1. Sufentanil as an strong analgesic is used widespreadly during laparoscopic cholecystectomy.
2. Lower dosages of sufentanil facilitated with dexmedetomidine can offer enough analgesia effect.
3. The investigator are planning use the most classical method for determining the ED50 or half effective concentration of sufentanil facilitated with dexmedetomidine using sequential method, which is also called" up and down method" and" ladder method".

The concordance of the current research is that lower dosages of sufentanil facilitated with dexmedetomidine can offer enough analgesia effect through operation. However, the dosages of sufentanil used in various researches vary greatly of operations. Previous studies have shown that the amount of sufentanil required for intraoperative analgesia is smaller than that required while dexmedetomindine facilitated. Although the appropriate dosage of sufentanil required for operation can ensure the analgesic effect of operation, excessive dosage can cause severe choking during intubation, postoperative nausea and vomiting, hyperalgesia, and chronic postoperative pain. The investigator planning use the most classical method for determining the ED50 or half effective concentration of sufentainl combined with dexmedetomidine using sequential method, which is also called " up and down method" and " ladder method".

Inclusion Criteria:

1. ASA I ~ II level;
2. BMI22-30kg/m2;
3. Those who intend to undergo laparoscopic cholecystectomy;
4. Age 18~65 years.

Exclusion Criteria:

1. Has been allergic to the drugs used in this study;
2. Patients with a history of chronic use of alcohol, smoke or other sedative drugs;
3. Patients took opioids and other analgesics before surgery;
4. Emergency operation;
5. Stress, anxiety resulting in secretion of catecholamine;
6. Patients with neuropsychiatric disorders, coma, depression, cognitive impairment and Alzheimer's disease;
7. Patients with epilepsy, autonomic nervous dysfunction and other diseases with abnormal EEG results;
8. SBP< 85 mmHg or HR< 45次/min;
9. Has implanted pacemaker;
10. Patients were unwilling or disable to finish whole study;
11. Patients with acute upper respiratory infections, liver or kidney failure and severe cardiopulmonary dysfunction.

Patients in Xuanwu Hospital for elective laparoscopic cholecystectomy are selected. Before surgery, visitors recorded basic information such as the patient's sex, age, BMI, ASA grade in the ward.