Small Doses of Dexmedetomidine for Emergence Agitation

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