Patients With Mild Asthma*

Background: Conflicting results from previous studies on the effect of bronchial challenge on ven¬ tilatory patterns in asthmatics may be due to airflow obstruction present before induction of bron¬ chospasm onset, as well as the different degrees of bronchoconstriction induced. Purpose and methods: We examined the ventilatory response to stepwise increments in respiratory resistance (Rrs) induced by continuous methacholine inhalation in eight patients with mild stable asthma without airway obstruction and eight normal subjects. Methacholine was inhaled continu¬ ously during tidal breathing until a nearly two-fold increase in Rrs was observed. Respiratory pa¬ rameters, including tidal volume (Vt), respiratory frequency (f), inspiratory ventilation (Vi), mean inspiratory flow (Vt/Ti), and duty ratio (Ti/Tt), were measured simultaneously by respiratory inductive plethysmograph (RIP). Arterial oxygen saturation (Sa02) was examined by pulse oximetry. The end-tidal CO2 fraction (FETCO2) was measured with a rapid-response infrared analyzer at the mouthpiece. Results: Rrs, Sa02, FETCO2, and respiratory parameters, measured after saline solution inhalation, were not different between the two groups. Inhaled methacholine first decreased and then increased Vi in both groups. However, changes in Vi occurred earlier and to a greater extent in asthmatics than