Evaluation of Bronchial Responsiveness by Pharmacological Challenges in Asthma

Bronchial challenges with bronchoconstrictive drugs have supplied important information on some relevant pathophysiological aspects of bronchial asthma, i.e. nonspecific bronchial hyperresponsiveness. Until now histamine and methacholine have been the most widely used inhalation tests. Recently, propranolol, given by inhalation, has been proposed and evaluated for possible application as a bronchial provocation challenge both in clinical and experimental setting. In asthmatic subjects with bronchial hyperresponsiveness to histamine and methacholine, inhaled propranolol induces a dose-related bronchoconstriction. In a group of 25 asthmatic patients, the mean value of provocative doses, in μmoles of propranolol that causes a 20% drop in FEV1 (PD20FEV1), is thirteen times greater than that of methacholine. In the same study, there is no correlation between the bronchial responses to the two drugs. Moreover, none of 9 normal subjects of the control group responds to inhaled propranolol. The mechanisms of propranolol-induced bronchoconstriction are poorly understood. A cholinergic reflex is probably involved, but it is likely that propranolol has some different and rather peculiar pathways. Bronchial response to inhaled propranolol is a reliable, reproducible, safe, and well-tolerated method. Bronchoconstriction induced by propranolol appears to be a very promising technique in the investigation of the mechanisms of nonspecific bronchial hyperresponsiveness.