High-flow Nasal Cannula Oxygenation Decrease Hypoxia in Gastroscopy Sedated by Propofol

Inclusion Criteria: * patients undergoing gastroscope * age over 18 years old * Signed informed consent form Exclusion Criteria: * Coagulation disorders or a tendency of nose bleeding; * An episode/exacerbation of congestive heart failure (CHF) that requires a change in medication, diet or hospitalization from any cause in the last 6 months; * Severe aortic stenosis or mitral stenosis; * Cardiac surgery involving thoracotomy (e.g., coronary artery bypass graft (CABG), valve replacement surgery) in the last 6 months; * Acute myocardial infarction in the last 6 months; * Acute arrhythmia (including any tachycardia - or bradycardia) with hemodynamic instability; * Diagnosed chronic obstructive pulmonary disease or current other acute or chronic lung disease requiring supplemental chronic or intermittent oxygen therapy); * Increased intracranial pressure; * ASA \>III; * Mouth, nose, or throat infection; * Fever, defined as core body temperature \> 37.5oC ; * Pregnancy, breastfeeding or positive pregnancy test * Emergency procedure