Evaluation of the antinociceptive effect of xylazine after systemic or lumbosacral injection in dogs anesthetized with isoflurane and the effect of atipamezole injection on postoperative analgesia

The alpha 2-selective adrenergic agonist xylazine has a long lasting antino ciceptive effect (> 4 hours) after lumbosacral injection in dogs (Rector, 1 996). The present study was performed to find out if epidurally administere d xylazine acts locally as well as systemically. In a clinical investigation 30 dogs anesthetized with isoflurane in oxygen (1.9 Vol.% ET) were examined before and after epidural and intramuscular in jection of xylazine (0.25 mg/kg) during surgery and over a 240-minute posto perative period. All dogs underwent surgery caudal the costal arch. The surgical patients were divided into three groups: group I: xylazine (0, 25 mg/kg) epidural and aqua pro injectione i.m. (n = 10 dogs); group II: aq ua pro injectione epidural and xylazine (0,25 mg/kg) i.m. (n = 10 dogs); gr oup III: aqua pro injectione epidural and aqua pro injectione i.m. (n = 10 dogs). The division of xylazine epidural or i.m. or aqua pro injectione only was r andomized. Prior to surgery, in all three groups somatic stimuli were exerted by press ure on the nailbed of a hind- and a forelimb before and after the epidural injection under general anesthesia. Heart rate, mean arterial pressure and rate of ventilation were used to determine the analgesic effect. During sur gery, heart rate and mean arterial pressure were measured every 15 minutes. At the end of the operation, all patients were treated with the (alpha 2-s elective adrenergic -antagonist atipamezole. During a 240 minute post operative examination heart rate, mean arterial pr essure, rate of ventilation and reaction to pressure on the surgery site we re used to determine the analgesic properties of xylazine. In this study it could be shown, that the concurrent epidural (group I) or intramuscular injection(group II) of xylazine in isoflurane anesthetized do gs leads to a better analgesic effect than isoflurane alone (group III) aft er somatic stimulation of ii hind- and forelimb. From the antinociceptive e ffect in the forelimb after epidural administration of xylazine it was conc luded that xylazine acts both locally as well as systemically. This was als o confirmed by the hernodynamic changes, which were similar in group I (xyl azine epidurally) and group IT (xylazine i.m.). In the two groups treated w ith xylazine (group I and II), bradycardia and AV-bloch I and II were obser ved in three dogs during the first 30 minutes after epidural and intramuscu lar injection of xylazine. Hemodynamic changes were seen at the time of max imal plasma xylazine concentrations. One dog in the control group also had a bradycardia in connection with an AV-block II. During surgery no hemodynamic differences could be observed between the thr ee groups. Heart rate was within normal limits and mean arterial pressure s howed a slight hypotension. In-agreement with the investigation of Rector ( 1996) it was shown in this study that xylazine has a long lasting (< 4 hour s) antinociceptive effect after lumbosacral injection in the epidural space (group I). This analgesic effect is of local, spinal cord origin, as it wa s impossible to antagonize the analgesia by systemic application of atipame zole. In contrast to this, atipamezole reversed all analgesic properties to tally after systemically administered xylazine in group II. However, suffic ient analgesic plasma xylazine concentrations could only be detected in gro up II up to 180 minutes after injection. After this rime period, an analges ic effect could not be expected anyway, even without antagonization. It can be concluded that the epidural administration of xylazine offers adv antages in contrast to a systemic administration, as a longer lasting analg esic effect can be observed (after the epidural application), and systemic side effects can be reversed without effecting spinal analgesia.