Conventional Mechanical Ventilation (CMV) Strategy Patients eligible for the study will be initially on CMV ([Siemens 300C ventilator (Siemens, Berlin, Germany), with the following FiO2/PEEP combinations: 0.5/10-12 cm H2O, 0.6/14-16 cm H2O, 0.7/14-16 cm H2O, 0.8/14-16 cm H2O, 0.9/16-18 cm H2O, 1.0/20-24 cm H2O. In patients with a body mass index of > 27 kg/m2 or with an intra-abdominal pressure of > 15 cmH2O, the positivity of end-expiratory transpulmonary pressure will be confirmed with the use of an esophageal balloon catheter (whenever feasible). Regarding patients with PaO2/FiO2 of less than 200 mmHg, the use of a high PEEP is consistent with recently published data on relevant survival benefits. The tidal volume will be 5.5-7.5ml/kg predicted body weight with a maximum allowable plateau pressure of ≤32 cmH2O. The goals for tidal volume and plateau pressure will be 6.0 ml/Kg predicted body weight and 30 cmH2O, respectively, provided that the below-mentioned gas exchange goals are achievable. Respiratory rate will be adjusted so that arterial pH (pHa) will range from 7.20 to 7.45 and the inspiratory to expiratory time (I/E) ratio will be 1:2. Oxygenation targets will be as follows: SaO2=90-95% or PaO2=60-80 mmHg. The target pHa will be >7.20. In cases of pHa of <7.20, breathing circuit deadspace will be reduced by substituting the routinely used catheter mount (Mallinckrodt Dar, Mirandola, Italy) with a short angular connector. Additional measures for pHa control may include increasing the tidal volume up to 8.0 mL/Κg predicted body weight and the respiratory rate up to 35/min, and starting a continuous bicarbonate infusion or implementing extracorporeal removal of CO2.