An evaluation of epidural oxymorphone and bupivacaine during halothane anesthesia in dogs

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Torske, Kristine E.
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University of Guelph

The effects of epidural oxymorphone (0.1 mg/kg)/bupivacaine (1 mg/kg) (O/B) on intra-operative halothane requirements, cardiopulmonary data and post-operative analgesia were compared to epidural bupivacaine (1 mg/kg) (BUP) and IV oxymorphone (0.05 mg/kg) (IVO) in 24 dogs undergoing hindlimb surgery. Anesthesia was induced with thiopental IV(10 mg/kg), following premedication with acepromazine (0.02 mg/kg) and meperidine (5 mg/kg) and was maintained with halothane. Treatments were administered 30-60 minutes prior to surgery in a randomized blinded manner. Data collected at baseline and every 15 minutes after, included heart rate (HR), respiratory rate, arterial blood pressure, temperature, endtidal CO\sb2 and halothane (%), and arterial blood gases. Fifteen minutes after surgery began, the endtidal halothane % was reduced by 0.2% every 5-10 minutes until dogs were as light as possible yet still in a stable plane of anesthesia. Post-operative analgesic requirements were recorded. Based on halothane requirements in phase 1 of this study, cardiovascular effects of O/B were compared to epidural oxymorphone (OXY) and epidural saline (SAL) using a randomized crossover design in halothane anesthetized dogs ventilated to eucapnia. Systemic and pulmonary arterial pressures, HR, central venous pressure (CVP), cardiac output (CO) and arterial blood gases were collected at baseline and every 15 minutes for 90 minutes. Cardiac index, stroke volume, stroke index, systemic vascular resistance (SVR) and left ventricular work were calculated. At 90 minutes, all groups received glycopyrrolate (0.01 mg/kg IV) and measurements were repeated 5 minutes later. Arterial blood samples were collected for serum opiate measurement and were compared to serum concentrations following IM oxymorphone (0.1 mg/kg) administration. Phase 1 revealed an insignificant difference in halothane requirements among O/B, BUP and IVO. O/B provided superior post-operative analgesia. The administration of O/B and IVO resulted in decreased HR and increased PaCO\sb2 compared to BUP. The cardiovascular effects of O/B and OXY were decreased HR and increased SVR and CVP compared to SAL. Glycopyrrolate administration reversed these changes and increased CO in O/B and OXY compared to SAL. The cardiovascular effects of O/B and OXY may be due to systemic absorption of oxymorphone since serum concentrations following epidural administration were equivalent to those following IM administration.

epidural, oxymorphone, bupivacaine, halothane anesthesia, dogs, hindlimb surgery