Surgical pain refers to pain that occurs immediately after surgery, including physical pain and visceral pain. Thus, it severely challenges the proper use of analgesics for patients undergoing laparoscope gastrointestinal surgery to clinicians.

Nalbuphine is a mixed agonist-antagonist opioid. The investigators hypothesized that the clinical effect of nalbuphine in combination with dexmedetomidine might be different from that of sufentanil in combination with dexmedetomidine.

So, the investigators performed a nalbuphine and dexmedetomidine dose finding study, for the patient controlled anaesthesia (PCA) after the laparoscopic gastrointestinal surgery, to establish their 95% effective dose (ED95). The investigators then compared the clinical effect and adverse events of the newly established dosing regimen of nalbuphine combined with dexmedetomidine, to the equivalent dosing of sufentanil combined with dexmedetomidine, in the same patient population.

Surgical pain refers to pain that occurs immediately after surgery, including physical pain and visceral pain. Appropriate perioperative analgesia is a fundamental component of enhanced recovery after surgery. Especially, 45% of postoperative patients experience inadequate pain after gastrointestinal surgery, and uncontrolled postoperative pain prompts respiratory distress, delays wound healing, and a potentially eventual transition from acute to chronic pain problems. Thus, it severely challenges the proper use of analgesics for patients undergoing laparoscope gastrointestinal surgery to clinicians.

Sufentanil is one of the most common opioid used in patient-controlled analgesia (PCA), it may induce many adverse events including respiratory depression, nausea, vomiting, constipation, urinary retention, pruritus, and drowsiness. Many drugs have been combined with dexmedetomidine in PCA to augment analgesic effect or to reduce the adverse events.

Nalbuphine is a mixed agonist-antagonist opioid. Nalbuphine derives its analgesic and sleep-producing effects through agonism at the kappa-opioid receptor with fewer opioid-induced adverse effects. some articles show that it also has the potential to attenuate the mu-opioid receptor-related adverse events. The investigators hypothesized that the clinical effect of nalbuphine in combination with dexmedetomidine might be different from that of sufentanil in combination with dexmedetomidine. Unfortunately, the optimal dosing of nalbuphine combined with dexmedetomidine for the PCIA after the laparoscopic gastrointestinal surgery, has not been determined.

So, the investigators performed a nalbuphine and dexmedetomidine dose finding study, for the PCIA after the laparoscopic gastrointestinal surgery, to establish their 95% effective dose (ED95). The investigators then compared the clinical effect and adverse events of the newly established dosing regimen of nalbuphine combined with dexmedetomidine, to the equivalent dosing of sufentanil combined with dexmedetomidine, in the same patient population.

Inclusion Criteria:

1. age ≥18 years and ≤75 years;
2. scheduled to undergo laparoscopic gastrointestinal surgery, under general anaesthesia;
3. 18.5 kg/m2≤BMI≤27.9kg /m2;
4. American Society of Anesthesiology (ASA) I-III;
5. agree to participate, and give signed written informed consents;
6. Verbal or reading ability, able to understand and complete the questionnaire independently or with the help of doctors.

Exclusion Criteria:

1. patients who were allergic to the drugs in this study, who refused to use analgesic devices and analgesic drugs after operation, or who refused to sign the consent forms were not included;
2. severe hypertension, severe psychiatric disease and mental system diseases, severe respiratory diseases, hyperthyroidism, severe liver and kidney dysfunction, alcohol or drug abuse, Severe heart dysfunction or pulmonary insufficiency,Previous history of schizophrenia, epilepsy, myasthenia gravis or delirium;
3. Patients who had a history of drug abuse, patients undergoing chemotherapy and radiotherapy one month before operation, patients unwilling to cooperate with the treatment or with disabilities, were excluded from the study;
4. Patients with a definite diagnosis of chronic pain syndrome, or substance use disorder, patients who used sedatives, antiemetics, or anti-pruritic agents within 24 h before operation;
5. participating in other clinical studies in recent 3 months.