The investigators hypothesize that the incidence of postoperative cognitive dysfunction (POCD) is not different in patients received intravenous anesthetics propofol or sevoflurane for their cardiac valvular surgery. Two groups of general anesthetics (propofol or sevoflurane) are used to explore the possible difference of the postoperative outcome and the incidence of POCD. After cardiac valvular surgery with CPB, patients' POCD tests at 7 days after the surgery, and postoperative delirium, etc., are evaluated by different questionnaires. Also the blood sample of these patients are collected for detection of some stress hormones.