TIMING OF THE FORCED VITAL CAPACITY

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Abstract
Based on analysis of more than 2500 human forced vital capacity (FVC) maneuvers, 2 aspects of spirometry standards concerning the timing of the FVC maneuver were examined. The forced expiratory volume in 1 s (FEV1) when timing was initiated by the back extrapolation method was compared with FEV1 when timing was initiated by flow or volume threshold to determine whether the latter offered a reliable eqiuvalent. Although the mean differences appear to be relatively small, because of variability in initiating expiratory effort, neither alternative is likely to offer a uniformly accurate numeric equivalent to backward extrapolation. The volume expired before the initiation of the timing as a per cent of FVC was measured to determine whether 10% was a reasonable limit to apply for acceptability of a test. Although 2 SD from the mean of 4.35% were well within this limit, the technician''s judgment of acceptability is probably sufficient in most cases.

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