A novel epidural delivery regimen was developed: Variable volume automated mandatory bolus (AMB) (VVAMB) will advance individualisation of labour epidural analgesia, by which a larger volume of bolus may contribute to better spread of the local anaesthetics within brief period and thereby reduces the chances of motor blockade that could reduce instrumental deliveries.

Epidural analgesia is the gold standard of pain relief for labour pain. Despite this, up to 50% of parturients continue to experience pain leading to suffering and increased caregiver workload. There is also higher risk of motor blockade found in those receiving epidural analgesia, and these factors is associated with dysfunctional labour requiring obstetric intervention (instrumental delivery). Automated mandatory bolus (AMB) of variable-frequency (VAMB) has been shown to provide better pain relief as compared with conventional patient-controlled epidural analgesia (PCEA) with basal infusion, however its long lockout time per hour is associated with unsuccessful patient bolus requests, with similar motor block to conventional regimens.

The investigators therefore proposes to develop a novel epidural delivery regimen: Variable volume AMB (VVAMB) will advance individualisation of labour epidural analgesia, by which a larger volume of bolus may contribute to better spread of the local anaesthetics within brief period and thereby reduces the chances of motor blockade that could reduce instrumental deliveries. This algorithm development including pilot and clinical trial will compare VVAMB with VAMB regimens, in reducing the incidence of motor block in 216 term women requesting for labour epidural analgesia.

Inclusion Criteria:

• Healthy (American Society of Anesthesiologists (ASA) physical status 1 and 2) primiparous parturient at term (≥36 weeks gestation);
• Singleton fetus;
• In early labor stage (cervical dilation ≤5cm);
• Request labor epidural analgesia and able to administer combined spinal epidural analgesia (CSEA) according to protocol.

Exclusion Criteria:

• Non-cephalic fetal presentation;
• Obstetric (e.g. pre-eclampsia, premature rupture of amniotic membranes for more than 48 hours, gestational diabetes on insulin, pregnancy-induced hypertension on medication) and uncontrolled medical (e.g. cardiac disease) complications;
• Have contraindications to neuraxial blockade or have received parenteral opioids within last 2 hours;
• Dural puncture/ suspected dural puncture at initiation of CSEA.

Only females having natural delivery in KKH will be recruited.