The measurement of response to bronchodilators by recording the change in forced expiratory volume in 1 second (F.E.V.1·0) can underestimate response unless the correct time for postbronchodilator tests is chosen. In 18 patients given isoprenaline, the time of peak response varied, but a 30-minute testing time gave the closest approximation to peak response. In order to test whether repeated doses would increase the response, inhalations of isoprenaline were given at 30-minute intervals, the F.E.V.1·0 being measured after each dose. There was a further response after successive doses until a plateau was reached, usually after the third dose. The maximum response obtained was often considerably greater than the response after the first administration of isoprenaline, the average increase in F.E.V.1·0 being 600 ml. after the first dose and 840 ml. after repeated doses. A similar response was obtained in four patients using orciprenaline spray administered with a metered aerosol, the dose being repeated at hourly intervals. Four patients were given repeated inhalations of isoprenaline until the response reached a plateau, and then 250 mg. of aminophylline was injected intravenously. None of these patients showed any further improvement with aminophylline. The significance of these findings is discussed.
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