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.

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.

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.

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).

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.

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:

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).

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.

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.

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.