DMCO, Vc and 1/theta

Roughton and Forester’s seminal paper from the 1950’s showed that DLCO was a function of two resistances: the alveolar-capillary membrane and the rate of CO uptake by red blood cells. This relationship is shown by:

Formula 1 DLCO conductances

Roughton and Forster also showed that the membrane diffusing capacity (DMCO) and pulmonary capillary blood volume (Vc) could be calculated by performing the DLCO test at different oxygen concentrations and then plotting the results.

Modifed from: Pulmonary Function Testing Guidelines and Controversies, Jack Clausen ed., page 166.

Modifed from: Pulmonary Function Testing Guidelines and Controversies, published 1982, Jack Clausen ed., page 166.

Since the 1950’s DMCO and Vc have been measured for research fairly often. I first performed this test around 30 years ago mostly because I was interested in the technical aspects. I’ve tried to keep current with the research using DMCO and Vc ever since and have come to realize that there are several important details with a significant effect on how this test is performed and calculated.

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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.



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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Mixing doesn’t always make a match

Recently I was reviewing a spirometry report and noticed that the FVC was below normal. A low FVC can suggest a restrictive lung disease but the reported expiratory time was only about 4 seconds. I took a look at the graphics included with the report and the volume-time curve showed the effort ended well before 6 seconds so my first thought was that the reduced FVC was more likely because of suboptimal patient effort than anything else.

I always try to review spirometry results whenever there is anything questionable so I pulled up the raw test results and immediately saw that the reported FVC was actually a composite. The ATS-ERS statements on Spirometry and Interpretation say that the highest VC regardless of which test it came from (which even includes the slow vital capacity test from lung volume measurements and the inspiratory volume from a DLCO) should be used when reporting spirometry results. In this case the FVC came from one effort, the FEV1 and everything else came from a different effort. The interesting thing was that the effort the FVC came from was about 10 seconds long which shows it actually was an adequate effort. The FEV1 effort on the other hand was only about 4 seconds long and showed an abrupt and early termination of exhalation.

The technician who performed the tests selected the correct efforts to make a composite. The patient had made five spirometry efforts and the selected FVC was significantly larger than all of the other efforts but the FEV1 from the same effort was significantly lower than several other efforts. Our criteria for selecting FEV1 does not just go by the largest FEV1, we also look at the peak flow (PEF) and whether there has been any back extrapolation and the effort the FEV1 came from had the highest peak flow and no back extrapolation. So, a good choice had been made on both efforts.

When it comes to selecting values from different spirometry efforts there are only a limited number of results that our lab software allows us to mix and match. The FVC, the FEV1 and the graphics (flow-volume loop and volume-time curve which are linked to each other) can all be selected individually, but everything else, which includes the expiratory time, PEF, FEF25-75, MEF50 etc. etc. can only be selected as a group.

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