Impact on Mortality of a Strategy Including Continuous Positive Airway Pressure Plus High Flow Nasal Cannula Oxygen Therapy Versus High Flow Nasal Cannula Oxygen Therapy Alone in Patients With de Novo Acute Hypoxemic Respiratory Failure: a Prospective, Randomized Controlled Trial.

Inclusion Criteria: 1. Adults (≥ 18 years old) 2. Admitted in the ICU for ≤ 24 hours 3. De novo hARF defined by the three following criteria: 1. Respiratory rate \> 25 breaths/min or signs of respiratory distress such as labored breathing or paradoxical inspiration 2. PaO2/FiO2 ≤ 200 mmHg under HFNC with flow ≥ 45 L/min, actual FiO2 being considered. 3. Uni or bilateral pulmonary infiltrate on chest X-ray or CT scan 4. Informed consent from the patient or her/his next of kin or another substitute decision maker, or inclusion procedure in emergency if the patient is unable to consent Exclusion Criteria: 1. Refusal of study participation by the patient or the proxy 2. Anatomical factors precluding the use of a nasal cannula or CPAP 3. Long term oxygen 4. Home CPAP or NIV or CPAP or NIV initiated prior to ICU admission 5. Hypercapnia indicating NIV (PaCO2 \> 45 mmHg) 6. Isolated cardiogenic pulmonary oedema indicating NIV 7. Known pregnancy or breastfeeding 8. Absence of coverage by the French statutory health care insurance system (including AME) 9. Abdominal, thoracic or cardiac surgery within the last 6 days 10. Use of vasopressors (norepinephrine\>0.3 mcg/kg/min) 11. Glasgow coma scale \< 13 12. Urgent need for endotracheal intubation 13. Patients at an end-of-life stage receiving compational oxygenation 14. Exacerbation of a chronic respiratory disease 15. Legal protection (curatorship or tutorship) 16. Undrained pneumothorax