Composite-, plain-auditory evoked potentials index and bispectral index to measure the effects of sevoflurane

The composite auditory evoked potentials index (cAAI) uses both cortical electroencephalogram (EEG) and response to auditory stimuli, while the bispectral index (BIS) uses only the cortical EEG and auditory evoked potentials index (AAI) uses only response to auditory stimuli. We expected that the cAAI was more useful to monitor anesthetic effect of sevoflurane than the BIS and AAI. The present study compared the changes of cAAI, AAI, and BIS in different sevoflurane concentration. Forty-five adult patients were anesthetized with sevoflurane in 50 % nitrous oxide. AAI (AEP version 1.4), cAAI (AEP version 1.6), and BIS (A-2000) were compared (each 15 patients in AAI, cAAI, and BIS groups) before induction, just before and after intubation, at 10 min since sevoflurane was set to 1.0, 1.5 and 2.0 %, and after extubation. All three indices decreased significantly before intubation. The cAAI was significantly higher than the AAI at sevoflurane 1.0 and 1.5 %. The AAI and BIS were significantly lower at sevoflurane 2.0 % than those at sevoflurane 1.0 %, but the cAAI did not. The cAAI had the largest and AAI had the smallest inter-individual variation. In sevoflurane-nitrous oxide anesthesia, cAAI was inferior to AAI and BIS to discriminate different anesthetic effect. The cAAI had larger inter-individual variation than the AAI and BIS.