Acute respiratory distress syndrome accounts for 23% of mechanically ventilated patients and is associated with high mortality rate. Although life-saving, mechanical ventilation may worsen lung injury through two main mechanisms: lung overdistension and atelectrauma. Indeed, the cyclic opening and closure of airways during tidal ventilation may cause lung and bronchial injuries as suggested by animal models and autopsy findings. Complete airways closure has recently been described in 40% of patients with acute respiratory distress syndrome, and setting positive end-expiratory pressure above the airway opening pressure may limit atelectrauma. However, animal and mathematical models suggest that above the airway opening pressure, more distal airways open unevenly according to their own opening pressure, resulting in an "avalanche"-like phenomenon during lung inflation. This phenomenon has never been described in humans. A better understanding of the opening of airways in acute respiratory distress syndrome may help to limit ventilation-induced lung injury and to improve outcomes.

Inclusion Criteria:

• Age ≥ 18
• Moderate-to-severe acute respiratory distress syndrome within the first 72 hours after meeting the Berlin definition criteria:
• Within 1 week of a known clinical insult or new or worsening respiratory symptoms;
• Bilateral opacities (not fully explained by effusions, lobar/lung collapse, or nodules);
• Respiratory failure not fully explained by cardiac failure or fluid overload;
• PaO2/FiO2 ≤ 200 mmHg with positive end-expiratory pressure ≥ 5 cmH2O.
• Absence of spontaneous breathing efforts
• Consent to participate to the study from the patient and/or its surrogate

Exclusion Criteria:

• Pneumothorax
• Broncho-pleural fistula
• Tracheostomy
• Hemodynamic instability
• Severe hypoxemia
• Suspected of proven intracranial hypertension
• Chronic obstructive lung disease
• Pacemaker or defibrillator
• Decision to withhold of withdraw life-sustaining measures
• Under protection

Patients with moderate-to-severe acute respiratory distress syndrome will be included within the first 72 hours after meeting the Berlin definition criteria.

After ruling out exclusion criteria and obtaining informed consent from their relatives, patients will be included in the study.