Biphasic (early and late) asthmatic responses to exercise in children with severe asthma, resident at high altitude.

Twenty-three children with severe asthma who had been resident at high altitude for at least 3 months, considerably reduced their requirement for regular anti-asthma therapy. On a standard exercise challenge in low humidity, ten had no response, seven had isolated immediate bronchospasm and six had biphasic responses with immediate bronchospasm followed 4-10 h later by a late reaction sustained for at least 2 h. The existence of biphasic responses after exercise, similar to those observed after antigen challenge, supports the concept of mast cell involvement in exercise-induced asthma.