The forced expiratory time as a measure of small airway resistance

1. In five subjects, ten consecutive forced expiratory spirograms were recorded on each of 6 different days and the total forced expiratory time (FET) on each spirogram was measured. The mean overall coefficient of variation of the FET was 11.3% and of the ratio forced vital capacity (FVC)/FET was 8%. 2. Twenty-two subjects with probable small (less than 2 mm diameter) airway dysfunction as manifested by a reduced (less than 75% predicted) maximum mid-expiratory flow (MMEF) in the middle 50% of the FVC with normal FVC and FEV1 (greater than 80% predicted) were studied before and after bronchodilatation. There was no significant correlation between change in FET and change in MMEF (r = 0.36, P greater than 0.01), but between change in MMEF (deltaMMEF) and change in FVC/FET (deltaFVC/FET) there was a positive correlation (r = 0.78, P less than 0.001). 3. In six of these twenty-two subjects, the deltaFVC/FET and deltaMMEF on bronchodilatation correlated well with the change in residual volume (deltaRV). but neither measurement correlated well with the change in the 'closing volume' measurement expressed as the ratio of phase 4/VC(%). The deltaFET did not correlate well with either deltaRV or delta[phase 4/VC(%)]. 4. In eighty-one selected normal subjects (FVC, FEV1 and MMEF all greater than 80% predicted) there was better correlation of age with the FVC/FET (r = -0.676) than with the FET (r = +0.43); both correlations were significant (P less than 0.001). 5. These results do not support the use of the FET as a suitable test of small airway function; however, the mean flow rate expressed as FVC/FET may, in the presence of a normal FEV1/FVC ratio, be an adequate test of small airway function.