Effect of PEEP Titration on the EELV Measured by the Nitrogen Dilution Technique in ARDS

Inclusion Criteria: * Patients admitted to intensive care with a diagnosis of moderate to severe ARDS in accordance with the Berlin criteria, i.e. a PAFI \<200 and PEEP \>5cm H20 ratio * Diagnosis of ARDS made within 72 hours * Age over 18 years * Informed consent of the patient and/or trusted person where applicable Exclusion Criteria: * Start of mechanical ventilation more than 72 hours prior to inclusion. * SDRA evolving for more than 72 hours * Presence of major hemodynamic instability with mean blood pressure \<60mmhg, and/or heart rate \<45 bpm or \>150bpm with an increase in vasopressor amine dosage of more than 20% over the last 6 hours. * Intracranial hypertension with CPP\<60mmhg * Massive hemoptysis requiring immediate surgical or interventional radiology procedure * Tracheal surgery (except intensive care tracheotomy) or sternotomy within the previous 15 days * Trauma or surgery of the face in the previous 15 days. * Deep vein thrombosis treated for less than 2 days * Pacemaker implantation in the last 2 days * Unstable fracture (spine, femur or pelvis) * Respiratory reasons * use of extracorporeal oxygenation * nitric oxide * pleural drainage system with bronchopleural gap * pulmonary transplantation * Poor respiratory tolerance per procedure with desaturation Spo2\<85%. * Poor hemodynamic tolerability per procedure combining hypotension with MAP\<65mmhg and a 20% increase in norepinephrine dosage. * Lack of patient consent to proceed * minor patient * lack of consent