Effect of PEEP on Lung Recruitment and Homogeneity Over Time in Moderate to Severe ARDS

Inclusion Criteria: * Age ≥ 18 years and \< 80 years * Weight \> 35 Kg and BMI \< 40 * Informed consent according to local regulations * Hemoglobin ≥ 70 g/dl * Haemodynamically stable \> 4 hours * Moderate to severe ARDS (PaO2/FIO2 \< 26.6 kPa with positive end-expiratory pressure (PEEP) \> 5 cmH2O) as per Berlin definition of ARDS Exclusion Criteria: * Expected survival \< 72 hours * Suspected pregnancy (negative pregnancy test required for women of child-bearing potential) * Open abdomen * Documented or suspected raised intracranial pressure * Active air leak (pneumothorax, pneumomediastinum, subcutaneous emphysema) * Morbid obesity BMI \> 40 * Recent \< 1 week cardiac or thoracic surgery * Unstable thorax and sternum with paradoxical chest wall movement * Severe Chronic Respiratory Disease (COPD) - GOLD 3 or 4 emphysema with bullae * Severe smoking (\> 40 pack-year history) * Liver Failure: Child-Pugh Class C * Massive ascites * Lung fibrosis * Severe cardiac disease (one of the following): New York Heart Association Class III or IV, acute coronary syndrome or persistent ventricular tachyarrhythmias * Sickle cell disease