1. The relationship between volumes expired and the maximum intrathoracic pressures developed within the first second of expiration was examined in a group of healthy subjects and patients with airways obstruction who exhaled with varying degrees of effort.

2. The body plethysmographic method was used to ensure that expirations started from total lung capacity.

3. Three different types of volume-effort curves could be distinguished: Type I; in this the volume expired increased with greater effort. Type II; after a phase in which the volume expired increased with effort, a plateau was reached after which increasing effort made no difference to the volume expired. Type III; this resembles Type II except that instead of a plateau being established at maximum expired volume, increasing effort resulted in diminishing expired volume. Correction for the degree of compression of the intrathoracic gas failed to account for this phenomenon. Type III curves were found in healthy subjects as well as in patients.

4. In most cases the maximum Forced Expiratory Volume in 1 s (FEV1) was not found to coincide with the maximum oesophageal pressure (Poes), nor was there a significant difference between the mean and the maximum FEV1 produced with an effort greater than 75% of maximum.

5. Blowing ‘half as hard’ is an unnatural procedure; all subjects tend to blow nearly as hard as possible or very gently. In the latter case, the resulting FEV1 tracing is easily recognizable as technically unsatisfactory.

6. We conclude that for practical purposes, measurements of FEV1 depend for their reliability upon ensuring that they are started from full inspiration; insistence upon maximum effort is less important because FEV1 is independent of effort for the greater part of the effort range.

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