Control and evaluation of high-frequency jet ventilation: mechanical lung model.

Ventilation systems that operate at high-frequency and deliver small volumes have the potential to provide adequate alveolar ventilation without excessive pulmonary pressures. One way of producing high-frequency ventilation is by use of jet bursts of an input gas through a cannula controlled by a solenoid valve. This high-frequency jet ventilation has yet to be quantitatively analysed for optimal clinical use. From an analysis of the jet-producing device, we obtained a quantitative relationship which allowed us to predict the gas volume of a jet burst (Vjet) from the driving pressure (Pd), and the jet duration (tI). The device was applied to a mechanical lung model (a tube attached to an elastic bag corresponding to the lung airway and alveolar space). We examined how the control variables of the jet ventilation system changed the bag (alveolar) volume with respect to Vjet, the volume of entrained gas, and the volume of shunted gas. Using a nitrogen washout analysis, we evaluated the operating lung volume, effective dead-space volume (Veds), and effective ventilation rate (Veff). We found that Veds is independent of the individual effects of jet cycle frequency, duty cycle, cannula diameter, and entrainment fraction. While Veds was not affected significantly by the shape of the airway, it did depend on the distance of the jet cannula tip to the ventilated bag (or alveolar region) and on the tidal volume.