The Acute Respiratory Distress Syndrome (ARDS) is defined by a recent (within 1 week) respiratory failure, not fully explained by cardiac failure or fluid overload. ARDS is also characterized by bilateral opacities at the chest imaging, with an alteration of the oxygenation while positive end-expiratory pressure equal or greater than 5 cmH2O is applied. Severe ARDS is characterized by a high mortality. In the most severe ARDS patients, venovenous extracorporeal membrane oxygenation (vv-ECMO) is increasingly accepted as a mean to support vital function, although not free from complications.

In patients with severe ARDS, prone position has been used for many years to improve oxygenation. In these patients, early application of prolonged (16 hours) prone-positioning sessions significantly decreased 28-day and 90-day mortality. More recently, prone position and ECMO have been coupled as concurrent treatment. Indeed, the addition of prone positioning therapy concurrently with ECMO can aid in optimizing alveolar recruitment, and reducing ventilator-induced lung injury. Nowadays, few data exist on respiratory mechanics modifications before and after the application of prone position in patients with severe ARDS receiving vv-ECMO. The investigators have therefore designed this observational study to assess the modifications of mechanical properties of the respiratory system, ventilation and aeration distribution, and hemodynamics occurring during ECMO before and after prone position in patients with severe ARDS.

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.

All patients will receive deep sedation and continuous infusion of neuromuscular blocking agents. Protective mechanical ventilation will be set in volume-controlled mode.

A pulmonary artery catheter (PAC) will be also positioned and connected to a dedicated system for hemodynamic monitoring.

After enrolment, a silicon 16-electrode belt of proper size will be positioned around the patient's chest between the 4th and 6th intercostal spaces and connected to the EIT device. The position of EIT belt will be therefore marked on the skin with a dermographic pencil to avoid its displacement. Invasive mechanical ventilation will be applied with a ventilator connected to the EIT device through a RS232 interface.

Deindividualized data will be share on reasonable request to the Principal Investigator after study completion and publication

Data will be available after study completion and publication

On reasonable request