Objectives HFOV is an optimal lung protective ventilation strategy aimed at an open lung. Spontaneous breathing during conventional mechanical ventilation improves lung aeration of dependent lung regions. However, in larger patients spontaneous breathing during HFOV ventilation is not well tolerated. The aim of this animal study was to evaluate lung aeration and spatial distribution of ventilation during HFOV with spontaneous breathing maintained and muscular paralysis respectively. HFOV was applied using continuous fresh gas flow (CF) and a custom-made demand flow system (DF) in order to facilitate spontaneous breathing.

Methods At the university animal laboratory 8 pigs (50 kg) were used. After spontaneous breathing was restored HFOV was applied in runs of 30 minutes with CF or DF. In the end animals were paralyzed. End-expiratory lung volume (EELV) and regional distribution of ventilation were quantified using electrical impedance tomography (EIT).

Results Comparison of EELV, by evaluation of change in relative deltaZ, showed that lung volume was best preserved when spontaneous breathing was maintained during HFOV compared to HFOV with muscular paralysis. This volume was preserved in dependent lung regions. A significant shift in ventilation toward the dependent lung regions was observed when HFOV with demand flow was applied compared to HFOV and spontaneous breathing suppressed.

Conclusions This study demonstrates that spontaneous breathing during HFOV preserves lung volume and improves ventilation of the dependent lung areas. These results underline the importance of maintaining spontaneous breathing during HFOV. These results support efforts to optimize HFO ventilators to allow patients spontaneous breathing.