[Evaluation of the antinociceptive effect of xylazine after epidural administration in dogs under general anesthesia with isoflurane].

Seven dogs anaesthetized with isoflurane in oxygen (1.9 Vol.-% ET) were examined before and after lumbosacral injection of xylazine (0.25 mg/kg) into the epidural space. Over a 240 minute period the dogs were first stimulated in a visceral manner (extension of the colon descendens by a balloon-catheter) every 15 minutes. In a second part, the dogs were stimulated somatically by pressure to the interdigital skin of a hindlimb. Heart rate and mean arterial pressure were used to determine the analgesic properties of epidurally injected xylazine. Furthermore, plasma xylazine concentration was controlled in both series. Before epidural anaesthesia, heart rate and mean arterial pressure increased during stimulation significantly in both groups. After epidural injection of xylazine, hemodynamic reactions were distinctly diminished, in the somatically stimulated group more than in the viscerally stimulated group. Up to 30 minutes after epidural injection, plasma xylazine concentrations of about 100 ng/ml were found in both groups. Up to 60 minutes after epidural injection, plasma xylazine concentrations of more than 60 ng/ml still were found. From the 120th minute on, xylazine concentration decreased below 30 ng/ml. From these results, it can be concluded that there is a well-suited analgesic effect of epidurally applicated xylazine for approximately 240 minutes.