High flow nasal cannula oxygenation (HFNC) offers high flow and concentration oxygen delivery, providing excellent non-respiratory oxygenation. As a relatively new oxygen delivery method, it has gained widespread use. We have demonstrated that high flow nasal cannula oxygenation reduce the incidence of hypoxia during sedated gastrointestinal endoscopy in patients with American Anesthesiologist Rating (ASA rating) grades 1 to 2 and obesity. We hypothesized that HFNC could mitigate the risk of hypoxia in critical patients during sedated gastrointestinal endoscopy. To confirm this, we selected critical patients with ASA grades 3 to 4 who were scheduled for gastrointestinal endoscopy. We observed and compared the incidence of hypoxia (75%≤SpO2 < 90% and < 60S), severe hypoxia (SpO2<75% for any duration or 75%≤SpO2 < 90%, ≥60s), subclinical respiratory depression (90%≤SpO2 < 95%), respiratory-related adverse events, sedation-related adverse events, and complications associated with high flow nasal cannula oxygenation using HFNC or regular nasal cannula during the sedated gastrointestinal endoscopy.

Inclusion Criteria:

1. 18≤ age ≤80, gender is not limited;
2. Patients undergoing elective gastroenteroscopy or treatment;
3. ASA grade Ⅲ~Ⅳ;
4. 18 kg/m2≤BMI≤28kg/m2;
5. The operation time of gastroenteroscopy is expected to be no more than 60min;
6. Clearly understand, voluntarily participate in the study, and have informed consent from myself or my family members.

Exclusion Criteria:

1. Patients with nasal congestion, nasal bleeding, recent nasal trauma, recent nasal surgery, increased intracranial pressure and skull fracture, etc., who can not perform high-flow nasal catheter oxygen;
2. acute respiratory infections and asthma attacks;
3. Patients diagnosed with COPD;
4. Clear difficult airway;
5. Acute upper gastrointestinal bleeding with shock;
6. Gastrointestinal obstruction with gastric content retention;
7. Allergic to sedatives such as propofol;
8. Persons with no capacity for civil conduct such as cognitive dysfunction.