Journal of Applied Physiology, Vol 57, Issue 2 493-506, Copyright © 1984 by American Physiological Society
Gas mixing during high-frequency ventilation: an improved model
M. C. Khoo, A. S. Slutsky, J. M. Drazen, J. Solway, N. Gavriely and R. D. Kamm
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.
