Postoperative Analgesic Effect of Esketamine

Inclusion Criteria: * nulliparous parturients who scheduled for elective caesarean delivery under spinal-epidural anaesthesia; * parturients who had requested postoperative analgesia; * parturients aged between 20-35 years old; * parturients who had a full-term pregnancy; * parturients who were identified as having a singleton pregnancy; and * parturients who were categorised as having an American Society of Anaesthesiologists (ASA) physical status Exclusion Criteria: * parturients with severe internal, surgical or obstetric comorbidities (including spinal deformities, hypertension, placental abruption, cholestasis in pregnancy, asthma, heart disease and abnormal coagulation parameters) * parturients with a known allergy to the drugs used in this study * parturients with severe mental illness who could not comply with doctors' instructions * parturients with chronic pain syndrome, which is defined as pain that persists for a period longer than 3 months