Use of midlatency auditory-evoked potentials as indicator of unconsciousness in the dog: characterisation of the effects of acepromazine-thiopentone, medetomidine-thiopentone and medetomidine-butorphanol-midazolam combinations.

Middle latency auditory-evoked potentials were measured in sedated and anaestetised dogs to determine their possible usefulness in monitoring of unconsciousness during anaesthesia and to compare the effects of anaesthetic protocols. There were three groups of five dogs: group I received acepromazine; groups 2 and 3 received medetomidine; 30 minutes later, groups 1 and 2 received thiopentone and group 3 received midazolam and butorphanol. Groups 2 and 3 received atipamezole 60 minutes after medetomidine was administered. Auditory-evoked potentials were recorded at time 15, 40 and 75 minutes. Thiopentone administration resulted in a profound modification of the pattern of response, and several peaks were no longer identified. In group 3, the administration of midazolam-butorphanol tended to increase the latency of the different peaks, but lesser than thiopentone did. Middle latency-evoked potentials appeared to be potentially useful in the monitoring of unconsciousness in the dog.