Inclusion Criteria:  all adult patients with a diagnosis of severe Acute Respiratory Distress Syndrome receiving veno-venous ECMO  Exclusion Criteria:  mechanical ventilation for 7 days or longer pregnancy body mass index (BMI) > 45 kg/m2 chronic respiratory failure with long-term oxygen therapy or domiciliary non-invasive ventilation cardiac failure resulting in veno-arterial ECMO history of heparin- induced thrombocytopenia cancer with a life expectancy of less than 5 years moribund condition or a Simplified Acute Physiology Score (SAPS-II) value of more than 90; current non drug- induced coma after cardiac arrest or presence of an irreversible neurologic injury decision to withhold or withdraw life--sustaining therapies presence of pneumothorax and/or pulmonary emphysema recent (1 week) thoracic surgery presence of chest burns inclusion in other research protocols refusal of consent.