Anesthesia and surgery All patients underwent a standardized general anesthesia procedure and none of them received any preanesthetic medication. Induction used intravenous propofol (2 mg/kg body weight) and fentanyl (3-5 mcg/kg). Vecuronium bromide (0.1 mg/kg) was used to facilitate tracheal intubation. Anesthesia was maintained with 2-3% sevoflurane and 66% nitrous oxide in oxygen. Ventilation was controlled mechanically and maintained constant throughout surgery using an anesthetic and respiratory gas analyzer for monitoring.