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The alpha 2-selective adrenergic agonist xylazine has a long lasting antinociceptive effect (> 4 hours) after lumbosacral injection in dogs (Rector, 1996). The present study was performed to find out if epidurally administered 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 injection of xylazine (0.25 mg/kg) during surgery and over a 240-minute postoperative 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: aqua pro injectione epidural and xylazine (0.25 mg/kg) i.m. (n = 10 dogs); group 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 randomized. Prior to surgery, in all three groups somatic stimuli were exerted by pressure 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 surgery, 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-selective adrenergic-antagonist atipamezole. During a 240 minute post operative examination heart rate, mean arterial pressure, rate of ventilation and reaction to pressure on the surgery site were 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 dogs leads to a better analgesic effect than isoflurane alone (group III) after somatic stimulation of a hind- and forelimb. From the antinociceptive effect in the forelimb after epidural administration of xylazine it was concluded that xylazine acts both locally as well as systemically. This was also confirmed by the hemodynamic changes, which were similar in group I (xylazine epidurally) and group II (xylazine i.m.). In the two groups treated with xylazine (group I and II), bradycardia and AV-blocks I and II were observed in three dogs during the first 30 minutes after epidural and intramuscular injection of xylazine. Hemodynamic changes were seen at the time of maximal 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 three groups. Heart rate was within normal limits and mean arterial pressure showed a slight hypotension. In agreement with the investigation of Rector (1996) it was shown in this study that xylazine has a long lasting (> 4 hours) antinociceptive effect after lumbosacral injection in the epidural space (group I). This analgesic effect is of local, spinal cord origin, as it was impossible to antagonize the analgesia by systemic application of atipamezole. In contrast to this, atipamezole reversed all analgesic properties totally after systemically administered xylazine in group II. However, sufficient analgesic plasma xylazine concentrations could only be detected in group II up to 180 minutes after injection. After this time period, an analgesic effect could not be expected anyway, even without antagonization. It can be concluded that the epidural administration of xylazine offers advantages in contrast to a systemic administration, as a longer lasting analgesic effect can be observed (after the epidural application), and systemic side effects can be reversed without effecting spinal analgesia.
Adrenergic alpha-Agonists↗