Airway reactivity in infants: a positive response to methacholine and metaproterenol.

Because the presence of bronchial smooth muscle reactivity in infants remains controversial, airway reactivity was assessed in 10 normal, asymptomatic male infants less than 15 mo of age by measuring the changes that occurred in the maximal expiratory flows at functional residual capacity (VmaxFRC) during a methacholine bronchial challenge test. Sleeping infants inhaled doubling concentrations of methacholine by 2 min of tidal breathing, starting with a concentration of 0.075 mg/ml, and the bronchial challenge was stopped when VmaxFRC decreased by at least 40%. The threshold concentration of methacholine required to produce a decrease in VmaxFRC by 2 SD's of the control value was 0.43 mg/ml (0.11-0.90). By a methacholine concentration of 1.2 mg/ml, all infants decreased VmaxFRC by at least 40% (range 40-75%), and the mean dose required to produce a 40% decrease was 0.72 mg/ml. The airway reactivity was not related to base-line flows. During the methacholine challenge, no infant developed wheezing, but the percent oxygen saturation for the group decreased significantly (P less than 0.05) from 94 to 92%. Following the methacholine, the infants inhaled the bronchodilator metaproterenol, and 10 min later, VmaxFRC returned to base line. This study demonstrates that infants exhibit airway reactivity as evidenced by bronchoconstriction with methacholine and the subsequent bronchodilation with metaproterenol.