A review of clinical approaches to antagonism of alpha2-adrenoreceptor agonists in the horse

Abstract

Alpha2-adrenoceptor agonists xylazine, romifidine, detomidine and, in some cases, medetomidine and dexmedetomidine, are fundamental drugs used in equine practice. There are situations where the undesirable pharmacodynamic effects (ataxia, prolonged sedation, bradycardia and ileus) or accidental overdose of these drugs may need to be antagonised. The α2-adrenoceptor antagonists tolazoline, yohimbine and atipamezole can be used to antagonise undesirable effects. However, despite being effective, α2-adrenoceptor antagonists are also not without undesirable pharmacodynamic effects. Excitement, muscle trembling and triggered inappropriate stress responses are a few of the more serious undesirable effects. Horses demonstrate a variable response to the antagonists thus recommending dose rates become fraught with difficulty. It is therefore recommended that the α2-adrenoceptor antagonist should be titrated to the desired clinical effect. Consequently, other reversal agents, such as anticholinergics (atropine, glycopyrrolate and hyoscine), have been administered for the treatment of α2-adrenoceptor agonist-induced bradycardia. Anticholinergics cannot be recommended for routine use in horses due to the undesirable cardiovascular effects and potentiation of α2-adrenoceptor agonist-induced gastrointestinal hypomotility. Novel peripheral acting α2-adrenoceptor antagonists, such as MK-467, are currently under scrutiny in veterinary anaesthesia in an effort to antagonise the undesirable effects of α2-adrenoceptor agonists without compromising on the level of sedation. This review examines the current literature on the α2-adrenoceptor antagonists used in horses and makes recommendations on how to use these drugs safely in an attempt to prevent undesirable pharmacodynamic effects.