Responsiveness of the upper airways to inhalation of methacholine in normal and asthmatic subjects

Upper airway resistance (Ruaw) and respiratory resistance below the larynx (Rlaw) were measured simultaneously by 3Hz forced oscillatory technique during inhalation of methacholine in three normal and seven asthmatic subjects. The tracheal lateral pressure was measured with an 18 gauge needle inserted into the trachea just below the crycoid cartilage. Ruaw was calculated from the translaryngeal pressure and mouth flow, and Rlaw from the tracheal lateral pressure and mouth flow. Provocation was induced from 0.049mg/ml in asthmatics, and 12.5mg/ml in normal subjects by continuous inhalation of methacholine at stepwise increased concentration until about twice the control Rlaw. In normal subjects, Ruaw increased with increase of Rlaw at high cumulative doses of methacholine. On the other hand, in asthmatic patients Ruaw did not change in spite of marked increase of Rlaw at a lower cumulative dose of methacholine. These findings suggest that the hyperresponsiveness of asthmatics to methacholine occurs mainly in the lower airways.