# VA, two ways

One of the recommendations in the 2017 ERS/ATS DLCO standards was that VA should be calculated using a mass balance equation. I’ve discussed this approach previously, but basically the volume of the exhaled tracer gas is accumulated over the entire exhalation and the amount of tracer gas presumed to remain in the lung is used to calculate VA. The conceptual problem with this for DLCO measurements is that VA is calculated using the entire exhalation but CO uptake is based solely on the CO concentration in the alveolar sample. Since VA calculated using mass balance tends to be larger than VA calculated traditionally in subjects with ventilation inhomogeneities this mean that DLCO calculated with a mass balance VA is also going to be proportionally larger as well.

This problem has concerned me for a while but what wasn’t clear was what difference should be expected in the VA (and DLCO) when it is calculated both ways. In order to figure this out I’ve taken a real-world example of a subject with severe COPD and calculated the difference in VA and DLCO.

Fortunately, my lab software lets me download the raw data for DLCO tests (volume, CH4, CO at 10 msec intervals) into a spreadsheet. The PFT results for the subject looked like this:

 Observed: %Predicted: FVC (L): 2.39 97% FEV1 (L): 0.66 36% FEV1/FVC: 27 38% TLC (L): 6.11 126% FRC (L): 4.84 174% RV (L): 4.04 171% DLCO: 9.21 57% VA (L): 3.19 68% Vinsp (L): 2.32

In order to use the mass balance approach with the spreadsheet I found that I could determine the start of exhalation after the breath-holding period but determining where the alveolar plateau started was much more difficult. For this reason I had to include the dead space but made adjustments for this when calculating VA.

To start off with, using the inspired volume and concentration of CH4 in the DLCO test gas mixture, the volume of inhaled CH4 was:

2.32 L x 0.003 = 6.96 ml.

# The effect of errors in Inspiratory Volume on DLCO.

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.

# The importance of an earnest SVC

A report came across my desk today and at first glance it looked fairly straightforward. There was a mildly reduced TLC and FVC, and although the SVC was slightly lower than the FVC it looked like this patient had mild restriction.

 Observed: %Predicted: Predicted: FVC: 1.73 68% 2.56 FEV1: 1.23 65% 1.89 FEV1/FVC: 71 97% 73 TLC: 3.58 73% 4.89 FRC: 2.07 75% 2.78 RV: 1.94 83% 2.33 RV/TLC: 54 114% 48 SVC: 1.69 66% 2.56 IC: 1.51 72% 2.11 ERV: 0.13 30% 0.45

In addition, the flow-volume loop looked fairly typical for restriction, with a normal peak flow and a reduced volume.

When I looked at the DLCO results however, I suddenly got a different picture. Specifically, the VA from the DLCO was larger than the TLC and the inspired volume (Vinsp) was significantly larger than both the FVC and the SVC.

 Observed: %Predicted: Predicted: DLCO: 13.51 83% 16.23 VA: 3.87 82% 4.73 Vinsp: 2.26

# VA, DLCO and COPD

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.