The aim of the study is to compare the efficiency on respiratory failure regression of high-flow nasal oxygen therapy versus standard oxygen in patients admitted to the ED for de novo acute respiratory failure.

Prospective observational before-after study at the ED in two French centers.

Selection of patients with acute hypoxemic respiratory failure at the ED.

Application of oxygen strategy according to the period:

• before period: standard oxygen delivered through nasal cannula, face mask or non-rebreathing reservoir mask, with a flow rate adjusted to maintain a pulse oxymetry of at least 92%.
• after period: high-flow oxygen therapy continuously applied via large-bore nasal prongs with a gas flow rate of 50 L/min and a FiO2 adjusted to maintain a pulse oxymetry at least 92%.

Inclusion Criteria:

• de novo acute respiratory failure cowith the following criteria: a respiratory rate > 25 b/min, or signs of increased work of breathing.

Main exclusion Criteria:

• cardiogenic pulmonary edema,
• acute exacerbation of chronic lung disease, respiratory acidosis (pH < 7.35 and PaCO2 > 50 mm Hg),
• hemodynamic instability
• Glasgow Coma Scale score of 12 points or less,
• an urgent need for endotracheal intubation,