Bolus effective dose of ketamine for preventing withdrawal movement on injection of rocuronium in paediatric patients.

The bolus effective dose of ketamine required to prevent withdrawal movement on injection of rocuronium was determined in 27 paediatric patients undergoing elective surgery. A predetermined dose of ketamine was given intravenously on arrival in the operating room and anaesthesia (2.5% thiopental, 5 mg/kg) was administered 1 min later. After loss of consciousness, 1% rocuronium at 0.6 mg/kg was injected over 5 s and the presence or absence of withdrawal movement recorded. The effective dose of ketamine was determined using a modified Dixon up-and-down method with a step size of 0.1 mg/kg, successful prevention of withdrawal movement being defined as no response or movement at the wrist only. The bolus effective dose of ketamine for preventing withdrawal movement after injection of rocuronium following thiopental anaesthesia in 50% of paediatric patients (ED(50)) was 0.21 mg/kg according to the modified Dixon up-and-down method. Probit analysis indicated an ED(50) of 0.18 mg/kg and an ED(95) of 0.33 mg/kg. The latter was the most satisfactory dose in the clinical setting.