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, administered into the epidural space of the sacrococcygeal joint on perineal analgesia, sedation, ataxia, and respiratory and cardiovascular function in standing mares.

DESIGN

Randomized, blinded study, using xylazine (treatment) and 0.9% NaCl (controls). At least 2 weeks elapsed between the treatments.

ANIMALS

Eight healthy mares.

PROCEDURE

Blood samples were drawn. Systemic hemodynamics were determined, including cardiac output and pulmonary arterial, systemic arterial, and right atrial pressures. Two-way ANOVA with repeated measures was used to detect significant (P < 0.05) differences between mean scores of analgesia, sedation, ataxia, and cardiorespiratory variables before and during a 3-hour testing period. Analgesia was determined by lack of sensory perception to electrical stimulation at the perineal dermatome and no response to needle prick stimulation extending from coccyx to S3 dermatomes. Sedation was determined by head ptosis.

RESULTS

Epidurally administered xylazine induced variable bilateral caudal analgesia extending from coccyx to S3, with minimal sedation, ataxia, and cardiovascular and respiratory depression in standing mares. Analgesia was attained at 15 +/- 6 minutes and lasted for 165 to over 180 minutes. Heart and respiratory rates, systolic, diastolic, and mean arterial blood pressure, PCV, hemoglobin concentration, arterial oxygen content, and oxygen transport were decreased after xylazine, but not 0.9% NaCl, treatment. Cardiac output, stroke volume, mean right atrial pressure, mean pulmonary artery pressure, systemic vascular resistance, pulmonary vascular resistance, arterial and mixed venous pH and gas tensions (PO2 and PCO2), oxygen consumption, blood temperature, and rectal temperature did not change significantly (P < 0.05) after epidural administration of xylazine or 0.9% NaCl.

CONCLUSIONS

Caudal epidurally administered xylazine (0.25 mg/kg in 6 ml of 0.9% NaCl) can be given safely to induce prolonged (>2 hours) caudal analgesia with minimal sedation, ataxia, and circulatory and respiratory disturbances in conscious, standing mares.