Breathing 30 per cent oxygen attenuates bronchial responsiveness to methacholine in asthmatic patients.

The effect of breathing 30% oxygen on bronchial responsiveness to inhaled methacholine was studied in 30 asthmatic patients using a direct graphical recording method. An aerosol of methacholine solution was inhaled during tidal breathing by measuring respiratory resistance (Rrs) continuously, and dose-response curves were obtained on two separate days during air and 30% oxygen inhalation. Oxygen inhalations for 20 min caused no change in baseline Rrs. The threshold dose of methacholine, the dose which increased Rrs from the baseline value while breathing 30% oxygen, increased significantly compared with that while breathing air (p less than 0.01). No significant difference was found between the mean values of the slopes of the dose-response curves of the two groups. End-expiratory oxygen concentration (FETO2) during air breathing decreased significantly from 13.7% to 12.0% at the point where methacholine dosage reached threshold. This study suggests that hyperoxia attenuates the bronchial responsiveness to methacholine in asthmatic patients.