 The investigators have recently demonstrated the beneficial effects of combined high-frequency oscillation HFO and tracheal gas insufflation TGI on the respiratory physiology 1,2 of patients with severe acute respiratory distress syndrome ARDS NCT00416260. Preliminary short-term survival results were also encouraging. Consequently, in the present clinical trial, the investigators intend to increase the size of the studied population, in order to evaluate the effect of HFO-TGI on survival with adequate statistical power. Furthermore, the investigators intend to elucidate the mechanism of the HFO-TGI-related physiological benefit. 