To compare the refractory period that follows exercise and isocapnic hyperventilation, 10 asthmatic children performed two pairs of challenge tests in random order at least six hours apart. In pair A a hyperventilation challenge was followed by an exercise challenge and in pair B the order was reversed. Both pairs of tests were done while the children were breathing cold dry air. Tests were matched in terms of work load, ventilation, and end tidal carbon dioxide tension (PCO2). The mean percentage fall in FEV1 (delta FEV1) after the first challenge (hyperventilation) of pair A and the first challenge (exercise) of pair B were the same (30% (SEM 2%)) and 30% (4%) respectively). The mean delta FEV1 of the exercise test following hyperventilation in pair A and of hyperventilation following exercise in pair B was 22% (4%) and 18% (4%) respectively. Both these latter results were significantly lower than the respective delta FEV1 when the challenge was the first test of the pair. Although the mean refractoriness index (reduction in induced asthma in the second test of each pair compared with the first test) was greater when exercise was the first challenge, the difference was not significant.
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