Effect of Lung Airway Branching Pattern and Gas Composition on Particle Deposition in Bronchial Airways: III. Experimental Studies with Radioactively Tagged Aerosol in Human and Canine Lungs

Abstract

Particle penetration into lung airways during normal respiration is affected by the exchange of inspired air and residual gas. In this study, particle penetration during the inspiratory phase was investigated using γ-tagged monodisperse particles (0.70, 0.90, 0.96, and 1.44 μm) suspended in various carrier gases having a wide range of kinematic viscosity. A 40-mL bolus of tagged aerosol was drawn into excised human and dog lungs at the end of a tidal breath, followed by a long breathhold to allow for complete particle deposition by sedimentation. The lungs were then fixed, sectioned, and autoradiographed to determine tidal front locations. In human lungs, particles suspended in He-O₂ penetrated deeper than particles suspended in air; particles penetrated least in SF₆-O₂. Dog lungs, which have more asymmetrical airway branching patterns than human lungs, had no significant particle penetration differences associated with carrier gas composition. It is concluded that particle penetration during the inspiratory phase is dependent on factors that determine flow profile development, such as branching pattern and the Reynolds number of the carrier gas. The bolus front at the end of an air inspiration extended into about 10% of human lung airways of 1 mm diameter, and into about 0.1% of 0.5-mm airways. It is concluded that rapid particle penetration to 1-mm airways during high-frequency oscillatory ventilation of lung casts is due to cumulative axial core transport during multiple breathing cycles. Similarly, the dispersion of an aerosol bolus from large airways to small airways during in vivo breathholding studies appears to be due to oscillatory flow created by the heartbeat.