Introduction: Acute respiratory failure is a life-threatening condition frequently found in the emergency department. High-flow nasal oxygen (HFNO) is increasingly used in emergency departments for patients with hypoxaemic acute respiratory failure. However, despite the increasing number of studies, its potential advantages regarding the need for therapeutic escalation and mortality have not been precisely evaluated. Our objective is to compare conventional oxygen therapy to HFNO when they are initiated during the first hour following the patient's arrival at the emergency department, with the hypothesis that HFNO would reduce the need for ventilatory therapy escalation.

Methods and analysis: This is a multicentric, prospective, open and randomised superiority study. 500 inpatients will be randomised (1:1) to receive conventional oxygen therapy or HNFO. The primary outcome is a failure in the oxygen therapy defined as the need for a therapeutic escalation within 4 hours after therapy initiation.

Ethics and dissemination: The study has been submitted and approved by the Comité de Protection des Personnes Nord Ouest IV (20 October 2020). As required, a notification was sent to the Agence nationale de sécurité du médicament et des produits de santé (22 October 2020). The research results will be published in peer-reviewed publications and presented at international conferences.

Trial registration number: NCT04607967.