Study objectives: The inhalation of hypoxic gas has been reported to enhance the airway responsiveness to methacholine in some animal models. However, the data on humans have so far been conflicting. We attempted to examine the effect of hypoxic gas inhalation on the airway responsiveness to methacholine.
Design: We evaluated the airway responsiveness to methacholine by continuously measuring respiratory conductance with the forced oscillation method under normoxia or hypoxia in a single-blind, randomized, crossover fashion (2 days for each).
Participants: Twelve asymptomatic male volunteers (mean +/- SD age, 27 +/- 4 years) with airway hyperresponsiveness to methacholine. Two of the 12 volunteers had a history of bronchial asthma.
Setting: The participants inhaled either normoxic or hypoxic gas with continuous inhalation of aerosolized methacholine in incremental doses with a sustained respiratory rate of 15 breaths/min. The arterial oxygen saturation was kept to 90% on the hypoxic days.
Results: There were no significant differences in any indexes of airway responsiveness to methacholine (the cumulative dose of methacholine at the threshold and the point of 35% decrease of the respiratory conductance, and the slope factor of the dose-response curve) between the hypoxic days and the normoxic days.
Conclusion: The inhalation of mildly hypoxic gas does not enhance the airway responsiveness to methacholine in humans with airway hyperresponsiveness.