Acute respiratory distress syndrome is an acute form of lung injury. The most commonly used classification criteria for this syndrome are Berlin's Criteria. The actual literature underlines the advantages of prone position in mild or severe forms of ARDS in association with invasive mechanical ventilation. The hypothesis of this study is to investigate the effective ventilation and perfusion modifications during pronation assessed with clinical parameters and with the aid of the electrical impedance tomography.

Inclusion Criteria:

• mild or severe ARDS according to Berlin's Criteria
• recruitment maximum 72 hours from the diagnosis of ARDS

Exclusion Criteria:

• other non-ARDS respiratory system pathologies
• hemodynamic instability (mean arterial pressure < 60 mmHg)
• pneumothorax
• endocranial hypertension
• skin lesions or burns
• presence of pacemaker or implantable defibrillator
• pregnancy

Patients' anthropometric data were collected. Ventilation parameters, electric impedence tomography parameters and blood gas analysis values were collected four times: patient supine (T0), one hour after pronation (T1), sixteen hours after pronation (T2), one hour after supination (T3).