum oxygen consumption (~10%) and for many individuals walking is more natural than riding a bicycle.

Static reports, dynamic world

Sun, 17 May 2015 00:00:00 +0000

Reports are how patient test results are distributed. Paper versions have become less common because reports are now stored electronically in hospital information systems. Even if the way in which a report’s image is now stored, retrieved and distributed has changed, reports are still generated by our lab’s software systems and the ways in which this is done have not changed in any significant way for quite a while.

Reports are the public face of any pulmonary function lab and they should be designed to be readable and pertinent. It is critically important for any lab to create and manage reports correctly. So why does our lab software make it so hard to do this?

Transgender PFTs

Sun, 10 May 2015 00:00:00 +0000

I was reviewing PFT reports today and noticed that a patient appeared to have had the wrong gender entered in their demographic information. Specifically, the patient had an unambiguously masculine name but had been entered as female. Just to be sure I checked the patient’s on-line medical record and there he was listed as male. I had noticed from the trend report that the patient had been in the PFT lab numerous times. Since the basic patient demographics (name, date of birth, height, gender, etc.) are automatically forwarded into a new demographics record when a new PFT lab visit is created it struck me as odd that after all this time we had somehow managed to make a mistake with something as basic as gender. For this reason I thought it would be a good idea to see how far back this problem existed and started going back through the patient’s PFT records. About four visits ago the patient’s name suddenly changed to one that was unambiguously feminine.

How does a CPET show a Cardiac Limitation?

Sun, 03 May 2015 00:00:00 +0000

Recently a patient was referred to my lab for a CPET by his oncologist. The patient had been complaining of excessive shortness of breath particularly when climbing only a few stairs. The patient recently had a full PFT panel (spirometry, lung volumes, DLCO) and the results had showed mild restriction with a mild gas exchange defect. The patient’s shortness of breath symptoms were far more severe than could be explained by his PFTs however, so he had been referred to Cardiology and had an ECG stress test. The stress test results were normal so Cardiology told the oncologist that the patient’s problems were probably not cardiac.

What’s normal about MIP and MEP?

Sun, 26 Apr 2015 00:00:00 +0000

The static respiratory pressures, Maximal Inspiratory Pressure (MIP or PIMAX) and Maximal Expiratory Pressure (MEP or PEMAX) are a way to non-invasively assess respiratory muscle strength. Respiratory muscle weakness is present in a number of conditions, most notably neuromuscular diseases and disorders, but also malnutrition, cardiovascular disease, polymyositis, sarcoid and COPD. Strictly speaking, the maximal inspiratory and expiratory pressures are not generated solely by the respiratory muscles but also by the elastic recoil. The elastic recoil of the lung at TLC contributes up to 40 cm H2O towards MEP and the elastic recoil of the chest wall at RV contributes up to 30 cm H2O towards MIP. Even so, an individual cannot reach TLC or RV without the use of their respiratory muscles so the measurements are still valid regardless of how the pressures are generated.

The VA/TLC ratio

Sun, 19 Apr 2015 00:00:00 +0000

I was reading James Hansen’s textbook on pulmonary function testing (one of my more interesting reads lately) and in passing he mentioned using the VA/TLC ratio as a way to measure ventilation inhomogeneity. The VA/TLC ratio has also been called the Va_,eff/VA_ ratio and the VA’/VA ratio by different researchers but regardless of what it is called it is the ratio between a single-breath TLC measurement (VA) taken from a DLCO test and a multi-breath (helium dilution or N2 washout) or plethysmographic TLC.

Vocal cord dysfunction

Sun, 12 Apr 2015 00:00:00 +0000

Recently I reviewed a set of completely irreproducible spirometry results. The patient had made eight attempts and the FVC, FEV1 and Peak Flow were different every time. In particular, there were frequent stops and starts during exhalation. I’ve always wondered why some patients have so much difficulty with what should be a simple test and although in this particular case it could simply be glottal closure I wondered if it could be Vocal Cord Dysfunction (VCD). For this reason I spent some time reviewing the literature.

Tidal flow-volume loops

Wed, 08 Apr 2015 00:00:00 +0000

I was reviewing a spirometry report and noticed something odd about the flow-volume loop, or more specifically the tidal loop, and this got me to thinking about what tidal loops can tell us about test quality, patient physiology and the ability of the technician to coach a spirometry test.

There are at least a couple things wrong with this FVC test effort. First the exhalation time was only about 3 seconds so the FVC volume was likely underestimated by a fair amount. Second, it wasn’t reproducible and this was actually the patient’s the best test effort. What I noticed however, was that the tidal loop was shifted almost completely to the left.

Washout volume, transit time and DLCO

Sun, 29 Mar 2015 00:00:00 +0000

Recently while reviewing PFT reports I ran across a test from a patient who had been having spirometry, lung volume and DLCO tests performed at regular intervals for the last several years. Compared to the last several set of tests the most recent DLCO had decreased significantly while the FVC, FEV1 and TLC hadn’t changed. I took a closer look at the raw data from the DLCO test and when I did I saw that the washout volume was not correct.

Diagnosing Mitochondrial Myopathies

Sun, 22 Mar 2015 00:00:00 +0000

I’ve been reviewing my CPET textbooks trying to get a better idea of how to differentiate between different cardiovascular limitations. The other day I ran across an article on a related subject and thought it might be instructive.

The hallmark of cardiovascular limitations is the inability to deliver enough oxygenated blood to the exercising muscles. Another limitation that has similarities to this (and one that is infrequently diagnosed) is the inability of the exercising muscle to utilize the oxygen delivered to it. The best examples of this type of exercise limitation are mitochondrial myopathies (MM).

The effects of Obesity on lung function

Sun, 15 Mar 2015 00:00:00 +0000

Obesity has become far more commonplace than it was a generation ago. The reasons for this are unclear and have been attributed at one time or another to hormone-mimicking chemicals in our environment, altered gut biomes, sedentary lifestyles or the easy availability of high calorie foods. Whatever the cause, obesity affects lung function through a variety of mechanisms although not always in a predictable manner.

Spirometry:

Many investigators have shown a relatively linear relationship between an increase in BMI and decreases in FVC and FEV1. These decreases are small however, and FVC and FEV1 tend to remain within normal limits even in extreme obesity. The decreases in FEV1 and FVC tend to be symmetrical which is shown by the fact that the FEV1/FVC ratio is usually preserved in obese subjects without lung disease. Several studies have shown that the decreases in FVC and FEV1 are reversible since a decrease in weight showed a corresponding increase in FVC and FEV1.

When TLC, RV and VC don’t add up

Sun, 08 Mar 2015 00:00:00 +0000

I thought I was done with lung volume issues for at least a little while but a short time ago I was reviewing a report from another PFT lab and I ran across something that didn’t seem to make sense. What the report showed was a normal TLC (99% of predicted) with a normal VC (101% of predicted) but the RV was 70% of predicted.

When I took a closer look, it was evident that the predicted VC came from the NHANESIII study and the predicted TLC and RV came from the ERS 1993 Statement. In my PFT lab our equipment manufacturer made the decision to use the predicted RV from whatever source the end-user selected (which in our case is the ERS93 study as well) but to re-calculate the predicted TLC using the predicted FVC, again from whatever source the end-user selected (which in our case was also NHANESIII). What this means is that for my lab:

The NHANESIII FEV1/FVC ratio and height, revisited

Sat, 28 Feb 2015 00:00:00 +0000

I was reading James Hansen’s textbook on pulmonary function testing and ran across a spot where he made a minor criticism of the NHANESIII (Hankinson et al) reference equations for the FEV1/FVC ratio. Specifically he noted that the NHANESIII equation ignored height and only used age as a variable but that when he compared the directly calculated FEV1/FVC ratio with one indirectly derived from predicted FEV1 and FVC there was a discrepancy across the normal ranges of height of up to 2.4%.

The CPET’s not over until it’s over

Sun, 22 Feb 2015 00:00:00 +0000

My guidelines for interpreting CPETs originally started as notes to myself about what needed to be on the report and what the normal values were. They grew into a more formal set of instructions that were given to the pulmonary fellows when they were reviewing CPETs. Lately I’ve been reviewing them and re-reading the source material in order to make sure what I had written was still correct and so I could add references.

There’s more than one way to determine AT

Sun, 15 Feb 2015 00:00:00 +0000

As workload increases during a progressive cardiopulmonary exercise test (CPET) there comes a point at which the amount of oxygen delivered to the exercising muscle is no longer able to meet its needs. This is the point at which lactic acid begins to accumulate, CO2 production increases and is the accepted definition of the Anaerobic Threshold (AT). The “gold” standard for determining AT is lactic acid measurements but these require sampling blood at regular intervals throughout the CPET. AT is far more commonly determined from respiratory parameters.

Substituting an FVC for an SVC in Lung Volume measurements

Tue, 10 Feb 2015 00:00:00 +0000

Recently I was reviewing test results from another PFT Lab that uses equipment from a different manufacturer than what my lab uses. When I came to the lung volumes it became evident that the FVC had been substituted for the SVC. I understand the point of using the largest vital capacity when calculating TLC and RV but there are some issues affecting these values when this is done.

Strictly speaking using the FVC is permitted by the ATS/ERS guidelines on lung volume testing, but how an FVC is to be used, as opposed to an SVC, is not addressed. The reason this is an issue is that all lung volume tests regardless of which method is used do not measure TLC and RV directly, they measure FRC. The preferred ATS/ERS TLC calculation is then:

When is it hyperinflation?

Sun, 01 Feb 2015 00:00:00 +0000

I was reviewing a PFT recently and noticed that the FEV1 was severely reduced and that the FRC and RV were both elevated. This is a pattern we associate with obstructive gas trapping but I’ve also been reviewing textbooks on pulmonary function interpretation and have found that there isn’t any kind of a universal definition for this.

Hyperinflation and gas trapping are used somewhat interchangeably but the distinction is that gas trapping causes hyperinflation. Gas trapping occurs to some extent in everybody but usually at lung volumes below FRC. The lung volume at which gas trapping occurs rises with age and with obstructive lung disease. Hyperinflation is usually considered to be an increase in FRC but FRC is a dynamic lung volume and there is a range in the response to increased gas trapping. The normal progression from mild to very severe COPD goes something like this:

What’s up with Peak Flow?

Sun, 25 Jan 2015 00:00:00 +0000

Two PFT reports came across my desk recently and comparing them got me to thinking about Peak Expiratory Flow (PEF). The FEV1 from both tests were mildly reduced with an FEV1/FVC ratio that was moderately reduced and an FVC that was within normal limits.

	Observed:	%Predicted:
FVC (L):	3.72	96%
FEV1 (L)	2.17	78%
FEV1/FVC (%):	58	80%
PEF (L/sec):	2.97	41%

	Observed:	%Predicted:
FVC (L):	2.46	88%
FEV1 (L)	1.66	75%
FEV1/FVC (%):	67	83%
PEF (L/sec):	7.65	128%

Both tests also showed mild airway obstruction but despite this the Peak Flows were quite different. One test had a PEF that was moderately to severely reduced and the other had a PEF that was elevated. It’s fascinating that two such completely different flow-volume loops are so numerically similar.

What’s a normal post-pneumonectomy CPET?

Sun, 18 Jan 2015 00:00:00 +0000

Recently a CPET report for an individual whose primary complaint was tachycardia and DOE with minimal activity came across my desk. Since the patient had had an pneumonectomy (one lung removed) about a year ago there wasn’t much doubt the results would be reduced, the question was whether they were reduced more than they should have been.

You might expect lung function to decrease by half following a pneumonectomy but because the remaining lung always expands to some extent FVC and TLC tend to be approximately 60%-65% of their pre-surgical volume. Although this increase in volume however does not increase the alveolar-capillary surface area the entire cardiac output needs to pass through the remaining lung which causes an increase in the pulmonary capillary blood volume. For this reason DLCO also tends to be about 65% of baseline.

What does it mean when Ve exceeds its predicted during a CPET?

Sun, 11 Jan 2015 00:00:00 +0000

When I review the results from a CPET I am used to considering a maximum minute ventilation (Ve) greater than 85% of predicted as an indication of a pulmonary mechanical limitation. Recently a CPET report came across my desk with a maximum minute ventilation that was 142% of predicted. How is this possible and does it indicate a pulmonary mechanical limitation or not?

It is unusual to see a Ve that is greater than 100% of predicted. We derive our predicted max Ve from baseline spirometry and calculate it using FEV1 x 40. We have tried performing pre-exercise MVV tests in the past and using the maximum observed MVV as the predicted maximum Ve but our experience with this has been poor. Patients often have difficulty performing the MVV test correctly and realistically even if it is performed well the breathing maneuver used during an MVV test is not the same as what occurs during exercise. Since both Wasserman and the ATS/ACCP statement on cardiopulmonary exercise testing recommend the use of FEV1 x 35 or FEV1 x 40 as the predicted maximum minute ventilation we no longer use the MVV.

Looking at the past, looking to the future

Wed, 31 Dec 2014 00:00:00 +0000

As New Year’s Day approaches it is a tradition for people look back to see what has happened during the last year and then look forward and guess what will happen during the next year. I’ve never done a New Year’s blog before but I’ve been mulling over a number of ideas for a while and this looks like a good place to explore them.

I’ve had the opportunity over the last several years to research the history of pulmonary function testing. There are a couple of interesting lessons from the past that may be useful, particularly when we are trying to guess what direction pulmonary function testing is heading towards in the future.

Where are your emergency plans?

Mon, 29 Dec 2014 00:00:00 +0000

Around 20 years ago I had to write the emergency evacuation plan for the pulmonary function lab. Like many other administrative duties I learned that I needed to do this when my new administrator asked where it was and whether I had documented that I had reviewed it with the lab staff. Since I didn’t even have a real procedure manual at the time (just reprints of pertinent articles and textbook chapters) I ended up getting a crash course in writing policies. Fortunately the manager of a nearby departments let me borrow their evacuation plan and I was able quickly to knock one out that met the requirements fairly quickly. Since then I’ve had to review it annually and update it every time the lab moved or when rooms were added or taken away.

Multi-breath Washout and the Lung Clearance Index

Mon, 22 Dec 2014 00:00:00 +0000

The March 1, 2014 issue of the American Journal of Respiratory and Critical Care Medicine had an article on the use of the Lung Clearance Index (LCI) with bronchiectasis. The study showed that the LCI was as good as high-resolution computed tomography and more sensitive than FEV1 when assessing changes in airway status. This is one of the few articles I’ve seen on the LCI that was specifically about adults and wasn’t about cystic fibrosis.

Closing Volume

Tue, 16 Dec 2014 00:00:00 +0000

Closing Volume (CV) is a measurement made from a single-breath nitrogen washout (SBNW) test. It was commonly performed decades ago and elevated values were considered to be an indication of small airways disease and an aid in the detection of the early stages of airways disease. It is hardly ever performed any more but I still occasionally see research studies that include this test and almost every test system that is capable of measuring lung volumes by nitrogen washout is also capable of performing a CV.

Standing height, ethnicity and the vital capacity

Mon, 08 Dec 2014 00:00:00 +0000

In 1844 John Hutchinson published his first paper describing his spirometer and his research on the Vital Capacity. He was the first person to use the word “spirometer” to describe his instrument and the first to use the term “vital capacity” to designate the maximum amount of air an individual can exhale after a maximal inhalation. Although he is remembered as the inventor of the spirometer, he was not the first person to use a gasometer to measure lung volumes nor was he the first to measure the vital capacity. What made his research different from those that came before him was partly the prodigious number of individuals whose vital capacity he measured but far more importantly that he was able to show a clear relationship between standing height, age and vital capacity which had not been previously apparent. This finding galvanized researchers in England, Europe and the United States and in many ways helped set the course of research into lung function for many decades to come.

When it’s FVC 1, EOT 2, volume comes out short

Sat, 29 Nov 2014 00:00:00 +0000

I was reviewing a pre- and post-bronchodilator spirometry report that showed a relatively large increase in FVC but the change in FEV1 was not significant. It’s not impossible for a patient to show this kind of a pattern following a bronchodilator but it is somewhat unusual. Usually when I see this it means that the patient exhaled a lot longer post-BD than they did pre-BD. When I looked however, I saw that just the opposite was true, the expiratory time was actually shorter for the post-BD effort than it was for the pre-BD effort.

Assessing the response to bronchodilators

Tue, 25 Nov 2014 00:00:00 +0000

The current ATS/ERS guidelines require that an individual have a post-bronchodilator increase in FEV1 or FVC of at least 12% and 200 ml in order for it to be considered a significant response. Numerous studies have shown however, that many patients that don’t meet these criteria, particularly those with COPD, do have a clinically significant improvement with bronchodilators.

The September 2014 issue of Chest had a point-counterpoint set of editorials on this standard and on assessing the response to bronchodilator in general. Both sides had a number of interesting things to say but to a large extent one side was talking apples (physiology) and the other side oranges (statistics). I actually think that both sides feel there are significant problems with the ATS/ERS standards, they just differ in what they think is wrong and in the best way to fix it.

Is it Dynamic Hyperinflation or something else?

Sun, 16 Nov 2014 00:00:00 +0000

Patients with COPD often have a ventilatory limitation as their primary limitation to exercise. A ventilatory limitation to exercise has traditionally been assessed by the breathing index or the breathing reserve:

breathing index = Peak Ve / Predicted MVV

breathing reserve = 1 – (Peak Ve / Predicted MVV)

which are basically two different ways of saying the same thing. In either case a breathing index greater than 85% or breathing reserve less than 15% is an indication that a patient has reached a ventilatory limit to exercise. There is some disagreement as to whether the predicted MVV should come from a MBC test performed by the patient or from the patient’s FEV1 x 40. I have tried both approaches and my experience has been that FEV1 x 40 is the best indicator for a patient’s predicted MVV. This is also Wasserman’s (my go-to source for exercise testing) recommendation so this is what we use.

Hidden FIVC

Sat, 08 Nov 2014 00:00:00 +0000

I was about to put a PFT report I’d been reviewing in my outbox when I noticed something odd about the flow-volume loop.

What I saw was that the final inspiration of the FVC maneuver had ended to the left of the initial inspiration. This means a couple of thing, first and foremost that the FIVC was larger than the FVC and that the FVC was likely underestimated because the patient hadn’t really taken a full inspiration prior to exhaling. I had already looked at the raw data for the patient’s spirometry results for other reasons but I pulled them up again to see if I had missed something.

Inspiratory Flow-Volume Loops

Fri, 31 Oct 2014 00:00:00 +0000

Recently I was discussing a flow-volume loop with one of our pulmonary physicians. His concern was whether the loop, which was from a patient with a cracked hyoid, was showing inspiratory obstruction or not. I had to point out that the inspiratory flows on the loop in question came from the pre-FVC inspiration that started at FRC, not RV, and that we don’t emphasize maximal inspiratory flow at that point in the test, just maximal inspiratory volume and for these reasons the flow-volume loop was not a reliable indicator of inspiratory obstruction.

When a Pulmonary Mechanical Limitation to exercise isn’t the real limitation

Sun, 26 Oct 2014 00:00:00 +0000

Recently I was reviewing a cardio-pulmonary exercise test (CPET) that at first glance seemed to show the patient had a pulmonary mechanical limitation. Specifically, the patient’s minute ventilation (Ve) at peak exercise was 93% of predicted. This was something you’d expect if a patient had an obstructive or restrictive lung disease but I could see right away that the patient’s baseline pulmonary function tests were actually pretty normal.

Creepy decimal points

Wed, 15 Oct 2014 00:00:00 +0000

This is one of my pet peeves. It started for me back in the 1970’s when the Intensive Care Units where I was working were evaluating thermal dilution cardiac output meters. This was at a time when digital displays were just starting to become common. One of the meters showed cardiac output with two digits after the decimal point (i.e. 0.12) and the other one had three digits after the decimal point (i.e. 0.123).

When an Inspiratory Volume really isn’t inspiring

Sat, 04 Oct 2014 00:00:00 +0000

I always like it when a patient does something during a test that makes me have to think about the basics of the test and what effect an error will have on the results. I was reviewing a report that had come across my desk and the technician performing the test had put “poor DLCO test reproducibility, fair quality in selected test” in the notes so of course I had to pull up the raw test data and take a look for myself.

Ve-VCO2 slope: Just to AT or all the way to the peak?

Fri, 26 Sep 2014 00:00:00 +0000

We’ve had some questions lately about some of our CPET guidelines. These questions were informational in nature not confrontational but they served to remind us that the reference values we use for CPET interpretation were developed and put in place at least ten years ago and it is past time they were reviewed. As a starting point I’ve been re-reading the ATS-ACCP and AHA statements on cardiopulmonary exercise testing. One sentence from the AHA statement concerning the Ve-VCO2 slope caught my eye. Specifically:

Exercise Oscillatory Ventilation

Sat, 20 Sep 2014 00:00:00 +0000

Recently I was trying to make some sense of an exercise test report that had come across my desk. Numerical results on our CPET reports are averaged over 30 second periods and there seemed to be a lot of variability from one time interval to the next. This isn’t uncommon in the first couple of minutes of an exercise test because patients often start off too hard and too fast, overshoot and then take a while to settle down into a steady pattern. This variability however, persisted throughout the entire test. I finally realized that what I was seeing was Exercise Oscillatory Ventilation (EOV).

COPD, BD and IC

Mon, 15 Sep 2014 00:00:00 +0000

The ATS/ERS recommendation for assessing the response to bronchodilator is based solely on changes in FEV1 or FVC. An FEV1 that does not improve significantly following bronchodilator inhalation is considered to be one of the hallmarks of COPD. Many individuals with COPD however, can have symptomatic relief and an improvement in their exercise capacity without a significant post-bronchodilator increase in FEV1. This means that FEV1 may not be the only criteria for assessing bronchodilator response.

Assessing MVV results

Mon, 08 Sep 2014 00:00:00 +0000

The Maximum Voluntary Ventilation (MVV) test was initially described in 1933. It was the first pulmonary function test that involved inspiratory and expiratory air flow in a significant way and for this reason it helped to set the stage both conceptually and technically for the FEV1, the FEV1/FVC ratio and our present understanding of obstructive lung diseases. MVV is reduced in a variety of conditions, including obstructive, restrictive and neuromuscular diseases, but a reduced MVV is non-specific and this limits its clinical utility. Nevertheless, it continues to be performed both in clinical labs and for research, and for this reason it would seem to be a good idea to know how to assess MVV results.

It isn’t a Raw without a TGV

Sat, 23 Aug 2014 00:00:00 +0000

One of the key reasons to perform spirometry is to measure expiratory flow rates. The flow rate of air through any system is primarily a function of driving pressure and resistance. Since there are limits to anybody’s ability to increase driving pressure (and physiological reasons why airflow does not continue to increase when driving pressure exceeds a certain threshold value) FEV1 is largely related to the amount of resistance in the airways.

Looking for help with an orphaned system

Thu, 21 Aug 2014 00:00:00 +0000

I was contacted recently by a reader who is trying to resurrect an older test system. It is a Spirotech S600, manufactured by Graseby-Andersen and outfitted for spirometry, helium dilution lung volumes and single-breath DLCO. It seems to be in reasonably good shape but he cannot test the components or use it in any way because he does not have the software for it. He is working on a shoestring budget and does not have funds to purchase a newer system.

Assessing FEV1 trends, re-visited

Tue, 19 Aug 2014 00:00:00 +0000

There are a couple of different ways to assess changes in FEV1 from one patient visit to another. For several decades my lab has used a change of >=10% and >=200 ml as the threshold for a significant change. Recently the ATS released standards for occupational spirometry that included an age-adjusted change in FEV1 of >=15% as the threshold for significant change. For the time being we have continued to use the 10% threshold when comparing results that are relatively close in time and are using the 15% threshold when they are separated by a much longer period. Since we haven’t actually gotten around to defining what is recent and what isn’t there is still a bit of uncertainty in how we apply this but even though there are differences in thresholds and how the numbers are calculated both approaches are essentially numerical. Recently a couple of reports crossed my desk that have caused me to wonder whether a qualitative change should also be a consideration.

What’s abnormal about FRC?

Sat, 09 Aug 2014 00:00:00 +0000

I’ve had a number of reports across my desk in the last couple of weeks with both elevated and reduced FRC’s that were associated with a more-or-less normal TLC. I reviewed the raw data from all of these tests (I review the raw data from all lung volume tests) and in only a few instances did I make any corrections to the report. This made me think however, about what, if anything, is an abnormal FRC trying to tell us?

DMCO, Vc and 1/theta

Thu, 31 Jul 2014 00:00:00 +0000

Roughton and Forester’s seminal paper from the 1950’s showed that DLCO was a function of two resistances: the alveolar-capillary membrane and the rate of CO uptake by red blood cells. This relationship is shown by:

Roughton and Forster also showed that the membrane diffusing capacity (DMCO) and pulmonary capillary blood volume (Vc) could be calculated by performing the DLCO test at different oxygen concentrations and then plotting the results.

When back-extrapolation goes astray

Sun, 13 Jul 2014 00:00:00 +0000

A spirometry report that looked very questionable came across my desk recently. The flow-volume loop was misshapen and the technician’s notes indicated that the results had been highly variable and to “interpret with caution”. I pulled up the raw test results and saw a series of test efforts with flow-volume loops that were all somewhat flattened and with no consistency in either the loops or the numerical results.

This kind of inconsistency can be an indication of poor patient effort but can also occur because of airway problems. The cardio-thoracic surgeons at my hospital have an active airway stenting program and so we see a fair number of patients with trachemalacia. One hallmark of tracheomalacia is that there is usually a flow limitation and that this means that there is usually a flat expiratory plateau in the flow-volume loops. These loops had peak flow-ish humps, but the humps seemed to appear in different locations in every loop and they seemed to have a relatively high frequency flutter.

Mixing doesn’t always make a match

Tue, 01 Jul 2014 00:00:00 +0000

Recently I was reviewing a spirometry report and noticed that the FVC was below normal. A low FVC can suggest a restrictive lung disease but the reported expiratory time was only about 4 seconds. I took a look at the graphics included with the report and the volume-time curve showed the effort ended well before 6 seconds so my first thought was that the reduced FVC was more likely because of suboptimal patient effort than anything else.

Seeing shouldn’t always be believing

Thu, 26 Jun 2014 00:00:00 +0000

Although the numerical results are of course important, visual inspection of the volume-time and flow-volume loop graphs from a spirometry test are a critical part of interpretation. Spirometry quality and performance issues that don’t show up in the numbers are often highly evident in the graphs. Choices we make in creating and configuring reports however, can hide important visual details and have the potential to decrease interpretation quality.

Recently I was inspecting the results for a spirometry test. There wasn’t anything particularly unusual about the numbers or the graphics on the report, I just like to make spot-checks on spirometry quality and wanted to make sure the best results had been selected. When I pulled up the raw test date on my computer screen I noticed an unusual wavering pattern in the volume-time curve. I don’t remember seeing a volume-time curve like this before and when I checked all of the patient’s efforts were similar and all showed similar oscillations.

Single-breath DLCO Breath-holding time (BHT)

Mon, 23 Jun 2014 00:00:00 +0000

The single-breath DLCO maneuver can rightly be criticized as being an artificial maneuver that bears little resemblance to normal breathing. It is only by standardizing the maneuver that clinically relevant and reproducible results can be obtained. One important aspect of this standardization is the breath-holding period.

The single-breath DLCO maneuver begins with a subject exhaling to RV, followed by an inhalation of the test gas mixture to TLC and then a 10-second breath-holding period, ending with an exhalation during which a sample of alveolar air is collected. The initial choice of a 10-second breath-hold period was largely arbitrary and was selected in order to strike a balance between being a short enough period that for most patients to hold their breath, long enough to minimize the inspiratory and expiratory phases and long enough to allow for a sufficiently measurable amount of carbon monoxide to be taken up.

Assessing change over a long period of time

Sun, 15 Jun 2014 00:00:00 +0000

Because our lab database goes back 24 years, we’ve started to see a certain number of patients who had last been seen ten or even twenty years ago fairly often. For this reason I’ve been thinking about what is a clinically significant change over that long a time period. The guidelines my lab uses for interpreting change in test results came about from a consensus among the department’s pulmonary physicians close to twenty years ago. As usual there are some discrepancies between our guidelines and those the ATS-ERS have published.

DLO2/Qc, SaO2 and CPETs

Tue, 03 Jun 2014 00:00:00 +0000

There are a number of simple observations that can be made during a cardio-pulmonary exercise test (CPET) that can point you immediately in a specific diagnostic direction. Recently I was reminded of this while reviewing the CPET results on patient with a complicated medical history whose test had been requested as part of a pre-operative assessment.

Most patients that are candidates for cardio-thoracic surgery do not need to have a CPET and that’s because it is usually straightforward to determine who is high risk and who is low risk from other routine tests. When risk is hard to determine or equivocal, the cardio-thoracic surgeons will order a CPET. They are primarily interested in the VO2 max and Ve-VCO2 slope since there are a number of widely accepted pre-op assessment algorithms that use these values. Even if the CPET results indicate the patient is high risk, the test details can help determine whether there is anything that can be done to improve the patient’s odds.

Gas solubility and why it matters

Mon, 26 May 2014 00:00:00 +0000

I have been searching through Pulmonary Function videos on YouTube in order to find ones I thought would be useful for technician education. So far what I’ve found have been intended either for medical students or for patients and not, in my opinion, particularly suitable for training technicians. Lately I’ve been looking at videos about lung volumes and have seen a half dozen presenters describe lung volume subdivisions using the same graph we’ve come to know and love with varying degrees of effectiveness and obfuscation.

How long should patient test data be retained?

Mon, 19 May 2014 00:00:00 +0000

In a recent post on the AARC Diagnostics forum a PFT Lab manager asked how long they need to keep database records. The ostensible reason for this was that they had too many years of records and had been having database problems.

The poster wasn’t specific about what kinds of problems they were having. Database problems can be hard to diagnose particularly when a database is networked but with a modern SQL database the number of records shouldn’t be an issue. SQL databases containing millions of records are routinely used in demanding multi-user applications. If this was thirty years ago when computers first started to be commonplace in the PFT Lab I could understand since PC-based databases were still in their infancy then. It was at least partly for this reason that a number of PFT equipment manufacturers developed their own proprietary databases. This is no longer the case and I have difficulty believing that there are any manufacturers at this time that don’t use a commercial SQL database of one kind or another.

What’s normal about FEV1 and how much does ethnicity matter?

Tue, 13 May 2014 00:00:00 +0000

When it comes to spirometry, it’s really all about FEV1. FVC and the FEV1/FVC ratio are also important of course, but because FVC is more likely to be underestimated than FEV1 they are less reliable.

Changes in FEV1 are critical in monitoring airway disease. The recent ATS guidelines on Occupational Spirometry indicate that a 15% decrease (adjusted for changes in age) is significant and cause for concern. For diagnosing airways disease however, it is important to know what a normal FEV1 is.

Personal spirometers, under $500

Sat, 03 May 2014 00:00:00 +0000

Pulmonary patients have been using personal peak flow meters for several decades and I started to seeing patients that had their own oximeter over 10 years ago. Within the last couple of years a number of different spirometers intended for personal use have become available. These devices have significant limitations when compared to regular office or lab spirometers but because there are many individuals with asthma or COPD for whom FEV1, and possibly FVC, are significantly better indicators of lung health than Peak Flow (PEF) alone, I think they can serve a useful purpose.

Vd/Vt, how accurate is it really?

Sat, 26 Apr 2014 00:00:00 +0000

My lab stopped inserting A-lines to get arterial blood samples during exercise testing well over 10 years ago. Our decision was partly based on the fact that we didn’t do them often enough to be good at it and partly based on the fact that we didn’t think that we were getting enough extra information from ABG’s to be worth the effort. Since another local hospital (a competitor but part of the same medical school network so we share pulmonary fellows with them) routinely performs level II and level III exercise tests we felt we could refer any patients that really needed ABG’s to the lab there. We don’t regret the decision and don’t feel that it has compromised the quality of our exercise testing.

Because we don’t obtain ABG’s, one of the values we don’t calculate is the deadspace/tidal volume ratio (Vd/Vt). Recently I was reading an article that related Ve-VCO2 to Vd/Vt and I was reminded of some the issues I had with Vd/Vt when I calculated it in the past. We’ve gone through two different exercise test systems since that time so I’m not sure if some of these problems still exist but I thought it would be a good idea to review both the problems and the literature on Vd/Vt to see if I could make some sense of them.

As a reminder, the original Bohr equation for Vd/Vt was:

The first problem I had run into was that mixed-expired CO2 (PECO2) is routinely calculated from CPET data as:

But it was also reported as a separate value by the test system’s software and the two values did not match.

The second problem was that the test system software reported a non-zero inspired CO2 concentration (PICO2). The PICO2 ranged from 10 mmHg at rest to 6 mmHg at peak exercise and when I reviewed some of the data from that time it is apparent that the PICO2 closely matched the difference between the reported and calculated PECO2.

It wasn’t a leak

Sun, 20 Apr 2014 00:00:00 +0000

The most common problem we have with helium dilution FRC tests are leaks. Although the system tubing and spirometer bell leak occasionally, we do have valve failures relatively frequently. Valve failures are usually obvious but they sometimes only fail partially so leak checks are regularly performed on these test systems. We can’t perform leak checks on patients except while they are being tested however, and patient leaks are far more common than system leaks.

Pneumotach accuracy

Sun, 13 Apr 2014 00:00:00 +0000

The first reasonably accurate flow-measuring device was the Fleisch pneumotachograph which was developed in 1925. Originally the Fleisch pneumotach bounced a light beam off a mirror mounted on a diaphragm and from there onto photographic film, all of which made it difficult to use. World War II saw the development of sensitive pressure transducers, amplifiers and recorders and by 1950 the pneumotachograph went totally electronic and began to be commonly used in routine pulmonary research.

Student spirometers, what can they teach?

Fri, 04 Apr 2014 00:00:00 +0000

It’s been several decades since I last saw water-seal bell spirometers being used in a Pulmonary Function lab. They have been displaced mostly by systems that use flow sensors of one type of another and the small handful of equipment manufacturers that still make volume-displacement spirometers use rolling seals. This isn’t to say that this kind of spirometer isn’t being manufactured any more and in fact there appears to be a modestly thriving market in water-seal spirometers intended for use by students.

OUES, a useful sub-maximal CPET indicator of maximum VO2

Mon, 31 Mar 2014 00:00:00 +0000

The primary goal of a Cardio-Pulmonary Exercise Test (CPET) is to determine an individual’s maximum oxygen consumption (VO2), minute ventilation (Ve) and heart rate (HR). An adequate CPET is usually indicated by a Respiratory Quotient (VCO2/VO2) that is greater than 1.10, a heart rate greater than 85% of predicted or a Ve greater than 85% of predicted. There are a variety of reasons why patients are unable to exercise to their maximum. Although these reasons can of course include poor motivation, factors such a musculo-skeletal limitations or concerns about patient safety due to EKG changes can cause patients have a sub-maximal test.

An IC shows it’s probably not restriction

Sat, 22 Mar 2014 00:00:00 +0000

For the last couple of years it seems that I’ve had more problems than usual with lung volume tests. Even though this seems to date from the time that my lab went through its hardware and software upgrade and we started performing N2 washouts I suspect that these problems have been around for a long time and these events just heightened my awareness of lung volume testing problems.

My lab performs helium dilution, N2 washout and plethysmographic lung volume tests. When you are assessing the quality of lung volume tests the first problem for the helium dilution and plethysmographic techniques is whether or not the Functional Residual Capacity (FRC) was accurately measured and for N2 washout, it’s whether or not the Residual Volume (RV) was accurately measured. Leaks are always an issue for any of these measurement techniques and for helium dilution and N2 washout leaks will almost always cause the Total Lung Capacity (TLC) to be overestimated. For plethysmography the picture is less clear since leaks can cause TLC to be either over- or under-estimated.

Measuring Thoracic Gas Compression

Mon, 17 Mar 2014 00:00:00 +0000

During exhalation air flow occurs because of the pressure difference between the alveoli and the surrounding atmosphere. The increase in alveolar pressure acts to compress the air inside the lung and because of this compression the decrease in lung volume during exhalation is always going to be greater than the volume of exhaled air. This effect is known as Thoracic Gas Compression (TGC).

The flow rate that occurs for a given alveolar pressure depends primarily on airway resistance. When an individual has airway obstruction their airway resistance is increased and they often attempt to increase their expiratory flow by increasing the amount of force they apply during exhalation. This increased force further compresses the air inside the lung and increases TGC. Numerous researchers have shown that there is usually a substantial differences in TGC between subjects with normal lungs and those with airway obstruction.

It’s all about FEV1, except when it isn’t.

Thu, 06 Mar 2014 00:00:00 +0000

A number of physicians and researchers I’ve known and respected have said that in spirometry it always comes down to FEV1 since it is the primary indicator for airway obstruction. Certainly FVC and the FEV1/FVC ratio are important but because patients can stop exhaling early for any number of reasons FVC can be underestimated which in turn can cause the FEV1/FVC ratio to be overestimated so they are not quite as reliable as FEV1.

One more DLCO technique: DLCO measured during exhalation (Intrabreath DLCO)

Thu, 27 Feb 2014 00:00:00 +0000

There have been numerous criticisms of the single-breath DLCO technique, many of them quite valid. In particular, the standard equation for calculating DLCO makes no consideration for the inspiratory and expiratory phases of the maneuver when lung volume and the alveolar capillary surface area are changing. Some investigators have devised ways of correcting for inhalation and exhalation, however other investigators have sidestepped the issue entirely by showing that DLCO can instead be calculated from information acquired only during exhalation.

Selecting a DLCO test in order to show airway obstruction

Sat, 22 Feb 2014 00:00:00 +0000

When DLCO tests are performed my lab’s standard policy to average two or more results that meet the criteria for quality and reproducibility. It is not unusual for us to perform three DLCO tests and have all of them meet quality criteria but to have one test result that is higher than the other two. Unlike spirometry tests, bigger isn’t necessarily better for DLCO, so in a circumstance like this we will average the two closest results rather than choose the highest result. Even though the higher test results can come from a DLCO test with good quality, I think that reproducibility trumps this and that choosing by reproducibility gives us results that are more clinically reliable.

How should predicted TLC and RV be derived?

Sat, 15 Feb 2014 00:00:00 +0000

The ATS-ERS standards on lung volume measurements says that measured TLC and RV can be calculated either by

RV = FRC – ERV then TLC = RV + SVC

or by

TLC = FRC + IC then RV = TLC – SVC

with the preference going to the first method. Strictly speaking, given the same FRC and SVC measurements either method is going to end up with exactly the same calculated TLC and RV values. Conceptually speaking I believe that TLC = FRC + IC is a more relevant way to think about TLC but this is only because I think that patients find it easier to perform a quality IC maneuver than a quality ERV maneuver.

LOINC, and why it matters to your HIS Interface

Sat, 08 Feb 2014 00:00:00 +0000

The Hospital Information Systems (HIS) at different medical centers have grown up mostly in isolation from each other. Even when an HIS is installed by a national vendor, each individual hospital has tended to make its own customizations and to follow past conventions. This is changing and it is changing because there are a number of issues driving rapid improvements in inter-hospital communication. The Meaningful Use (MU) Act is major factor and one that has been helping to set the pace, but because improved communication lowers costs and improves the quality of care insurers and medical institutions have been moving in this direction for their own reasons as well.

Using an HIS Interface as your report manager

Sun, 02 Feb 2014 00:00:00 +0000

The last several decades has seen a complete transition to the use of computers in pulmonary function testing. This has improved Lab efficiency, but it is also the new baseline. Further improvements in technology may improve the reliability and accuracy of test equipment and test results, but it is unlikely to improve PFT Lab efficiency any more than it already has.

Report management, which is really information management, has started but hasn’t yet completed the same technological transition and it is here that significant improvements can still be made. These improvement will not only improve the efficiency of the pulmonary function lab, but also its clinical effectiveness for the physicians and patients that are the lab’s customers.

Exhaled air temperature and Asthma

Sat, 01 Feb 2014 00:00:00 +0000

One of the hallmarks of chronic asthma is airway inflammation. This frequently causes an increase in the perfusion of the airways which in turn can appear as an increased DLCO in routine PFTs. A number of investigators have noted that this inflammation can also cause an increase in exhaled air temperature. This increase in exhaled air temperature is not due to an increase in body temperature but to increase in the rate of heat exchange between the airways and respired air due to the increased airway perfusion.

DLCO: Sample balloon versus real-time gas analysis

Sun, 26 Jan 2014 00:00:00 +0000

The manager of a nearby PFT Lab made an interesting statement recently, and that was that DLCO measurements made with a sample balloon were superior to those made with a real-time gas analyzer. I think that he is biased to some extent by the fact that the manufacturer of his lab’s testing systems only supports the sample balloon approach to DLCO testing and so that is what he is used to. The same can be said of me however, because all of my lab’s equipment performs DLCO tests using real-time gas analysis and that is what I am used to.

The GLFI TLCO Task Force is open for business

Sun, 26 Jan 2014 00:00:00 +0000

Late in 2012 the Global Lung Function Initiative (GLFI) released their reference equations for spirometry. Although not without some criticism this was an important step towards the development of a single spirometry reference equation that can be used by PFT Labs worldwide. The GLFI recently announced that they were accepting data for their TLCO Task Force. This is project similar to what was done for spirometry, and is intended to create a set of reference equations for TLCO (DLCO) that is applicable to all ages and ethnicities.

What’s normal about DLCO?

Thu, 02 Jan 2014 00:00:00 +0000

The lungs are the gas exchange organ of the body. The mechanical aspects of the lung, which do of course have a bearing on gas exchange, can be assessed by spirometry and lung volume tests but for a complete assessment of an individual’s lung function a diffusing capacity test (DLCO) must be performed as well. The actual gas exchange rate at any moment can be highly variable and depends on a number of factors such as cardiac output, pulmonary capillary blood volume and ventilation-perfusion matching that are difficult to measure. For this reason the diffusing capacity test, more so perhaps than any other pulmonary function test, must be performed in a highly standardized way in order to produce results that can be meaningfully trended over time and meaningfully compared to other individuals.

Sawtooth pattern on the flow-volume loop

Sun, 22 Dec 2013 00:00:00 +0000

One of the recognized abnormalities of a flow-volume loop is a sawtooth profile due to flow oscillations that are superimposed on either the maximal expiratory or inspiratory flow curve, or the tidal loop.

Sawtooth pattern on a flow-volume loop

The sawtooth pattern occurs in only a small fraction of patients but it is quite noticeable when you see it. Estimates of the number of individuals with flow oscillation range from 1.4% to 13.4% with the higher estimates being observed primarily with inspiratory loops. Since many spirometry efforts are concentrated solely on exhalation this means that it may frequently go unrecognized. Recently, I had several reports with distinct sawtooth flow-volume loops come across my desk within a short time period and for this reason thought it might be interesting to re-visit this old subject. I call it old only because most of the research on sawtooth profiles was done in the 1970’s and 1980’s and not much has been published since then.

What’s wrong with this picture?

Fri, 20 Dec 2013 00:00:00 +0000

I had mentioned previously that my PFT Lab has been questioned why the percent predicted Residual Volume (RV) measurements on some research patients were coming out so much lower at my lab than at some other PFT Labs. At that time the researchers had not shared the results they had from these patients so we could only speculate that either our RV’s were actually lower or that the predicted RV values used by the other PFT Labs were different than ours. We finally got a copy of the PFT report for one of these patients and it turns out that both answers were correct.

Zero offset in DLCO: system error or patient physiology?

Thu, 12 Dec 2013 00:00:00 +0000

I’ve noticed for a while that there has often been more variance between DLCO tests than I’d like to see. Some of this is of course attributable to differences in the way the patient performs each test. I am not overly surprised to see tests with different inspired volumes, different breath-holding times, different inspiratory times etc. etc. produce different results (in fact I am surprised that so many tests that have been performed differently frequently end up with almost identical results).

FRC baseline shift affects TLC and RV

Sat, 07 Dec 2013 00:00:00 +0000

The plethysmographic technique for measuring lung volumes determines FRC first and then uses a slow vital capacity maneuver to calculate TLC and RV. FRC is defined as the volume of the lung at the end of a normal exhalation. The two components of the SVC maneuver that are used to calculate TLC and RV are the Inspiratory Capacity (IC) and the Expiratory Reserve Volume (ERV) and they too are measured relative to FRC. It would therefore seem to be important to have an accurate notion as to where FRC is in relation to TLC and RV when measuring lung volumes.

CO2 response testing, still crazy after all these years.

Sun, 24 Nov 2013 00:00:00 +0000

I’ve had several exercise tests come across my desk lately where the patient had an elevated Ve-VCO2 slope. An elevated Ve-VCO2 slope during exercise is usually taken as a sign of pulmonary vascular disease however these patients had a normal DLCO so I have been reviewing the literature to try to get a better understanding of what the Ve-VCO2 slope is trying to tell us in these cases.

Although the majority of the literature on Ve-VCO2 response indicates that it is likely due to some form of pulmonary vascular disease (micro-fracturing of the pulmonary capillaries, increased pulmonary vascular resistance, V-Q mismatching) there are some investigators that feel that in some individuals it is more likely due to an increased ventilatory chemosensitivity to CO2. It has been over 25 years since I last performed a CO2 response test and at that time there was no particular consensus on how the test should be performed. Since chemosensitivity may have a distinct bearing on Ve-VCO2 slope I thought it would be a good idea to also review the literature on CO2 response and see what has happened in the meantime.

What does Peak Flow have to do with back-extrapolation?

Wed, 13 Nov 2013 00:00:00 +0000

Well, everything, actually. Surprisingly enough they are intimately related to each other.

One of my current projects is to develop specifications for computer software designed to analyze spirometry results. Determining the “real” start of a spirometry effort using back-extrapolation is a critical part of accurately measuring FEV1 and all other timed values (FEV3, FEV6, TET). The ATS-ERS statement on spirometry includes recommendations for the back-extrapolation process, but this explanation shows its roots in old-school volume-time oriented spirometry:

What’s normal about RV and what does this have to do with TLC?

Fri, 25 Oct 2013 00:00:00 +0000

A physician associated with my PFT lab has become an investigator for a device study intended for patients with severe COPD. One of the major criteria for patients to be able to enroll in this study is a severely elevated Residual Volume (RV). Patients who have met this criteria at other PFT labs in New England have been referred to this study but when they have been re-tested in my lab their Residual Volumes are coming out lower and almost none of these patient have met this criteria. We have been asked why this is the case because they are now having difficulty finding patients that qualify for the study.

DLCO overestimated from a pre-test leak

Sun, 13 Oct 2013 00:00:00 +0000

We’ve been aware of this particular issue for several years. My PFT lab has had some turnover lately and the newer staff aren’t familiar with this problem so it has re-appeared in some of the reports I have been reviewing.

Our lab has a mix of flow-based and volume-based test systems. This problem is peculiar to only the volume-based systems that have a vertically mounted volume-displacement spirometer and is due in part to mechanical issues but also to some underlying assumptions made by the test software.

The FEV3/FVC ratio, a useful tool for assessing early and mild airway obstruction

Sun, 06 Oct 2013 00:00:00 +0000

The FEV1 and FEV1/FVC ratio seems to have become the predominant, if not the sole factor for determining the presence of airway obstruction. It is true that a reduced FEV1/FVC ratio provides a strong and reliable signal for this purpose but its limitations have also been recognized for quite a while. The most obvious one is that the FEV1/FVC ratio will be falsely elevated when the FVC is underestimated. This is the primary factor driving the interest in FEV6 and the FEV1/FEV6 ratio. Less well appreciated is the fact that there are many causes and sites within the airways that can be involved in airway obstruction and that the focus on the FEV1/FVC ratio may cause certain forms of airway obstruction to be overlooked.

The ERV Effect

Wed, 02 Oct 2013 00:00:00 +0000

I used to think that spirometry and diffusion capacity tests were hard and that lung volumes were easy. That may have been true in terms of getting patients to do the tests but I’ve long since come to the conclusion that it is easier to assess the quality of spirometry and diffusing capacity tests and know whether you have reasonably accurate results than it is to do this for lung volumes regardless of which lung volume measurement technique you use.

SNIP when you can’t MIP

Sun, 22 Sep 2013 00:00:00 +0000

The MIP test is used to assess the strength of the inspiratory muscles and is commonly performed in patients with neuromuscular disease. Results however, are often low because the sensation involved in performing a maximal inspiratory effort against an occluded airway is unpleasant and because the MIP requires coordination, cooperation and motivation to be performed correctly. In addition patients with neuromuscular disease frequently lack the muscular strength necessary to grip the mouthpiece and can therefore leak around it. For these reasons although a normal value will rule out significant muscular weakness, a low value can be difficult to interpret.

What’s normal about a 6-minute walk?

Sun, 15 Sep 2013 00:00:00 +0000

The distance attained during a 6-minute walk (6MWD) has been used extensively to assess the functional capacity of patients with a variety of diseases and conditions. It is relatively easy to perform and requires a minimum of equipment. Changes in 6MWD before and after rehabilitation, surgery, or medications are often used to signal the success or failure of these therapies. For the last dozen years every drug and device research study my lab has been involved with has used the 6MWD as one of their outcomes.

When spirometry was amusing

Sat, 07 Sep 2013 00:00:00 +0000

The late 1800’s through the early 1900’s saw the birth and spread of amusement parks, penny arcades and nickelodeons. Although this was due in part to an increase in the number of people living in cities and to an increase in disposable income, it was also in large part due to the invention of mass transit. In a bid to increase ridership many railroad, trolley and subway lines built or sponsored amusement parks.

Marketing your PFT Lab

Mon, 02 Sep 2013 00:00:00 +0000

A writer posed an interesting question on the AARC Diagnostics forum several weeks ago and that was how to market their PFT Lab. I don’t think they got much of a response but I have been thinking about this since then.

I think that any good lab manager wants to see their lab succeed and grow. I’ve always felt that pulmonary function testing is an essential component of preventive care but that despite this PFT Labs are underutilized. In order to market your PFT Lab effectively you need to understand your customers and target your message accordingly. You also need to understand that you can’t get something for nothing. Marketing requires that you expend resources, whether it is just your time or includes lab budget money, in order to get any payback.

When the spirometer was Quack Medicine

Thu, 08 Aug 2013 00:00:00 +0000

The beginning of pulmonary function testing can best be dated to 1846 when John Hutchinson invented the spirometer. Although similar devices had been used by previous scientists he turned it into a precision instrument and approached the study of lung function systematically. His work inspired innumerable other researchers and inventors, and within a few years improved versions of his spirometer were appearing across Europe and America. By my count within twenty years there were at least four other methods for measuring expiratory volume (at least one of which is still in use) had been applied to a new generation of spirometers.

SVC and the FEV1/FVC Ratio

Fri, 02 Aug 2013 00:00:00 +0000

The Slow Vital Capacity (SVC) maneuver is usually performed as part of lung volume measurements. It is not unusual for the SVC to be larger than the FVC, particularly in patients with airway obstruction. This can have a bearing on the FEV1/FVC ratio and in fact the ATS-ERS recommendations for PFT interpretation say that the largest vital capacity value regardless of which test it comes from should be used to calculate the FEV1/VC ratio. When I review a full panel of tests (FVC, lung volumes, DLCO) I always check to see if the SVC or IVC (from the DLCO test) are larger than the FVC and then re-calculate the FEV1/FVC ratio and its percent predicted if they are. Test results that at first glance look normal will instead show airway obstruction often enough when this has been done that the time spent going through this process is worthwhile.

Don’t ignore office spirometry

Wed, 31 Jul 2013 00:00:00 +0000

My PFT Lab has recently been asked by several doctor’s offices and clinics to advise them on the purchase of an office spirometry system. I am not a fan of office spirometry because I think the test quality is often low. Office spirometry is usually performed by poorly trained office staff using poorly maintained equipment and under these conditions quality is going to suffer. Despite my misgivings the reality is that office spirometry is not going away and in fact its use is probably expanding.

How to HAST

Mon, 22 Jul 2013 00:00:00 +0000

Patients that have problems with oxygenation at sea level are going to have even more problems at higher elevations where the barometric pressure and oxygen partial pressure are lower. During commercial aircraft travel the cabin pressure is required by U.S. Federal regulations to be pressurized to at least 565 mm Hg which is the equivalent of 8000 feet altitude. It has been reported that most airliners are pressurized to an equivalent altitude of between 5000 and 8000 feet but this will depend on both the airplane and the airline in question.

Height and the GLFI FVC, FEV1 and FEV1/FVC ratio

Tue, 25 Jun 2013 00:00:00 +0000

Although the authors of the Global Lung Function Initiative (GLFI) study acknowledge the effect of height on their reference equations the range and distribution of heights in its study populations was not included in the report. This was a similar problem for the NHANESIII reference equations since the height range was never reported within the text of the original report however it did include scatter graphs showing the range of heights. These graphs imply the height range was 162 to 194 cm (64” to 76”) for caucasian males and 145 to 180 cm (57” to 71”) for caucasian females. Using the extremes of these height ranges it is interesting to see how the GLFI reference equations compare to the NHANESIII reference equations.

PETCO2 during exercise, a quick diagnostic indicator

Thu, 20 Jun 2013 00:00:00 +0000

I was reviewing a cardiopulmonary exercise test (CPET) recently. The test was part of a pre-op workup for a patient with lung cancer who also had a diagnosis of COPD. I had looked at the spirometry results first (we always do spirometry pre- and post-exercise) and seeing that the patient had severe airway obstruction (FEV1 < 50% of predicted) assumed the review would be relatively straightforward. I then saw just one exercise test value and knew immediately that this wasn’t going to be an ordinary test. That test value was the PETCO2 at anaerobic threshold, which happened to be 40.

GLFI and the FVC, FEV1 and FEV1/FVC Ratio

Sat, 15 Jun 2013 00:00:00 +0000

The Global Lung Function Initiative (GLFI) was established by the European Respiratory Society in 2008 with the goal of establishing a truly worldwide set of reference equations for spirometry. Its results were released in the December 2012 issue of the European Respiratory Journal. Although the reference equations presently apply only to Caucasians, African-Americans and northern and southern Asians, it will likely be updated with Hispanic, African and Polynesian data within the next couple of years.

The difficulties in reporting multiple spirometry post efforts.

Sat, 08 Jun 2013 00:00:00 +0000

It’s always fascinating how a simple problem can blossom into something much more complicated. Ninety-five percent of the time when we need to report post test results for spirometry they are for post-bronchodilator. The remaining times they are for supine or post-exercise spirometry. At least once or twice a month however, we need to report two post results, not just one and this is where we run into problems.

The most common scenario for this is when we need to report a baseline result and then both a post-exercise result and then a post-bronchodilator result. We need to do this when a patient bronchospasms during or following an exercise test and we find need to give them a bronchodilator after performing the post-exercise spirometry. Less commonly we are occasionally requested to perform both supine and post-bronchodilator spirometry as part of the same visit. Whenever either of these situations occurs we need to perform a number of work-arounds that end up taking extra time and effort.

The origin of FEV1 and the FEV1/FVC ratio

Mon, 03 Jun 2013 00:00:00 +0000

Over forty years ago when I started to learn about pulmonary function testing I was taught about the nuts and bolts of the tests, not where the tests came from or how they came into existence. Spirometry and the FVC, FEV1 and FEV1/FVC ratio have always appeared to be the core elements of pulmonary function testing and seemed most likely to have been scratched on the wall of the first paleolithic PFT lab cave. I have been reviewing a lot of older research papers lately, including several historical reviews, and was quite surprised to find out how recent both FEV1 and the FEV1/FVC ratio really are.

DLCO overestimated from an apparent zero offset error

Thu, 23 May 2013 00:00:00 +0000

I’ve had some concerns for a while now about how the CO and CH4 concentrations are being calculated from the DLCO analyzer calibration zero offsets and gains on our test systems. For this reason I’ve been looking carefully at all of the raw data from our DLCO tests and today I came across an oddball test result. There are several reason why this is probably not the best example for this particular problem that I could come up with but it illustrates an important point and it’s in front of me so I’ll go with it.

Tidal Expiratory Flow Limitation and NEP

Mon, 20 May 2013 00:00:00 +0000

Expiratory flow limitation exists when increasing expiratory effort fails to increase the expiratory flow rate. This can been seen when a patient exhales along the same flow-volume curve during quiet, tidal breathing as they do during a forced expiratory effort. Tidal expiratory flow limitation (EFLt) is a key concept in understanding dyspnea, hyperinflation and orthopnea in patients with COPD and in assessing the functional severity of COPD.

The notion that EFLt could be detected through flow-volume loops was proposed in the 1960’s but subsequent research has shown this approach to be unreliable for a variety of moderately esoteric reasons which include the differences in static lung recoil and airway resistance that occur following a deep inhalation, differences in time-dependent lung emptying and the viscoelastic forces in the lung. When tidal flow limitation as seen on flow-volume loops is compared to other techniques it is apparent that flow-volume loops overestimate the presence of EFLt by approximately one-third.

Odd results from a leak during a DLCO test

Wed, 15 May 2013 00:00:00 +0000

One of our testing systems has been leaking during the breath-holding period of DLCO tests. This has been an on-again, off-again problem caused by a leaking balloon valve. The odd thing about this is that the results from tests that show a patient leak are often higher than the results where they don’t.

This is not the pattern I would have expected. My initial thought would be that in the DLCO tests with a leak, the patient is spending more time at a lung volume below TLC which in turn means a lower surface area. A lower surface area should lead to a lower DLCO but this doesn’t appear to be happening.

Should PFT Lab staff be credentialed?

Fri, 10 May 2013 00:00:00 +0000

The issue of PFT staff licensure or certification has recently been a topic of discussion on the LinkedIn Pulmonary Function Studies group and the Diagnostics section of the AARConnect discussion board. The overwhelming majority of those posting comments have been in favor of some form of licensure or certification. CPFT (Certified Pulmonary Function Technician) and RPFT (Registered Pulmonary Function Technician) certification is discussed most often but not surprisingly on the AARConnect board most posters feel that RRT (Registered Respiratory Therapist) certification should be required as well.

FEV6

Tue, 07 May 2013 00:00:00 +0000

FEV6 (the volume of air forcefully exhaled after 6 seconds) has been proposed as a replacement or surrogate for FVC in spirometry. Given that using FEV6 would simplify and speed up the spirometry test this is a seductive notion.

The use of an expired volume with a fixed expiratory time as a replacement for FVC was proposed at least 25 years ago, although at that time FEV7 was proposed as being slightly more accurate than FEV6. The first reference values for FEV6 however, were not available until the results from NHANESIII study were made available in 1999 and most studies of FEV6 post-date that.

QC and the DLCO Simulator

Wed, 10 Apr 2013 00:00:00 +0000

I’ll start by saying that I am not associated with Hans Rudolph Incorporated in any way. I just think they make a bunch of good products, and more specifically in this case, a DLCO simulator.

For as long as I can remember I have always had intermittent problems with DLCO testing and I suspect that every PFT Lab has had them at some time. One of my current concerns has to do with how our test system’s software is calculating exhaled CO and CH4 concentrations from calibration data, but I’ve had other concerns at other times too. One way around to verify that your test system is actually performing the way it should is biological QC but a more precise way is to use a DLCO simulator.

Chronotropic Index and O2 Pulse

Sun, 24 Mar 2013 00:00:00 +0000

One of the things that I enjoy most about reviewing cardiopulmonary exercise tests is that I always have to reach back into the basics of physiology in order to get a sense of what the results are trying to say. Oftentimes it isn’t so much the absolute or percent predicted value of a given parameter but its relationship with other parameters that is revealing. One of the bits of human physiology that has always struck me as fascinating is the relationship between heart rate and oxygen consumption.

DLCO Dilemma

Sat, 23 Mar 2013 00:00:00 +0000

For the last several months I’ve noticed what appears to be a greater than normal number of patient test results where the VA volume from the DLCO test was greater than the TLC. This is not impossible of course, but it usually tends to be more on the rare side and when I’ve seen this in the past and inspected the results closely there were usually either problems with the lung volume test or the difference was only a few percent and within the error bar for both tests. We’ve been seeing VA’s that were larger than TLC more frequently lately but when I look at the results closely most of the time I have been unable to see anything wrong with either the lung volume or the DLCO test. At the same time, we have a number of patients that are frequent fliers and have seen what looks to be bigger differences in DLCO from visit to visit than usual as well as a number of patients that have had larger DLCO results than we would have expected.

Equipment is not detecting glottal closure and cough

Sat, 16 Mar 2013 00:00:00 +0000

In my last blog on personal spirometers I mentioned that one of the university projects has developed software that assesses spirometry results for ATS-ERS standard including cough, glottal closure and early termination of exhalation. The specific project that claimed this also claimed to have attained a 70% detection rate using waveforms from the NHANESIII study. This should be taken as a challenge by the pulmonary function equipment manufacturers to improve their existing software.

Personal spirometers

Mon, 04 Mar 2013 00:00:00 +0000

Peak flow meters, both mechanical and electronic, have been available to asthma patients for years. A number of inexpensive spirometers that meet ATS-ERS standards are now available for little as $500. Although these spirometers are primarily intended for a doctor’s office and not intended for self-monitoring by asthmatics, a number of even less expensive spirometers intended for personal use have recently appeared on the market.

Additionally, in the last couple of years there have been at least four different university engineering projects to develop a low-cost spirometer, with a goal of costing substantially less than $100 and are intended for self-monitoring or for use in third world countries. Although these could be considered to be demonstration projects, several have the potential to become viable products.

Is it time to scuttle the FEF25%-75%?

Mon, 25 Feb 2013 00:00:00 +0000

When we went through our hardware and software upgrade last August, one of the changes we made was to stop reporting the FEF25%-75% (AKA MMEF, MMFR, MMF). The pulmonary physicians had long since stopped using this value when assessing spirometry results and we had kept it on our reports as long as we did only for inter-laboratory compatibility. Along with other changes we made at that time we decided it was time to drop the FEF25%-75% off our reports.

Problem selecting an FEV1 highlights a training failure

Fri, 22 Feb 2013 00:00:00 +0000

A report of a patient’s complete set of PFTS came across my desk yesterday. There were a lot of inconsistencies in the report and I ended up looking at the raw data for every single test. When I looked at the spirometry results I was surprised to see which effort the technician had selected to report the FEV1.

The patient’s spirometry efforts were highly variable and not terribly reproducible. The FEV1 that had been selected came from an effort with an expiratory pause that occurred before the first second and with a number of cough artifacts. There was another spirometry effort effort however, that did not have a pause and had a larger FEV1. The reason the other effort had not been selected was that it did not meet ATS-ERS criteria for back-extrapolation whereas the first effort did.

PACO and DLCO

Sat, 16 Feb 2013 00:00:00 +0000

Patients are advised not to smoke prior to DLCO testing, primarily because it increases carboxyhemoglobin. The effect of COHb on DLCO has been well studied, but COHb is not often measured before DLCO testing. Alveolar carbon monoxide (PACO) can be measured however, and there is a good correlation between PACO and COHb.

The single-breath DLCO calculation assumes that PACO is zero. At first glance this is a reasonably good approximation since a non-smoker will normally have a PACO of less than 5 ppm. This is no more than 0.17 percent of the 0.3% CO concentration (3000 ppm) used for testing which is negligible. Even so, smokers can have significantly elevated COHb levels and COHb increases during testing.

DLCO, by the numbers

Sat, 09 Feb 2013 00:00:00 +0000

I was recently contacted by a physician looking for an illustration or diagram to help make gas exchange and DLCO more understandable. We’ve all seen the diagram of the alveoli with a capillary stretched around it and with oxygen and carbon dioxide exchanging across the membrane. I think I first saw it in Comroe’s ‘The Lung” published in the 1960’s, but it may well be older than that. It’s hard to improve on this and dozens of versions have been made of it.

Should complete PFTs always be done on a first visit?

Thu, 31 Jan 2013 00:00:00 +0000

This is not something I have any real influence over because the tests ordered on a patient’s first visit to the PFT lab are going to be determined by the ordering patterns of the referring physician and not by what I think. It’s still a worthwhile question, however.

There are no standards for PFT ordering. There are recommendations from the ATS, ERS, ACCP and NIH regarding patient diagnosis and treatment for a variety of pulmonary diseases and buried inside of them are some guidelines for PFT tests. What I’ve seen, however, is that these guidelines are honored far more in the breach than in their observance. As an example, for asthmatics the NIH recommends spirometry during an initial visit, after asthma has been stabilized, during an exacerbation, after an exacerbation and at least every 1 to 2 years otherwise. How often do you see this guideline followed in more than spirit?

VA versus TLC, what this can tell you about test quality

Sat, 19 Jan 2013 00:00:00 +0000

I’ve had a bug lately about lung volumes and I guess that today is no different. A report with some odd lung volume results came across my desk and I’ve spent some time trying to figure out what the numbers are telling me about test quality.

What concerned me was the 15% discrepancy between the VA from the DLCO and the TLC measured by plethysmography. VA is measured from the insoluble component of the DLCO gas mixture (methane in this case) and is almost always less than TLC. VA is a single-breath measurement and for VA and TLC to be close this usually means the patient must have very good gas mixing inside their lungs. Even when the quality of the DLCO and lung volume tests are good however, VA is almost always less than TLC. So why was VA so much larger than TLC in this case?

VA, DLCO and COPD

Mon, 14 Jan 2013 00:00:00 +0000

Although the technology used to perform the single-breath DLCO test has improved since it was first developed in the 1950’s the essential concepts and equations have not changed significantly. Probably the most important advance has been the introduction of rapid response real-time gas analyzers in the 1990’s. Prior to that time the patient’s washout and sample volumes had to be preset which always involved a certain amount of guesswork when a patient was significantly obstructed or restricted. With a real-time gas analyzer it is possible inspect the exhaled gas tracings after the test has been performed in order to determine when washout has occurred and then select the appropriate location for the sample volume. This has improved the single-breath DLCO test quality but at the same time it has also exposed some of its limitations.

Open-access on-line medical journals

Thu, 10 Jan 2013 00:00:00 +0000

I have been researching different pulmonary function topics for quite a few years. The medical libraries I’ve frequented were, of course, originally all paper-based and to be able to find an article the library or one of the department’s physicians had to subscribe to the journal in question. In the last fifteen years we have all seen an explosion in on-line publishing and it has become much easier to research articles. I vigorously applaud the pulmonary medicine journals (Chest, Amer__ican J__ournal of Resp__iratory and Crit__ical Care Med__icine_, Eur__opean_ Resp__iratory J__ournal_, J__ournal of_ Appl__ied Physiol__ogy_, Thorax)_ for having opened part or all of their archives to anybody who wants to search and download articles.

N2 washout drift throws another curve.

Thu, 03 Jan 2013 00:00:00 +0000

Just when we thought it was safe to go back in the water, we’ve run into another N2 washout-related problem. Although it probably affects the TLC and RV calculations in a minor way, it was actually noticed in relation to spirometry.

When spirometry is reviewed in my lab the FVC is compared to the SVC, if one has been performed. If the SVC is greater than FVC, then the FEV1/VC ratio is recalculated using the SVC. This is in line with the ATS recommendations on interpreting spirometry and does occasionally throw up a patient with airway obstruction that otherwise would not have been detected.

Re-breathing DLCO, another almost unknown technique

Fri, 28 Dec 2012 00:00:00 +0000

Re-breathing DLCO may not be well known because this technique has not been used in routine clinical testing. It has instead been primarily used to research diffusing capacity during exercise because it is able to do this without the need to interrupt or significantly alter a subject’s breathing pattern.

Re-breathing DLCO is probably best thought of as a hybrid between the single-breath DLCO and steady-state DLCO techniques. The gas mixture and calculations are from the single-breath world but the breathing pattern is more from the steady-state side of the family.

It’s after midnight, do you know where your reports are?

Wed, 26 Dec 2012 00:00:00 +0000

After the tests themselves, the second most important thing that a Pulmonary Function Lab needs to do is to report results. Like a tree falling in the woods, if a report hasn’t gotten in front of the person that ordered the tests, has anything happened? A successful PFT Lab needs to manage its reports effectively.

An obstacle that many labs will need to overcome is the reporting software that comes with their testing systems. I think that for many of the PFT equipment manufacturers reports are an afterthought at best. The ability to format reports to a specific lab’s needs is often limited and is just as often is a difficult and time-consuming process.

Steady-State DLCO, an almost-forgotten technique

Sat, 22 Dec 2012 00:00:00 +0000

Recently I have been reviewing a lot of early pulmonary function research. I’m not feeling nostalgic but I think that re-visiting some of the older methods and technology may be interesting. Almost all Pulmonary Function laboratories presently use the single-breath technique to measure diffusing capacity. There are a number of reasons why this is case but I suspect that many technicians and physicians aren’t aware that there are several alternative methods for measuring diffusing capacity and that there was a time when at least one of these, the steady-state DLCO, was routinely used in clinical labs.Diffusing capacity, regardless of technique, is determined by the rate of CO uptake, divided by the driving (alveolar) pressure:

Busted for speeding during an N2 washout

Mon, 17 Dec 2012 00:00:00 +0000

Nitrogen washout lung volumes are still relatively new to my PFT Lab. The number of problems we’ve encountered has decreased substantially but we are still learning some of the idiosyncrasies of the system. Recently while trying to understand a test with odd results we were reminded by the manufacturer that during the washout period a patient’s inspiratory and expiratory flow rates should not exceed 1.5 liters/second. The reason this “speed limit” is necessary highlights some of the limitations of modern open-circuit lung volume measurements.

Plethysmography and “trapped air” demystified

Sat, 01 Dec 2012 00:00:00 +0000

Recently I was reading the blog of someone who teaches Pulmonary Function testing and they stated:

“…in emphysema and air trapping, the VTG (thoracic gas volume) will be higher than a FRC (functional residual capacity) measured by Helium dilution and Nitrogen washout. This is because VTG is the volume of gas contained in the thorax, whether in communication with the airways or trapped in the thorax.”

This unfortunately is not correct, but it is a common misconception and since there was a time when I believed it myself I find it difficult to fault the author too much. I think it’s time to return to the basics of plethysmographic lung volumes however, and show why this is not true.

What’s normal about the FEV1/FVC ratio?

Sun, 25 Nov 2012 00:00:00 +0000

The FEV1/FVC ratio is used to estimate the presence and degree of airway obstruction. For well over thirty years my lab has used an FEV1/FVC ratio of 95% of predicted as the cutoff for normalcy. This value (carved onto a stone tablet by the way) had been brought to the lab by a founding physician who had come to the department from the NIH in the 1970’s. Since the software and hardware upgrade this summer our PFT Lab has switched to the NHANES III spirometry reference equations but we have so far resisted changing our 95% cutoff to the lower limit of normal (LLN). This is due in part to inertia but also in part to a mistrust in the concept of LLN. We have been steadily re-evaluating all of our testing criteria and have turned again to the FEV1/FVC ratio with the question as to whether our 95% cutoff is over-zealous or whether the LLN is too lax.

N2 washout switch-in error

Thu, 15 Nov 2012 00:00:00 +0000

I have been taking a close look at the raw data from all lung volume tests lately in large part because N2 washouts are still relatively new to my PFT Lab and we’re continuing to learn from our mistakes. When I saw this N2 washout test I knew that there was something wrong with it. The patient had performed three N2 washout tests and the TLC, FRC and RV for this one test were significantly larger than for the other tests. The most common problem we’ve been having with N2 washouts has been with patient leaks during the washout period which almost always show up as an upwards drift in the tidal baseline. This test did not show any drift however, and it took me a little while before I could see what was wrong with it.

Height and arm-span

Wed, 07 Nov 2012 00:00:00 +0000

The PFT Lab I am associated with has been making a point of having the technicians re-measure patient height with each visit. Part of the reason for this is that several years ago the medical assistants in the pulmonary outpatient clinic were tasked with obtaining patient heights, weights and blood pressures. For a period of time the technicians used these heights when entering the patient demographic information but it was soon noticed that patient heights often changed by several inches from visit to visit. For this reason we have asked the technicians to re-measure patient height instead.

Peak VO2 and low body weight

Wed, 31 Oct 2012 00:00:00 +0000

The PFT Lab I am associated with performs cardiopulmonary exercise tests for pre-op cardiothoracic surgery patients with lung cancer. Surgeons have to make a decision to operate or not based at least in part on the amount of function they think will remain after a lung resection and this depends on which and how many lobes are involved. This calculation is done by taking a percentage based on the amount of lung tissue that will be lost which is weighted using ventilation-perfusion scan results off the baseline DLCO and vital capacity. When the result is inconclusive or there are other medical factors that can affect a patient’s prognosis the results from a CPET can help with the surgeon’s decision.

Plethysmographic Puzzle

Fri, 26 Oct 2012 00:00:00 +0000

This PFT report came across my desk the other day. At first glance it looked wrong because when was the last time you saw a patient with a FRC that was 165% of predicted?

Lung volumes were done by plethysmography and I took a close look at the TGV loops (mouth pressure vs box volume) and they had excellent quality and reproducibility, about as good as you can ever expect. When I looked at the volume data however, it was apparent that the patient leaked during the panting maneuver while the shutter was closed.

Poor quality test results. When do you accept them?

Wed, 24 Oct 2012 00:00:00 +0000

The ATS/ERS have published standards for most Pulmonary Function tests. These standards include the criteria that are used for assessing test quality. What these standards don’t address however, is how poor a test’s quality can be yet still be acceptable for reporting.

We’ve all had patients that aren’t able to perform tests correctly. There can be many reasons why this happens. Lack of comprehension due to cultural or language barriers. Dementia. Extreme shortness of breath. Fatigue. Neuromuscular disease. Whatever the reason, you’ve done the tests with a patient and gotten the best you think you can get, how bad do they have to be before you don’t report them?It could be argued that any test results that do not meet ATS/ERS criteria should not be accepted or reported. If this was the case, however, many of our patients would never have any test results reported at all so there has to be some leeway. An opposing argument could be that any test result should be accepted because it at least shows the minimum a patient is capable of. But because poor quality results can imply the presence of a serious lung disease when it may not actually be present there has to be a point at which results should not be accepted.

What do you say to your patients and how do you say it?

Mon, 22 Oct 2012 00:00:00 +0000

Pulmonary Function testing is almost unique among medical tests in that it requires maximum effort and cooperation from the patient for quality results. The better you are able to communicate with a patient the more likely you are to be able to get good quality test results. This is one of the more interesting and the more difficult parts of being a technician.

Performing tests means being a cheerleader to some extent and over the years I’ve seen a number of different approaches. At one extreme, which thankfully I haven’t seen for a while, is what I call the “Nazi” approach which is mostly yelling and telling the patient as condescendingly as possible that they aren’t doing the test right and to do it again. At the other extreme is the “Flower Child” who is chatty, sympathetic, cheerful and can never, ever bring themselves to tell the patient they are doing anything wrong.

DL/VA is really K in disguise

Thu, 18 Oct 2012 00:00:00 +0000

One misconception about DLCO results that I’ve heard over and over is that DLCO/VA is DLCO “adjusted” for lung volume and that using it is a way of “normalizing” DLCO for lung volume. This is not true and the information that DL/VA can give you about a patient’s lung without also considering VA as well is very limited and can be misleading.

WARNING, MATH AHEAD!

The equation for calculating DLCO is:

Evaluating spirometry predicted equations for the elderly

Fri, 12 Oct 2012 00:00:00 +0000

I’ve mentioned previously that the PFT Lab I am associated with recently went through a major hardware and software update. As part of this update we decided to change spirometry predicted equations to NHANESIII. The lab has used the Morris equations for at least the last 25 years and this has caused us to revisit a number of issues associated with interpretation of results, one of which is age.

The software update included the NHANESIII equation set but when we selected it we found that the software would not calculate predicted values for patients over the age of 80. The manufacturer said that this was because that was as far as the age range went in the original NHANESIII study and for this reason they could not extend it. Furthermore, their recommendation was to use the Crapo or Knudsen equations for ages above 80 because they were “more linear”.Our PFT Lab has a substantial population of patients who are older than 80 and I suspect that this is true for many other PFT Labs as well. Our decision was to use the NHANESIII equations for all ages because the study they came from has the largest number of well characterized subjects and that the results were extensively analyzed. We also felt that using a different equation set above 80 would lead to discontinuities in the predicted values.

Is the MVV clinically relevant?

Wed, 10 Oct 2012 00:00:00 +0000

The respiratory system is in part a mechanical pump or bellows. The Maximum Voluntary Ventilation test (MVV, aka Maximum Breathing Capacity, MBC) is intended to measure the maximum ventilation a patient is capable of. As such the results are dependent on a patient’s lung volume, respiratory muscle strength and endurance, airway resistance and overall inertia of the thoracic cage.

When I started doing PFT’s in the early 1970’s the MVV was a standard part of a complete workup. This has long since changed and I have not performed the MVV test routinely in over 25 years but I’ve always wondered what the MVV test is actually supposed be measuring in a clinical sense.

The FIVC can reveal expiratory obstruction

Sat, 06 Oct 2012 00:00:00 +0000

This spirometry report came across my desk today.

Obs: %Pred: Pred:

FVC (L): 5.14 101% 5.10

FEV1(L): 4.07 105% 4.07

FEV1/FVC: 79 104% 76

On the face of it, this looks like an eminently normal spirometry test. If you look carefully at the flow-volume loop however, you will see that the inhaled volume is greater than the exhaled volume. The test system did not report the inspiratory vital capacity (FIVC) but by eyeball I would estimate the difference to be about 0.60 liters. The ATS recommends that the largest vital capacity, regardless of where it comes from should be used to calculate the FEV1/VC ratio. This means that the FEV1/FIVC ratio is actually about 70.9 not 79.2 which is 93% of predicted, not 104% and that this is probably mild airway obstruction.

Supine spirometry

Thu, 04 Oct 2012 00:00:00 +0000

If you are not already performing supine spirometry you should consider adding it to your arsenal of tests. Other than an exam table no new equipment is needed to perform it and it is a simple technique that can provide useful information towards diagnosing and monitoring diaphragmatic dysfunction. It is non-invasive when compared to a transdiaphragmatic pressure test (which requires an esophageal balloon), does not require ionizing radiation (fluoroscopy) and is likely more accurate and better tolerated than MIP and MEP tests. Candidates for this test include patients with neuromuscular diseases, suspected or known diaphragmatic paralysis or any patient complaining of dyspnea that cannot be explained by other routine testing.

Lung volume measurements and software smarts.

Wed, 03 Oct 2012 00:00:00 +0000

Many years ago I used to think that lung volume measurements were the easy part of PFTs. As time has gone on I’ve seen that getting accurate lung volume measurements is actually more difficult than getting accurate spirometry and DLCO results mostly because the errors tend to be subtle.

The errors that occur in lung volume measurements tend to cause an overestimation of lung volumes. This often means that restrictive diseases can be unrecognized or that hyperinflation and gas trapping can be diagnosed where it does not exist.

Why haven’t computerized interpretations gotten any better?

Tue, 25 Sep 2012 00:00:00 +0000

Almost all pulmonary function test systems seem to come with a module that can perform a computerized interpretation of PFT results. Their accuracy has been studied occasionally, often by the developers of a particular algorithm and just as often a rosy picture is painted. Given their limited (and likely pre-cleaned) data sets I am sure this is accurate as far as it goes. I have done my own admittedly very unscientific comparison and would say that for two-thirds of the patients tested the results are probably okay. The other third? Varying degrees of not so much.

FEV1/FVC ratio and height

Fri, 21 Sep 2012 00:00:00 +0000

The PFT Lab I work with has recently gone through a major software and hardware upgrade. As part of this process we made the decision to switch our spirometry predicted equations to NHANESIII. The lab has been using the Morris predicteds for at least the last 25 years and this switch has led us to re-visit some of the issues involved in interpreting spirometry results.

More than one person that I’ve known and respected has said that spirometry is all about FEV1 and I think this is a true statement. There is a lot of other information you can get from a Forced Vital Capacity but it always comes back to FEV1.

Mouthpieces and test quality

Sun, 16 Sep 2012 00:00:00 +0000

Mouthpieces serve at least two purposes. The first is to prevent cross-contamination among patients using PFT equipment. The ATS Statement on General Considerations for Lung Function Testing, (Eur Resp J, 2005; 26: 153-161) discusses this and it is clear that mouthpieces should be used whenever and wherever pulmonary function tests are performed.

The second purpose however, is to prevent patients from leaking air during testing. Leaks are a chronic problem for all pulmonary function tests and my experience has been that when they occur it is almost always the patient that is leaking and not the equipment.

Single-breath TLC measurements

Sun, 16 Sep 2012 00:00:00 +0000

I was reviewing the specifications of different pulmonary function test systems recently and saw that several manufacturers were advertising that some of their test systems are able to measure TLC, FRC and RV from a single-breath maneuver. This is true, but only to a very limited degree and I think it is reasonable to ask how accurate and clinically useful these measurements are and whether it is legitimate to bill for the test.

Why isn’t PAO2 measured during DLCO tests?

Sun, 16 Sep 2012 00:00:00 +0000

Most pulmonary technicians and physicians are aware that a patient’s hemoglobin and carboxyhemoglobin levels are factors that affect DLCO test results. What may be less well appreciated is the degree to which DLCO varies with the alveolar oxygen concentration.

Oxygen and carbon monoxide both compete for uptake by hemoglobin so when alveolar O2 is low or high the measured DLCO will also vary accordingly. The relationship between DLCO and PAO2 has been studied both in a regards to FIO2 and to barometric pressure. Both empirically and mathematically DLCO is estimated to change by approximately 0.35% per mmHg PAO2 change.