• alpha-2 adrenergic agonist;
  • antagonist;
  • cardiac arrhythmias;
  • detomidine;
  • equine;
  • yohimbine


Objective  To describe selected pharmacodynamic effects of detomidine and yohimbine when administered alone and in sequence.

Study design  Randomized crossover design.

Animals  Nine healthy adult horses aged 9 ± 4 years and weighing 561 ± 56 kg.

Methods  Three dose regimens were employed in the current study. 1) 0.03 mg kg−1 detomidine IV, 2) 0.2 mg kg−1 yohimbine IV and 3) 0.03 mg kg−1 detomidine IV followed 15 minutes later by 0.2 mg kg−1 yohimbine IV. Each horse received all three treatments with a minimum of 1 week between treatments. Blood samples were obtained and plasma analyzed for detomidine and yohimbine concentrations by liquid chromatography-mass spectrometry. Behavioral effects, heart rate and rhythm, glucose, packed cell volume and plasma proteins were monitored.

Results  Yohimbine rapidly reversed the sedative effects of detomidine in the horse. Additionally, yohimbine effectively returned heart rate and the percent of atrio-ventricular conduction disturbances to pre-detomidine values when administered 15 minutes post-detomidine administration. Plasma glucose was significantly increased following detomidine administration. The detomidine induced hyperglycemia was effectively reduced by yohimbine administration. Effects on packed cell volume and plasma proteins were variable.

Conclusions and clinical relevance  Intravenous administration of yohimbine effectively reversed detomidine induced sedation, bradycardia, atrio-ventricular heart block and hyperglycemia.