To compare in ARDS patients 2 ventilatory Strategies : One based on Lung Morphology (Focal versus non-Focal ARDS) versus ARDS networks Guidelines (PEEP based on FiO2). In Focal ARDS prone position will be promote early, with low PEEP and moderate Vt. In non-Focal ARDS, Recruitment maneuvers, high PEEP and low V twill be used.