Hidden FIVC and FVC. When all the data is relevent.

For the first dozen or so year that I worked in a pulmonary function lab it was with counter-weighted, volume-displacement water-seal spirometers more or less like this:

Spirometer_Collins_13_5_Liter_Respirometer_1967

Patients would do a series of tests and I’d end up with a bunch of pen traces on kymograph paper that I’d have to measure with a ruler and use a desktop calculator (it was about a foot square, weighed a couple of pounds and had a nixie tube digital display) to create a hand-written report. I’m not going to suggest that these spirometers were in any way better than what we’re using now but I have to say that I would have seen the following problems more or less immediately.

Recently I was reviewing a report from a patient with very severe obstruction and noticed something a bit off about the flow-volume loop. Specifically, the end-exhalation of the tidal loop looked like it was at a significantly higher volume than the end of the FVC effort.

Hidden_FIVC_2_FVL

Because the high-frequency sawtooth pattern (from the patient, not the equipment) makes it a little hard to see if this is what was really happening, I downloaded the raw data and re-graphed the volume-time curve with a spreadsheet.

Hidden_FIVC_2_VT2

When I did this it was first evident that the patient started the inhalation for the FVC maneuver before they’d finished exhaling. Next, despite exhaling for 13.8 seconds (the entire trace spans about 44 seconds) the patient did not exhale all the way back to the same end-exhalation level they’d had while breathing tidally. This is actually a sign of gas trapping and relatively common with severe obstruction. It also means that the inhaled volume from the lowest end-exhalation to the maximum inhalation was about a half a liter larger than the reported FVC.

The reason that I say that this is something I would have noticed on older equipment is that with our lab software the volume-time curve looks like this:

Hidden_FIVC_2_VT

which only shows one second of test data before the start of the FVC’s maximal exhalation. This means the baseline shift that occurred during the FVC maneuver does not show up at all.

Strictly speaking I doubt that the FIVC volume derived from the more complete volume-time tracing meets the official criteria for an inspiratory vital capacity, particularly since it occurs over more than one breath. Even so, it is “real” and the gas trapping it shows is an important clinical finding. Even more importantly I don’t know of any test systems where this would have been evident.

So, if that flow-volume loop showed gas trapping, what about this one?

Hidden_FIVC_3_FVL

Again, the end-exhalation of the tidal breath is at a higher volume than the one at the end of the FVC maneuver. This happened for a completely different reason than the first example however, and an important clue is that it was performed on one of our test systems that has volume displacement spirometer.

When I downloaded the raw data for this test and graphed it using a spreadsheet, what I got was this:

Hidden_FIVC_3_VT_2

The real difference is that the end of exhalation for all of the tidal breaths showed a consistent drift and what was most likely happening was that the patient was leaking around the mouthpiece. The weight of the spirometer bell is enough to maintain a small amount of positive pressure inside the spirometer and if there is a leak then the bell will drop during testing.

When I analyzed the trace more carefully what I found was:

Hidden_FIVC_3_VT_3

There was a more-or-less constant drift of about 0.085 L/sec. That means that during the ~10 seconds the patient exhaled, the FVC was probably underestimated by about 0.85 L. This has to be a guesstimate however, since there is no way to be sure the drift was constant.

Again, this is a problem that would have showed up immediately on the kymograph paper of an older spirometer but wasn’t evident at all in my lab’s testing systems. To some extent the problem is related to the use of a test system with a volume-displacement spirometer but patient or equipment leaks can occur on any test system and their effect on the results are probably going to be less evident than they were here.

I’m not advocating that we return to the “good old days” of counter-weighted, volume-displacement water-seal spirometers and kymograph paper. What I do feel however, is that all too often our test systems and the choices made by engineers, programmers and manufacturers end up limiting instead of enhancing what we can see. There were clues in the flow-volume loops that something wasn’t right but I had to do some serious digging in order to find out what was actually happening. I don’t expect our test systems to be able to correct the results for gas trapping or for a leak but I also don’t expect for them to make it more difficult than it should be to detect that these things were happening.

I’d like to suggest that all test systems come with a “raw data” or “diagnostic” mode where all of the unprocessed information collected during a test is quickly and easily available. To people like myself and anybody who needs to troubleshoot problems, this is a complete no-brainer, but how useful would it be to regular staff? I’ve visited a lot of PFT labs and met a lot of technicians and many of them seem uninterested in how test results are derived. The possible reasons for this could fill a couple blogs but I have to wonder if at least part of the reason is that for last several decades all our test systems have been a “black box”. Patient test data goes in one side, numerical and graphical results come out the other side and it isn’t possible to see what happens in-between. How can staff ever be interested and knowledgeable when the processes they work with every day are opaque? Quick access to “raw” data would be a great teaching tool but if not that then at the very least it would allow staff to easily get a better look at questionable results.

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3 thoughts on “Hidden FIVC and FVC. When all the data is relevent.

  1. Hi Richard,

    I’m not sure I’m talking about the same thing you are, but the saw-tooth pattern seen on these F/V loops has an almost 100% correlation with patients who snore, but not necessarily the other way around. The saw-tooth pattern indicates a weak musculature towards the back of the throat and manifests as a “vibration,” so to speak, under forced conditions.

    If the musculature is particularly weak, combined with a patient who has has severe dynamic airway collapse, the extra-thoracic component can lead to an almost complete block on th expiratory curve and cut off volume prematurely.

    Hope this helps.

    Best,

    Ira

    • Ira –

      I agree that the sawtooth profile is a sign of a flaccid upper airway but that it also has very limited diagnostic utility. As you said all patients with a sawtooth profile snore but only a small fraction of people who snore have a sawtooth profile. I’m not sure what correlation a sawtooth profile has however, with gas trapping and dynamic hyperinflation. I’m sure it contributes a bit to any underlying intrathoracic airway obstruction (there has to be an energy loss of some kind going on) but I would have thought that the positive pressure gradient between the extrathoracic airways and the surrounding tissue would keep any of the additional obstruction in that area to a minimum. Now during inspiration, however….

      – Richard

  2. In an obstructive lung disease, airway obstruction causes an increase in resistance. During normal breathing, the pressure volume relationship is no different from a normal lung. However, when breathing rapidly, greater pressure is needed to overcome the resistance to flow, and the volume of each breath gets smaller. The increase in the effort to breathe can cause an overdistention of the lungs.

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