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ABSTRACT.Objective. There is a potential use for spectral entropy or bispectral index (BIS) for controlling level of anesthesia, but it is not known how these EEG monitors relate during steady state anesthesia. We compared Response Entropy (RE) and BIS during anesthesia for laparoscopic gastric banding with RE targeted to 45. Methods. Forty patients undergoing laparoscopic gastric banding were randomly assigned to receive either fentanyl or dexmedetomidine infusion, with desflurane concentration adjusted to maintain RE at 45. During anesthesia the average RE and BIS was determined in each patient and the RE-BIS difference plotted as a function of RE every 10 seconds. Fifteen of 40 patients showed activation of RE above 60 during surgery. In these patients RE, BIS and the electromyogram (EMG) were evaluated for the period 10 minutes before and including the peak change in RE. Results. In fentanyl and dexmedetomidine treated patients the average RE was 44-47 with no statistical difference between anesthesia groups or between RE and BIS. In each patient there was a linear relationship between the RE-BIS difference and RE during anesthesia. RE and BIS were similar at a level of 41-44 and RE showed a greater range at higher and lower values compared to BIS. When RE activation was identified during surgery in 15 patients, it was associated with an increase in BIS and EMG. Conclusion. Within the range of 41-44, RE and BIS are equal but the gain of RE is 0.5 greater than BIS with deeper or lighter anesthesia. This is not likely due to increased smoothing with BIS. Identifying periods of RE activation show that BIS, EMG and RE increase together.

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