[Effect of nitrous oxide on the bispectral index during sevoflurane anesthesia].

We studied the effects of nitrous oxide on the relationship between end-tidal sevoflurane concentration and bispectral index (BIS) in patients undergoing abdominal surgery. Anesthesia was maintained with combination of epidural and sevoflurane anesthesia in air (control group; n = 15) or with 67% nitrous oxide (nitrous oxide group; n = 15). The end-tidal sevoflurane concentration was increased by 0.5% every 15 min to 3% and BIS values were recorded at each step. In both groups, sevoflurane decreased BIS values in a dose-dependent manner and the decrease in BIS reached plateau at 2% of sevoflurane. Nitrous oxide with sevoflurane caused more reduction in BIS in comparison with sevoflurane alone. The sevoflurane concentration for BIS at 50 in the nitrous oxide group (0.9 +/- 0.4%) was significantly lower than that in the control group (1.2 +/- 0.4%). The results suggest that the hypnotic effect of sevoflurane was enhanced by the addition of nitrous oxide during abdominal surgery.