Arterial oxygen pressure (PaO2 in mmHg) will be measured using a blood gas analyzer (ABL-510 analyzer, Radiometer, Copenhagen, Denmark) and will be related to the inspiratory O2 fraction to express the oxygenation index (PaO2/FIO2). In addition, chest radiograph scores (number of quadrants with >50% involvement with an alveolar filling process) will be recorded on POD0 (arrival in HDU) and POD1 (end of the study).