The EVLW for Set the Positive End Expiratory Pressure (PEEP) in the Acute Respiratory Distress Syndrome (ARDS)

* ARDS is a severe pathology with high mortality and morbidity.Actual ventilatory management is clear for the set of the tidal volume and for the survey of the plateau pressure for the patients who require mechanical ventilation. * The set of the PEEP (low or high levels) remains unclear : it seems that some patients need low levels of PEEP whereas anther need high levels; but there is no validated data that can discriminate them. * We hypothesized that patients with low levels of EVLW ( measured with the Picco® system ) need low level of PEEP to ameliorate their oxygenation ( measured with the PaO2/FiO2 ratio ) whereas patients with high levels of EVLW) need high levels of PEEP.