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.
