Use of the bispectral index as a monitor of anaesthetic depth in cats anesthetized with isoflurane

Abstract

Objective - To determine whether the prestimulation bispectral index (BIS) value or relative change in BIS after noxious stimulation can be used to assess the depth of isoflurane anesthesia in cats. Animals - 17 healthy female cats. Procedure - Electroencephalogram (EEG) patterns and BIS values were examined in cats that received increasing end-tidal (ET) isoflurane concentrations. Subsequently, BIS values were determined before and after either a noxious somatic or visceral stimulus in cats that received ET isoflurane concentrations ranging from 1.8% to 2.4%. Electrical stimuli of the tail base and bladder distension to 50 cm of water were the somatic and visceral stimuli, respectively. Results - The resting BIS at ET isoflurane concentrations from 1.4% to 1.9% steadily decreased concurrently with increasing degrees of EEG suppression. Prestimulation BIS values, however, were not related to 1.8% to 2.4% ET isoflurane concentrations and not useful for prediction of BIS values or hemodynamic and movement responses after a noxious stimulus. The poststimulation BIS value and the difference between mean BIS values before and after stimulation were inversely correlated with increasing ET isoflurane concentrations. Poststimulation BIS values > 60 were observed at ET isoflurane concentrations greater than those associated with a movement response after a stimulus. Conclusions and Clinical Relevance - The prestimulation BIS value has limited use in assessing anesthetic depth in cats during isoflurane anesthesia. The change in BIS values after a noxious somatic or visceral stimulus was a reliable measure of anesthetic depth and may be a useful measure of early arousal from the hypnotic state.