Because the presence of bronchial smooth muscle reactivity in infants remains controversial, airway reactivity was assessed in 10 normal, asymptomatic male infants <15 mo of age by measuring the changes that occurred in the maximal expiratory flows at functional residual capacity (V̇maxFRC) 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 V̇maxFRC decreased by at least 40%. The threshold concentration of methacholine required to produce a decrease in V̇maxFRA 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 V̇maxFRC by at least 40% (range 404-75%), 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 of the group decreased significantly (P < 0.05) from 94 to 92%. Following the methacholine, the infants inhaled the bronchodilator metaproterenol, and 10 min later, V̇maxFRC returned to the base line. This study demonstrates that infants exhibit airway reactivity as evidenced by bronchoconstriction with methacholine and the subsequent bronchodilation with metaproterenol.
ASJC Scopus subject areas
- Physiology (medical)