Comparison of midazolam and diazepam as co-induction agents with ketamine for anaesthesia in sedated ponies undergoing field castration.

OBJECTIVE: To compare intravenous (IV) midazolam and diazepam administered with ketamine for induction of anaesthesia in ponies, already sedated with detomidine, undergoing field castration. STUDY DESIGN: Prospective, randomised, 'blinded', clinical study. ANIMALS: Twenty Welsh pony yearlings. METHODS: After IV injection of detomidine (20 µg kg(-1) ) and phenylbutazone (4.4 mg kg(-1) ) ponies were allocated to receive either IV midazolam (group M) or diazepam (group D) (both 0.06 mg kg(-1) ) with ketamine (2.2 mg kg(-1) ) for induction of anaesthesia. Using simple descriptive scales, quality of sedation, induction, endotracheal intubation, surgical conditions and recovery were scored by observers blinded to treatment. Time from sedation to induction of anaesthesia, IV injection to lateral recumbency, induction to start of surgery, induction to first head lift and to standing, and total surgical time were measured. Cardiorespiratory function was assessed every 5 minutes. Time, number and total quantity of additional IV ketamine as well as any adverse effects were documented. Data were tested for normality and analysed using two-way anova with Bonferroni post hoc tests, unpaired t-tests and Mann-Whitney U tests as appropriate. Significance was set at p < 0.05. RESULTS: There were no significant group differences in any of the measured variables except bodyweight (mean ± SD: group M 163 ± 12 kg; group D 150 ± 7 kg; p = 0.01). One pony in group M required ketamine 15 minutes after induction of anaesthesia. Surgical conditions were good in all cases; time from induction to standing was 50 ± 11 minutes in group M and 48 ± 12 minutes in group D. There were no adverse effects. Recoveries were uneventful with minimal ataxia. CONCLUSIONS AND CLINICAL RELEVANCE: Midazolam and diazepam at 0.06 mg kg(-1) can be used interchangeably in combination with ketamine for IV induction of short term anaesthesia in ponies.