Cardiopulmonary effects of ephedrine in halothane-anesthetized horses.

The cardiopulmonary effects of intravenous (i.v.) administration of the sympathomimetic drug ephedrine during two different levels of halothane anesthesia [end-tidal concentration of 1.37% (light anesthesia) and 2.1% (deep anesthesia)] were studied in eight horses. Anesthesia was induced and maintained using only halothane in O2. Ventilation was controlled to maintain a Paco2 of 38-42 mmHg. Following instrumentation and stabilization of the horse at the halothane concentration being studied, baseline measurements of cardiac output (Q), arterial blood pressure (AP), pulmonary artery pressure, heart rate, Pao2, Paco2 and pH were made. Ephedrine was then administered (0.06 mg/kg i.v.) and these measurements repeated at 10, 20, 30, 45 and 60 min after injection. At both doses of halothane there was a significant (P less than 0.05) increase in Q, stroke volume (SV), and systolic AP following ephedrine administration. In addition, at 2.1% halothane, ephedrine administration resulted in a significant (P less than 0.05) increase in mean AP and Pao2 and a decrease in total peripheral resistance. The increase in systolic AP, Q, and SV was significantly (P less than 0.05) greater at 2.1% halothane than at 1.37% halothane. Ephedrine administration to horses during both light and deep halothane anesthesia results in an increase in AP that is due to an increase in Q and SV.