Dry air-induced constriction in lung periphery: a canine model of exercise-induced asthma.

We studied the effects of the flow of dry air on collateral tone in the lung periphery. A bronchoscope was wedged in sublobar segments of anesthetized dogs, and measurements of collateral resistance (Rcs) were recorded before and after flow was increased from 200 to 2,000 ml/min for a 5-min period. Five minutes after exposure was completed, Rcs increased by an average of 117 +/- 25.2% (SE) over control. Maximum Rcs occurred 5 min after the challenge was concluded and required 48 +/- 10.5 min to return to base line. When flow rate was held constant and exposure period varied, Rcs increased with increased stimulus duration. With exposure times held constant, the response of the collateral system was positively associated with changes in stimulus strength (flow rate). No refractory period was observed with repetitive challenges. Finally, when dry air (delivered at 22 degrees C) and conditioned air (i.e., delivered at 28 degrees C; relative humidity = 80%) challenges were alternated in the same wedged segment, dry air produced a mean increase in Rcs of 93.2%, whereas challenge with warm moist air increased Rcs only 33.5%. Regardless of which challenge was presented first, dry air consistently produced a greater constrictor response. This response is similar to that observed in cold air- and exercise-induced asthma and indicates that the lung periphery in dogs, like larger airways in asthmatic subjects, has the potential to increase tone when exposed to dry air. Peripheral airways in dogs thus constitute a model that can be used for the investigation of exercise-induced asthma.