The relationship between change in airway calibre and change in airway reactivity after administration of bronchodilator drugs has been investigated by comparing the effect of increasing doses of inhaled salbutamol and ipratropium bromide on the forced expiratory volume in one second (FEV1), specific airways conductance (sGaw), and the dose of histamine causing a 20% fall in FEV1 (PD20) in six subjects with mild asthma. On each of 10 occasions measurements were made of baseline FEV1, sGaw, and PD20 after 15 minutes' rest, and followed one hour later, when the FEV1 had returned to baseline, by a single nebulised dose of salbutamol (placebo, 5, 30, 200 and 1000 micrograms) or ipratropium (placebo, 5, 30, 200 and 1000 micrograms) given in random order. Measurements of FEV1, sGaw, and PD20 were repeated 15 minutes after salbutamol and 40 minutes after ipratropium. Salbutamol and ipratropium caused a similar dose related increase in FEV1 and sGaw, with a mean increase after the highest doses of 0.76 and 0.69 litres for FEV1 and 1.15 and 0.96 s-1 kPa-1 for sGaw. Salbutamol also caused a dose related increase in PD20 to a maximum of 2.87 (95% confidence interval 2.18-3.55) doubling doses of histamine after the 1000 micrograms dose, but ipratropium bromide caused no significant change in PD20 (maximum increase 0.24 doubling doses, 95% confidence interval -0.73 to 1.22). Thus bronchodilatation after salbutamol was associated with a significantly greater change in airway reactivity than a similar amount of bronchodilatation after ipratropium bromide. This study shows that the relation between change in airway reactivity and bronchodilatation is different for two drugs with different mechanisms of action, suggesting that change in airway calibre is not a major determinant of change in airway reactivity with bronchodilator drugs.
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