Schematic representation of the study protocol. CMV, conventional mechanical ventilation; RM, recruitment maneuver; HFO, high-frequency oscillation; TGI, tracheal gas insufflation; mPaw, mean airway pressure; f, oscillation frequency; ΔP, oscillatory pressure amplitude; minV, minute ventilation; FiO2, fractional inspired oxygen. *Includes the (1) confirmation of correct positioning and patency of tracheal tubes by chest radiography and 10-second or less fiberoptic endoscopy, respectively [19–21, 23]; (2) introduction of a TGI catheter (through a dedicated circuit adapter) and positioning of the TGI catheter tip at 0.5 to 1.0 cm beyond the tracheal tube tip, as previously described [[18–20, 22]; Additional file 1]; and (3) minor ventilatory adjustments aimed at further, concurrent optimization of PaCO2, intracranial pressure, and plateau pressure (Additional file 1). This patient preparation was carried out once, immediately after study enrolment. †Period duration was as illustrated on study day 1; on a subsequent study day, it constituted a 60-minute pre-HFO-TGI CMV period that followed the 11-hour post-HFO-TGI CMV period of the preceding study day. §Depending on tracheal tube inner diameter (9.0, 8.5, or 8.0 mm) , the HFO mPaw was set at 10, 12, or 15 cm H2O (respectively) above preceding CMV mPaw . ‡Performed by pressurizing the HFO breathing circuit at 40 to 45 cm H2O for 20 seconds with oscillator piston off. **Causing a 3- to 5-cm H2O decrease in mPaw, which was reversed by adjusting the mPaw valve; the tracheal tube cuff leak was placed immediately after the first RM. #PaCO2 of HFO-TGI was to be maintained within 30 to 50 mm Hg.