Bronchial responsiveness to methacholine and effects of respiratory maneuvers

Eight asthmatic and six normal subjects had methacholine chloride inhalation tests on two visits. On first assessment the provocative concentration causing a 20% fall in forced expiratory volume in 1 s (PC20FEV1) was measured. On the second visit the provocative concentration causing a 35% fall in specific lung conductance (PC35sGL) was obtained in addition to the PC20FEV1. Lung resistance was continuously monitored to evaluate the bronchomotor effect of FEV1 maneuver. Results of PC20FEV1 were within one single twofold concentration on the two visits. In all subjects but one PC35sGL was lower than PC20FEV1, and in 10 instances this difference was superior to a single twofold concentration. The bronchodilator effect of FEV1 maneuver was significantly although loosely related to base-line airway caliber and excitability. However, we were unable to show a significant relationship between the differences between PC20FEV1 and PC35sGL and this bronchodilator effect. This suggests that other factors may explain the greater sensitivity of PC35sGL compared with PC20FEV1.