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

Roman T. Skarda From the Department of Veterinary Clinical Sciences, The Ohio State University, Columbus, OH 43210–1089.

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 Dr med vet, PhD
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William W. Muir III From the Department of Veterinary Clinical Sciences, The Ohio State University, Columbus, OH 43210–1089.

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 DVM, PhD

Abstract

Objective

To determine effects of IV administered yohimbine on perineal analgesia, cardiovascular and respiratory activity, and head and pelvic limb position in healthy mares following epidural administration of detomidine hydrochloride solution.

Animals

8 healthy mares.

Procedure

Each mare received detomidine hydrochloride (0.06 mg/kg of body weight), administered in the caudal epidural space, followed 61 minutes later by yohimbine (0.05 mg/kg; test) or sterile saline (0.9% NaCl) solution (control), administered IV, in a randomized, crossover study design with ≥ 2 weeks between treatments. Analgesia was determined by lack of sensory perception to electrical stimulation of perineal dermatomes and needle-prick stimulation of coccygeal to 15th thoracic dermatomes. Arterial pH, Paco2, Pao2, heart and respiratory rates, rectal temperature, arterial blood pressure, and cardiac output were determined, and mares were observed for sweating and urination. Mean scores obtained for test and control groups were compared.

Results

Intravenously administered yohimbine significantly reduced mean scores of detomidine-induced perineal analgesia, head ptosis, changes in pelvic limb position, and sweating and diuresis; antagonized detomidine-induced decreases in heart rate and cardiac output; but did not affect detomidine-induced decrease in respiratory rate.

Conclusions and Clinical Relevance

Most effects of epidurally administered detomidine, except bradypnea, were antagonized by yohimbine, suggesting that detomidine may influence respiratory rate by mechanisms other than stimulation of α2-adrenoceptors, or that yohimbine induces respiratory depressant effects. Yohimbine may be an effective α2-adrenoceptor antagonist for all but respiratory depression following epidural administration of detomidine to mares. (Am J Vet Res 1999;60:1262–1270)

Abstract

Objective

To determine effects of IV administered yohimbine on perineal analgesia, cardiovascular and respiratory activity, and head and pelvic limb position in healthy mares following epidural administration of detomidine hydrochloride solution.

Animals

8 healthy mares.

Procedure

Each mare received detomidine hydrochloride (0.06 mg/kg of body weight), administered in the caudal epidural space, followed 61 minutes later by yohimbine (0.05 mg/kg; test) or sterile saline (0.9% NaCl) solution (control), administered IV, in a randomized, crossover study design with ≥ 2 weeks between treatments. Analgesia was determined by lack of sensory perception to electrical stimulation of perineal dermatomes and needle-prick stimulation of coccygeal to 15th thoracic dermatomes. Arterial pH, Paco2, Pao2, heart and respiratory rates, rectal temperature, arterial blood pressure, and cardiac output were determined, and mares were observed for sweating and urination. Mean scores obtained for test and control groups were compared.

Results

Intravenously administered yohimbine significantly reduced mean scores of detomidine-induced perineal analgesia, head ptosis, changes in pelvic limb position, and sweating and diuresis; antagonized detomidine-induced decreases in heart rate and cardiac output; but did not affect detomidine-induced decrease in respiratory rate.

Conclusions and Clinical Relevance

Most effects of epidurally administered detomidine, except bradypnea, were antagonized by yohimbine, suggesting that detomidine may influence respiratory rate by mechanisms other than stimulation of α2-adrenoceptors, or that yohimbine induces respiratory depressant effects. Yohimbine may be an effective α2-adrenoceptor antagonist for all but respiratory depression following epidural administration of detomidine to mares. (Am J Vet Res 1999;60:1262–1270)

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