Particle deposition in the pulmonary region of the human lung: A semi-empirical model of single breath transport and deposition

Abstract

Particle deposition has been observed in the distal pulmonary regions of human airways, where their clearance is lower and thus their potential health effects greater. Whereas gases are transported convectively to proximal pulmonary regions and then diffuse to the distal ones, particles have a very low diffusivity so only few can be transported to the two or three most distal generations by convection in a single tidal breath. Yet models of particle deposition in human airways invariably employ a single breath to characterize regional pulmonary deposition. To study particle transport and deposition during multiple breathing cycles, it is essential to develop a single breath model that describes aerosol bolus dispersion and deposition for the entire lung. Here we present a simple semi-empirical model for whole lung aerosol bolus dispersion, and use bolus dispersion data from the literature to identify model parameters. The model assumes that the particle transport on inhalation and exhalation differs due to secondary flow asymmetries at the bifurcations. By adjusting model parameters to measurements, the effective particle transport profiles averaged over the entire lung and mixing intensity at different lung depths are obtained. The average inspiratory particle transport profile is found to be slightly sharper than parabolic, whereas on exhalation it is relatively blunt. The mixing intensity due to secondary flows is large in the conducting airways ( ∼ 95 ml of lung depth) but drops sharply due to the rapid decrease in Dean number in the deeper lung depths. Therefore, significant mixing due to secondary flows occurs mainly in the proximal airways during exhalation. A set of parameters is identified such that modeled bolus standard deviation, mode shift, and skewness agree with the measurements. Particle loss due to deposition was calculated in each generation and the resulting predictions of particle deposition fractions were in a good agreement with measurements. Although this model does not simulate detailed flow mechanics in the lungs, it does represent overall particle transport phenomena that lead to bolus dispersion observations and it provides a framework for the simulation of pulmonary particle transport during multiple breaths, as presented in an accompanying study of this work.