The purpose of this study is to explore the effectiveness of dexmedetomidine as an adjunctive analgesic, combined with dezocine and flubiprofen, used in intravenous Patient-Controlled Analgesia (PCA) after open colorectal surgery.

Dexmedetomidine is a selective alpha-2 adrenoceptor agonist, which has been demonstrated to have anaesthetic, sedative and analgesic-sparing effects. Besides, Dexmedetomidine has been reported to cause sedative effects and reduce opioid requirements in the perioperative period. Many studies had been conducted to reduce the side effect of opioid analgesic. The current trend of reducing opioid analgesic side effect is direct combination of other drugs in PCA. The advantages of this method are convenient in clinical use and preventive for side effects.

In this study, 50 patients who is undergoing elective colorectal surgery will be randomly allocated into two groups (Control group and Dexmedetomidine group). Patients in both groups will be given a patient-controlled analgesia (PCA) pump in post anesthetic recovery unit (PACU) after surgery.

The PCA protocol of Control group is dezocine 0.6mg/kg, flubiprofen 3mg/kg, diluted into 120ml and administer at a background infusion of 2ml/h, and a bolus of 2ml, with a lock-out of 15min. The PCA protocol of Dexmedetomidine group is dezocine 0.6mg/kg, flubiprofen 3mg/k and dexmedetomidine 6μg/kg diluted into 120ml and administer at a background infusion of 2ml/h, and a bolus of 2ml, with a lock-out of 15min.

Inclusion Criteria:

1. American Society of Anesthesiologists(ASA) Ⅰ-Ⅱ patient undergoing open colorectal surgery
2. Written informed consent from the patient or the relatives of the participating patient.
3. BMI:18～30kg/m2

Exclusion Criteria:

1. Mental illness or cannot communicate.;
2. A second operation during the study;
3. Slow-type arrhythmias or hypotension;
4. Lung infection or sleep apnea syndrome;
5. Renal failure;
6. Alcohol or drug abuse;
7. Already taking gabapentin, pregabalin, benzodiazepin or antidepression drug;
8. Long-term use of analgesics,sedatives or non steroidal anti-inflammatory drugs history;