Increasing halothane concentrations reduce nitroprusside dose requirement

There has been no description of the hemodynamic dose-response relationship between halothane and sodium nitroprusside (SNP), although these drugs are used together frequently for induction of deliberate hypotension. Utilizing aortic root cannulation and thermister-tipped pulmonary artery catheterization, this relationship was studied in 6 beagles receiving a standard 100 microgram/kg infusion of SNP solution administered at 3 different infusion rates (5, 10, and 20 microgram/kg/min) while anesthetized with 3 different concentrations of halothane (0.5, 1, and 2%). Sodium nitroprusside infusion resulted in dose-related reductions in mean arterial pressure, systemic vascular resistance, and left ventricular stroke work. Increasing concentrations of halothane significantly potentiated the hypotensive effects of SNP. Cardiac output increase as the SNP infusion rate increased, whereas increasing the halothane concentration resulted in a reduction of cardiac output at each SNP infusion rate studied. Pulmonary artery wedge pressure was significantly reduced by SNP infusion at all 3 halothane concentrations, whereas mean pulmonary artery pressure was unchanged. Arterial pH fell in response to each SNP infusion, from 7.46 at the beginning of the study to 7.32 at the end (p less than 0.001). Sodium nitroprusside predictably induced hypotension during halothane anesthesia at the cost of a dose-related metabolic acidosis. Increasing the depth of halothane anesthesia afforded a greater percentage reduction in arterial pressure at each SNP infusion rate studied. Metabolic acidosis, however, developed no more rapidly at 2% halothane than it did at 0.5 or 1%.