[BIS monitoring is useful for reliable intraoperative cortical mapping during brain tumor operations].

Poor detection of muscle motor evoked potential (m-MEP) by electrical cortical stimulation has often been encountered during neurosurgical operations, although widely known optimal factors for reliable detection have been put into practice. We hypothesized that the depth of anesthesia influenced the detection of m-MEP. Bispectral Index (BIS) monitoring was used to assess the depth of anesthesia for 16 patients during brain tumor operations. The doses of intravenous anesthetics, propofol, were titrated according to BIS indices during operations. When BIS indices were 40 or below, detection was difficult and higher intensities of electrical stimulation were necessary to evoke m-MEPs. Furthermore, risk of seizure was relatively high due to stronger electrical stimulation. Our results suggested that a BIS index above 50 and below 70 was optimal range. BIS monitoring is a useful method for setting the optimal dose of anesthetics for reliable intraoperative cortical mapping.