Use of S+Ketamine During Target-Controlled Intravenous Anaesthesia After Abdominal Hysterectomy

The investigators want to investigate the effect of low dose S+ ketamine compared to placebo on cumulative morphine consumption at 24 hours in 90 women undergoing open abdominal hysterectomy with remifentanil-propofol target controlled infusion (TCI) in KK Women's and Children's Hospital. The secondary aims are to investigate the use of low dose S+ ketamine on the incidence of nausea, vomiting, pruritus (opioid side effect), sedation score and psycho mimetic assessment compared to placebo group. The investigators propose to conduct a double blinded, randomized controlled study in women undergoing open abdominal hysterectomy with remifentanil-propofol TCI. (1) Treatment Group: intravenous ketamine 0.5 mg/kg at the beginning and 0.5 mg/kg 20 minutes before extubation. (2) Control Group: intravenous normal saline (as placebo) at the beginning and 20 minutes before extubation.