Anesthesia Depth's Influence on Postoperative Delirium

The investigators,according to the eligibility criteria,plane to choose 80 elderly anticipates who will undergo selective hip replacement surgery.This participants will be randomly divided into 2 groups:light anesthesia depth group(L Group) and deep anesthesia depth group (D Group).Before anesthesia induction,each participant will be given lumbar plexus and sciatic nerve block in order to control pain of interoperative and postoperative.During operation period,the L Group will be kept in light anesthesia depth(BIS:50-60),while the D group will be kept in deep anesthesia depth(BIS:35-45)--The BIS values in the range of 30-60 have been recommended in general anesthesia.After operation,a PCIA(patient controlled intravenous analgesia) analgesia pump will be given to each participant to control postoperative pain. The investigator will evaluate the mental state of participant with MMSE(mini-mental state examination) before surgery and review each participant with a CAM-CR for 3 days after surgery to assess the situation of postoperation delirium.intraoperative awareness,postoperative pain will also be investigated in postoperative days 1, 2, 3.