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 almost everyone there is a linear relationship between heart rate and oxygen consumption. When you plot them against each other you can put a ruler on the plotted points and see that they form a straight line. Chronotropic Index and O2 pulse are two ways of analyzing this relationship.
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.
The problem is that these apparent problems are really just suspicions with very little real evidence. I’ve been paying very close attention to lung volumes since our hardware and software upgrade last summer so my paranoia level is on the high side and I may well be overreacting. Late last week however, I found myself on the horns of a dilemma. The test results for a patient with a helium dilution TLC that was 68% of predicted but at the same time with a VA that was 93% of predicted and a DLCO that was 129% of predicted came across my desk.
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.
The test systems I am familiar with are pretty much limited to being able to determine back-extrapolation and whether end-expiratory flow rates meet ATS-ERS end-of-test criteria. So far they have not attempted to assess spirometry efforts for the cough or glottal closure that are also part of the ATS-ERS criteria for test quality. Here is a good example of glottal closure:
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.