To find out whether low dose of Ketamine can provide effective and safe analgesia during labour

INTERVENTION: Intervention1: Ketamine: 0.4 mg/kg/hr over 30 min, 0.2 mg/kg/hr infusion Control Intervention1: Normal saline: saline infusion at same rate CONDITION: Pain and analgesia during labour PRIMARY OUTCOME: To evaluate the efficacy of low dose ketamine in providing obstetric analgesia‐‐‐‐‐‐Timepoint: 10 point visual analogue scale SECONDARY OUTCOME: To observe if this regimen has any side effects on the mother, the progress of labor and the fetus‐‐‐‐‐‐Timepoint: Mother‐ progress of labor,haemodynamic parameters, sedation, hallucinations and sleep disturbances. Baby‐ APGAR score at 1 and 5 minutes,the cord blood pH INCLUSION CRITERIA: All parturient, with no antenatal risk factors and expected to have a normal vaginal delivery will be included in the study