Comparison of morphine and butorphanol as pre-anaesthetic agents in combination with romifidine for field castration in ponies

Objective The aim of this study was to compare two different alpha(2) agonist-opioid combinations in ponies undergoing field castration. Study design Prospective double-blind randomized clinical trial. Animal population Fifty-four ponies undergoing field castration. Materials and methods The ponies were randomly allocated to receive one of three different preanaesthetic medications [intravenous (IV) romifidine 100 mug kg(-1) and butorphanol 50 mug kg(-1); romifidine 100 mug kg(-1) and morphine 0.1 mg kg(-1) IV, or romifidine 100 mug kg(-1) and saline IV] before induction of anaesthesia with ketamine 2.2 mg kg(-1) IV. Further doses of romifidine (25 mug kg(-1)) and ketamine (0.5 mg kg(-1)) were given when required to maintain anaesthesia. Quality of sedation, induction of anaesthesia, maintenance of anaesthesia, recovery, and surgical condition were assessed using a visual analogue scale scoring system and compared. The effects of the different drug combinations on heart and respiratory rate were evaluated and the recovery time was recorded. Results Anaesthesia was considered adequate for surgery in all ponies. No anaesthetic complications were observed. Quality of sedation was significantly better in the butorphanol group compared with the control group (p = 0.0428). Overall quality of anaesthesia was better in the butorphanol group compared with morphine (p = 0.0157) and control (p < 0.05) groups. Quality of induction of anaesthesia and recovery were not significantly different between groups, nor were the surgical conditions, recovery time and the number of repeated anaesthetic doses required during the procedure. Muscle twitches were observed in both the control and morphine groups. Maintenance of anaesthesia was judged to be smoother in the butorphanol group compared with the morphine and control groups (p = 0.006). Heart rate decreased significantly (p < 0.01) in all groups after administration of sedatives but did not differ significantly between groups at any time point. Conclusion The combination of butorphanol and romifidine was found to provide better sedation compared with the other drug combinations.