The Influence of Airway Tree Geometry and Ventilation Frequency on Airflow Distribution.

The human lung is known to be asymmetric and heterogeneous which leads to an inhomogeneous distribution of air. Within the scope of this paper the influence of the upper airway tree geometry on ventilation distribution and the differences between conventional mechanical ventilation (CMV) and high frequency oscillatory ventilation (HFOV) will be analyzed. The comparison is carried out under the assumption of positive pressure ventilation. Thereby, the mechanics of lung tissue is expected to play a minor role. Oscillatory flow is therefore generated numerically at a 3D model geometry of the upper human airways. For large enough frequencies in the range of HFOV (here 7 Hz) the shape of the velocity profiles changes, but this had no measurable influence on the flow distribution. The flow division is rather governed by airway tree geometry, i.e., branch length, curvature, and tortuosity. A convective net transport of fresh air to the distal branches occurs due to the relocation of mass during ins-/expiration driven by secondary flow. However, a mixing by secondary flow plays a minor role as was suggested by the visualization of particle pathlines. The phenomenon of steady streaming is further investigated by calculating the mean flow of one breathing cycle. Streaming was found to contribute only to a minor percentage to the overall mass transport in the upper lung airways.