A prospective, randomised, blinded controlled study was performed to determine the anaesthetic and cardiorespiratory effects of a constant‐rate infusion (CRI) of alfaxalone in 12 sheep anaesthetised with desflurane, and undergoing experimental orthopaedic surgery. Sheep were sedated with dexmedetomidine (4 μg/kg, intravenously) and butorphanol (0.3 mg/kg, intravenously). Anaesthesia was induced with alfaxalone (1 mg/kg/minute to effect, intravenously) and maintained with desflurane in oxygen and alfaxalone 0.07 mg/kg/minute or saline for 150 minutes (range 150–166 minutes). The anaesthetic induction dose of alfaxalone, the desflurane expiratory fraction required for anaesthetic maintenance, cardiorespiratory measurements and blood‐gases were recorded at predetermined intervals. Quality of sedation, anaesthetic induction and recovery were assessed. The alfaxalone induction dose was 1.7 mg/kg (1.2 to 2.6 mg/kg). The desflurane expiratory fraction was lower (22 per cent) in sheep receiving alfaxalone CRI (P = 0). Also, heart rate (P = 0), cardiac index (P = 0.002), stroke index (P = 0) and contractility (P = 0) were higher, and systemic vascular resistance (P = 0.002) was lower. Although respiratory rate tended to be higher with alfaxalone, there was no difference in PCO ₂ between the groups. Recovery times were significantly longer in sheep given alfaxalone (25.4 v 9.5 minutes) but recovery quality was similar. Alfaxalone reduced requirements of desflurane and maintained similar cardiorespiratory function, but recovery time was more prolonged.

Ten New Zealand White rabbits were used in a crossover experimental study: 200 µg/kg medetomidine and 1 (M1) or 2 mg/kg (M2) morphine were administered intramuscularly. After preoxygenation, anaesthesia was induced with alfaxalone at 10 mg/kg intravenously. The trachea was intubated and 60 per cent oxygen provided. Heart and respiratory rates (HR and RR), direct arterial pressures (APs), arterial pH (pHa), PaO ₂ , PaCO ₂ and SaO ₂ were taken at baseline, after premedication and every 10 minutes during the 90 minutes following induction. The times to return the ear pinch, toe pinch and righting reflexes were recorded. Data were analysed using a two‐way analysis of variance and a paired samples t test. Compared to baseline values, HR, RR, APs, PaO ₂ and SaO ₂ decreased significantly after premedication in both groups. Postinduction apnoea of 20±10 with M1 and 27±18 minutes with M2 was experienced following alfaxalone administration, intermitent positive pressure ventilation was applied until spontaneous breathing efforts appeared. Cardiovascular variables, RR and pHa remained below, and PaCO ₂ over baseline values during the anaesthetic period. No significant differences were observed in the recovery times. Morphine at 1 or 2 mg/kg combined with medetomidine and alfaxalone in rabbits produced a suitable level of anaesthesia, although profound cardiorespiratory depression was found.