Site of action of inhaled histamine in asymptomatic asthmatic patients

Histamine inhalation provocation-tests were performed in twenty-two young asthmatics with normal lung-function tests with progressive, increasing doses of a pressurized aerosol of histamine phosphate. Airway resistances (Raw) and N2 washout-curves were recorded. Two different types of response have been observed: (1) in thirteen cases, there was an increase of both Raw and the N2 phase III slope; and (2) in eight cases, there was only an increase in Raw (in one subject there was an increase in the N2 phase III slope only). Comparing the two groups of patients there was no difference in the inhalation procedure, the dose of histamine delivered or the smoking habits and lung-function data, except a slightly higher residual volume/total lung capacity (RV/TLC) ratio in the first group. The histamine-induced changes could not be related to any of the control lung-function data. We conclude that histamine inhalation may induce either peripheral bronchoconstriction only, or central bronchoconstriction with or without peripheral bronchoconstriction. A local and/or peripheral vagal-mediated bronchoconstrictor effect could account for the different site of airway response.