Alfaxalone and medetomidine intravenous infusion to maintain anaesthesia in colts undergoing field castration

Summary Reasons for performing the study The use of alfaxalone and medetomidine administered as an i.v. infusion to maintain anaesthesia has not previously been reported in the horse. Objectives To investigate the use of alfaxalone in hydroxpropyl‐beta‐cyclodextrin ( A lfaxan) and medetomidine infusion as a field anaesthetic for short‐term surgical procedures in the horse. Hypothesis Alfaxalone–medetomidine anaesthesia is suitable for short‐term field anaesthesia in horses. Methods Eleven healthy colts underwent 45 min of anaesthesia with an i.v. infusion of alfaxalone (2 mg/kg bwt/h) and medetomidine (5 μg/kg bwt/h) for routine field castration. Horses were premedicated with i.v. acepromazine (0.03 mg/kg bwt), medetomidine (7 μg/kg bwt) and guaiphenesin (35 mg/kg bwt) before i.v. induction with alfaxalone (1 mg/kg bwt). Colts were intubated with an endotracheal tube and 100% oxygen insufflated at 10  l /min. The physiological variables monitored included pulse rate, respiratory rate, direct arterial blood pressure, arterial blood gases and the quality of the inductions and recoveries were scored. Results Overall, the anaesthetic period and surgical conditions were acceptable and the quality of the anaesthetic inductions and recoveries was good to excellent. All colts stood on their first attempt (mean ± s.d. ) 37 ± 13.5 min after the infusion was stopped. During anaesthesia, cardiopulmonary data, presented as range of mean values at each time point were: heart rate: 45–47 beats/min; mean blood pressure: 104–112 mm H g; respiratory rate: 8 breaths/min; PaO 2 : 117–172 mm H g; PaCO 2 : 50–56 mm H g and pH  7.34–7.37. Conclusions and potential relevance The co‐administration of alfaxalone and medetomidine as an i.v. infusion after anaesthetic induction with alfaxalone was suitable for short‐term field anaesthesia in the horse.