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.