Maximal Expiratory Flow at FRC (V′max_FRC): Methods of Selection and Differences in Reported Values

We compared three methods of reporting maximal expiratory flow (V′max_FRC) measured in partial expiratory flow-volume curves (PEFVCs) at the point of functional residual capacity (FRC). PEFVCs were obtained with the rapid thoracoabdominal compression technique (RTC) on a total of 446 occasions in 281 HIV-negative, asymptomatic infants (4.8-28.1 months old). Three different expressions of V′max_FRC were recorded: 1) the highest measured flow (maxV′_FRC), 2) the mean of the three highest flows (mean3V′FRC), and 3) the flow at FRC in a composite curve (compV′_FRC) consisting of PEFVCs, obtained at different jacket pressures and superimposed at their distal limb. The numerical value of maxV′_FRC was 7.4% (±5.6%) higher than the mean3V′_FRC, and 11.9% (±17.7%) higher than the compV′_FRC; the mean3V′_FRC was 5% (±18.3%) higher than the compV/FRC. Bland-Altman analysis was used to evaluate the agreement between the three indices. The mean difference and 95% limits of agreement were: maxV′_FRC-mean3V′ _FRC, 14 ± 18 ml/sec; maxV′_FRC - compV′_FRC, 23 ± 58 ml/sec; and mean3V′ _FRC - compV′_FRC, 10 ± 52 ml/sec. The differences between the slopes of the three indices (regressed against height) were statistically significant, although clinically unimportant. We conclude that despite their high correlation, the mean3V′_FRC and maxV′_FRC should not be used interchangeably, and that the composite analysis, although useful, does not improve the reproducibility of V′max_FRC, and thus it cannot be recommended for routine use in its current form.