Mild Isocapnic Hypoxia Enhances the Bronchial Response to Methacholine in Asthmatic Subjects

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Abstract
We studied the effect of mild isocapnic hypoxia (FIO2 = 15.5%) on lung mechanics, heart rate, circulating plasma catecholamines, and bronchial responsiveness to methacholine in ten asthmatic adults. Hypoxia did not alter lung mechanics (i.e., dynamic pulmonary compliance [Cldyn], pulmonary resistance [RL]) nor did it increase plasma catecholamines, but it significantly increased bronchial responsiveness to aerosolized methacholine, as assessed by the fall in forced expiratory volume in one second (FEV1: 1.2 .+-. 0.18 versus 0.9 .+-. 0.14 L/s, p < 0.05), the rise in RL (RL: 19.1 .+-. 1.4 versus 8.4 .+-. cm H2O/L/s, p < 0.05), and the steeper slope of the dose-response curve to methacholine. We concluded that the hypoxic characteristic of asthmatic attacks may aggravate airflow obstruction.