This spirometry report came across my desk today.
Obs: %Pred: Pred:
FVC (L): 5.14 101% 5.10
FEV1(L): 4.07 105% 4.07
FEV1/FVC: 79 104% 76
On the face of it, this looks like an eminently normal spirometry test. If you look carefully at the flow-volume loop however, you will see that the inhaled volume is greater than the exhaled volume. The test system did not report the inspiratory vital capacity (FIVC) but by eyeball I would estimate the difference to be about 0.60 liters. The ATS recommends that the largest vital capacity, regardless of where it comes from should be used to calculate the FEV1/VC ratio. This means that the FEV1/FIVC ratio is actually about 70.9 not 79.2 which is 93% of predicted, not 104% and that this is probably mild airway obstruction.
This spirometry effort met all ATS/ERS criteria. It was longer than 6 seconds and met end-of-exhalation flow rate criteria. Is this gas trapping? Given the FEV1, probably not in the same way that somebody with COPD might have gas trapping, but maybe yes. The patient was 68 years old and since closing volume rises with age a certain amount of gas trapping is a normal consequence of aging and I think this may be what this spirometry effort is showing. Or, despite meeting end-of-exhalation criteria maybe the patient should have exhaled longer.
This spirometry effort also got me curious about why the test system did not report the FIVC. The PFT Lab I am associated with does not normally report FIVC. For those patients whose inspiratory flows are important we tend to look solely at the contour of the flow-volume loop and not at any specific inspiratory flow rate numbers or ratios. There were no settings in the software for FIVC and the test system’s manual did not address this issue at all so with some simple experimentation I found that it would report an FIVC only if the inspiratory effort was performed immediately following the FVC effort.
This is a point of some concern because the ATS/ERS statement on spirometry says you can perform an FIVC maneuver either before or after a forced expiratory effort and that as already mentioned the largest VC should be used for the FEV1/VC ratio. Because the software for our testing system (and I suspect many other test systems) only allows the FIVC to be measured after the FVC it is missing, at least in those cases like this one, an accurate assessment of the patient’s largest VC.
In addition, from a procedural point of view I suspect that some patients may be able to exhale further towards RV and therefore produce a larger inspiratory vital capacity when the exhalation starts with a steady exhalation from FRC than immediately after a prolonged forced expiratory effort and our test system will not allow FIVC to be measured this way.
Strictly speaking, forced inspiratory flow rates and vital capacity are not values that normally need to be measured. Inspiratory airway obstruction is much less common than expiratory airway obstruction and in addition my experience has been that patient inspiratory efforts tend to be far more variable and irreproducible than expiratory efforts. This particular spirometry effort was unusual in that the reported values were essentially normal and this kind of discrepancy between inspiratory and expiratory vital capacity is something that is more commonly seen with moderate to severe COPD.
References:
Miller MR, et al. ATS/ERS Task Force: Standardization of Spirometry. Eur Respir J 2005; 26: 319-338.
Pellegrino R, et al. ATS/ERS Task Force: Interpretive strategies for lung function tests. Eur Respir J 2005; 26: 948-968.
PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
I found this on a google search because I’ve been on my own exploration of the the FIVC but that’s not the FIVC in your graph. The FIVC is the inspiratory volume after the forced expiration, not before.
Bobby –
The 2005 ATS/ERS standards for spirometry say that an FIVC can be performed either before or after the forced expiratory maneuver. In one sense you’re correct since a proper FIVC maneuver should be performed with maximal effort in order to measure PIF, MIF50 etc. Even if it isn’t however, it’s still a vital capacity measurement, and that was the point.
Regards, Richard
Richard,
The standards don’t mention FIVC the way that you’ve described it. If I’m missing or overlooked something, please refer me to that section. What the standards do mention is the acceptance of using and IVC or EVC maneuver for a VC value.
Also, the flow volume example you give above is an IC maneuver, more like a forced IC maneuver. It would be interesting to see what this person could have produced in an IVC maneuver. Regardless, this person looks normal with either VC that chosen. The ratio that you’ve calculated is still above 0.7 and the FEV1 is over 100% predicted. It should be mentioned that ATS may suggest using the highest VC or FVC but the GOLD guidelines for COPD severity rely solely on FEV1/FVC.
-Bobby
Bobby –
You’re right. I apparently mis-read a section of the spirometry standard describing an FVC maneuver that started with an exhalation to RV (pg. 327, second paragraph after ‘test procedure’). However, I also couldn’t find any official description of how an FIVC should be performed other than saying there should be a maximal inhalation after the maximal exhalation.
The problem with this definition of the FIVC (if that is what it is) is that it assumes that the FVC exhalation is truly maximal (i.e. to RV) and that frequently isn’t the case, particularly in those individuals with COPD. In fact there are some patients with severe airway obstruction who, because of severe gas trapping, cannot exhale to their FRC, even with a relatively prolonged expiratory time. If you assume that the FIVC maneuver must always follow the FVC maneuver then these individuals can never have an FIVC greater than their FVC, even though the initial IC may be greater than the FVC.
I will be the first to admit that the example in the posting isn’t perfect. I’ve run across many better ones in the years since but since I’d already addressed the topic I didn’t feel the need to correct it. My main points for this problem are that (1) the FVC can be smaller than the IVC (or IC) and that this can occur either because of gas trapping or because of an expiratory leak. (2) As the ATS/ERS interpretation standard says, the FEV1/VC ratio should use the largest VC. (3) In these instances, the FEV1/VC ratio using the IVC (or IC) will be lower than the FEV1/FVC ratio. (4) My lab software (and all lab software I’ve seen) does not measure the initial IVC (or IC) so this problem can be overlooked unless it is observed and manually corrected. (5) Unless it is corrected there will be some individuals who meet the FEV1/VC criteria for airway obstruction whose report will not reflect this fact (regardless of which standards you use to assess airway obstruction).
A problem with using the FIVC and any of the inspiratory flows (MIF50 etc) in any way however, is that although maximal inspiratory flows are mentioned in the ATS/ERS spirometry and interpretation guidelines not only is there no clear definition of the FIVC maneuver there are also no criteria for assessing the quality of an FIVC maneuver other than saying “It is critical that the patient’s inspiratory and expiratory efforts are near maximal and the technician should confirm this in the quality notes.” (ATS/ERS interpretation guidelines, pg. 960, upper right paragraph).
Regards, Richard