Figure 1

Schematic representation of the study protocol. CMV, conventional mechanical ventilation; RM, recruitment maneuver; HFO, high-frequency oscillation; TGI, tracheal gas insufflation; mP_aw, 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) [17], the HFO mP_aw was set at 10, 12, or 15 cm H₂O (respectively) above preceding CMV mP_aw [20]. ‡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 mP_aw, which was reversed by adjusting the mP_aw 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.