The incidence of postoperative cognitive dysfunction (POCD) is high and POCD affect the life quality and the prognosis of patients. Geriatrics is the independent risk factor of POCD, and POCD is also correlated with many other factors such as type of surgery, the duration of anesthesia and the anesthesia drugs used et al. So, the prevention and treatment of POCD in geriatrics is important.

Dexmedetomidine is found to have the effect of neuro-protection, but it is controversy whether Dexmedetomidine has the effect of neuro-protection in geriatrics, especially the prolonged surgery.

The purpose of this study is to explore the effect of Dexmedetomidine to the cognitive function at prolonged surgery in geriatrics in geriatrics.

Inclusion Criteria:

1. Written informed consent
2. Scheduled for elective abdominal surgery
3. Age≥65 yrs
4. Weight is within the range of ±20% of standard weight [standard weight=Height(cm)-80)×70﹪ for male and(Height(cm)-70)×60﹪] for female
5. American society of anesthesia classification I~III
6. Expected time of surgery is more than 4 hours -

Exclusion Criteria:

1. Systolic blood pressure≥180 mm Hg or <90 mm Hg，diastolic blood pressure≥110 mm Hg or < 60 mm Hg；
2. Serious cardiac,liver,kidney,lung, endocrine disease or sepsis.
3. Allergy to trial drug or other contraindication；
4. Difficult airway occurred before,or difficult airway possibly occurred and difficult to extubation
5. History of mental illness and cerebral vascular disease
6. History of unstable angina or myocardial infarction
7. Education level<7 yrs
8. Factors existed that affect cognition assessment such as language, visual sense,auditory sense disorders.
9. Abuse of narcotic analgesia or suspected;
10. Neuromuscular diseases;
11. Mentally unstable or has a mental illness;
12. Pregnant or breast-feeding women;
13. Attended other trial past 30 days; -