Gas mixing during high-frequency ventilation: an improved model

Abstract

A model for gas transport during high-frequency ventilation incorporating recently derived empirical forms for the effective diffusivity in oscillatory gas flow through a symmetrical branching network is proposed. The model accounts for the movement of gas among airways with changing cross-sectional area by using a moving-reference-frame analysis. The analysis technique incorporates the convective purging of the bias flow at the airway opening. The model predicts that although the cycle-averaged CO2 elimination rate (VCO2) depends most strongly on the product of frequency and tidal volume (VT), VT has an effect on its own, a finding consistent with published observations. This "VT effect" is due primarily to the oscillatory movement of gas from more central regions into peripheral regions where large cross-sectional areas promote efficient CO2 transport by molecular diffusion. Although the VT effect exists independent of the presence of a bias flow, placing the bias flow near the main carina can enhance the VT effect substantially. As VT is increased to values in the range of ordinary tidal breaths, VCO2 predicted by the model achieves close agreement with VCO2 deduced from conventional gas exchange theory.