A change that probably isn’t a change

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:

Observed: %Predicted: Predicted:
FVC: 1.19 50% 2.38
FEV1: 0.64 35% 1.79
FEV1/FVC: 53 71% 76
TLC: 3.21 76% 4.22
FRC: 2.34 96% 2.43
RV: 2.11 113% 1.85
RV/TLC: 66 150% 44
SVC: 1.15 48% 2.37
IC: 0.87 48% 1.80
ERV: 0.25 41% 0.58
DLCO: 6.59 38% 16.18
VA: 1.78 43% 4.12
IVC: 1.04

Change_that_isnt_change_2015_FVL_redacted_2

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The ERV Effect

I used to think that spirometry and diffusion capacity tests were hard and that lung volumes were easy. That may have been true in terms of getting patients to do the tests but I’ve long since come to the conclusion that it is easier to assess the quality of spirometry and diffusing capacity tests and know whether you have reasonably accurate results than it is to do this for lung volumes regardless of which lung volume measurement technique you use.

I was reviewing a set of plethysmographic lung volume tests when I noticed something very odd about the reported results. I usually look at just the VTG loops and the volume-time graphs in order to assess test quality. The testing software automatically selects and averages all VTG efforts and when I reviewed them there were a couple loops that were poor quality and I manually de-selected them. I was reviewing this report because the reported lung volume results didn’t quite match what the spirometry results were saying so this time I also took a close look at the numbers after I removed the low-quality loops. That’s when I realized that the reported TLC was larger than the two tests it was averaged from.

Pleth Math

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