On-line analysis of middle latency auditory evoked potentials (MLAEP) for monitoring depth of anaesthesia in laboratory rats

In laboratory animals as well as in human beings a depth of anaesthesia, wh ere the subject has no pain or recall of events from the surgery, should be provided. Haemodynamic parameters such as heart rate and blood pressure ar e not a guarantee for an optimal depth of anaesthesia, especially when usin g neuromuscular blocking agents (NMBA). A number of studies suggest that th e Middle Latency Auditory Evoked Potentials (MLAEP) contain information abo ut the state of consciousness in humans. The purpose of this study was to e xamine whether the AEP could serve as an indicator of depth of ansesthesia in rats. The AEP was elicited with a click stimulus and monitored in an XO ms window synchronised to thp stimulus. The AEP was extracted applying an A uto Regressive Model with Exogenous Input (ARX-model) from which a Depth of Anaesthesia Index (DAI) was calculated. DAI was normalised to 100 while aw ake and decreasing gradually to a level between 50 and 20 as the rat was an aesthetised. Nine rats were anaesthetised and included in the study. Four d oses of Hypnorm vet.(R) and Dormicum(R) were given as a total, each with 5 minutes interval. Clinical signs of the level of anaesthesia were observed simultaneously with the AEP. The results showed that in four rats DAI decre ased to a lever below 30 while anaesthetised. In the remaining five rats th e AEP was only decreased to a level below 45. The results indicated that a simple dosing regimen based on weight was unable to give the same depth of anaesthesia in individual rats. The decrease in the DAI correlated well wit h the loss of stimulus response. in conclusion, MLAEP could be used as an i ndicator of depth of anaesthesia in rats during Hypnorm vet.(R) and Dormicu m(R) administration, However studies applying other anaesthetic drugs shoul d be carried out, before a conclusion of the general utility of the method can be made. (C) 1999 IPEM. Published by Elsevier Science Ltd. All rights r eserved.