Maximum expiratory flows (MEFx%, FEFx%)

Maximum expiratory flow when x% of the FVC has been exhaled (FEFx%) or x% of the FVC remain to be exhaled (MEFx%, now deprecated) is assessed in either of three ways:

1. Reproducible flow-volume curves with the 3 largest reproducible FVCs are superimposed at the starting point (TLC). The largest flow values at 25%, 50% and 75% of the largest FVC are reported (envelope method) (ref. 1).
2. Expiratory flow at 25%, 50% and 75% of the FVC is taken from three flow-volume curves that are reproducible with respect to shape and size. Report the largest value derived from the maneuver the FVC of which differs less than 5% from the largest FVC (ref. 2).
3. Report the value taken from a curve with the largest sum of FEV1 and FVC (ref. 3 and 4).

The reproducibility of forced expiratory flows diminishes in the cited order of recommendations in adults and adolescents. In practice the third recommendation (ATS) is most frequently used, but it cannot be applied in young children.

If the maneuver is performed properly, forced expiratory flows between 25% and 75% of the FVC are effort independent because they are determined by a flow limiting segment developing in intrathoracic airways. Effort independence has led to the misconception that forced expiratory flows would provide more reliable and more ‘sensitive’ indices of airway obstruction than for example the FEV1. The opposite is the case:

• As measurements are performed at a specified percentage of the FVC, variability in FVC contributes to that in the measured forced expiratory flow, unlike the FEV1.
• The shape of the flow-volume curve varies markedly between healthy subjects of the same age, standing height and gender. Forced expiratory flow at x% of the FVC is sensitive to such variations, which have little biological consequence.
• It is more difficult to measure an instantaneous flow reproducibly as it is more sensitive to small and meaningless fluctuations than the FEV1, where ‘white noise’ is filtered as it is a mean flow over 1 second. The ATS/ERS recommendation on standardization of spirometry (ref. 4) no longer mentions instantaneous flows, except FEF25-75%.
• The above factors contribute to the fact that the explained variance of predictions of FEFx% on the basis of age, height and gender are very small, and that the residual standard deviation of prediction equations is large. For example, depending on age, sex and ethnic group, the between subject CoV varies between 27-89% for FEF75% (ref. 5).

On that account predicted values of FEFx% are of little value, but the shape of flow-volume curves provides very valuable information (see characteristic flow-volume curves). If FEV1 is within normal limits but forced expiratory flow below the ‘limit of normal’, disregard the latter observation (ref. 5).

MEF50%FVC correlates very highly (explained variance 98%) with the MMEF = FEF25-75%, so that one of these indices suffices in practice; the logarithmic transformation in the right part of the illustration was necessary to more or less stabilize the variance around the regression line. MEFx% is assessed at a specific percentage of the FVC and therefore an unsuitable index of airway obstruction or bronchodilatation if the FVC is abnormal (obstructive and restrictive ventilatory defects), as explained in the paragraph about the MMEF (FEF25-75%).

Measurements of FEF25-75% and FEF75 do not contribute any useful information that is not available from measurements of the FEV1, FVC and FEV1/FVC ratio and therefore have no clinical utility (ref. 6).