Relationship of bispectral index to minimum alveolar concentration during isoflurane, sevoflurane or desflurane anaesthesia

Abstract

Objective To determine bispectral index (BIS) values produced by equipotent concentrations of commonly used volatile anaesthetics. Methods Female patients undergoing thyroidectomy were randomly assigned to receive isoflurane, sevoflurane or desflurane anaesthesia. After induction, anaesthesia was maintained by the volatile agent at 1 minimum alveolar concentration and supplemented with remifentanil infusion. BIS values were recorded during 1 h surgical anaesthesia after a 15 min equilibrium phase. Results Time-averaged BIS value during the study period was significantly lower in the desflurane group ( n = 29) than the sevoflurane group ( n = 27) (37.0 ± 4.9 vs 41.5 ± 5.9). Duration of deep hypnosis (BIS < 40) was significantly longer in the desflurane group than the sevoflurane group (40.2 ± 20.7 vs 24.3 ± 22.5 min). There were no significant differences in any parameter between the isoflurane group ( n = 27) and any other group. Conclusions Desflurane produces a greater hypnotic effect than sevoflurane during equipotent anaesthesia. Management of volatile anaesthesia using predetermined minimum alveolar concentration targets can lead to an unnecessarily long duration of deep hypnosis.