Hemodynamic effects of pirbuterol in conscious dogs

Abstract

We studied a new synthetic “selective” beta 2 catecholamine, pirbuterol (PBL), in eight conscious, chronically instrumented dogs. PBL was administered as 10-minute intravenous infusions of 0.125, 0.5, 2, 4, and 8 μg/kg/min. At 2 μg/kg/min, changes from control (all significant at p < 0.01) were heart rate (HR) + 118 ± 13 (SEM) bpm, left ventricular (LV) maximum rate of pressure rise ( dP dt max ) + 110 ± 16%, and mean arterial pressure (AP) − 16 ± 4 mm Hg. Cardiac output (CO) increased 114 ± 36% (p < 0.05), and systemic vascular resistance decreased 59 ± 4% (p < 0.01). LV end-diastolic diameter decreased 15 ± 3%, and LV end-systolic diameter decreased 24 ± 5% in four dogs. The magnitude of all changes increased only slightly at the higher infusion rates. When HR was maintained constant by electrical pacing, responses to PBL infusion were similar to results without pacing. All PBL effects were blocked by propranolol (1 mg/kg intravenously) except at the infusion rates of 4 and 8 μg/kg/min. Practolol (5 mg/kg intravenously) blocked the inotropic effects of PBL, but the chronotropic and vasodilator effects were still apparent. When PBL and isoproterenol were compared in four dogs at doses that increased LV dP dt max by 50%, the double product of HR and systolic AP increased in three dogs approximately 50% with PBL vs 81% with isoproterenol. In the fourth dog, however, isoproterenol increased the double product less (13%) than PBL (50%). Thus PBL has both beta 2 and beta 1 effects, and compared with isoproterenol at doses that give equivalent increases in myocardial contractility, PBL produced less increase in an index of myocardial oxygen consumption in three of the four dogs studied. On the basis of these data PBL may be useful for treating low CO syndromes without provoking myocardial ischemia.