Effects of intravenously administered yohimbine on antinociceptive, cardiorespiratory, and postural changes induced by epidural administration of detomidine hydrochloride solution to healthy mares

Objective-To determine effects of IV administered yohimbine on perineal ana lgesia, cardiovascular and respiratory activity, and head and pelvic limb p osition in healthy mares following epidural administration of detomidine hy drochloride solution. Animals-8 healthy mares. Procedure-Each mare received detomidine hydrochloride (0.06 mg/kg of body w eight), administered in the caudal epidural space, followed 61 minutes late r by yohimbine (0.05 mg/kg; test) or sterile saline (0.9% NaCl) solution (c ontrol), administered IV, in a randomized, crossover study design with grea ter than or equal to 2 weeks between treatments. Analgesia Was determined b y lack of sensory perception to electrical stimulation of perineal dermatom es and needle-prick stimulation of coccygeal to 15th thoracic dermatomes. A rterial pH, Paco(2), Pao(2), heart and respiratory rates, rectal temperatur e, arterial blood pressure, and cardiac output were determined, and mares w ere observed for sweating and urination. Mean scores obtained for test and control groups were compared. Results-Intravenously administered yohimbine significantly reduced mean sco res of detomidine-induced perineal analgesia, head ptosis, changes in pelvi c limb position, and sweating and diuresis; antagonized detomidine-induced decreases in heart rare and cardiac output; but did not affect detomidine-i nduced decrease in respiratory rate. Conclusions and Clinical Relevance-Most effects of epidurally administered detomidine, except bradypnea, were antagonized by yohimbine, suggesting tha t detomidine may influence respiratory rate by mechanisms other than stimul ation of alpha(2)-adrenoceptors, or that yohimbine induces respiratory depr essant effects. Yohimbine may be an effective alpha(2)-adrenoceptor antagon ist for all but respiratory depression following epidural administration of detomidine to mares.