The investigators try to conduct a study to assess the impact of changing patients' lateral decubitus position on BIS readings during esophageal cancer operation. Also, the investigators evaluate the effects of one-lung ventilation and total lung ventilation on BIS value with inhalation agent.

Bispectral index (BIS) was developed to monitor a patient's level of consciousness under general anesthesia. Several factors have been found to alter BIS readings without affecting the depth of anesthesia.Changing a patient's position (head up and head down) significantly affects the BIS values, which might affect the interpretation of anesthetic depth.

During esophageal cancer surgery, there were several factors which could affect to BIS values.First, the surgical positions are consisted with supine and lateral decubitus position more than 1 h. Thus, the investigators compare the BIS value of both side on supine and lateral decubitus position by both side approach. Second, the one lung ventilation was applied for surgical filed. It alters the oxygenation and the cerebral flow. Because of these effects, the BIS monitoring can differ by the period of surgery. Thus, in this study, the BIS value of both side monitor throughout the surgery.

Inclusion Criteria:

the patients who are planned the esophageal cancer operation

Exclusion Criteria:

• patient who has dermatitis
• patient who refuse to participate in this study