The investigators want to compare the performance of anesthetic depth monitoring for sedation (ADMS) to the bispectral index (BIS) as electroencephalographic measures of sevoflurane effect using two combined sigmoidal Emax models via population pharmacodynamic approach.

Intraoperative awareness under general anesthesia is rare, but it may cause serious psychological sequelae. Anesthetic depth monitoring system based on electroencephalography(EEG) or evoked potential are broadly used methods. Bispectral index (BIS) monitor is most popular anesthetic depth monitoring system. Anesthetic depth monitoring for sedation (ADMS) is developed in Korea as an electroencephalographic monitor for measuring the depth of anesthesia. The investigators want to compare the performance of ADMS to the bispectral index (BIS) as electroencephalographic measures of sevoflurane effect using two combined sigmoidal Emax models via population pharmacodynamic approach.

Inclusion Criteria:

• adult patients scheduled for an elective operation and postoperative epidural PCA with ASA physical status 1,2,3 who have signed the wrtten informed consent.

Exclusion Criteria:

• preventive anticoagulation treatment
• Significant liver, kidney, coagulation dysfunction
• Clinically significant ECG abnormalities
• administration of psychologic durgs or opioid within 14 days before surgery