Assessing bronchodilator responsiveness in infants using partial expiratory flow-volume curves.

Our objective was to compare the effectiveness of maximum forced expiratory flow measured at functional residual capacity (V'maxFRC) and the ratio of flow at 75% of the forced expiratory volume to peak forced expiratory flow (FEF(75)/FEF(peak)) for detecting bronchodilator-related changes in wheezy infants. In 55 infants (mean age, 7.8 +/- 3.1 months) with a history of recurrent wheezing, V'maxFRC and FEF(75)/FEF(peak) were measured at baseline and 15 min following nebulized albuterol. Mean results from 4 baseline and 4 postalbuterol partial expiratory flow-volume curves were compared at baseline and following bronchodilator challenge. The strength (relative effect size) of each measure for assessing change was quantified by dividing the mean of the pre- to postdifferences by the standard deviation of the differences. Mean percent predicted V'maxFRC was 41.3 +/- 34.3% at baseline and 44.4 +/- 34.0% following albuterol. Mean FEF(75)/FEF(peak) was 26.7 +/- 13.4% at baseline and 35.8 +/- 14.3% following albuterol. The mean percent change from baseline [(post-pre)/pre] in percent predicted V'maxFRC was 18.3 +/- 39.3, and for FEF(75)/FEF(peak), it was 44.1 +/- 36.8. The change in FEF(75)/FEF(peak) following albuterol was significantly greater than the change in V'maxFRC (P < 0.0001). The relative effect size for mean percent change from baseline in V'maxFRC was 0.47, and for FEF(75)/FEF(peak), 1.20. Changes in FEF(75)/FEF(peak) appear to differentiate changes in airway function following administration of a bronchodilator better than do changes in V'maxFRC.