Although the technology used to perform the single-breath DLCO test has improved since it was first developed in the 1950’s the essential concepts and equations have not changed significantly. Probably the most important advance has been the introduction of rapid response real-time gas analyzers in the 1990’s. Prior to that time the patient’s washout and sample volumes had to be preset which always involved a certain amount of guesswork when a patient was significantly obstructed or restricted. With a real-time gas analyzer it is possible inspect the exhaled gas tracings after the test has been performed in order to determine when washout has occurred and then select the appropriate location for the sample volume. This has improved the single-breath DLCO test quality but at the same time it has also exposed some of its limitations.

I have been researching different pulmonary function topics for quite a few years. The medical libraries I’ve frequented were, of course, originally all paper-based and to be able to find an article the library or one of the department’s physicians had to subscribe to the journal in question. In the last fifteen years we have all seen an explosion in on-line publishing and it has become much easier to research articles. I vigorously applaud the pulmonary medicine journals (Chest, Amer__ican J__ournal of Resp__iratory and Crit__ical Care Med__icine_, Eur__opean_ Resp__iratory J__ournal_, J__ournal of_ Appl__ied Physiol__ogy_, Thorax)_ for having opened part or all of their archives to anybody who wants to search and download articles.

Just when we thought it was safe to go back in the water, we’ve run into another N2 washout-related problem. Although it probably affects the TLC and RV calculations in a minor way, it was actually noticed in relation to spirometry. When spirometry is reviewed in my lab the FVC is compared to the SVC, if one has been performed. If the SVC is greater than FVC, then the FEV1/VC ratio is recalculated using the SVC. This is in line with the ATS recommendations on interpreting spirometry and does occasionally throw up a patient with airway obstruction that otherwise would not have been detected.

Re-breathing DLCO may not be well known because this technique has not been used in routine clinical testing. It has instead been primarily used to research diffusing capacity during exercise because it is able to do this without the need to interrupt or significantly alter a subject’s breathing pattern. Re-breathing DLCO is probably best thought of as a hybrid between the single-breath DLCO and steady-state DLCO techniques. The gas mixture and calculations are from the single-breath world but the breathing pattern is more from the steady-state side of the family.

After the tests themselves, the second most important thing that a Pulmonary Function Lab needs to do is to report results. Like a tree falling in the woods, if a report hasn’t gotten in front of the person that ordered the tests, has anything happened? A successful PFT Lab needs to manage its reports effectively. An obstacle that many labs will need to overcome is the reporting software that comes with their testing systems. I think that for many of the PFT equipment manufacturers reports are an afterthought at best. The ability to format reports to a specific lab’s needs is often limited and is just as often is a difficult and time-consuming process.

Recently I have been reviewing a lot of early pulmonary function research. I’m not feeling nostalgic but I think that re-visiting some of the older methods and technology may be interesting. Almost all Pulmonary Function laboratories presently use the single-breath technique to measure diffusing capacity. There are a number of reasons why this is case but I suspect that many technicians and physicians aren’t aware that there are several alternative methods for measuring diffusing capacity and that there was a time when at least one of these, the steady-state DLCO, was routinely used in clinical labs.Diffusing capacity, regardless of technique, is determined by the rate of CO uptake, divided by the driving (alveolar) pressure:

Nitrogen washout lung volumes are still relatively new to my PFT Lab. The number of problems we’ve encountered has decreased substantially but we are still learning some of the idiosyncrasies of the system. Recently while trying to understand a test with odd results we were reminded by the manufacturer that during the washout period a patient’s inspiratory and expiratory flow rates should not exceed 1.5 liters/second. The reason this “speed limit” is necessary highlights some of the limitations of modern open-circuit lung volume measurements.

Recently I was reading the blog of someone who teaches Pulmonary Function testing and they stated: “…in emphysema and air trapping, the VTG (thoracic gas volume) will be higher than a FRC (functional residual capacity) measured by Helium dilution and Nitrogen washout. This is because VTG is the volume of gas contained in the thorax, whether in communication with the airways or trapped in the thorax.” This unfortunately is not correct, but it is a common misconception and since there was a time when I believed it myself I find it difficult to fault the author too much. I think it’s time to return to the basics of plethysmographic lung volumes however, and show why this is not true.

The FEV1/FVC ratio is used to estimate the presence and degree of airway obstruction. For well over thirty years my lab has used an FEV1/FVC ratio of 95% of predicted as the cutoff for normalcy. This value (carved onto a stone tablet by the way) had been brought to the lab by a founding physician who had come to the department from the NIH in the 1970’s. Since the software and hardware upgrade this summer our PFT Lab has switched to the NHANES III spirometry reference equations but we have so far resisted changing our 95% cutoff to the lower limit of normal (LLN). This is due in part to inertia but also in part to a mistrust in the concept of LLN. We have been steadily re-evaluating all of our testing criteria and have turned again to the FEV1/FVC ratio with the question as to whether our 95% cutoff is over-zealous or whether the LLN is too lax.

I have been taking a close look at the raw data from all lung volume tests lately in large part because N2 washouts are still relatively new to my PFT Lab and we’re continuing to learn from our mistakes. When I saw this N2 washout test I knew that there was something wrong with it. The patient had performed three N2 washout tests and the TLC, FRC and RV for this one test were significantly larger than for the other tests. The most common problem we’ve been having with N2 washouts has been with patient leaks during the washout period which almost always show up as an upwards drift in the tidal baseline. This test did not show any drift however, and it took me a little while before I could see what was wrong with it.