One of the things that I enjoy most about reviewing cardiopulmonary exercise tests is that I always have to reach back into the basics of physiology in order to get a sense of what the results are trying to say. Oftentimes it isn’t so much the absolute or percent predicted value of a given parameter but its relationship with other parameters that is revealing. One of the bits of human physiology that has always struck me as fascinating is the relationship between heart rate and oxygen consumption.

For the last several months I’ve noticed what appears to be a greater than normal number of patient test results where the VA volume from the DLCO test was greater than the TLC. This is not impossible of course, but it usually tends to be more on the rare side and when I’ve seen this in the past and inspected the results closely there were usually either problems with the lung volume test or the difference was only a few percent and within the error bar for both tests. We’ve been seeing VA’s that were larger than TLC more frequently lately but when I look at the results closely most of the time I have been unable to see anything wrong with either the lung volume or the DLCO test. At the same time, we have a number of patients that are frequent fliers and have seen what looks to be bigger differences in DLCO from visit to visit than usual as well as a number of patients that have had larger DLCO results than we would have expected.

In my last blog on personal spirometers I mentioned that one of the university projects has developed software that assesses spirometry results for ATS-ERS standard including cough, glottal closure and early termination of exhalation. The specific project that claimed this also claimed to have attained a 70% detection rate using waveforms from the NHANESIII study. This should be taken as a challenge by the pulmonary function equipment manufacturers to improve their existing software.

Peak flow meters, both mechanical and electronic, have been available to asthma patients for years. A number of inexpensive spirometers that meet ATS-ERS standards are now available for little as $500. Although these spirometers are primarily intended for a doctor’s office and not intended for self-monitoring by asthmatics, a number of even less expensive spirometers intended for personal use have recently appeared on the market. Additionally, in the last couple of years there have been at least four different university engineering projects to develop a low-cost spirometer, with a goal of costing substantially less than $100 and are intended for self-monitoring or for use in third world countries. Although these could be considered to be demonstration projects, several have the potential to become viable products.

When we went through our hardware and software upgrade last August, one of the changes we made was to stop reporting the FEF25%-75% (AKA MMEF, MMFR, MMF). The pulmonary physicians had long since stopped using this value when assessing spirometry results and we had kept it on our reports as long as we did only for inter-laboratory compatibility. Along with other changes we made at that time we decided it was time to drop the FEF25%-75% off our reports.

A report of a patient’s complete set of PFTS came across my desk yesterday. There were a lot of inconsistencies in the report and I ended up looking at the raw data for every single test. When I looked at the spirometry results I was surprised to see which effort the technician had selected to report the FEV1. The patient’s spirometry efforts were highly variable and not terribly reproducible. The FEV1 that had been selected came from an effort with an expiratory pause that occurred before the first second and with a number of cough artifacts. There was another spirometry effort effort however, that did not have a pause and had a larger FEV1. The reason the other effort had not been selected was that it did not meet ATS-ERS criteria for back-extrapolation whereas the first effort did.

Patients are advised not to smoke prior to DLCO testing, primarily because it increases carboxyhemoglobin. The effect of COHb on DLCO has been well studied, but COHb is not often measured before DLCO testing. Alveolar carbon monoxide (PACO) can be measured however, and there is a good correlation between PACO and COHb. The single-breath DLCO calculation assumes that PACO is zero. At first glance this is a reasonably good approximation since a non-smoker will normally have a PACO of less than 5 ppm. This is no more than 0.17 percent of the 0.3% CO concentration (3000 ppm) used for testing which is negligible. Even so, smokers can have significantly elevated COHb levels and COHb increases during testing.

I was recently contacted by a physician looking for an illustration or diagram to help make gas exchange and DLCO more understandable. We’ve all seen the diagram of the alveoli with a capillary stretched around it and with oxygen and carbon dioxide exchanging across the membrane. I think I first saw it in Comroe’s ‘The Lung” published in the 1960’s, but it may well be older than that. It’s hard to improve on this and dozens of versions have been made of it.

This is not something I have any real influence over because the tests ordered on a patient’s first visit to the PFT lab are going to be determined by the ordering patterns of the referring physician and not by what I think. It’s still a worthwhile question, however. There are no standards for PFT ordering. There are recommendations from the ATS, ERS, ACCP and NIH regarding patient diagnosis and treatment for a variety of pulmonary diseases and buried inside of them are some guidelines for PFT tests. What I’ve seen, however, is that these guidelines are honored far more in the breach than in their observance. As an example, for asthmatics the NIH recommends spirometry during an initial visit, after asthma has been stabilized, during an exacerbation, after an exacerbation and at least every 1 to 2 years otherwise. How often do you see this guideline followed in more than spirit?

I’ve had a bug lately about lung volumes and I guess that today is no different. A report with some odd lung volume results came across my desk and I’ve spent some time trying to figure out what the numbers are telling me about test quality. What concerned me was the 15% discrepancy between the VA from the DLCO and the TLC measured by plethysmography. VA is measured from the insoluble component of the DLCO gas mixture (methane in this case) and is almost always less than TLC. VA is a single-breath measurement and for VA and TLC to be close this usually means the patient must have very good gas mixing inside their lungs. Even when the quality of the DLCO and lung volume tests are good however, VA is almost always less than TLC. So why was VA so much larger than TLC in this case?