Oscillatory flow and quasi-steady behavior in a model of human central airways.

We studied biased oscillatory flow in a model of human central airways to examine under what conditions oscillatory flow deviated from steady flow. Although the steady flow resistance of the model was 25% less than the oscillatory flow resistance of the model at 15Hz, the overall inertance of the model did not change over the range of frequencies from 5 to 15 Hz, suggesting that frequency-dependent inertial distortion of velocity profiles did not alter central airway pressure-flow relationships over this frequency range. In a given terminal branch of the model, instantaneous oscillatory flow at 2 and 5 Hz could be predicted well from the steady flow distribution, but with increasing frequency the oscillatory flow from the branch deviated more from the steady flow predictions. A significant component of this deviation was due to a phase shift between predicted and measured oscillatory flow. We conclude that the major frequency-dependent behavior flow in the human central airways is a phasic redistribution of flow above 7 Hz, resulting from the asymmetric distribution of inertances in this structure.