 Just prior to HFO-TGI initiation, a TGI catheter internal external diameter 1.0 2.0 mm, respectively will be introduced into the endotracheal tube. TGI catheter tip will be placed 0.5-1 cm beyond the endotracheal tube tip. For TGI administration, the proximal end of the TGI catheter will be connected to a variable-orifice O2 flowmeter. The Sensormedics 3100B high-frequency ventilator will be connected to the endotracheal tube. Initial HFO settings will be as follows: 1 FiO2 100, later-on titrated toward the FiO2 of preceding CMV; 2 bias flow 30-40 Lmin-1; 3 oscillation frequency 4.0-5.0 Hz; 4 oscillatory pressure amplitude arithmetical PaCO2 value during preceding CMV 20-30 cm H2O, maximal acceptable value 95-100 cm H2O 1; 5 inspiratory to expiratory time ratio 1:2; and 6 mPaw adjusted to 9-10 cm H2O above preceding CMV mPaw corresponding to an HFO-mean tracheal pressure of 2-3 cm H2O above the mean tracheal pressure of the preceding CMV. Three-to-five mins after HFO initiation, a recruitment maneuver will be performed by pressurizing the HFO circuit at 40-45 cm H2O for 30-40 secs with oscillator piston off. HFO will then be resumed, and a 3-5 cm H2O endotracheal tube cuff leak will be placed. Immediately thereafter, continuous forward thrust TGI flow 50 of the minute ventilation of the preceding CMV 1 will be superimposed on the HFO. The mPaw control knob will then be adjusted to return mPaw to its originally set value. Fifteen min thereafter, arterial blood gas analysis will be performed and oscillatory pressure amplitude and oscillation frequency will be adjusted, in order to achieve a PaCO2 of 10-15 mm Hg above the PaCO2 of the preceding CMV and to keep pHa 7.20. At 90 min after HFO-TGI initiation, physiologic measurements i.e., blood gas analysis and hemodynamics will be conducted, and then, mPaw will be gradually reduced estimated reduction rate: 0-2 cm H2Oh-1 toward 6 cm H2O below its initially set value corresponding to an HFO-mean tracheal pressure of 2-3 cm H2O below the mean tracheal pressure of the preceding CMV. Subsequently, TGI will be discontinued and standard HFO will be continued for 30 min. 