Optimization of Arterial Oxygen Transport by Positive End-expiratory Pressure Variation in Acute Respiratory Distress Syndrome

Inclusion Criteria: * patient presenting a Acute Respiratory Distress Syndrone (ARDS) definite by : acute respiratory failure, bilateral alveolo-interstitial infiltrate on chest radiography, absence of increased left ventricular filling pressure, partial pressure of oxygen in arterial blood (PaO2)/Fraction of inspired oxygen(FiO2) \<200 and Positive End-Expiratory Pressure (PEEP) \> or equal to 5 cmH2O * Need to invasive mechanical ventilation * Precedent criteria persisting at least 6 hours * Haemoglobin \> 8g/dl * Ramsay score at 6 Exclusion Criteria: * Participation to en other study on the Acute Respiratory Distress Syndrone (ARDS) with the same end-point * Presence of external circulatory assist * Left Ventricular Ejection fraction (LVEF)or Left ventricular shortening fraction (LVSF)\< 40% after correction of hypovolemia and/or vasoplegia * Infusion of inotrope * Presence of Acute Pulmonary Heart * Following procedure : ExtraCorporeal Membrane Oxygenation(ECMO), prone position, inhalation of Nitrogen dioxyde * Presence of a chest tube * Pregnant or nursing mother * Less than 18 years old * Complete arrhythmia by atrial fibrillation * Person under justice protection