Inspiratory limb carbon dioxide entrainment during high-frequency oscillatory ventilation: characterization in a mechanical test lung and swine model.

BACKGROUND High-frequency oscillatory ventilation (HFOV) has been utilized as a rescue oxygenation therapy in adults with ARDS over the last decade. The HFOV oscillating piston can generate negative pressure during the exhalation cycle, which has been termed active exhalation. We hypothesized that this characteristic of HFOV entrains CO(2) into the inspiratory limb of the circuit and increases the total dead space. The purpose of this study was to determine if retrograde CO(2) entrainment occurs and how it is altered by HFOV parameter settings. METHODS An HFOV was interfaced to a cuffed endotracheal tube and connected to a mechanical test lung. Negative pressure changes within the circuit's inspiratory limb were measured while HFOV settings were manipulated. Retrograde CO(2) entrainment was evaluated by insufflating CO(2) into the test lung to achieve 40 mm Hg at the carina. Inspiratory limb CO(2) entrainment was measured at incremental distances from the Y-piece. HFOV settings and cuff leak were varied to assess their effect on CO(2) entrainment. Control experiments were conducted using a conventional ventilator. Test lung results were validated on a large hypercapnic swine. RESULTS Negative pressure was detectable within the inspiratory limb of the HFOV circuit and varied inversely with mean airway pressure (P(-)(aw)) and directly with oscillatory pressure amplitude (ΔP). CO(2) was readily detectable within the inspiratory limb and was proportional to the negative pressure that was generated. Factors that decreased CO(2) entrainment in both the test lung and swine included low ΔP, high mean airway pressure, high oscillatory frequency (Hz), high bias flow, and endotracheal tube cuff leak placement. CO(2) entrainment was also reduced by utilizing a higher bias flow strategy at any targeted mean airway pressure. CONCLUSIONS Retrograde CO(2) entrainment occurs during HFOV use and can be manipulated with the ventilator settings. This phenomenon may have clinical implications on the development or persistence of hypercapnia.