Background and aimInfusion of ketamine and midazolam can maintain prolonged anaesthesia, but delayed recovery is a limitation. We aimed to develop an approximation regimen for the infusion of ketamine and midazolam to obtain steady-state anaesthesia with acceptable recovery.MethodsThirty-one patients undergoing radical cystectomy were studied. The initial regimen was calculated from drug pharmacokinetic variables and tailored in a pilot study (15 patients) to develop the approximation regimen dosage. Anaesthesia was induced with midazolam (150 µg kg−1) and ketamine (2 mg kg−1). Tracheal intubation and ventilation with oxygen enriched air (FiO2= 0.35) were facilitated by muscle relaxants. Anaesthesia was maintained by the approximation regimen doses. Routine monitoring was used for all patients, but pulmonary artery catheters were inserted in 11 patients, to obtain haemodynamic and oxygenation variables.ResultsSteady-state anaesthesia was obtained with minimal deviations in the regimen in some patients followed by reasonable recovery.ConclusionIt is concluded that infusion of ketamine and midazolam in the approximation regimen doses can be used to maintain anaesthesia for prolonged abdominal surgery.