BACKGROUND: There have been some conflicting reports showing that the muscle relaxant affect the level of bispectral index (BIS) during various condition of anesthesia. The purpose of this study was to investigate whether muscle relaxant affects the level of BIS during propofol anesthesia.

METHODS: Eighty patients (ASA class I or II , 20-55 yrs) scheduled for elective surgery under general anesthesia were randomly assigned to four groups. Anesthesia was maintanined at a BIS level of 60 by propofol target controlled infusion (TCI) with air-oxygen (2 L/min : 2 L/min). After maintaining BIS 60 with spontaneous breathing, normal saline 0.05 mg/kg was iv injected in group I and vecuronium bromide 0.1 mg/kg was iv injected in group II. After maintaining at a BIS level of 40 with assisted ventilation, normal saline 0.05 mg/kg was iv injected in group III and vecuronium bromide 0.1 mg/kg was iv injected in group IV. The BIS, mean arterial pressure, and heart rate were measured before anesthetic induction and up to 10 min after normal saline or vecuronium bromide injection in each group.

RESULTS: BIS was not changed in group I, but BIS significantly decreased in group II (P < 0.05). In group III and IV, BIS were not changed in both group. There were no hemodynamic changes among the four groups.

CONCLUSIONS: These result suggest that the effect of muscle relaxant on BIS vary according to the level of BIS during propofol anesthesia.