The use of the dose-response curve in the assessment of normal and asymptomatic asthmatic patients.

We studied bronchial reactivity, the slope of the dose-response curve to inhaled methacholine, in normal subjects (N), young asthmatics with normal respiratory resistance (Rrs) (YAL), young asthmatics with high Rrs (YAH), and old asthmatics with high Rrs (OAH) by three different methods: linear reactivity (slope of the linear cumulative dose-conductance (Grs = 1/Rrs) curve), timed reactivity (slope of the log concentration-Grs or time-Grs curve), and log reactivity (slope of the log cumulative dose-Grs curve). There were significant differences in linear reactivity between the N and the three asthmatics but no difference among the three asthmatic groups. On the other hand, both timed and log reactivities showed significant differences between the three asthmatic groups. Log and timed reactivities corrected by baseline Grs did not differentiate the YAL from the YAH but showed a significant difference between the YAH and OAH. This study suggests that timed and log reactivities may be better indices of bronchial reactivity than linear reactivity because they differentiate asthmatics, and that bronchial reactivity in asthmatics may be dependent on not only baseline airway caliber in asthmatics but also other factors such as atopic status or age.