During sedated gastroscopy, the insertion of the fiberscope and gastric distension required to perform the examination may induce respiratory depression, airway obstruction, and decreased chest wall compliance. Patients with obesity, especially visceral fat, have poor lung and chest wall compliance, lower lung capacity and functional residual capacity, and an unbalanced ventilation-to-perfusion ratio. Thus, obese patients are at a high risk of hypoxemia.

Increasing evidence supports the use of High-flow nasal cannula (HFNC) oxygenation in obese patients during sedated gastrointestinal endoscopy. Obesity, especially visceral obesity, is an established risk factor associated with all-cause mortality. Body roundness index (BRI) is a newer anthropometric measure associated with identification of high-risk individuals. Owing to the limited evidence, we designed this unblinded randomized controlled trial to assess whether HFNC, compared to standard mask oxygenation, improves oxygenation at the end of the procedure (primary endpoint) in patients with visceral obesity.

Inclusion Criteria:

• ASA class ≤Ⅲ
• Visceral obesity（body mass index≥28 and BRI ≥5.46）
• Patients who will undergo sedated gastroscopy

Exclusion Criteria:

• Life-threatening heart disease or acute myocardial infarction within 6 weeks
• Presence of pneumothorax or pulmonary bullae, pulmonary embolism, pulmonary oedema
• Upper respiratory tract infection
• Presence of tracheostomy
• Nasal or nasopharyngeal diseases
• Coagulation disorders or a tendency of nose bleeding
• Pregnancy
• Recent (within 1 week) thoracic surgery
• Emergent procedure or surgery
• Allergy to drugs used during the procedure
• Unwillingness to participate in the study