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

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.

We have not been given access to the original PFT reports for these patients and have not been able to actually compare results on a case by case basis. For this reason we can only offer two possible reasons. First, that my lab may not be using the same reference equations for RV that other labs are. Second, that these patient’s RV’s may have been overestimated at other labs because of errors in testing.

To compare predicted RV’s I was able to find a dozen different reference equations for RV in adult males and females. These equations are mostly for Caucasian populations, but I was also able to find at least one reference equation each for Black, Asian, Indian, Iranian and Brazilian populations as well.

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DLCO overestimated from a pre-test leak

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.

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

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 FEF25-75 (aka MMEF) was originally proposed as way to determine the presence of small airways disease but it has since been shown to be an unreliable indicator. Most of the pulmonary physicians I work with have expressed doubt that there is such a thing as small airways disease but that doesn’t mean that some patients don’t have mild airway obstruction that is not evident when assessed solely by the FEV1/FVC ratio.

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The ERV Effect

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.

I was reviewing a set of plethysmographic lung volume tests when I noticed something very odd about the reported results. I usually look at just the VTG loops and the volume-time graphs in order to assess test quality. The testing software automatically selects and averages all VTG efforts and when I reviewed them there were a couple loops that were poor quality and I manually de-selected them. I was reviewing this report because the reported lung volume results didn’t quite match what the spirometry results were saying so this time I also took a close look at the numbers after I removed the low-quality loops. That’s when I realized that the reported TLC was larger than the two tests it was averaged from.

Pleth Math

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