Moderate/severe ARDS will be ventilate with protective ventilation (low tidal volume and PEEP set to limit plateau pressure under 30cmH2O). Markers of inflammation will be measured in the broncho Alveolar Lavage the first day and in the blood the following days. Strain will be measured at the bedside using the nitrogen washout-washin. Markers of inflammation and strain will be analysed with ventilatory and oxygenation.