The purpose of this multicenter study is to evaluate the effectiveness and safety of dexmedetomidine combined with sufentanil for patient-controlled analgesia after caesarean section

Inclusion Criteria:

1. Parturients undergoing elective caesarean delivery under spinal anaesthesia
2. American Society of Anesthesiologists class I and II parturients aged 18-45 years, with term singleton pregnancies
3. Parturients with the ability to understand verbal and written mandarin
4. Parturients who want to use PCA intravenous analgesia and can use the pump correctly
5. Parturients whose written informed consent have been obtained

Exclusion Criteria:

1. A history of allergy to dexmedetomidine or other study drugs
2. A long use history of opioid analgesic, NSAIDs, tranquilliser use
3. Psychiatric disorders
4. A history of neuromuscular and endocrine disease or allergic disease
5. A history of lower abdominal surgery
6. Preoperative heart rate (HR) less than 50 bpm, SBP less than 100 mmHg or cardiac conduction or rhythm abnormalities
7. spinal anaesthesia was unsuccessful or an epidural catheter had to be used to achieve adequate anaesthesia
8. A second operation is required during the study.
9. Participating other study during the latest three months
10. Body mass index more than 30 kg/m2
11. Difficult airway ( Mallampati score more than 3 and anatomic deformity )
12. Not suitable for participation estimated by investigation