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.

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).

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.

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):

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.

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.

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.

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.

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.

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.