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Flow dynamics using high-frequency jet ventilation in a model of bronchopleural fistula.

BACKGROUND
Positive pressure ventilation in patients with a bronchopleural fistula (BPF) is associated with variable, unpredictable gas leaks that can impair gas exchange. The optimum settings for high-frequency jet ventilation in this scenario are unclear. We investigated flow dynamics with BPFs of 2 and 10 mm, at various positions and with different jet ventilator settings in a bench-top model.


METHODS
A 2 or 10 mm length fistula was created at proximal, middle, or distal sites in standard artificial ventilator 'test' lungs and cadaveric porcine lungs. The effects of alterations in frequency, applied pressure, and on entrained, expired, and leak volumes were determined using gauge and differential pressure sensors.


RESULTS
Entrained, delivered, and leak volumes were affected markedly by ventilator settings, particularly frequency: leaks were much greater at frequencies <100 min(-1). The leak/expired volume ratio varied between 0% and 92%. Leak and entrained volumes increased progressively with more proximally situated fistulae, whereas the measured expired volume decreased. Leak volumes with a 2 mm fistula were approximately half that of a 10 mm fistula across all ventilator frequencies. All volumes increased with increased driving pressure. The optimum injection time varied depending on BPF position and the accepted compromise between leak and expired volumes. Entrained volume contributed up to 50% of the total tidal volume.


CONCLUSIONS
These data suggest that gas leak will be minimized and ventilator volumes maintained during jet ventilation using frequencies >200 min(-1) and lower driving pressures, but confirmatory clinical studies are required. Values displayed by the jet ventilator are unreliable.