The study was approved by the appropriate Institutional Review Board. After written informed consent 63 ASA I-III patients undergoing elective hip or knee replacement surgery under general anesthesia were included. Patients were randomly organized into 3 groups (20-22 each). Anesthesia was induced with intravenous anaesthetics. After tracheal intubation MIRUS automatically adjusted the end-tidal VA concentration to 1.0 MAC. Patients were ventilated with the Puritan Bennett 840 ICU ventilator. After 1 h of anaesthesia with 1.0 MAC the ventilator mode was switched from SIMV V+ (totally controlled ventilation, "passive patient", with a tidal volume of 8 ml/IBW) to proportional assist ventilation (PAV) with 50% support ("active patient"). At the end of surgery the MIRUS system was stopped (MAC set to 0.0) and recovery times were measured.