Medetomidine-ketamine anaesthesia induction followed by medetomidine-propofol in ponies: infusion rates and cardiopulmonary side effects

Reasons for performing study: To search for long-term total i.v. anaesthesia techniques as a potential alternative to inhalation anaesthesia. Objectives: To determine cardiopulmonary effects and anaesthesia quality of medetomidine-ketamine anaesthesia induction followed by 4 h of medetomidine-propofol anaesthesia in 6 ponies. Methods: Sedation consisted of 7 mug/kg bwt medetomidine i.v. followed after 10 min by 2 mg/kg bwt i.v. ketamine. Anaesthesia was maintained for 4 h with 3.5 mug/kg bwt/h medetomidine and propofol at minimum infusion dose rates determined by application of supramaximal electrical pain stimuli. Ventilation was spontaneous (FIO2>0.9). Cardiopulmonary measurements were always taken before electrical stimulation, 15 mins after anaesthesia induction and at 25 min intervals. Results: Anaesthesia induction was excellent and movements after pain stimuli were subsequently gentle. Mean propofol infusion rates were 0.89-0.1 mg/kg bwt/min. No changes in cardiopulmonary variables occured over time. Range of mean values recorded was: respiratory rate 13.0-15.8 breaths/min; PaO2 29.1-37.9 kPa; PaCO2 62-6.9 kPa; heart rate 312-40.8 beats/min; mean arterial pressure 90.0-120.8 mmHg; cardiac index 44.1-59.8 mI/kg bwt/min; mean pulmonary arterial pressure 11.8-16.4 mmHg. Recovery to standing was an average of 31.1 mins and ponies stood within one or 2 attempts. Conclusions: In this paper, ketamine anaesthesia induction avoided the problems encountered previously with propofol. Cardiovascular function was remarkably stable. Hypoxaemia did not occur but, despite FIO2 of >0.9, minimal PaO2 in one pony after 4 h anaesthesia was 8.5 kPa. Potential relevance: The described regime might offer a good, practicable alternative to inhalation anaesthesia and has potential for reducing the fatality rate in horses.