In this study, the investigators investigated the efficacy, usefulness and analgesic consumption of three different patient-controlled analgesia(PCA) programmes:bolus dose alone without background infusion, bolus dose with low background infusion and bolus dose with high background infusion to evaluate postoperative analgesia for patients after laparoscopic colorectal surgery.

There have been many studies using patient-controlled analgesia (PCA) and opioids for postoperative analgesia.Patient-controlled analgesia with opioids is now widely used after surgery for the treatment of acute postoperative pain. This technique provides effective pain treatment, adjusts the dose according to personal need and keeps plasma analgesic levels at a constant level,but the incidence of opioid-induced side-effects still be reported frequently.A proper PCA programme may increase efficiency of analgesic and decrease incidence of analgesic-induced side-effects.

In this study, 90 patients who is undergoing elective laparoscopic colorectal surgery will be randomly allocated into three groups (no background infusion group(Group B0),low background infusion group(Group B1) and high background infusion group(Group B2)). Patients in all groups will be given a patient-controlled analgesia (PCA) pump in post anesthetic recovery unit (PACU) after surgery.The PCA programme of Group B0 is dezocine 0.6mg/kg, flubiprofen 3mg/kg, diluted into 120ml and administer at a background infusion of 0ml/h, and a bolus of 4ml, with a lock-out of 15min. The PCA protocol of Group B1 is dezocine 0.6mg/kg,flubiprofen 3mg/k diluted into 120ml and administer at a background infusion of 1ml/h, and a bolus of 2ml, with a lock-out of 15min.The PCA protocol of Group B2 is dezocine 0.6mg/kg,flubiprofen 3mg/k 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 laparoscopic 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. Lung infection or sleep apnea syndrome;
4. Renal failure;
5. Alcohol or drug abuse;
6. Already taking gabapentin, pregabalin, benzodiazepin or antidepression drug;
7. Long-term use of analgesics,sedatives or non steroidal anti-inflammatory drugs history .