A comparison of the asthmatic response to methacholine and exercise.

The airway responses to methacholine and to exercise challenges were compared in 45 young adults with asthma. The spirometric response to five minutes of treadmill exercise was first documented. On a separate day methacholine dose-response relationships were determined. All asthmatics had an abnormal response to methacholine, and 36 had an abnormal response to exercise. Methacholine sensitivity and exercise-induced asthma were significantly related (r = 0.69, p less than 0.001), but the relationship was nonlinear; the increased response to exercise related to the logarithm of the methacholine response. Between asthmatics with generally unreactive airways, small variation in methacholine sensitivity was associated with large variations in the severity of exercise-induced asthma; between more responsive asthmatics, there was a smaller effect. It is suggested that exercise-induced asthma is dependent on two factors: a stimulus generated during exercise and a response from abnormal bronchi. The bronchial response may be a limiting factor in asthmatics with less responsive airways.