Sedation for endoscopic submucosal dissection places patients at risk of desaturation, and high-flow nasal oxygen may reduce the risk. The aim of this study is to evaluate the role of high-flow nasal oxygen during endoscopic submucosal dissection. We will compare the incidence of hypoxemia (defined as SpO2 lower than 90%) of conventional nasal oxygen cannula group and that of high-flow humidified oxygen-delivery system group during the procedure.

Inclusion Criteria:

• patients who will undergo endoscopic sumucosal dissection under remimazolam-based sedation

Exclusion Criteria:

• dementia or cognitive dysfunction
• altered mental status
• intubated patients or tracheostomy
• pregnancy
• recent history of nasal bleeding
• contraindication for positive pressure ventilation
• patients under oxygen therapy
• illiteracy or foreigner

Patients will be randomly allocated to 2 groups using computer-generated randomization method. Neither participants nor physicians will be blinded to the groups because of organizational reasons.