Lateral Positioning and Prone Positioning in ARDS Patients

Inclusion criteria: * Age of at least 18 years * ARDS evolving for less than 5 days with a PaO2/FiO2 ratio (P/F ratio) \< 150 after optimization of PEEP (P/V curve, R/I ratio), curarization and without prior use of the PP since orotracheal intubation. * Obtained written consent of the patient, one of his relatives or the previously designated trusted person. As soon as possible, the patient will be informed and his written consent will be obtained * Beneficiary of health insurance coverage * Possibility of participating in any other studies whose evaluation criteria do not interfere with those of the study Non-inclusion criteria: Patients with at least one of the following criteria will not be eligible: * Refusal to participate * Pregnant, parturient or breastfeeding women * Intracranial pressure \> 30 mm Hg or cerebral perfusion pressure \< 60 mmHg * Severe chronic respiratory disease with oxygen therapy or mechanical ventilation at home (except CPAP/BIPAP for obstructive sleep apnea) * Chronic interstitial lung diseases * Patients on ECMO * Weight \> 100 kg * Severe liver disease Child-Pugh score 12-15 * Pneumothorax * SAPS II score \> 75 at inclusion * Unstable spinal fracture * Contraindications to EIT (pacemaker, implantable defibrillator, skin lesions between the 4th and 5th ribs). * Withdrawal (except for ECMO) or witholding treatment decision * Any other reason which, according to the investigator, could interfere with the evaluation of the study objectives * Person under legal protection measure (guardianship, curatorship, etc.) * Person deprived of liberty by a judiciary or administrative decision Exclusion criteria: \- Patients who will be unable to complete the 25-hour intervention due to worsening requiring ECMO, death or organizational problems will be excluded from the analysis.