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.

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).

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.

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:

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%.

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.

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.

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:

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:

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.