Subjects assigned to Dex group will receive a continuous dexmedetomidine infusion (0.4 ug/kg/h) after anesthesia induction until dural closure, and then received an intravenous analgesia pump with dexmedetomidine(0.08ug/kg/h), sufentanil and antiemetic until 48 hours postoperatively

Subjects in the Placebo group were given comparable volumes of normal saline during the surgery, and intravenous analgesia pump also contains sufentanil and antiemetic, but no dexmedetomidine used until 48 hours postoperatively.

In Dex group, dexmedetomidine will continue to be used during and after surgery, with a infusion of 0.4ug/kg/ h from anesthesia induction to dural closure, and then 0.08ug/kg/ h to 48 hours postoperatively.In placebo group, equivalent normal saline will be injected during operation, and the intravenous analgesia pump will not contain dexmedetomidine after operation

Using the Confusion Assessment Method for Intensive Care Unit (CAM-ICU) for critical care patients, or the 3-min Diagnostic interview for Confusion Assessment Method (3D-CAM) for ward assessment,combined with the Richmond Agitation Sedation Scale (RASS). Delirium assessments will be only conducted in patients with RASS sedation score exceeding -4.The CAM-ICU and the 3D-CAM describes four main characteristics of delirium: acute altered mental state ,fluctuating level of consciousness, inattention, and confusion of thought. If both the first and second features are present, and both the third or fourth features are present, the patient is diagnosed with postoperative delirium.

Using the Delirium Rating Scale-Revised -98 (DRS-R-98) to assess.There are 3 diagnostic items (0~2 or 0~3 points each) and 13 severity assessment items (0~3 points each), for a total of 46 points. The higher the score, the more severe the delirium.