Effect of circadian rhythm on bronchomotor tone after deep inspiration in normal and in asthmatic subjects.

Bronchomotor tone after deep inspiration and bronchial responsiveness to methacholine were studied at 4:00 A.M. and 4:00 P.M. in 14 normal and 13 asthmatic subjects. Bronchomotor tone was assessed with respiratory resistance (Rrs) measured by the forced oscillation method. Bronchial responsiveness to methacholine and baseline Rrs were higher at 4:00 A.M. than at 4:00 P.M. in both normal and asthmatic subjects (p less than 0.01). The difference in methacholine threshold between 4:00 P.M. and 4:00 A.M. In asthmatics was similar to that in normal subjects. Immediately after deep inspiration, Rrs decreased more at 4:00 A.M. than at 4:00 P.M. In normal subjects. In contrast, asthmatic subjects showed a significantly greater immediate increase in Rrs after deep inspiration at 4:00 A.M. than at 4:00 P.M. It is suggested that the bronchoconstrictive effect after deep inspiration is a distinguishing characteristic of asthmatics.