Tracheal gas insufflation combined with high-frequency oscillatory ventilation

Objectives: To determine the efficacy of tracheal gas insufflation delivered by two different catheter designs on CO₂ elimination when used in conjunction with high-frequency oscillatory ventilation. Design: A nonrandomized before and after trial. Each animal served as his own control. Subjects: Ten mongrel dogs weighing 20.9 ± 1.9 kg. Four animals were assigned to a normal lung group and six animals underwent lung injury by large volume saline lavage. Intervention: Permissive hypercapnia was allowed to occur by selecting oscillator settings that would lead to alveolar hypoventilation. Proximal mean airway pressure was kept constant. Tracheal gas was insufflated at 1 cm above the carina for 30-min periods at gas flows of 5 to 15 L/min. Measurements and Main Results: Carinal pressure, hemodynamic parameters (cardiac output, mean arterial pressure, pulmonary arterial pressure, pulmonary artery occlusion pressure), and gas exchange parameters (PaCO₂, PaO₂, PaO₂/FiO₂, shunt fraction, DO₂) were measured. For the normal dogs, st catheter flow of 15 L/min, the forward thrust catheter increased carinal pressure and PaO₂/FiO₂ by 30% (p < .003) and 105% (p < .005), respectively. The forward thrust catheter reduced PaCO₂ by 40% (p < .04). The reverse thrust catheter increased PaO₂/FiO₂ by 102% (p < .001) and decreased carinal pressure and PaCO₂ by 44% (p < .001) and 34% (p < .003), respectively. For the injured dogs, at catheter flow rate of 15 L/min, the forward thrust catheter increased carinal pressure, PaO₂, and PaO₂/FiO₂ by 6% (p < .001), 23% (p < .001) and 24% (p < .02), respectively. The forward thrust catheter reduced PaCO₂ by 29% (p < .002). The reverse thrust catheter increased PaO₂ and PaO₂/FiO₂ both by 11% (p < .02) and reduced carinal pressure and PaCO₂ by 23% (p < .001) and 18% (p < .002), respectively. Conclusions: Tracheal gas insufflation is capable of improving oxygenation and ventilation in acute lung injury when combined with high-frequency oscillatory ventilation. The addition of this second gas flow at the level of the carina raises or lowers distal airway pressure, the magnitude of which is dependent on the direction and rate of gas flow. The beneficial effects of tracheal gas insufflation may be tempered by the long- term effects of altering distal airway pressure; lowering distal airway pressure may lead to atelectasis, whereas raising distal airway pressure may lead to an auto-positive end-expiratory pressure (auto-PEEP) effect.