Yesterday while reviewing reports I ran across an interesting error in the Inspiratory Volume (VI) from a DLCO test. I’ve probably seen this before but this time I realized what effect it could have on DLCO. Specifically, what I saw was that at the start of the DLCO test the subject had not finished exhaling and although the technician had started the test, the subject continued to exhale.
What makes this interesting is that the software used the subject’s volume at the start of the test as the initial volume. This means that the software measured the VI from the initial volume to the end of inspiration, not from the point at which the subject stopped exhaling to the end of inspiration. This also means that the VI was underestimated by 0.20 L and this affects both VA and the calculated DLCO.
A low FEV1/VC ratio is the primary indication for airway obstruction.
From ATS/ERS Interpretive Strategies for Lung Function tests, page 956.
The ATS/ERS statement on interpretation says
“The VC, FEV1, FEV1/VC ratio and TLC are the basic parameters used to properly interpret lung function (fig. 2). Although FVC is often used in place of VC, it is preferable to use the largest available VC, whether obtained on inspiration (IVC), slow expiration (SVC) or forced expiration (i.e. FVC).”
I understand and in general agree with the idea of using the largest VC regardless of where it comes from and this is because the FVC is often underestimated for any number of good (and not so good) reasons. When this happens the FEV1/FVC ratio will be overestimated and airway obstruction will be under-diagnosed. However the ATS/ERS statement is also grounded in the notion that all vital capacities (FVC, SVC, IVC) are the same and this isn’t necessarily true. The problem comes from the fact that the predicted values and lower limit of normal (LLN) for the FEV1/VC ratio always come from reference equations for FEV1/FVC ratios. Because the SVC (and IVC) are usually larger than the FVC this means there is at least the potential for airway obstruction to be over-diagnosed.
Recently a report came across my desk from a patient being seen in the Tracheomalacia Clinic. The clinic is jointly operated by Cardio-Thoracic Surgery and Interventional Pulmonology and among other things they stent airways. The patient had been stented several months ago and this was a follow-up visit. Given this I expected to see an improvement in spirometry, which had happened (not a given, BTW, some people’s airways do not tolerate stenting), but what I didn’t expect to see was a significant improvement in lung volumes and DLCO.
When I took a close look at the results however, it wasn’t clear to me that there really had been a change. Here’s the results from several months ago:
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