The effects of medetomidine on the action of vecuronium in dogs anaesthetized with halothane and nitrous oxide

ObjectiveTo quantify the effects of medetomidine on the onset and duration of vecuronium-induced neuromuscular blockade in dogs.Study designRandomized, prospective clinical study.AnimalsTwenty-four, healthy, client-owned dogs of different breeds, aged between 6 months and 10 years and weighing between 5.0 and 40.0 kg undergoing elective surgery.MethodsDogs were randomly allocated to two groups. Pre-anaesthetic medication in group M+ was intramuscular acepromazine (ACP) 25 μg kg^?1, morphine 0.5 mg kg^?1 and medetomidine 5 μg kg^?1. Group M? received ACP and morphine only, at the same dose rate. After induction with thiopental, anaesthesia was maintained with halothane in oxygen and nitrous oxide. End-tidal halothane concentration was maintained at 1.1%. Neuromuscular blockade was produced with intravenous vecuronium (50 μg kg^?1) and monitored using a train of four stimulus applied at the ulnar nerve. The times taken for loss and reappearance of the four evoked responses (twitches T]) were recorded. Normal and nonparametric data were analysed with an independent t-test and Mann-Whitney's U-test, respectively.ResultsThe fourth twitch (T4) disappeared at similar times in each group: 107 ± 19; 72–132] (mean ± SD; range]) seconds in M+ and 98 ± 17 72–120] seconds in M? dogs. The first twitch (T1) was lost at 116 ± 15; 96–132] seconds in group M+ and 109 ± 19; 72–132] seconds in M?. The fourth twitch returned significantly earlier in M+ dogs: 20.8 ± 3.8 14–28] minutes compared with 23.8 ± 2.7 20–27] minutes (p = 0.032). The duration of drug effect (T4 absent) was significantly shorter (p = 0.027) in M+ (18.9 ± 3.7 minutes) compared with M? dogs (22.2 ± 2.9 minutes). The recovery rate (interval between reappearance of T1 and T4) was significantly more rapid (p = 0.0003) in medetomidine recipients (3.0 ± 1.2 versus 5.2 ± 1.3 minutes).Conclusion and clinical relevance Medetomidine 5 μg kg^?1 as pre-anaesthetic medication shortened the duration of effect of vecuronium in halothane-anaesthetized dogs and accelerated recovery, but did not affect the onset time. These changes are of limited clinical significance.