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 never used to think about this too much but several years ago I had a long conversation with a Pulmonary lab manager at a tertiary care hospital in Australia. One of the things he said was that all patients newly referred to the Pulmonary division there always had a complete set of PFTs, including post-bronchodilator spirometry, MIP & MEP and an ABG before they even saw a pulmonary physician. The ABG may be a bit of overkill, but since that time I now spend a lot less time on the front lines and lot more time reviewing PFT reports. I have a more global view of patient management (or at least I like to think I do) and I have to wonder if complete PFTs on a first visit shouldn’t be a standard approach.
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?
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.
The single-breath DLCO test attempts to simplify what is actually a very complex process. One of the key assumptions of the single-breath DLCO calculations is that the inspired gas mixture is evenly distributed throughout the lung. This is not really true even for patients with normal lungs and in general, inspired gas follows the last in-first out rule. In patients with lung disease this inhomogeneous filling and emptying can be magnified and a maldistribution of ventilation is often most evident in patients with COPD.
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, American Journal of Respiratory and Critical Care Medicine, European Respiratory Journal, Journal of Applied Physiology, Thorax) for having opened part or all of their archives to anybody who wants to search and download articles.
There are a number of other journals, however, that remain entirely behind paywalls. I was reminded of this recently while looking for an article on diffusing capacity from the 1970’s only to find that it would cost me $31 to access it (and then only for 24 hours). Although strictly speaking this does not prevent anyone from accessing these articles, anybody who does has to have deep pockets and this is acting as a significant barrier to the ability of individuals and institutions to obtain and to share information.
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.
I have been reviewing the raw data from all of the lung volume tests lately. My lab has a mix of equipment and performs lung volumes with helium dilution, nitrogen washout and by plethysmograph. I’ve mentioned previously that we went through a major upgrade in equipment and software over the summer and this extra scrutiny on lung volumes is in part because of the problems we’ve had with the nitrogen washout test.