To determine whether refractoriness to histamine induced bronchoconstriction occurs, 20 asthmatic subjects aged 19-50 years were tested. Subjects underwent two histamine challenge tests (1 and 2) on the same day, the second one being given 45-60 minutes after the first, once the FEV1 after test 1 had returned spontaneously to within 90% of baseline. A further "control" histamine challenge test was carried out on a different day at the same time (+/- 2 hours) as test 1. Bronchial responsiveness was recorded as the cumulative dose (microgram) of histamine provoking a 20% fall in FEV1 (PD20), and the ratio PD20 test 2:PD20 test 1 was used to assess refractoriness. The median value of this ratio (2.20) was significantly greater than 1 (p = 0.003), indicating refractoriness at the time of test 2. By contrast the median ratio PD20 control:PD20 test 1 of 1.03 was not significantly different from 1. Refractoriness could not be accounted for by failure to regain the initial baseline FEV1, though such failure may have exaggerated the effect. An increase in PD20 with the second test was observed uniformly in subjects with moderate or high initial PD20 values but not in those with low values. This suggests that there may be a PD20 threshold of the order 25-100 micrograms for refractoriness to occur. Refractoriness could exert an important confounding effect in investigations in which repeated histamine tests are carried out at short intervals.
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