Making Assumptions about TGV and FRC

When lung volumes are measured in a plethysmograph the actual measurement is called the Thoracic Gas Volume (TGV). This is the volume of air in the lung at the time the shutter closes and the subject performs a panting maneuver. Ideally, the TGV measurement should be made at end-exhalation and should be approximately equal to the Functional Residual Capacity (FRC). For any number of reasons in both manual and automated systems this doesn’t happen and the point at which the TGV is measured is either above or below the FRC.

Testing software usually corrects for the difference in TGV and FRC by determining the end-exhalation baseline that is present during the tidal breathing at the beginning of the test. Using this value the software can determine where the TGV was measured relative to the tidal breathing FRC and then either subtracts or adds a correction factor to derive the actual FRC volume.

One problem with this is that leaks in either the subject or the mouthpiece and valve manifold can occur during the panting maneuver and the end-exhalation baseline can shift and this will affect the calculation of RV and TLC. I’ve discussed this previously and as a reminder, RV is calculated from:

RV = [average FRC] – [average ERV]

where the FRC is determined from the corrected TGV and ERV is determined from SVC maneuvers. TLC is then calculated from:

TLC = RV + [largest SVC]

When the post-shutter FRC baseline shifts upwards (higher lung volumes relative to the pre-shutter FRC):

ERV is underestimated, which in turn causes both RV and TLC to be overestimated. When the post-shutter FRC baseline shifts downwards (lower lung volumes relative to the pre-shutter FRC):

ERV is overestimated, which in turn causes both RV and TLC to be underestimated.

I’ve been aware of this problem for quite a while and use this as a guideline when selecting the FRCs and SVCs from specific plethysmograph tests. All of these assumptions are based on the fact that FRC is derived from the pre-shutter end-exhalation tidal breathing. Well, you know what they say about assuming…

Our lab software was updated a while back, ostensibly with only the changes we needed to perform electronic signing by the pulmonary physicians. I’ve reviewed the information we got concerning the update and there is nothing in it that indicates that any changes were made in any of the testing modules. Recently I was reviewing the plethysmographic lung volume for a patient and suddenly realized that the FRC baseline was being determined by the post-shutter end-exhalation level.

In this particular instance this means is that even though the shutter closed near the end-exhalation level of the pre-shutter tidal breathing, the reported FRC was being corrected to a much lower lung volume. This was causing both RV and TLC to be underestimated. Interestingly, for this patient their TGV measurements were well within +/- 5% (the 2005 ATS/ERS standard for plethysmographic FRC measurements), but the FRCs were not and this was all due to post-shutter baseline shifts.

Fortunately, our software lets me mix and match SVC and TGV results, and to manually correct the FRC. This allowed me to take the highest quality TGV measurements and correct the FRC and then to match them with the highest quality SVC measurements (also corrected for FRC). So for this patient I was able to report what I believe to be reasonably accurate lung volume measurements.

But just to keep me confused, a couple of reports later another patient had also had their lung volumes measured plethysmographically, and also had a post-shutter baseline shift in FRC, but this time the software was using the pre-shutter end-exhalation level to determine FRC (and yes, there was a negative ERV).

Looking carefully at all the plethysmographic lung volumes I’ve reviewed since then I’ve seen that FRC is being determined occasionally by the post-shutter end-exhalation level and more often by the pre-shutter end-exhalation level. I haven’t been able to determine why one or the other is being used so I have no idea what criteria the software is using.

Other than mentioning that the SVC maneuver should be linked to the FRC measurements there are no guidelines in the ATS/ERS 2005 lung volume standards as to how this linking should be done. Although it would seem to be more logical for the pre-shutter end-exhalation level to be used to determine FRC (since they appear to be more closely related to the actual TGV measurement) this is left to individual manufacturers.

Note: I will also mention that there are no guidelines for the number of tidal breaths and the repeatability of the end-exhalation volumes that are used to determine FRC and this is also left to the individual manufacturers.

In a sense, the real problem is that leaks are occurring while the shutter is closed. I suspect that it’s the patients that are leaking rather than the valve manifold because leaks don’t occur with every patient or with every test (although that doesn’t rule out an intermittent valve problem). It’s hard to know how to address this since we routinely instruct the patients to keep their lips tight and we already use the largest mouthpiece the patient is comfortable using.

Baseline shifts are more often downwards than they are upwards. This implies that leaks are more likely during the compression (exhalation) part of the panting maneuver but I suspect that this is because patients probably don’t generate as much negative pressure as they do positive pressure during panting.

Regardless of where or when the leak is occurring it’s also unclear to me what effect a leak has on the actual TGV measurement. A leak will cause lung volume to change during the panting period and may also dampen the amplitude of the mouth pressure signal but I suspect that these effects are small. Our plethysmographs accumulate between 1 and 4 pants for a TGV measurement but any difference between individual pants (other than the normal pant-to-pant variations) in patients with baseline shifts is not readily apparent.

We maintain our plethysmographs well and try to instruct our patients properly but leaks and baseline shifts during plethysmographic lung volume measurements still happen. For these reasons I suspect that this is not an uncommon problem regardless of which manufacturer’s equipment is being used. How evident this problem is however, depends on the way each manufacturer’s software displays the TGV results graphically and I’ve seen examples from some systems where a baseline shift would be difficult to detect.

As importantly, our ability to detect a baseline shift also depends on the procedure we follow. Specifically, for my lab a TGV test consists of:

  • tidal breathing
  • shutter closes
  • panting
  • shutter opens
  • tidal breathing
  • SVC maneuver

I’ve seen results from labs where the subject goes directly into an SVC maneuver as soon as the shutter opens and without a second period of tidal breathing a baseline shift would probably be hard to detect.

Strictly speaking the periods of tidal breathing pre-shutter and post-shutter serve different purposes. Pre-shutter the end-exhalation level of tidal breathing is used to determine where FRC is in relation to the TGV. Post-shutter the end-exhalation level is used to determine IC and ERV from the SVC maneuver. I would like to see these facts addressed the next time the ATS/ERS updates the standards for lung volume measurements and that pre-shutter and post-shutter tidal breathing periods be mandated and FRC determined independently for both.

Our software (and I suspect those from many other manufacturers as well) assumes that the pre-shutter tidal end-exhalation level is the same as post-shutter (ore vice-versa). It’s clear that this isn’t always true and that when it isn’t it can affect the reported RV, FRC and TLC. So you know what they say about assuming…

References:

Brusasco V, Crapo R, Viegi G. ATS/ERS task force: Standardisation of lung function testing. Standardisation of the measurement of lung volumes. Eur Respir J 2005; 26: 511-522.

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10 thoughts on “Making Assumptions about TGV and FRC

  1. Hello Richard,

    Can this not be corrected by going from end-expiratiory baseline into FRC pleth and directly into either expiratory VC and or inspiratory VC without tidal breathing between FRC pleth and SVC?

    This is what we do in our lab with little to no difficulty for the patient.
    What I find best with the linkled maneouver, done in such a way, is that if my patient produces 3 FRC’s within 5%, I have a much greater chance of producing IC’s and ERV’s that are for the most part repeatable.

    Regards,
    Mo

    • Mo –

      If you go directly from FRC pleth to the SVC it will be difficult to determine if there has been a baseline shift during the time the shutter was closed. Many patients that do have a baseline shift will do so relatively reproducibly so FRC, RV, TLC, IC and ERV will also be reproducible. Not accurate but reproducible. This is why I think that a tidal breathing period is necessary after the shutter is opened and that the FRC level should be determined separately for the TGV and the SVC.

      Regards, Richard

      • Hi Richard,

        If end expiratory level is established and the patient pants at FRC or VTG and no leaks (no open ends in the panting graphics) during the panting, then would it not be fair to say that no drift has occurred, therefore no issues going directly into the SVC?

        On a side note, do we have the ability to upload any graphics?
        Regards,
        Mo

        • Mo –

          It’s not clear to me whether there is any sign of leaking in the panting loops. I’ve looked at loops from patients with baseline shifts and from those without any and I don’t see anything particularly different but I may well be missing something. Open ended loops on my system appear to largely be an artifact of the software, not of the panting maneuver. My feeling is that without a period of tidal breathing before you perform the SVC, you won’t ever know if there’s been a baseline shift or not.

          I can download graphics and some raw data from my test systems if that’s what you’re asking.

          Regards, Richard

          • Hi Richard,

            With the software system that we run, an open ended panting loop equals a leak during the panting. If the drift is of concern, would it then not be best to do the FRC pleth and SVC separately? This could be an option, though not my preferred method. Assuming that the patient breaths at true end-expiratory level (FRC) for all the trials should resolve any issues with a drift.

            With regards to being able to up load graphics, would I be able to on my end?

            Mo

          • Mo –

            I am less certain than you that an open loop means leaking or drifting since I see these all the times in patients without any baseline shift, and in fact most our test system’s loops are open. It’s great if you can rely on the presence of open or closed loops to indicate whether or not a patient is leaking and/or drifting but my concern is that what you see comes from your test system’s software and may or may not truly reflect what is happening. It’s also possible that the baseline shifts my lab sees on a somewhat regular basis are due to software or mechanical issues specific to my lab’s equipment and that for you a baseline shift is very rare. However, without actually verifying this by performing post-shutter tidal breathing, you can’t say for certain it’s not happening. So my point is that without performing pre-shutter and post-shutter tidal breathing you can’t be certain that the FRC derived from the TGV or the IC and ERV from the SVC are correct.

            Regards, Richard

    • Sue –

      Oh heavens, there are so many possibilities, where do you start?

      The first thing I’d look at is a discrepancy between the SVC from the lung volume test and the Inspired Volume from the DLCO. If the SVC is a lot lower than the Inspired Volume that in itself may be the reason the TLC is low. If they are about the same then I’d look for errors in the lung volume test that would cause FRC to be underestimated and there it depends on how you are measuring FRC (Helium Dilution, N2 Washout or Plethysmography) as to what kind of errors I’d be looking for. Most problems (leaks) with Helium Dilution and N2 Washout tests tend to cause FRC to be overestimated but if the washout/washin period is too short then FRC will be underestimated. I’ve also seen the FRC underestimated occasionally when the switchin is performed incorrectly. For plethysmography I’d look at whether the angle measured from the loop was correct (TGV) and whether there was a baseline shift (downwards most likely) during the time the shutter was closed.

      If the lung volume test quality was good then I’d look at the exhaled gas waveforms in the DLCO test and there I’d be look for gas analyzer problems. I’ve seen marked shifts in gas analyzer baselines from one test to another and techs often choose the largest DLCO as the “best” DLCO even though the results are wacky (real high VA compared to TLC and/or real high DLCO for somebody that has COPD or ILD). I’ve also seen that VA is occasionally overestimated (although not by a lot) when the inspired volume is lower than it should and I believe this is due to the fact that the lung does not fill or empty homogenously. Finally, even though they tend to be relatively rare I’ve seen computer glitches of all kinds so I’d be looking for consistency (reproducibility) for both TLC and DLCO measurements, and would be suspicious of both overly high and overly low values.

      Warm regards, Richard

  2. Do we need to review the idea that the slow VC “should” be linked to the FRC manoeuvre? Other than it taking slightly longer, could we not simply perform slow VC manoeuvres and the TGV measurement entirely independently, and then importing the slow VC results to calculate the remaining static lung volumes? Assuming, of course, that both the slow VC and FRC manoeuvres are preceded by a stable end-expiratory level.

    • Ron –

      I’d prefer to keep the SVC + FRC linked whenever possible but in this case the link is being broken by the baseline shift and the problem rests in the fact that the software is using the end-exhalation of the pre-shutter tidal breathing as the FRC level. Strictly speaking the post-shutter SVC should be preceded by its own tidal breathing and the FRC level for the SVC derived from that. The volume of the TGV relative to the FRC should be derived from the pre-shutter tidal breathing.

      Having said all that, we are able to, and often do, perform just SVC maneuvers and then use the software to link them to good TGV maneuvers, so we’re already doing un-linked maneuvers whenever we think we need to. It’s hard to say whether performing un-linked TGV + SVC maneuvers should be routine or not. Although the ATS/ERS Lung Volume guidelines recommend performing linked maneuvers I don’t think that the difference in lung volumes with linked vs un-linked maneuvers has ever been studied.

      Regards, Richard

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