Acute respiratory distress syndrome (ARDS) is a diffuse inflammation of the lungs that occurs in a variety of diseases. According to the Berlin definition, ARDS is characterized by diffuse lung damage in patients with predisposing factors. Understanding the physiology of ARDS has led to improved ventilatory management, which must be protective to ensure adequate oxygenation and CO2 clearance. Prone position (PP) is a technique that can reduce mortality in patients with severe ARDS. PP results in a more homogeneous distribution of pulmonary stress and strain, helping to protect the lung against ventilator-induced lung injury (VILI). It also increases the PaO2/FiO2 (P/F) ratio, improves the pulmonary ventilation-perfusion ratio, decreases PaCO2 and promotes ventilation of the dorsal lung regions. This technique should be offered to all patients with severe ARDS for 16 consecutive hours, to improve survival and weaning success from mechanical ventilation. However, PP has adverse effects. A meta-analysis showed an increased risk of pressure sores, possibly linked to generalized acute inflammation associated with significant cytokine discharge and diffuse lesions of the vascular endothelium. PP also increased the risk of obstruction and displacement of the endotracheal tube. Final positioning in PP, (i.e., the position imposed on the patient for the duration of the PP session) varies from one ICU to another, and is rarely described in scientific articles. There are two main variants:

1. prone , with arms alongside the body
2. prone, swimmer's position

The aim of our study is to show that the "swimmer" PP reduces the occurrence of stage 3 or higher pressure sores, compared with the "arms alongside the body" PP (standard care) at Day 28 post inclusion.

Inclusion Criteria:

• Intubated patient on invasive mechanical ventilation with moderate to severe acute respiratory distress syndrome according to the BERLIN classification with a P/F ratio < 150, requiring prone position.
• Express consent of the patient or representative or in the absence of this, emergency inclusion procedure
• Health insurance coverage

Exclusion Criteria:

• Patient with 2 (or more) Prone position sessions
• Patient in whom one of the two positions could not be achieved: (Joint limitation; Neck size that would prevent head rotation; Orthopaedic spinal or segmental trauma; BMI greater than 45)
• Presence of stage 2 or higher pressure ulcers on the anterior parts of the body at screening
• Presence of extracorporeal membrane oxygenation (ECMO)
• Patient already included in the study
• Pregnant or breastfeeding woman
• Patient under legal protection