ANALGESIC, HEMODYNAMIC, AND RESPIRATORY EFFECTS OF CAUDAL EPIDURALLY ADMINISTERED XYLAZINE HYDROCHLORIDE SOLUTION IN MARES

Objective-To examine effects of 0.25 mg of xylazine/kg of body weight diluted to a total volume of 6 ml/450 kg with sterile 0.9% NaCl, admin istered into the epidural space of the sacrococcygeal joint on perinea l analgesia, sedation, ataxia, and respiratory and cardiovascular func tion in standing mares. Design-Randomized, blinded study, using xylazi ne (treatment) and 0.9% NaCl (controls). At least 2 weeks elapsed betw een the treatments. Animals-Eight healthy mares. Procedure-Blood sampl es were drawn. Systemic hemodynamics were determined, including cardia c output and pulmonary arterial, systemic arterial, and right atrial p ressures. Two-way ANOVA with repeated measures was used to detect sign ificant (P < 0.05) differences between mean scores of analgesia, sedat ion, ataxia, and cardiorespiratory variables before and during a 3-hou r testing period. Analgesia was determined by lack of sensory percepti on to electrical stimulation at the perineal dermatome and no response to needle prick stimulation extending from coccyx to S3 dermatomes. S edation was determined by head ptosis. Results-Epidurally administered xylazine induced variable bilateral caudal analgesia extending from c occyx to S3, with minimal sedation, ataxia, and cardiovascular and res piratory depression in standing mares. Analgesia was attained at 15 +/ - 6 minutes and lasted for 165 to over 180 minutes. Heart and respirat ory rates, systolic, diastolic, and mean arterial blood pressure, PCV, hemoglobin concentration, arterial oxygen content, and oxygen transpo rt were decreased after xylazine, but not 0.9% NaCl, treatment. Cardia c output, stroke volume, mean right atrial pressure, mean pulmonary ar tery pressure, systemic vascular resistance, pulmonary vascular resis tance, arterial and mixed venous pH and gas tensions (Po-2, and Pco(2) ), oxygen consumption, blood temperature, and rectal temperature did n ot change significantly (P < 0.05) after epidural administration of xy lazine or 0.9% NaCl. Conclusions-Caudal epidurally administered xylazi ne (0.25 mg/kg in 6 ml of 0.9% NaCl) can be given safely to induce pro longed (> 2 hours) caudal analgesia with minimal sedation, ataxia, and circulatory and respiratory disturbances in conscious, standing mares .