Background : Patient with hypoxemic respiratory failure treated with NIV receive between NIV session oxygen therapy. Gaz exchange disorder with a decrease of the ratio between Partial Pressure of Arterial Oxygen (PaO2) and Inspired Fraction of Oxygen (FIO2) are noticed when NIV is stopped at the end of the session du to alveolar derecruitment Optiflow is a high flow oxygen delivery system used a heated humidifier and heated breathing circuit. In observational studies, Optiflow increase oxygenation of patients with hypoxemic respiratory failure. Oxygenation is better than under high-concentration mask and work of breathing is reduced.

The aim of the study is to compare in acute hypoxemic respiratory failure, optiflow to oxygen therapy under high-concentration mask, on patients oxygenation between NIV sessions (measured as the difference between PaO2/FiO2 ratio at the beginning and at the end of the session), during the first two NIV sessions

Study design : Prospective, randomized, controlled, multicentric, open clinical trial with two groups:

• control group with conventional clinical management, oxygen therapy and sequential used of NIV
• Optiflow group with high flow oxygen delivery system, conventional clinical management and sequential used of NIV for a period of 36 hours at least.

Number of subjects: 100 (50 patients per group) patients admitted in intensive Care Unit for hypoxemic respiratory failure.

Inclusion Criteria:

• Patient at least 18 years old
• Patients affiliated to or benefiting of the French social welfare system
• Patients with ARDS on previously healthy lung. Acute respiratory insufficiency, defined as the PaO2/FIO2 ratio less than 200 mm Hg associated with the presence of lung infiltrates on chest radiograph
• Free and informed consent of patient
• Hospitalized in one of involved ICU

Exclusion Criteria:

• Existence of criteria for intubation.
• Refusal to participate in the study.
• Pregnancy
• Patient under guardianship or trusteeship
• Patients for whom a decision of non intubation has been decided.
• Patients who have received one or several NIV session for ARDS before hospital admission
• Home noninvasive ventilation
• Tracheostomy
• Hemodynamic instability
• Hypercapnia (PaCO2>45 mm Hg).
• left ventricular failure (cardiogenic pulmonary acute edema).
• Ineffective coughing.
• Recent gastric or oesophageal surgery
• Severe ventricular rhythm disorder
• High digestive haemorrhage
• Lack of collaboration
• Pernicious vomiting
• Upper airway obstruction
• Severe sepsis.
• Undrained pneumothorax.