Acute lung injury and ARDS (acute respiratory distress syndrome) are characterized by lung inhomogeneity, leading to a different distribution of the tidal volume (and pressure) within the lung. The quasi-static PV curve is a useful bedside tool to set mechanical ventilation, but it reflects a global behaviour of the lung. The electrical impedance tomography (EIT) is a non-invasive and radiation-free tool, monitoring dynamic changes in gas distribution. Images from EIT can be divided in several regions of interest, allowing to measure regional changes in compliance. The regional derived-EIT PV curve could provide valuable information on airway closure and AOP (airway opening pressure). Recent studies suggest that AOP measured by the ventilator seems to correspond to the AOP of the lowest injured lung. The investigators will perform one pressure-volume (PV) curve with a low-flow insufflation of 5 L/min starting from 0 cmH2O to a maximal airway pressure corresponding to the plateau pressure. During the low-flow insufflation, both ventilator and EIT-derived PV curves will be recorded. All PV curves will be analysed offline by the investigator to detect complete and regional airway closures, and measure AOPs.

Inclusion Criteria:

• Adult patients (≥18 years old).
• Patients with PaO2/FiO2 ratio <300 mmHg.
• Volume- or pressure-controlled ventilation.
• Sedated, with or without infusion of neuromuscular blockage.
• Patients in supine position

Exclusion Criteria:

• Pneumothorax and bronchopleural fistula.
• Severe hemodynamic instability (>30 % increase in vasopressors in the last 6 hours or norepinephrine > 0.5 µg/kg/min).
• PaO2/FiO2 ratio < 80 mmHg.
• Severe or very severe chronic obstructive pulmonary disease (COPD) according to the GOLD criteria (stage III: FEV1 30-50% predicted; stage IV: FEV1 < 30 % predicted).
• Known or highly suspected elevated intracranial pressure (>18 mmHg).
• Impossibility to correctly position the EIT belt (e.g., burns chest drainage, etc.).
• Contraindications to EIT (e.g., implantable cardiac defibrillator, pacemaker, instable spinal lesions, etc.).
• Clinical judgement of the attending physician.
• Pregnant or breastfeeding woman
• Patient under guardianship, curators or safeguard of justice