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

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When back-extrapolation goes astray

A spirometry report that looked very questionable came across my desk recently. The flow-volume loop was misshapen and the technician’s notes indicated that the results had been highly variable and to “interpret with caution”. I pulled up the raw test results and saw a series of test efforts with flow-volume loops that were all somewhat flattened and with no consistency in either the loops or the numerical results.

This kind of inconsistency can be an indication of poor patient effort but can also occur because of airway problems. The cardio-thoracic surgeons at my hospital have an active airway stenting program and so we see a fair number of patients with trachemalacia. One hallmark of tracheomalacia is that there is usually a flow limitation and that this means that there is usually a flat expiratory plateau in the flow-volume loops. These loops had peak flow-ish humps, but the humps seemed to appear in different locations in every loop and they seemed to have a relatively high frequency flutter.

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One plausible explanation for the inconsistent results is vocal cord dysfunction (VCD). VCD is characterized by the paradoxical closure of the vocal cords that results in wheezing or stridor and shortness of breath. The gold standard for diagnosing it is laryngoscopy while the patient is symptomatic but it can be difficult to make a definitive diagnosis since symptoms can often come and go. VCD can mimic asthma but patients usually don’t respond to bronchodilators and have negative challenge tests. Spirometry results like these can only be suggestive, however.

The real problem though, was that the spirometry effort that had been selected for reporting indicated the patient had moderately severe airway obstruction (FEV1 56% of predicted) and there were several efforts that had a significantly higher FEV1. When I checked the numerical values it was apparent that this effort had been selected because it was the effort with the highest FEV1 whose back-extrapolation met ATS-ERS criteria.

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It’s all about FEV1, except when it isn’t.

A number of physicians and researchers I’ve known and respected have said that in spirometry it always comes down to FEV1 since it is the primary indicator for airway obstruction. Certainly FVC and the FEV1/FVC ratio are important but because patients can stop exhaling early for any number of reasons FVC can be underestimated which in turn can cause the FEV1/FVC ratio to be overestimated so they are not quite as reliable as FEV1.

There are, of course, a number of factors that can cause FEV1 to be mis-estimated. It can be underestimated due to cough or glottal closure and it can be overestimated because of excessive back-extrapolation. Nevertheless, I think that overall the FEV1 tends to be the most accurate and reliable number obtained from spirometry.

This spirometry report came across my desk this morning: 

  Observed: % Predicted: Predicted:
FVC (L): 5.01 114% 4.39
FEV1 (L): 3.86 117% 3.30
FEV1/FVC: 77 103% 75
PEF (L/sec): 4.91 55% 8.99 

Because a reduced FEV1 is a reliable indicator of airway obstruction doesn’t that mean that a normal or as in this particular case, a slightly elevated FEV1 rules it out? Well, actually no, it doesn’t.

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