Sedative effects and changes in cardiac rhythm with intravenous premedication of medetomidine, butorphanol and ketamine in dogs.

Objective: To determine the sedative effects and characteristics of cardiac rhythm with intravenous (IV) premedication of medetomidine, butorphanol and ketamine in dogs.

Study Design: Prospective, blinded, randomized clinical trial.

Animals: A total of 116 client-owned healthy dogs undergoing elective surgery.

Methods: Dogs were randomly allocated one of four groups: group M, medetomidine 5 μg kg; group B, butorphanol 0.2 mg kg; group MB, medetomidine 5 μg kg and butorphanol 0.2 mg kg; or group MBK, medetomidine 5 μg kg, butorphanol 0.2 mg kg and ketamine 1 mg kg IV. Sedation was assessed using a numerical descriptive scale. Heart rate (HR) and rhythm were monitored; propofol dose (mg kg IV) to allow orotracheal intubation was documented. Data were analysed using anova, accounting for multiple testing with the Tukey honest significant difference test.

Results: Sedation scores varied significantly between all groups at all time points, except between groups MB and MBK at four time points. HR decreased in all groups: most in groups M and MB, least in group B. HR was initially higher in group MBK than in groups M and MB. Arrhythmias occurred in all groups: group B showed second-degree atrioventricular blocks occasionally, all other groups showed additionally ventricular escape complexes and bundle branch blocks. Dose of propofol required for orotracheal intubation was significantly higher in group B (5.0 ± 2.0 mg kg) than in group M (2.6 ± 0.6 mg kg). Although no difference could be demonstrated between groups MB (1.4 ± 0.6 mg kg) and MBK (0.9 ± 0.8 mg kg), both groups required significantly less propofol than group M.

Conclusion And Clinical Relevance: Medetomidine-based premedication protocols led to various bradyarrhythmias. Addition of subanaesthetic doses of ketamine to medetomidine-based protocols resulted in higher HRs, fewer bradyarrhythmias and fewer animals that required propofol for intubation without causing side effects in healthy dogs.