Assessment of airway responsiveness in infants with cystic fibrosis.

We compared the responses of cystic fibrosis (CF) (N = 14) and normal (N = 14) infants with inhaled methacholine. Airway function was assessed by forced expiratory flows at functional residual capacity (Vmax FRC) generated by the rapid compression technique, and methacholine responsiveness was quantitated as (1) TC: the threshold concentration to decrease Vmax FRC by 2 SD from baseline; (2) PC50: the provocative concentration to decrease Vmax FRC by 30%; and (3) SPC30; the slope of the dose-response curve between TC and PC30. There were no significant differences in age between CF and normal infants (16 +/- 8 versus 17 +/- 5 months, p greater than 0.3); however, the CF infants were shorter (74 +/- 10 versus 81 +/- 5 cm, p less than 0.05), had lower absolute Vmax FRC (241 +/- 103 versus 374 +/- 113 ml/s, p less than 0.001), and tended to have lower percentage of predicted flow values (87 +/- 13 versus 111 +/- 34%, p less than 0.10). Comparison of the indices of airway responsiveness revealed no difference in logTC; however, the CF infants had smaller, more negative values for logPC30 (-0.76 +/- 0.52 versus -0.22 +/- 0.53, p less than 0.02) and steeper slopes to their dose-response curves (logSPC30, 2.42 +/- 0.45 versus 1.88 +/- 0.74, p less than 0.025). Indices of airway responsiveness correlated significantly with baseline Vmax FRC (% of predicted). After the influence of baseline flow upon airway responsiveness was accounted for by multiple linear regression analysis, there was a tendency for CF infants to be more responsive than control infants.(ABSTRACT TRUNCATED AT 250 WORDS)