Background and Goal of Study: 
The minimum alveolar concentration (MAC) for maintaining the bispectral index below 50 (MAC-BIS50) of sevoflurane during a resting state has been reported. However, the MAC-BIS50 of sevoflurane during noxious stimulation has never been determined, and this index may provide clinically useful information. The depth of the anesthesia will decrease under noxious stimulation, and lead to a risk of awareness during the operation. It is reported that tetanic stimulation of 80 mA is equivalent to a skin incision. We investigated the MAC-BIS50 during tetanic stimulation (MAC-BIS50-tetanus) of sevoflurane.

Materials and methods:
 The study was performed following institutional review board approval and after written informed consent was obtained from each patient, without psychotic disease, neurological disorder, and dementia. H2 blocker was solely used as a premedication, and anesthesia was induced with sevoflurane and 50-60% of nitrous oxide in 40-50% of oxygen and air. Under an adequate neuromuscular blockade with 0.6 mg/kg of rocronium, supraglottic device (i-gel or LMA proseal) was introduced. Anesthesia was maintained with a predetermined end-tidal concentration of sevoflurane in oxygen and air, and we confirmed that the end-tidal nitrous oxide concentration was below 5%. Tetanic stimulation of 80 mA was applied for 10 seconds to the ulnar nerve with a TOF monitor, and the recorded BIS values for 3 minutes at 10-s intervals during before-and-after stimulation. Since our BIS monitors had a 30-s lag, so we defined that the average of the BIS values at 30-s after the stimulations was as the BIS value when tetanic stimulation was applied. The MAC-BIS50 of sevoflurane at in middle-aged patients has been reported to be 0.97%; we started at 1.2 % and determined the MAC-BIS50-tetanus by Dixon's up-and-down method.

Results and discussion: 
We studied 19 adult patients (41-69 yr). The MAC-BIS50-tetanus of sevoflurane was 1.38 % (SE 0.03, 95% CI 1.32-1.44 %).

Conclusion(s): 
The MAC-BIS50-tetanus of sevoflurane was 1.38 % (95% CI; 1.32-1.44 %). It was suggested that a half of patients during general anesthesia with a 1.38 % end-tidal concentrations of sevoflurane would have a BIS value more than 50 when noxious stimulation is applied.

The minimum alveolar concentration (MAC) of sevoflurane for maintaining the bispectral index below 50 during noxious stimulation