methods This is a prospective, randomized, parallel-group, single sample study ,which was approved by the Ethics Committee of Children's Hospital affiliated to Xi'an Jiaotong University.The experiment is expected to be completed between December 2018 and March 2019 in Xi'an children's Hospital, signed consent will be obtained from the parents or the guardian of the child participating in the study.The inclusion criteria are: children aged between 0 to 108 months, Weighed between 2 to 35 kg,with an American Standards Association (ASA )physical status Ι or Ⅱ, who are scheduled for elective surgery shorter than four hours under general anesthesia . In addition,the operation of supraglottic airway devices needs to be seamless during perioperative period. Children with below condition will be excluded form the experiment:a suspected difficult airway, a history of upper respiratory tract infection two weeks before operation ,being at risk of aspiration (such as non-fasting status , gastroesophageal reflux disease and hiatus hernia),lung diseases, limited head-neck movements, and those refusing to participate.The children with symptoms bradycardia and conduction block shown on EEG are also excluded from this experiment.The children were assigned into four groups based on their weights : size 1 for 2-5kg ,size 1.5 for 5-12 kg, size 2 for 10-25 kg, size 2.5 for 25-35 kg . The required device is prepared according to the child's weight and using the the manufacturer's guidelines . The children's group allocation were operated by a researcher who do not participate in post anesthesia care unit or outcome assessment. All children involved in the experiment, the children's parents, and the data analyzer are blind to the device used. In preparation for surgery,children are fasted 8h from solid food with fat content, 6h from semi-solid food and liquid food ,4h for breast milk and 2 h for clear water .Premedication is provided about 20 minutes before entering the operating room, by which children receive intranasal dexmedetomidine 1.5ug/kg, so that children can enter the operating room quietly. On arrival in the operating room, children were monitored using pulse oximetry, electrocardiography, non-invasive blood pressure, and capnography. General anaesthesia was induced with 100% oxygen 2-4 Liter(L)/min. Injection atropine 0.01mg/kg、fentanyl 2 μg/kg 、 propofol 2mg/kg 、vecuronium bromide 0.1 mg/kg intravenously (IV) in turn.After loss of consciousness, children are promptly positioned supine with an adequate sized roll underneath the neck to establish optimal position.Achieving appropriate anesthesia depth,children are randomised to have inserted i-gel ,The jaw thrust maneuver are used to confirm anesthesia depth whether it is suitable for placing the laryngeal mask and facilitating its placement. We do not administer neuromuscular blocking during intraoperative period. Two anesthesiologists came to the hospital for working at least one year and experienced in inserting supraglottic airway devices in at least 200 pediatric patients. The device was inserted according to the manufacturer's recommendations .Device insertion was considered successful by observing bilateral chest wall movement,as well as a stable square-wave capnogram trace ,the lung breath sound with no audible leak .Three failed insertion attempts are considered failed insertion of the device, and patients were withdrawn from the study.Tracheal intubation was considered after failure of three times of laryngeal mask airway placement.

Insertion time was measured from the moment a device was picked up until confirmation of the first wave on a capnogram，The ease of device placement was assessed using a subjective grading score of 1-4 (1: no resistance, 2: mild resistance, 3: moderate resistance, 4: inability to place the device) by the anesthesiologist. We also record the manipulations for improving device position, such as adjusting the insertion depth, conducting a jaw thrust maneuver, or flexion/extension of the head.

Oropharyngeal leak pressure and fiberoptic view through the device were assessed. Oropharyngeal leak pressure was determined by closing the adjustable pressure-limiting valve to 30 centimeter water column(cmH 2 O) at the fresh gas flow rate of 3 L/min, switching the mechanical ventilation to manual ventilation, and reading the airway pressure on the monitor in the anesthesia machine which an audible leak on auscultation over neck was defined as OLP. The OLP was not allowed to exceed 30 cmH 2 O to avoid barotrauma, If mean airway pressure is higher than OLP or insufficient ventilation (lesser than 5 ml/kg) occurred during the airway maintenance, proper manipulations (pushing or pulling of device, extension or flexion of the head, jaw thrust) were performed. If appropriate manipulation does not solve the problem, the device should be removed and endotracheal intubation be performed. The anatomical position of the LMA in relation to the glottis was evaluated by placing a fiberoptic bronchoscope through the device .The fiberoptic view is classified into four grades by observing the fiberoptic views (grade 1: vocal cords not visible, grade 2: vocal cords and anterior epiglottis visible, grade 3: vocal cords and posterior epiglottis visible, grade 4: only vocal cords visible). The 8-12 French size gastric tubes are used for those groups. A well-lubricated gastric tube is prepared before inserted into the stomach through gastric port of airway device. The assessment of correct placement is through detection of injected air by epigastric auscultation or by aspiration of gastric content .A grading scale(1:easy,2:difficult,3：failure) is used for assessing the ease of gastric tube insertion.Easy is defined as placing the tube successfully in the first attempt,difficult is defined as placing the tube successfully within two attempts, failure is defined as inability to establish correct placement within three placements.Anesthesia was maintained using 2%\~3% sevoflurane and remifentanil 0.2\~0.4ug/kg•min.1 ug/kg fentanyl is added again before the skin incision and suture. At the end of surgery, the anesthetics are terminated. The patient was then moved to the postanesthesia care unit (PACU).We recorded heart rate, mean blood pressure, oxygen saturation, peak inspiratory pressure and complications during the induction and maintenance of anesthesia. At the conclusion of the surgery, when the tidal volume is \>6 ml/kg or end-tidal carbon dioxide(CO2) is less than 50 mmHg,we can stop the mechanical ventilation and keep the patient's spontaneous respiration. The LMA is removed when evidence of adequate emergence including spontaneous eye opening, grimacing using forehead and/or eyebrow, and purposeful movement. In addition,we also observed the complication such as coughing, bronchospasm, laryngospasm, desaturation (SpO 2 \< 90%), regurgitation, aspiration, sore throat, blood-on LMA, postoperative nausea and vomiting (PONV), breath-holding and teeth or lip injury during anesthesia maintenance and emergence . A blinded investigator assessed all patients in the PACU and conducted a ward visit or telephone interview 24 h after the end of surgery. In our study, the primary outcome is oropharyngeal leak pressure，the secondary outcome are the rate of successful insertion at first attempt, fiberoptic view，insertion time，ease of insertion，ease of gastric tube insertion。 We recorded complications including coughing, breath-holding, desaturation, laryngospasm or bronchospasm，blood-on LMA, sore throat, postoperative nausea and vomiting (PONV), regurgitation, aspiration, teeth or lip injury.