A comparison of the SNAP II and BIS XP indices during sevoflurane and nitrous oxide anaesthesia at 1 and 1.5 MAC and at awakening

Background. Monitoring level of consciousness during anaesthesia,with the ability to predict the intentional or unintentionalreturn to consciousness, is desirable. The purpose of this studywas to compare two processed electroencephalographic depth ofanaesthesia monitors (SNAP II^TM and BIS XP^TM) during sevofluraneand sevoflurane/nitrous oxide anaesthesia. Methods. In total, 42 subjects received an interscalene block,followed by general anaesthesia with sevoflurane or sevoflurane/nitrousoxide. The indices were recorded at baseline, at 1.5 and 1.0minimum alveolar concentration (MAC) equivalents, and duringemergence. Results. The SNAP and BIS indices decreased from baseline at1.5 and 1.0 MAC equivalents, but there was no difference withingroups between subjects who received nitrous oxide and thosewho did not. The SNAP index returned to baseline by 1 min beforeawakening and was higher than baseline at eye opening, but theBIS index remained below baseline at awakening. There was abias of –1 (95% CI: –3 to 1) between the SNAP andBIS at baseline; this increased to 21 (95% CI: 19–23)during maintenance of anaesthesia and was 6 (95% CI: 4–8)at awakening. Conclusions. The SNAP index tracks loss of consciousness andemergence from sevoflurane and sevoflurane/nitrous oxide anaesthesia.There is significant bias between the SNAP and BIS indices andtherefore, the indices are not interchangeable. The SNAP indexreturns to baseline before awakening, whereas the BIS indexremains below baseline at awakening, suggesting that the SNAPindex may be more sensitive to unintentional awareness.