Effect of frequency on lung protection during high-frequency oscillation ventilation in a sheep acute respiratory distress syndrome model

Introduction

The objective was to evaluate the effect of frequency on the prevention of ventilation-induced lung injury during high-frequency oscillation ventilation (HFOV) in a sheep acute respiratory distress syndrome (ARDS) model.

Methods

Twenty-four adult sheep (38.3 ± 2.3 kg) were randomly divided into four groups (n = 6): three HFOV groups (3 Hz, 6 Hz, 9 Hz) and a conventional mechanical ventilation (CMV) group. After induction of the ARDS model (PaO₂ < 60 mmHg) by repeated NS lavage, step-by-step lung recruitment was performed in all groups, optimal alveolar recruitment as a PaO₂ > 400 mmHg. After this recruitment procedure, the optimal mean airway pressure was selected by decreasing 2 mmHg every 5 minutes until the PaO₂ decreased below 400 mmHg, and ventilation was continued for 4 hours. Hemodynamics, respiratory mechanics and gas exchange were measured throughout the experiment, and lung histopathological changes, lung wet/dry weight ratio, lung myeloperoxidase activity, lung and plasma IL-6 expression (ELISA) were determined.

Results

The heart rate, mean arterial pressure, cardiac output, central venous pressure and pulmonary arterial wedge pressure did not differ among the four groups in experiment (P > 0.05). The mean pulmonary arterial pressure was significantly higher in the HFOV group after 4 hours than in the CMV group (P < 0.05). After lung recruitment, sustained improvements in the oxygenation index were observed in all groups. Lung compliance and the intrapulmonary shunt (Qs/Qt) were significantly improved in the 6 Hz and 9 Hz HFOV groups after 4 hours of ventilation (P < 0.05). The amplitude of alveolar pressure was significantly lower in the 9 Hz HFOV group during the experiment (P < 0.05). Histologically, the lung injury score was significantly lower in the 9 Hz HFOV group than the other groups (P < 0.05). The lung wet/dry weight ratio did no differ among the four groups (P > 0.05). The lung MPO activity and expression of IL-6 in lung tissue and blood plasma significantly reduced in the 6 Hz and 9 Hz HFOV-treated animals (P < 0.05).

Conclusion

Compared with CMV and low frequency in HFOV, the higher frequency in HFOV results in less lung injury. HFOV may be an optimal lung-protective strategy.