I have been searching through Pulmonary Function videos on YouTube in order to find ones I thought would be useful for technician education. So far what I’ve found have been intended either for medical students or for patients and not, in my opinion, particularly suitable for training technicians. Lately I’ve been looking at videos about lung volumes and have seen a half dozen presenters describe lung volume subdivisions using the same graph we’ve come to know and love with varying degrees of effectiveness and obfuscation.

 In a recent post on the AARC Diagnostics forum a PFT Lab manager asked how long they need to keep database records. The ostensible reason for this was that they had too many years of records and had been having database problems. The poster wasn’t specific about what kinds of problems they were having. Database problems can be hard to diagnose particularly when a database is networked but with a modern SQL database the number of records shouldn’t be an issue. SQL databases containing millions of records are routinely used in demanding multi-user applications. If this was thirty years ago when computers first started to be commonplace in the PFT Lab I could understand since PC-based databases were still in their infancy then. It was at least partly for this reason that a number of PFT equipment manufacturers developed their own proprietary databases. This is no longer the case and I have difficulty believing that there are any manufacturers at this time that don’t use a commercial SQL database of one kind or another.

When it comes to spirometry, it’s really all about FEV1. FVC and the FEV1/FVC ratio are also important of course, but because FVC is more likely to be underestimated than FEV1 they are less reliable. Changes in FEV1 are critical in monitoring airway disease. The recent ATS guidelines on Occupational Spirometry indicate that a 15% decrease (adjusted for changes in age) is significant and cause for concern. For diagnosing airways disease however, it is important to know what a normal FEV1 is.

Pulmonary patients have been using personal peak flow meters for several decades and I started to seeing patients that had their own oximeter over 10 years ago. Within the last couple of years a number of different spirometers intended for personal use have become available. These devices have significant limitations when compared to regular office or lab spirometers but because there are many individuals with asthma or COPD for whom FEV1, and possibly FVC, are significantly better indicators of lung health than Peak Flow (PEF) alone, I think they can serve a useful purpose.

My lab stopped inserting A-lines to get arterial blood samples during exercise testing well over 10 years ago. Our decision was partly based on the fact that we didn’t do them often enough to be good at it and partly based on the fact that we didn’t think that we were getting enough extra information from ABG’s to be worth the effort. Since another local hospital (a competitor but part of the same medical school network so we share pulmonary fellows with them) routinely performs level II and level III exercise tests we felt we could refer any patients that really needed ABG’s to the lab there. We don’t regret the decision and don’t feel that it has compromised the quality of our exercise testing. Because we don’t obtain ABG’s, one of the values we don’t calculate is the deadspace/tidal volume ratio (Vd/Vt). Recently I was reading an article that related Ve-VCO2 to Vd/Vt and I was reminded of some the issues I had with Vd/Vt when I calculated it in the past. We’ve gone through two different exercise test systems since that time so I’m not sure if some of these problems still exist but I thought it would be a good idea to review both the problems and the literature on Vd/Vt to see if I could make some sense of them. As a reminder, the original Bohr equation for Vd/Vt was: The first problem I had run into was that mixed-expired CO2 (PECO2) is routinely calculated from CPET data as: But it was also reported as a separate value by the test system’s software and the two values did not match. The second problem was that the test system software reported a non-zero inspired CO2 concentration (PICO2). The PICO2 ranged from 10 mmHg at rest to 6 mmHg at peak exercise and when I reviewed some of the data from that time it is apparent that the PICO2 closely matched the difference between the reported and calculated PECO2.

The most common problem we have with helium dilution FRC tests are leaks. Although the system tubing and spirometer bell leak occasionally, we do have valve failures relatively frequently. Valve failures are usually obvious but they sometimes only fail partially so leak checks are regularly performed on these test systems. We can’t perform leak checks on patients except while they are being tested however, and patient leaks are far more common than system leaks.

The first reasonably accurate flow-measuring device was the Fleisch pneumotachograph which was developed in 1925. Originally the Fleisch pneumotach bounced a light beam off a mirror mounted on a diaphragm and from there onto photographic film, all of which made it difficult to use. World War II saw the development of sensitive pressure transducers, amplifiers and recorders and by 1950 the pneumotachograph went totally electronic and began to be commonly used in routine pulmonary research.

It’s been several decades since I last saw water-seal bell spirometers being used in a Pulmonary Function lab. They have been displaced mostly by systems that use flow sensors of one type of another and the small handful of equipment manufacturers that still make volume-displacement spirometers use rolling seals. This isn’t to say that this kind of spirometer isn’t being manufactured any more and in fact there appears to be a modestly thriving market in water-seal spirometers intended for use by students.

The primary goal of a Cardio-Pulmonary Exercise Test (CPET) is to determine an individual’s maximum oxygen consumption (VO2), minute ventilation (Ve) and heart rate (HR). An adequate CPET is usually indicated by a Respiratory Quotient (VCO2/VO2) that is greater than 1.10, a heart rate greater than 85% of predicted or a Ve greater than 85% of predicted. There are a variety of reasons why patients are unable to exercise to their maximum. Although these reasons can of course include poor motivation, factors such a musculo-skeletal limitations or concerns about patient safety due to EKG changes can cause patients have a sub-maximal test.

For the last couple of years it seems that I’ve had more problems than usual with lung volume tests. Even though this seems to date from the time that my lab went through its hardware and software upgrade and we started performing N2 washouts I suspect that these problems have been around for a long time and these events just heightened my awareness of lung volume testing problems. My lab performs helium dilution, N2 washout and plethysmographic lung volume tests. When you are assessing the quality of lung volume tests the first problem for the helium dilution and plethysmographic techniques is whether or not the Functional Residual Capacity (FRC) was accurately measured and for N2 washout, it’s whether or not the Residual Volume (RV) was accurately measured. Leaks are always an issue for any of these measurement techniques and for helium dilution and N2 washout leaks will almost always cause the Total Lung Capacity (TLC) to be overestimated. For plethysmography the picture is less clear since leaks can cause TLC to be either over- or under-estimated.