The bronchoprotective effect of deep inspiration is flow rate dependent

Background and study objective: Deep inspiration (DI) protects against methacholine-induced bronchoconstriction in healthy subjects. We hypothesized that this bronchoprotective effect of DI depends upon the inspiratory flow rate. Design: Prospective, controlled study. Setting: Pulmonary function laboratory within a large tertiary medical center. Participants: Ten healthy nonsmokers without asthma or rhinitis. Measurements: First, we performed a methacholine challenge in the absence of DI to determine the concentration sufficient to reduce FEV₁ by 20%. On two subsequent days, the challenge was repeated with the addition of either a fast or slow DI immediately before the threshold concentration of methacholine. We calculated the % reduction in FEV₁ and FVC from baseline. Results: Mean % reduction in FEV₁ and FVC was significantly less with a fast DI than with no DI (20±3% vs. 34±4% for FEV₁, p=0.02; 12±3% vs. 23±3% for FVC, p=0.03); slow DIs did not significantly affect the methacholine-induced reduction in lung function. Conclusion: A fast DI is bronchoprotective while a slow DI is not. Elucidating the conditions that maximize or alternatively, eliminate bronchoprotection in healthy subjects may ultimately provide insight into the pathophysiology of asthma.