For ECMO supported patients with severe ARDS (acute respiratory distress syndrome), usual care include use of "ultraprotective" mechanical ventilation with tidal volume and pressure reductions that might ultimately enhance lung protection of patients with ARDS. Although very low tidal might also cause pulmonary derecruitment. The aim of this study is to monitor effects of very low tidal volume on regression of overdistension and derecruitment using electrical impedance tomography.

Secondary aim is to describe the evolution of the optimal PEEP (Positive End Expiratory Pressure) during the decrease of the tidal volume

Patients requiring venovenous ECMO support for severe ARDS will be monitored with electrical impedance tomography during introduction of "ultraprotective" mechanical ventilation.

Monitoring will start with pre-ECMO ventilator setting, then the tidal volume will be lowered step by step. Other parameters of the ventilator will be kept constant. Then, cumulated collapse and overdistention percentages will be estimated for each tidal volume. Optimal tidal volume will be defined by the lowest sum of alveolar overdistension and collapse.

Also, at the end of each step, PEEP titration will be performed to identify the optimal PEEP setting that combines the best alveolar recruitment with minimal overdistention. The aim is to determine whether the derecruitment induced by the decrease in tidal volume can be reduced by the application of PEP. Then, comparison of the different tidal volumes with application of ideal PEEP will be performed: measurement of the percentage of collapsed and overdistended areas by impedancemetry method between the different levels of tidal volume with application of ideal PEEP.

Inclusion Criteria:

• aged ≥ 18 years
• patient with ARDS on venovenous (VV)-ECMO
• Written informed consent
• patient affiliated to a social security scheme

Exclusion Criteria:

• Pregnancy
• Adult patient subject to a legal protection measure (tutor, curator, etc.)
• Patients with a pacemaker, automatic implantable cardioverter defibrillator,
• contraindications to thoracic belt placement (e.g., thoracic or spinal cord trauma, recent thoracic surgery)
• undrained pneumothorax, bronchopleural fistula
• hemodynamic instability (i.e., use of intravenous fluids of more than 10 mL/kg or vasopressors 2 mg/h of norepinephrine or 0.5 mg/h of epinephrine)