The study will investigate the influence that extravascular lung water index (EVLWi) could have on the efficacy and persistance of efficacy of prone position in patients with acute respiratory distress syndrome.

Prone position will increase blood oxygenation in 75% of the cases and will be persistant in half of the cases. Unfortunately, no clinical criteria has been found correlated with efficacy. The quantity of lung edema, with increased lung weight, could be a determinant factor of efficacy and the persistance of the efficacy. EVLWi, assessed with the PiCCO2 device, reflects the quantity of fluid accumulated in interstitial and alveolar spaces.

The hypothesis is that patients with higher EVLWi will have less efficacy of prone position in oxygenation and also that the beneficial effects of prone position will last shorter compared to patients with lower EVLWi.

Inclusion Criteria:

• Age ≥18 years
• Presence of acute respiratory distress syndrome
• Decision to perform prone position according to the physician in charge of the patient
• Hemodynamic monitoring with a PiCCO2 device (Pulsion Medical Systems, Feldkirchen, Allemagne)

Exclusion Criteria:

• Absence of affiliation to the French Sociale security
• Patient under a tutelage measure or placed under judicial protection
• Known pregnancy
• Currently receiving ECMO therapy

Patients with moderate-to-severe ARDS in whom prone positioning is deemed necessary