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.

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.

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.

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.

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.

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: 

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.

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.

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.

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.