 Return to CMV will be considered at the following HFO settings: mPaw 3-4 cm H2O above preceding CMV mPaw corresponding to an HFO-mean tracheal pressure of 2-3 cm H2O below the mean tracheal pressure of the preceding CMV; and TGI 0 Lmin-1. Return to HFO-TGI will be considered necessary if after 12 h of CMV, PaO2FiO2 is 150 mm Hg at a PEEP of 8 cm H2O. In the HFO-TGI-group, patients will receive repeated daily sessions of HFO-TGI until they no longer satisfy the severe ARDS criterion during CMV for at least 48 h. Predicted minimum duration of HFO-TGI sessions is 6 h. During HFO-TGI sessions, physiologic measurements will be performed every 1.5-4 h, and their average values will be used in the subsequent statistical analysis. For HFO-TGI-group members, total HFO-TGI duration will have to be 12 h. 