The ATS/ERS spirometry standard defines the FEV1 as the

“maximal amount of air exhaled in the first second of a forced expiration from a position of full inspiration”.  

In addition the standard recommends that

“…the largest FEV1 (BTPS) should be recorded after examining the data from all the usable curves…”

but since there are factors that can cause FEV1 to be overestimated or underestimated and therefore a reported FEV1 needs to meet certain quality criteria.

In particular, the FEV1 measurement can be overestimated due to back-extrapolation and a submaximal Peak Expiratory Flow.  FEV1 can be underestimated due to coughs or hesitations,

Back-Extrapolation:

Since the FEV1 is a time-based measurement and since no individual is able to start exhaling instantaneously, there needs to be a standardized approach towards measuring the start of exhalation. This is the purpose of back-extrapolation.

Back-extrapolation has its origins from the time when spirometry was measured from pen tracings on kymograph paper and its primary definition, “the back extrapolation method traces back from the steepest slope on the volume-time curve”, reflects this.  This process is fairly clear when graphed in volume-time format:

Since all spirometry systems are computerized this definition has been updated to include “the largest slope averaged over an 80 millisecond period”.  By definition, this 80 millisecond period contains the Peak Flow, and when it is graphed in flow-volume format, the back-extrapolation process becomes significantly less clear:

Nevertheless, a standardized process for determining the beginning of exhalation is critical towards measuring FEV1 and the current ATS/ERS definitions for back-extrapolation works reasonably well.

An extrapolated volume larger 150 ml or 5% of the FVC (whichever is greater) indicates the presence of a suboptimal start of exhalation. When this occurs FEV1 is usually overestimated and for this reason spirometer software will usually bypass a spirometry effort for reporting when the extrapolated volume is too high. There may be reasons however, to select a spirometry effort with an elevated extrapolated volume such as when all efforts have an elevated extrapolated volume or when the FEV1 from other efforts is underestimated due to cough or hesitations.

Peak Expiratory Flow (PEF):

PEF in not officially included within the ATS/ERS standards as a criteria for selecting FEV1.  However the standards do included the phrase “maximal forced effort” as part of the definition for FVC and FEV1. It has been recognized for decades that the FVC and FEV1 from a submaximal spirometry effort are often higher than the FVC and FEV1 from a maximal effort.  Peak flow is an indicator of this maximal effort and for this reason many labs use PEF as a quality indicator.

As an example of this, these two efforts from the same patient testing session highlight this problem. Both spirometry trials meet the ATS/ERS all criteria for the start of the test:

	Blue:	Red:
FVC (L):	2.72	3.06
FEV1 (L):	1.73	1.99
PEF (L/sec):	6.28	3.82

The Blue spirometry trial had a significantly higher peak flow but its FVC and FEV1 are smaller than those from the Red trial.  The higher PEF however, indicates that it was performed with a notably more maximal effort than the Red trial.  For this reason, the FEV1 should be selected from the Blue trial and not the Red trial.

Coughs or Hesitations:

A cough or a hesitation during the first second of exhalation can cause the FEV1 to be underestimated.  This is evident when graphed in volume-time format:

But much less evident when graphed in flow-volume format:

This is because there is no time axis in a flow-volume loop, only volume and flow rates.  A notch like this in a flow-volume loop will signal that there is a discontinuity during an exhalation but cannot say how long it occurred or whether it affected the FEV1 or not.

an incorrect FEV1, regardless of the reason, has implications not just for itself but also for the FEV1/FVC ratio as well.

The amount by which the FEV1 is under- or over-estimated can only be guessed.  For this reason an interpretation has to consider the reported results in terms of the probability they are correct rather than assuming they are.

PFT Interpretation by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License