Different bronchoconstrictor effects of carbachol boluses inhaled near residual volume or total lung capacity

The purpose of this study was to determine if the lung volume at which aerosol inhalation begins (LVi) influences airway responses to bronchoconstrictor agents. We compared the effects of carbachol boluses (25, 50 and 100 ml), inhaled at high and low LVi (averaging 72.2 and 15.8% of vital capacity, respectively) on specific airway resistance (SRaw). In order to eliminate the possible influence of airway obstruction on aerosol distribution and deposition, we selected 5 asthmatic subjects with normal respiratory function (spirometry, SRaw, nitrogen washout and closing volume); furthermore, non-cumulative dose-response curves were obtained (i.e. the patients inhaled only one dose of carbachol on a given test day). Inhaling carbachol at low LVi yielded a significantly (p less than 0.01) larger degree of bronchoconstriction. Differences in bronchial responses were probably due to differences in the amount of particles deposited in the airways and/or to their distribution. These data suggest that LVi should be controlled for quantified inhalation provocation tests.