The use of positive end-expiratory pressure (PEEP) has been shown to prevent the cycling end-expiratory collapse during mechanical ventilation and to maintain alveolar recruitment, keeping lung portions open, increasing the resting end-expiratory volume. On the other hand PEEP may also overdistend the already open lung, increasing stress and strain. Theoretically high frequency oscillatory ventilation (HFOV) could be considered an ideal strategy in patients with ARDS for the small tidal volumes, but the expected benefits have not been shown yet. PEEP and HFOV should be tailored on individual physiology. Assuming that the esophageal pressure is a good estimation of pleural pressure, transpulmonary pressure can be estimated by the difference between airway pressure and esophageal pressure (PL= Paw - Pes). A PL of 0 cmH2O at end-expiration should keep the airways open (even if distal zones are not certainly recruited) and a PL of 15 cmH2O should produce an overall increase of lung recruitment. The investigators want to determine whether the prevention of atelectrauma by setting PEEP and mPaw to obtain 0 cmH2O of transpulmonary pressure at end expiratory volume is less injurious than lung recruitment limiting tidal overdistension by setting PEEP and mPaw at a threshold of 15 cmH2O of transpulmonary pressure. The comparison between conventional ventilation with tidal volume of 6 ml/Kg and HFOV enables us to understand the role of different tidal volumes on preventing atelectrauma and inducing lung recruitment. The use of non-invasive bedside techniques such as lung ultrasound, electrical impedance tomography, and transthoracic echocardiography are becoming necessary in ICU and may allow us to distinguish between lung recruitment and tidal overdistension at different PEEP/mPaw settings, in order to limit pulmonary and hemodynamic complications during CMV and HFOV.