Mechanical ventilation in ARDS requires protective ventilation with low VT and PEEP. PEEP titration can improve lung recruitment in the dependent lung but with a risk of overdistension in the non-dependant lung. EIT can measure the distribution of tidal ventilation and assess overdistension and collapse during a PEEP titration in any position (prone or not). This study aims to measure the best PEEP as a compromise between recruitment and overdistension during a PEEP trial with EIT before and just after PP.