 The subjects will give an informed consent prior to the participation in the study. All patients will receive 2 mg of midazolam for premedication. Anesthesia will be induced with the intravenous administration of fentanyl 5-6 μgkg and propofol 2-3 mgkg. Following tracheal intubation, anesthesia will be maintained with 70 nitrous oxide in oxygen supplemented with an infusion of propofol 120 -150 μgkgmin. Ventilation will be controlled to maintain normocapnia end-tidal carbon di oxide 35-40 mmHg. 