Central versus peripheral airway obstruction in bronchial responsiveness due to exercise.

The change in the site of maximal flow-limitation with time in exercise-induced bronchoconstriction was studied in 16 asthmatic children by repeated measurements of maximal flow-volume curves in air and helium-oxygen (80/20). Bronchoconstriction shortly after exercise is determined by flow-limitation in the more peripheral airways in nearly all of the patients. Bronchoconstriction then fades away, while the site of maximal flow-limitation returns towards the central airways. Both variables have a significant relationship (P less than 0.05), which differs with time and among patients. The degree of protection by the beta-sympathetic agonist is more pronounced (mean 28%) than that of the parasympatholytic (mean 3%), and its action increases along with the degree of shift in the maximal flow-limitation towards the periphery. The implications for the mechanism involved and the therapy are discussed.