Another post-BD FVC conundrum

Okay, this may be wrong but at the moment I’m can’t seem to find a reason why it should be. A report like this came across my desk a couple of days ago.

Observed: %Predicted: Post-BD: %Predicted: %Change:
FVC: 4.59 94% 4.87 100% +6%
FEV1: 3.38 89% 3.58 94% +6%
FEV1/FVC: 73.6 95% 73.5 95% 0

Not particularly unusual and it would usually be interpreted as being within normal limits without a significant post-BD change. If you calculate the FEV1/VC ratio using the pre-BD FEV1 and the post-BD FVC however, it’s 89% of predicted and this indicates mild airway obstruction. But you’re not supposed to use the post-BD FVC this way, are you?

Well, why not?

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When is it an expiratory plateau?

Over the last couple of weeks I’ve had an unusual number of patients with expiratory plateaus on their flow-volume loops. Expiratory plateaus are usually considered to be a sign of an intrathoracic central or upper airway obstruction and several of these patients had a diagnosis of tracheomalacia but many of them didn’t. Expiratory (and inspiratory) plateaus are mentioned in the ATS/ERS standards for interpretation but since there isn’t a specific definition (other than “plateau”), an expiratory plateau is a “know it when you see it” sort of thing.

The word plateau tends to imply that the flow-volume loop is both flat and level. Most textbook examples of an expiratory plateau tend to show a flow-volume loop that has been perfectly truncated, usually something like this:

or this:

but it usually isn’t that simple. An expiratory plateau is a consequence of a flow limitation, but during a forced exhalation the diameter of the airways decreases as the lung volume decreases from TLC towards RV. Depending on what is causing the flow limitation the plateau isn’t necessarily flat or level.

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Normal or obstruction?

I had finished reviewing a pre- and post-BD spirometry report yesterday and was about to toss it on my out pile when I noticed something a bit odd about the post-BD results. I pulled it back and spent some time trying to decide if the interpretation needed to be changed but after a lot of internal debate I finally let it go as it was. I’ve continued to think about it however, and although I’m not sure that was the right decision I still haven’t come up with a clear answer.

Here’s what I saw:

Observed: %Predicted: Post-BD: %Predicted: %Change:
FVC: 3.70 97% 3.91 103% +6%
FEV1: 2.82 94% 2.79 93% -1%
FEV1/FVC: 76 95% 71 89% -6%
PEF: 6.62 94% 7.19 102% +9%
Exp. Time: 10.92 11.15

The reported pre-BD and post-BD results were from good quality tests and met the criteria for repeatability. My problem is that the baseline results were normal but if I had seen the post-BD results by themselves I would have considered them to show mild airway obstruction.

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An unusual error in helium dilution lung volumes

Recently I was reviewing a report that included helium dilution lung volumes. What caught my eye was that the TLC and the FRC didn’t particularly fit in with the results from the other tests the patient had performed.

Test: Observed: %Predicted:
FVC: 2.83 114%
TLC: 3.03 71%
FRC: 0.88 39%
RV: 0.09 5%
SVC: 2.93 118%
VA: 3.64 88%

When compared to the FVC and the VA (from the DLCO test) the lung volumes are significantly lower. In particular the FRC and RV are markedly reduced. This is somewhat unusual for helium dilution lung volume since most errors usually cause FRC, RV and TLC to be over-estimated instead of being under-estimated. When I checked the other reports for the day I found that two other patients that had had their lung volumes measured on the same test system also had a TLC, FRC and RV that were noticeably reduced. Obviously we had some kind of equipment problem with that test system but it took a bit of sleuthing before I found out what had happened.

Like all lung volume tests, the helium dilution technique produces a lot of numbers, most of which are not included on the report. One of the first things I did was to call up the within-test data (our test systems store data every 15 seconds during the test and re-calculate FRC each time).

Time: FRC, Liters He conc. (%) Ve (L./min.) Vt, Liters
0:15 -1.00 9.71 6.16 0.21
0:30 0.06 8.87 10.1 0.59
0:45 0.43 8.61 11.76 0.78
1:00 0.69 8.44 9.05 0.72
1:15 0.76 8.39 8.18 0.74
1:30 0.79 8.37 8.32 0.59
1:45 0.82 8.36 8.15 0.62
2:00 0.83 8.35 7.79 0.65
2:15 0.86 8.33 5.51 0.62
2:30 0.87 8.32 5.34 0.63
2:45 0.88 8.32 0 0

When looking at this it was immediately evident there was a problem because the initial FRC was negative and this shouldn’t be possible. About the only way that helium dilution lung volumes can normally be underestimated is if the test is terminated way too early and the negative FRC ruled this out. It also narrows down the possible problems, but I had to think for a while and in doing so had to go back to the basics of the helium dilution test.

Helium dilution used to be the most common method for measuring lung volumes, but it requires a closed-circuit test system with a volume displacement spirometer. Most current test systems are open-circuit flow sensor-based systems and lung volumes are usually measured by nitrogen washout (or by plethysmography). Nevertheless, there are a couple of closed-circuit systems still being manufactured and there are a fair number of these systems still in service.

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