Cardiovascular effects of epidurally administered morphine and a xylazine-morphine combination in isoflurane-anesthetized dogs

Cardiovascular effects of epidurally administered morphine, a morphine-xylazine combination, and saline solution (control) during isoflurane-maintained anesthesia were assessed in 6 healthy dogs. Anesthesia was induced with isoflurane in O2 and was maintained at 2.0% end-tidal isoflurane concentration. Ventilation was controlled to maintain PaCO2 at 35 to 45 mm of Hg. The dorsal pedal artery was cannulated for measurement of systolic, mean, and diastolic pressures, and for blood sample collection. Arterial blood pH and gas tensions were determined every 30 minutes. Cardiac output was determined by thermodilution. The ECG, heart rate, body temperature, central venous pressure, mean pulmonary artery pressure, pulmonary capillary wedge pressure, end-tidal isoflurane concentration, and CO2 tension were monitored. Systemic and pulmonary vascular resistance, arterial HCO3(-) concentration, base excess, and cardiac index were calculated. After baseline measurements were taken, morphine (0.1 mg/kg of body weight) in 5 ml of isotonic saline solution, morphine and xylazine (0.1 mg of morphine and 0.09 mg of xylazine/kg) in 5 ml of isotonic saline solution, or 5 ml of isotonic saline solution was injected into the lumbosacral epidural space. Data were recorded at 5, 15, 30, 45, 60, 75, 90, 105, and 120 minutes after epidural injection. Statistical analysis included ANOVA for repeated measures. Significance was set at P < 0.05. None of the measured variables was significantly different among the 3 treatments at any time. Results of the study indicated that epidural administration of morphine or morphine and xylazine is not associated with significant cardiovascular side effects during isoflurane-maintained anesthesia in dogs.