Responses of laboratory animals to some injectable anaesthetics

Xylazine, ketamine, methohexitone and alphadalone/alphaxalone, were administered intraperitoneally, intramuscularly or intravenously to mice, rats, guinea pigs and rabbits. Times for disappearance and reappearance of reflexes were recorded, and duration of loss of reflex. Delivering a predetermined dose gave a varying individual response, ranging from inadequate anaesthesia to death. Using reflexes to assess depth of anaesthesia was of limited value. Reflex movements to noxious stimuli generally persisted even at dose rates that caused prolonged recovery times and death. Conversely, in rats there was no response to a cutaneous stimulus in some animals even though recumbency was almost restored.