Detomidine has been injected into the caudal epidural space of mares to induce selective analgesia for various obstetrical and surgical procedures performed in the anal, perineal, and pelvic region of standing mares. This study determined the antagonistic effects of yohimbine on caudal epidurally administered detomidine- induced perineal analgesia, head ptosis, changes in pelvic limb position, and cardiovascular and respiratory parameters in 8 healthy mares (475 ± 20 kg, mean ± SD). Each mare received detomidine (ED), 0.06 mg/kg, diluted with sterile 0.9% NaCl to a total volume of 8 ml/450 kg of body weight; in the epidural space at the first coccygeal intervertebral space (Co1- Co2), followed by intravenous administration of yohimbine, 0.05 mg/kg (test), or 0.9% NaCl (control) at 61 minutes in a randomized, blinded design study. At least 2 weeks elapsed between treatments. Two- way ANOVA with repeated measures was used to detect significant (P < 0.05) differences between variables of the two treatment groups before and at 15 minute intervals during a 3- hour testing period. ED produced variable degrees of analgesia as determined by lack of sensory perception to electrical stimulation (40 to 80 V, 0.5 ms) at the perineal dermatome and no response to needle prick stimulation from the coccyx to T15. Perineal analgesia was induced in 9.4 ± 4.2 minutes and lasted 135 to >180 minutes with head ptosis, changes in pelvic limb position, decreases of HR, RR, cardiac output, and PCV without significantly affecting arterial BP, pHa, PaO2, PaCO2, and rectal temperature. Yohimbine effectively reversed ED induced analgesia, head ptosis, and pelvic limb position score; increased cardiac output and PCV to baseline values, and did not affect RR after administration. These results suggest that detomidine (0.06 mg/kg, diluted to a volume of 8 ml with 0.9% NaCl) may be a desirable alpha-2 adrenoceptor agonist for analgesia in the perineal dermatomes with its effects being reversible with yohimbine (0.05 mg/kg IV).
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