The aim of the study is to assess, in patients with acute hypoxemic respiratory failure, the short term physiologic effects of the high flow oxygen nasal therapy (Optiflow), in term of inspiratory muscle effort, gas exchange, comfort and dyspnea

The administration of a known concentration of oxygen is an important part of routine care of the patient admitted in intensive care unit for acute hypoxemic respiratory failure. A new high flow oxygen delivery system (Fisher and Paykel Health care) has been developed (Optiflow). The system used a heated humidifier and heated breathing circuit via a nasal interface. High flow nasal therapy (Optiflow) is associated with the generation of significant positive airway pressure in healthy volunteers. Positive expiratory pressure may have a number of benefits in respiratory failure which include improved ventilation/perfusion matching with improved oxygenation, reduced airways resistance and reduced work of breathing. Moreover high flow nasal therapy may improve oxygen administration by decreasing oxygen dilution, decreasing death space and using high levels of humidification

Inclusion Criteria:

• Acute respiratory insufficiency, defined as the PaO2/FIO2 ratio of 300 mm Hg or less after breathing oxygen
• The presence of lung infiltrates on a posteroanterior chest radiograph

Exclusion Criteria:

• Face or cranial trauma or surgery
• Patients younger than 18 years
• History of COPD
• Acute respiratory acidosis (defined as a pH <7.30 and a PaCO2 >50 mm Hg)
• Hemodynamic instability with arterial pressure < 90mmHg
• Respiratory instability with PaO2/FiO2<100mmHg
• ventricular arrhythmias
• Excess respiratory secretions.
• Upper gastrointestinal bleeding
• Recent gastric or oesophageal surgery
• Tracheostomy or other airways disorders
• Pneumothorax
• Contraindication of gastric probe insertion
• Impossibility to insert the oesophageal probe