HFNO Reduces Hypoxia During Sedated Gastroscopy or Colonoscopy in High Risk Patients

Inclusion Criteria: 1. Age ranging from 18 to 80, both sexes 2. Patients receiving selective gastroscopy or colonoscopy or gastroscopy and colonoscopy procedure 3. ASA Ⅲ\~Ⅳ 4. 18 kg/m2\<BMI\<28kg/m2 5. The anticipated operation time of the procedure shall be less than 30min. 6. Patients should clearly understand and voluntarily participate in the study, with signed informed consent. Exclusion Criteria: 1. Patients with nasal congestion, epistaxis, recent nasal trauma, recent nasal surgery, increased intracranial pressure and skull fracture, etc. who can not tolerate high-flow nasal cannula. 2. Patients diagnosed POCD 3. Patients clearly identified difficult airway 4. Patients with mechanical ventilation 5. Patients with acute respiratory infection and asthma 6. Patients with acute upper gastrointestinal hemorrhage accompanied by shock, severe anemia, gastrointestinal obstruction accompanied by retention of gastric contents 7. Patients with aortic stenosis and carotid stenosis 8. Patients allergic to sedatives such as propofol 9. Patients without civil capacity such as cognitive dysfunction