The goal of this prospective observational study is to estimate the prevalence of brachial plexus injury after prone positioning in patients with ARDS and to evaluate the safety of swimmer position. The main questions it aims to answer are:

• Could arm positioning during pronation play a role in the development of any nerve injury at the brachial plexus level?
• Is swimmer position safe when adopted during prone positioning?

Participants will be studied at selective time points using EMG assessment.

Prone positioning will be performed according to the current guidelines. Particularly, face and limbs will be placed according to the "swimmer position", ensuring the face position in the direction of ventilator and limbs (one arm above head and opposite arm at side) positioned as to prevent abnormal extension or flexion against the shoulders and elbows. Patients will lie on low air loss pressure mattresses, thus avoiding the utilization of any thoraco-pelvic supports, which are not recommended. Electromyography (EMG) will be performed to evaluate the occurrence of brachial plexus injury (BPI).

Inclusion Criteria:

• Age ≥ 18 years old
• Admission to ICU for severe ARDS
• Administration of sedation and neuromuscular blocking agents
• Presence of endotracheal intubation and mechanical ventilation
• Use of prone positioning to treat hypoxemia

Exclusion Criteria:

• Extracorporeal membrane oxygenation
• Prone positioning performed in other centers
• Prone positioning contraindications
• Neurodegenerative disorders
• Previous known brachial plexus injury

Patients admitted in the ICU with severe ARDS, also secondary to pulmonary involvement of COVID-19, receiving invasive mechanical ventilation and requiring prone positioning to treat hypoxemia, as decided by the medical team.