Rescue therapy. In case of desaturation (SpO2 ≤90%) of a patient it will be crucial to exclude hemodynamic impairment as a possible cause. Also, airway obstruction and ventilator malfunction must be ruled out as possible causes. Provided those factors are excluded, a rescue step-up strategy is allowed as follows: institution of protective controlled mechanical ventilation (see above for settings) and performance of recruitment maneuvers at 40-50 cmH2O, PEEP ≥15 cmH2O, prone positioning, inhaled nitric oxide, extracorporeal membrane oxygenation. Patients undergoing rescue treatments will be reassessed at least every 8 hours and switched back to PSV or PSV+Sigh (to maintain study group assignment) with the abovementioned settings and adjustments as soon as all the above mentioned conditions will be met.