Pulmonary gas exchange responses to histamine and methacholine challenges in mild asthma

Histamine (HIST) produces greater changes in bronchial and pulmonary vascul ature, and so mag produce more gas exchange abnormalities, than methacholin e (MTH) after inhalational challenge. The goals of this study were to compa re the effects of HIST and MTR challenge on pulmonary gas exchange in patie nts with mild asthma at an equivalent degree of bronchoconstriction. Eleven patients were studied (mean+/-SEM age, 22+/-1 yr; forced expiratory volume in one second (FEV1), 91+/-5% pred) using a randomized, double-blind cross-over design. Respiratory system resistance (Rrs), arterial blood gas es, and ventilation-perfusion distributions were measured before and after HIST/MTH challenges when cumulative doses caused a 30% fall in FEV1. Compared with baseline, HIST and MTH provoked similar moderate to severe in creases in Rrs (p<0.005 each), and mild to moderate decreases in arterial o xygen tension (Pa,O-2) due to ventilation-perfusion abnormalities (dispersi on of pulmonary blood flow -log SDQ-, 0.40+/-0.03-0.71+/-0.08 and 0.47+/-0. 04-0.89+/-0.06; normal values <0.60-0.65), respectively, similar to those s hown in mild to moderate acute asthma, without differences between them. For the same degree of airflow obstruction, both histamine and methacholine bronchoprovocations induce, in patients with mild asthma, very similar dis turbances in ventilation-perfusion distribution and respiratory system resi stance, suggesting similar mechanisms of airway narrowing.