Inhaled nitric oxide - A bronchodilator in mild asthmatics with methacholine-induced bronchospasm

Nitric oxide (NO) reduces airway tone in the methacholine-treated guinea pig. We examined whether low levels of inhaled NO gas would relax airway smooth muscle tone in patients with mild asthma subjected to methacholine-induced bronchospasm. Thirteen adult volunteers with mild asthma inspired increasing concentrations of methacholine until their baseline forced expiratory volume in one second (FEV(1), 3.29 +/- 0.17 L, mean +/- SEM) decreased by greater than or equal to 20% (2.33 +/- 0.18 L, p < 0.01). Thereafter, they sequentially inhaled 100 parts per million (ppm) NO, 40% O-2; 40% O-2; and 100 ppm NO, 40% O-2 while spirometry was performed. Subsequent inhalation of isoproterenol returned the FEV(1) levels to baseline. Inhaling 100 ppm NO increased FEV(1) to 2.66 +/- 0.18 L (p < 0.01), and this increase was maintained after NO was discontinued. FEV(1) did not change during the second period of NO inhalation. Similar results were observed for vital capacity, but no significant effect was noted on forced expiratory flow at 25% of vital capacity or peak expiratory flow. Subjects were then divided into a responder subgroup, which showed a mean increase in FEV(1) after initial NO inhalation of 560 +/- 150 ml, and a nonresponder subgroup, which showed a mean increase in FEV(1) of 129 +/- 29 ml. Our data suggest that inhalation of nitric oxide by patients with mild asthma with methacholine-induced bronchospasm results in a minor but significant relaxation of airway tone.