Inhibition of the bronchial response to respiratory heat exchange by increasing doses of terbutaline sulphate.
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Ten asthmatic patients inhaled terbutaline sulphate (250, 500, or 1000 micrograms) or placebo on separate days, double blind and in random order, 30 minutes before isocapnic hyperventilation induced by cold air inhaled in doses that increased in a precisely controlled manner. The respiratory heat exchange (RHE) was calculated for each level of ventilation and the results were expressed as the RHE causing a fall in FEV1 of 10% (PD10RHE). The PD10RHE after placebo was highly reproducible. After terbutaline inhalation there was a highly significant shift in the RHE dose-response curves to the right and a trend towards a linear increase in PD10RHE with increasing doses of terbutaline. The subjects who were most responsive to the RHE required more terbutaline to inhibit the response completely. The results indicate that RHE dose-response curves are a precise method to examine the effects of drugs on hyperventilation-induced asthma, that increasing doses of terbutaline can produce increasing protection, and that the degree of protection is dependent on the level of increased bronchial responsiveness to RHE.
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