Aerosol Tracer Study of Gas Convective Transport to 0.1-cm Airways by High-Frequency Ventilation in a Human Lung Airway Cast

A cast of human tracheobronchial airways, which is complete to airways less than 0.1 cm in diameter, was employed for experimental studies of gas convective transport by high-frequency ventilation (HFV). The cast was ventilated with gases of different kinematic viscosity (He, air, and SF6) and tidal volumes of 10-60 mL. The convective transport through the cast was followed by labeling the tidal volume with 0.5-μm aerosol particles. Such particles undergo little diffusion and have a short relaxation time and, therefore, can serve as tracers of stream flow. The time of arrival of particles transported to isolated peripheral segments of the cast during HFV was measured with an optical particle counter at various oscillatory tidal volumes and frequencies. Distally directed particle transport was found to be substantial in He and air, but weak in SF6. The extent of transport increased with increasing tidal volume. These results provide evidence for a distally directed axial flow during quasi-steady-state tidal breathing of lung airways over a wide range of frequency. This superimposed distal flow along the axial core is consistent with (1) the demonstrated efficacy of O2-CO2 exchange during HFV, and (2) concentrations of particles deposited on bifurcations of alveolar ducts as observed during normal breathing in small animal inhalation studies