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?
The ATS/ERS standards for spirometry states:
“The largest FVC and the largest FEV1 (BTPS) should be recorded after examining the data from all of the usable curves, even if they do not come from the same curve.”
The ATS/ERS standards for interpretation further states:
“An obstructive ventilatory defect is a disproportionate reduction of maximal airflow from the lung in relation to the maximal volume (i.e. VC) than can be displaced from the lung.”
Nowhere does it say that the VC can’t come from a post-BD FVC. The vital capacity is a relatively fixed value based on rib cage dimensions and the distance the diaphram can travel and bronchodilators do not change this. Any increases that are seen in a post-BD FVC are almost always related to a reduction in airway resistance and gas trapping, and not to a change in lung volume. This being the case, a VC is a VC, regardless of when it is measured.
The ATS/ERS standards for interpretation seconds this point of view to some extent when it states:
“The VC, FEV1, FEV1/VC ratio and TLC are the basic parameters used to properly interpret lung function. Although FVC is often used in place of VC, it is preferable to use the largest available VC, whether obtained on inspiration (IVC), slow expiration (SVC) or forced expiration (i.e. FVC).”
So other than saying that’s not the way it’s supposed to be done, why can’t you use the post-BD FVC this way? If you accept the idea that the FEV1/IVC ratio and FEV1/SVC ratio are valid approaches to determining the presence of airway obstruction you are also accepting the idea that the FVC is often underestimated as a consequence of airway obstruction. If you accept this idea, then why isn’t the post-BD FVC as valid a measurement of VC as IVC and SVC?
I can’t think of a reason why not and the ATS/ERS standards for spirometry and interpretation don’t explicitly forbid it. They don’t explicitly approve it either, but once you accept the notion of using the FEV1/VC ratio instead of the FEV1/FVC ratio it becomes a relatively logical consequence of that train of thought.
The downside (or is it the upside?) of using the pre-BD FEV1/post-BD FVC ratio is that some individuals whose spirometry would otherwise be considered to be within normal limits would instead be considered to have mild airway obstruction. I’d point out however, that this is also what would probably happen if SVC was routinely measured and the FEV1/SVC ratio reported instead of the FEV1.
So, why isn’t the post-BD FVC used this way right now? I think the answer is mostly psychological. Pretty much from the beginning of modern spirometry in the 1950’s post-BD testing has been performed only to compare FVC to FVC and FEV1 to FEV1. Although the ATS/ERS standards mandated the use of the FEV1/VC ratio (with the VC coming from the SVC, IVC or FVC, whichever was largest) a dozen years ago, this is recent history and the idea still hasn’t trickled completely down to all PFT labs and all pulmonologists. Even with the mandate from the ATS/ERS SVC and IVC maneuvers are performed much less frequently than FVC maneuvers so most practitioners aren’t in the habit of making substitutions. This adds up to a sort of institutional blindness and I will admit to having been part of it because I’ve been in the field for over 40 years and this is the first time it ever occurred to me.
This issue really only applies when the baseline FEV1/FVC ratio is withn normal limits. I can’t find any logical reason not to insert a post-BD FVC into the FEV1/VC ratio and if it increases the number of individuals diagnosed with airway obstruction then I don’t think this is any different than it would be if we performed SVC maneuvers as part of routine spirometry.
References:
Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of lung function testing. Standardisation of spirometry. Eur Respir J 2005; 26: 319-338.
Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of lung function testing. Interpretive strategies for lung function tests. Eur Respir J 2005; 26: 948-969.
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