Behavioural and cardiovascular effects of medetomidine constant rate infusion compared with detomidine for standing sedation in horses.

Objective: To compare the efficacy of a medetomidine constant rate infusion (CRI) with a detomidine CRI for standing sedation in horses undergoing high dose rate brachytherapy.

Study Design: Randomized, controlled, crossover, blinded clinical trial.

Animals: A total of 50 horses with owner consent, excluding stallions.

Methods: Each horse was sedated with intravenous acepromazine (0.02 mg kg), followed by an α-adrenoceptor agonist 30 minutes later and then by butorphanol (0.1 mg kg) 5 minutes later. A CRI of the same α-adrenoceptor agonist was started 10 minutes after butorphanol administration and maintained for the treatment duration. Treatments were given 1 week apart. Each horse was sedated with detomidine (bolus dose, 10 μg kg; CRI, 6 μg kg hour) or medetomidine (bolus dose, 5 μg kg; CRI, 3.5 μg kg hour). If sedation was inadequate, a quarter of the initial bolus of the α-adrenoceptor agonist was administered. Heart rate (HR) was measured via electrocardiography, and sedation and behaviour evaluated using a previously published scale. Between treatments, behaviour scores were compared using a Wilcoxon signed-rank test, frequencies of arrhythmias with chi-square tests, and HR with two-tailed paired t tests. A p value <0.05 indicated statistical significance.

Results: Total treatment time for medetomidine was longer than that for detomidine (p = 0.04), and ear movements during medetomidine sedation were more numerous than those during detomidine sedation (p = 0.03), suggesting there may be a subtle difference in the depth of sedation. No significant differences in HR were found between treatments (p ≥ 0.09). Several horses had arrhythmias, with no difference in their frequency between the two infusions.

Conclusions And Clinical Relevance: Medetomidine at this dose rate may produce less sedation than detomidine. Further studies are required to evaluate any clinical advantages to either drug, or whether a different CRI may be more appropriate.