General anesthesia is one of precipitating factor of postoperative delirium which Its mechanism is still unknown. General anesthesia can affect on pattern of electroencephalograph. Previous studies showed that patients received BIS guided anesthesia could wake up faster, earlier extubation and stay in recovery room shorter compared to standard general anesthesia. Optimized level of anesthesia by BIS could reduce cognitive dysfunction in elderly patients and reduce biological marker of brain injury. Therefore, the investigators hypothesized that adjustment of general anesthesia by BIS guided could protect central nervous system and reduce incidence of postoperative delirium.