The 90% Effective Ventilation Pressure (EP90) for Esophageal Insufflation Avoidance During Anesthesia Induction

Inclusion Criteria: 1. Age: 18-65 years, regardless of gender; 2. ASA classification: I-III; 3. Scheduled for elective general anesthesia surgery; 4. BMI: 18.0-28.0 kg/m²; 5. Preoperative fasting: Solid food \>6 hours, liquid \>2 hours; 6. Less than two from five criteria predicting difficult mask ventilation as described by Langeron et al.(Prediction of difficult mask ventilation. Anesthesiology 2000; 92:1229-36); 7. No severe underlying conditions such as heart, lung, liver, or kidney disease; 8. Signed informed consent and ability to cooperate with the study protocol. Exclusion Criteria: 1. Pregnant or breastfeeding women; 2. History of upper gastrointestinal diseases such as gastroesophageal reflux disease, peptic ulcers, or hiatal hernia; 3. Recent (within 2 weeks) respiratory infections, chronic cough, similar symptoms, and other known or predictable respiratory system diseases; 4. Need for emergency surgery or airway obstruction after anesthesia induction requiring urgent intubation; 5. Inability to achieve adequate oxygenation during mask ventilation (e.g., SpO₂ \< 92% for 30 seconds, unresponsive to treatment); 6. History of contraindications or allergies to study medications; 7. Inability to understand the study content or refusal to cooperate; 8. Oropharyngeal or facial pathology; 9. with an indwelling gastric tube, and who had previously undergone gastric surgery.