Pressure-flow characteristics of the upper and central airways during high-frequency oscillation

Pressure-flow relationships were measured in a cast of human upper and central airways at high-frequency and low stroke volume. In a dimensionless Moody plot of peak pressure and peak flow, data obtained at various frequencies and stroke volumes fell along a single curve, but diverged from the quasisteady flow behavior. Plotted against Re/ beta (ratio between Reynolds and Womersley numbers), the normalized instantaneous pressure drop was generally smooth throughout the range of Re/ beta except for the appearance of a transition at the lowest frequency, suggesting the onset of turbulent flow. The larynx was found to increase airway resistance and to reduce the asymmetry between inspiratory and expiratory pressure drop. The phase lead of pressure relative to flow was smaller in the presence of the larynx than in its absence, indicating an increase in the resistive part of impedance by the larynx.<>