Airway responsiveness to histamine in patients refractory to repeated exercise.

To investigate the mechanisms contributing to refractoriness in exercise-induced asthma (EIA), airway responsiveness to histamine was studied in eight asthmatic patients. Patients were included in the study on the basis of their refractory response to multiple exercise challenges. Incremental challenges with inhaled histamine were performed at rest and 40 minutes after single and paired exercise tests. The geometric mean histamine concentration required to produce a 20 percent fall in FEV1 (PC20) for the challenge after paired exercise test (4.34 mg/ml) was significantly higher (p greater than 0.001) than those for the challenges after a single exercise (1.05 mg/ml) and for the challenge at rest (0.67 mg/ml). There was no significant difference between PC20 values at rest and after a single exercise test (p greater than 0.05). The results show that the response to histamine is reduced during the refractory period following paired exercise test.