Children scheduled for tonsillectomy surgery were recruited into the study. General anesthesia was induced with 8% sevoflurane inhaled with 8 l.min-1 of oxygen via mask, followed by adjustment of inspired sevoflurane to the target concentration based on the result in previous patient at which laryngoscopy and tracheal intubation were attempted and maintained for 12 min. All responses to tracheal intubation were assessed. At the end of the procedure, sevoflurane was titrated to the target concentration, which was kept constant for 12 min before a standard stimulus was applied to determine whether the children was awake. The Dixon's 'up and down'method was used to determine progression of subsequent concentrations.The aim of this study was to determine whether the minimum alveolar concentration of endotracheal intubation（MACEI）of sevoflurane in children with obstructive airway are different from that observed in normal children.

38 patients American Society of Anesthesiologists (ASA) class I or II aged 2-8 years undergoing tonsillectomy surgery were randomized to either the patient's head in the neutral position or 30 degree rotated lateral position. All patients were preoxygenated using 100% oxygen with a normal tidal volumen for 3 min. The circuit was primed with sevoflurane 8% at a fresh gas flow of 8 Litre/min （L/min） for 1 min. Anaesthesia was induced with inhaled sevoflurane up to 5% in oxygen via facemask with fresh gas flow at 6 L/min.After loss of consciousness, the inspired concentration of sevoflurane was maintained 10-12min a predetermined end-tidal concentration. A single experienced anaesthetist in the use of endotracheal devices inserted the devices according to the manufacturer's recommendations. Neuromuscular blocking agents and other intravenous anesthetics were not given. One nurse, who was blinded to the anaesthetic concentration, classified responses by the patient to tracheal intubation as either "movement" or "not movement". Assesment was made for jaw relaxation and graded with intubation score. The sum of intubation scores >4 was defined as a failure of insertion. The end-tidal (ET) sevoflurane concentration used for each patient was determined using the Dixon's up-and-down method. The ratio of the end-tidal to predetermined end-tidal concentrations was maintained at 0.95-1.0 for at least 10 minutes to establish equilibration before device insertion was attempted. The first patient received a 5.0% sevoflurane concentration and the step size of increase/decrease was 0.5%. If the endotracheal device insertion was successful, sevoflurane concentration for the next patient was decrease by 0.5%. If not, sevoflurane concentration was increased by the same amount for the next patient. For their comfort, patients experiencing movement received a 2 mg/kg bolus dose of propofol,fentanyl 2.0ug/kg and rocuronium 0.6mg/kg.If the endotracheal device insertion was successful,fentanyl 2.0ug/kg and rocuronium 0.6mg/kg was also administered.

Inclusion Criteria:

• class I or II
• children aged 2-8 years
• children with obstruction airway under general anesthesia undergoing tonsillectomy surgery
• signed informed consent

Exclusion Criteria:

• Patients with a history of adverse reactions to sevoflurane
• Patients with a potentially difficult airway (Mallampati III or IV, a limited mouth opening and/or cervical spine disease)
• Patients with reactive airway disease
• Signs of upper respiratory infection
• Violate experimental scheme
• refused to participate