Bispectral index as an indicator of anaesthetic depth during isoflurane anaesthesia in the pig

Abstract

SUMMARY The objective of the study was to examine the relationship between the ‘depth’ of anaesthesia - as determined by clinical signs - and the bispectral index (BIS). Electroencephalograms (EEG)s were recorded in 8 female and 8 castrated male, healthy Norwegian landrace pigs undergoing isoflurane anaesthesia, from which the bispectral index (BIS) was calculated. Isoflurane was delivered in pure oxygen at end-tidal concentrations of 1.6, 1.9, 2.2 and 2.5%, in randomised order, for 30 min after which the EEG was recorded over a 5 min period. Anaesthetic depth was evaluated on a visual analogue scale (VAS) by an experienced anaesthetist. The 95% confidence interval for the mean correlation coefficient between BIS and VAS was calculated to be -0.52–0.30. Confidence intervals (95%) for the mean change in the BIS obtained during the conscious state and that obtained during anaesthesia at different isoflurane concentrations was also calculated. There was a significant decrease in the BIS recorded during consciousness and after 1.6% isoflurane anaesthesia, and between readings after inhalation of 2.2% and 2.5% isoflurane. This indicates that BIS does not accurately reflect ‘depth’ at surgical levels of isoflurane anaesthesia in the pig, and is of no use for this purpose.