The main objective of this study is to evaluate the gas exchange (ventilation-perfusion ratio) with the system Optiflow® compared to facial mask among patients having an acute respiratory failure after cardiac surgery.

Inclusion Criteria:

• Patient with indication of coronary artery bypass
• Absence of preoperative respiratory failure
• Hypoxia after extubation defined as SpO2 < 96% with Venturi mask 50% 8L/min
• Age > 18 years
• Signed informed consent

Exclusion Criteria:

• Patients requiring an imminent intubation, in coma or respiratory exhaustion, in a state of shock or severe rhythm disorders
• Hemodynamic instability, low flow rate
• Pneumothorax or hemothorax
• Bleeding major, ischemic signs
• Ventricular arrhythmia
• Respiratory failure
• Non controlled hyperalgia
• Alteration of mental status
• Cardiac surgery patients in post-operative valve surgery associated with coronary artery bypass surgery or isolated valve surgery
• Pregnancy
• Major under guardianship