Tag Archives: Washout volume

Some DLCO errors the 2017 standards will probably fix

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Last week I ran across a couple errors in some DLCO tests that I don’t remember seeing before, or at least not as distinctly as they appeared this time. If I hadn’t been looking carefully I could have missed them but both sets of errors will be a lot more evident when the 2017 ERS/ATS DLCO standards are implemented.

The first error has to do with gas analyzer offsets. What alerted me was a set of irreproducible DLCO results.

Test 1: Test 2: Test 3: Test 4:
DLCO (ml/min/mmHg): 24.53 17.21 12.91 6.74
Inspired Volume: 1.99 2.06 2.32 2.26
VA (L): 3.83 3.52 3.63 2.60
Exhaled CH4: 43.27 49.19 54.80 74.14
Exhaled CO: 16.09 23.15 31.39 49.46

When I first looked at the graphs for each test, there wasn’t anything particularly evident until I pulled up the graph for the fourth DLCO test:

This graph showed that the baseline CH4 and CO readings were significantly elevated, but this hadn’t been evident in the previous tests.

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Washout volume, transit time and DLCO

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Recently while reviewing PFT reports I ran across a test from a patient who had been having spirometry, lung volume and DLCO tests performed at regular intervals for the last several years. Compared to the last several set of tests the most recent DLCO had decreased significantly while the FVC, FEV1 and TLC hadn’t changed. I took a closer look at the raw data from the DLCO test and when I did I saw that the washout volume was not correct.

Alveolar_Sample_Unadjusted_Cropped

Or more correctly, even though the washout volume matched the ATS/ERS standard for DLCO testing it was evident the expiratory gas sample was not taken from the alveolar plateau. The CO and CH4 concentrations at this point in the exhalation are higher than they are in the alveolar plateau and this means the reported DLCO was underestimated.

Alveolar_Sample_Adjusted_Cropped

When I re-adjusted the washout so the gas sample was taken from the alveolar plateau, the DLCO went from 18.56 ml/min/mmHg to 22.26 ml/min/mmHg, which is a 20% increase and far more in line with the patient’s prior DLCO test results.

This, however, increased the washout volume from 0.75 L to 1.34 L. Why was the washout volume so high? The answer is it probably wasn’t.

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VA, DLCO and COPD

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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.

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