Controlled ventilation will be set on volume mode with Vt 6-8 ml/kg PBW, RR to control pH, unchanged PEEP and FiO2. Controlled ventilation will be thereafter adjusted according to clinical evolution. Patients switched to controlled ventilation will be reassessed at least every 8 hours and they will be switched back to PSV or PSV+Sigh (to maintain study group assignment) targeting the abovementioned settings and adjustments as soon as all the following conditions will be met: