Comparison of metaproterenol, isoetharine and salbutamol in the relief of methacholine-induced bronchospasm in dogs.

We evaluated cardiovascular effects and effectiveness of isoetharine, metaproterenol and salbutamol, when administered intratracheally to relieve methacholine-induced bronchospasm in dogs anaesthetized with 50 per cent nitrous oxide, oxygen, halothane and mechanically ventilated. Methacholine 2 micrograms X kg-1 X hour-1 was administrated first followed by halothane (1 MAC) for 30 minutes (control), then metaproterenol, isoetharine or salbutamol. Metaproterenol (15 mg) significantly decreased transpulmonary pressure to 20.1 +/- 0.5 (SE) from 22.5 +/- 1.15 cmH2O (p less than 0.025) after three min and to 15 +/- 0.5 cmH2O (p less than 0.005) after 90 min. Isoetharine (2.5 mg) decreased transpulmonary pressure after five min to 22.1 +/- 1 from 24.5 +/- 1.5 cmH2O (p less than 0.05), and to 21.75 +/- 0.55 mmH2O after 90 min. Salbutamol 25 micrograms X kg-1 decreased transpulmonary pressure to 20.7 +/- 0.75 from 24.25 +/- 1.28 after three min and to 16 +/- .5 after 90 min. The peak effects on airway pressure occurred at 15 min for metaproterenol, 25 min for salbutamol and 20 min for isoetharine. Pulmonary vascular resistance was not significantly changed during halothane anaesthesia alone but decreased significantly after metaproterenol and isoetharine infusion. Heart rate increased ten per cent after metaproterenol, three per cent after isoetharine, and five per cent after salbutamol. No arrhythmias occurred in any group. Cardiac output increased significantly to 3.25 +/- 0.2 from 1.5 +/- 0.17 L X min-1 (p less than 0.025) after metaproterenol to 3.2 +/- .025 from 1.45 +/- .009 after salbutamol and was unchanged after isoetharine. Metaproterenol and salbutamol in the presence of 1 MAC halothane anaesthesia relieved methacholine-induced bronchospasm more rapidly than did isoetharine. The onset of effect was 3 +/- 0.05 min for metaproterenol and salbutamol and 5 +/- 0.01 min for isoetharine. The effect lasted 210 +/- 10.5 min for metaproterenol, 170 +/- 12.5 min for salbutamol and 90 +/- 4.75 min for isoetharine.