We studied the effects of repeated exercise and histamine challenge in asthmatic patients to determine the frequency and degree to which a state of refractoriness was induced by these stimuli. Twenty-nine patients performed three exercise tests, and on a separate day 16 of these patients had three histamine inhalational challenge tests. Forty minutes separated each challenge. Changes in airways resistance were measured using the peak expiratory flow rate (PEFR). The fall in PEFR (expressed as a percentage of the pre-challenge value) was used to quantify the response to challenge. Significant “protection” was defined as a fall in PEFR after a repeated challenge less than 50% of the fall observed on the first challenge. All patients had a fall in PEFR greater than 22% on the first challenge of the day. With repeated exercise 28 out of 29 patients had a fall in PEFR less than that observed on the first test and 12 had significant “protection”. The fall in PEFR after the third exercise challenge was not significantly different to the second challenge and a “plateau” effect was observed. There was no significant difference in the fall in PEFR after the first and second histamine challenge although two of the 16 patients were significantly protected. After the third histamine challenge five of the 16 patients were significantly protected from the effects of the same dose of histamine. The degree to which repeated exercise challenge induces a diminished response is variable. With repeated challenge the response to histamine remains relatively constant in most patients though 30% may be expected to be refractory after a third challenge.
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