Dexmedetomidine Hydrochloride is a kind of high selectivity alpha 2 agonists adrenaline, can inhibit the activity of sympathetic nerve, and reduce the adverse effects of stress reaction in anesthesia recovery period.Literatures have been reported that Dex can reduce the rate of emergence agitation of children and adults,especially in elderly patients(>64 years) after using sevoflurane anesthesia.

Inclusion Criteria:

1. obtain informed consent;
2. patients whose selective under general anesthesia colorectal cancer or esophageal cancer radical operation time > 2h;
3. the American society of anesthesiologists (ASA) class I-II;
4. age>64 years old;
5. BMI<28kg/㎡;

Exclusion Criteria:

1. systolic pressure≥180mm Hg or<90mm Hg, diastolic pressure≥110mm Hg or< 60mm Hg;
2. serious heart, brain, liver, kidney, lung, endocrine diseases or serious infections;
3. patients with difficult airway (such as Mallampati airway class III, airway abnormalities, etc.)
4. HR<50times/min
5. a history of mental illness or a long history of taking medicine of psychiatric drugs and chronic analgesics
6. a history of alcoholism
7. diseases of the neuromuscular
8. a tendency to malignant hyperthermia
9. allergy to test drugs or have other contraindications
10. participated in other clinical drug researches over the past 30 days