Imre Balásházy

Particle deposition in airway bifurcations–II. Expiratory flow

Abstract In the present analysis, local inhomogeneities of particle deposition are examined in a bifurcation geometry under realistic expiratory flow conditions, considering the simultaneous effects of inertial impaction, gravitational settling, Brownian motion and interception. The airflow field within the bifurcation during expiration is obtained by solving the Navier-Stokes equations on a three-dimensional computer mesh with a finite-difference technique. Knowledge of the velocity field allows us to simulate the trajectories of aerosol particles entrained in the airstream with Monte Carlo methods. The spatial deposition pattern within the bifurcation is then determined by the intersection of particle trajectories with the surrounding wall surfaces. The effects of particle size, flow rate, inlet flow profile, and bifurcation geometry on the spatial deposition pattern are investigated. In most cases, hot spots are found downstream of the central bifurcation zone. Model predictions are compared with the available experimental data and with other theoretical results.

Effect of particle mass size distribution on the deposition of aerosols in the human respiratory system

Elemental mass size distributions were experimentally determined in atmospheric aerosols collected at four different locations in Budapest, Hungary, comprising a urban background site, two downtown sites and a road tunnel. Based on these distributions, deposition fractions for the various elements in the respiratory system were calculated for a healthy Caucasian adult male, female and 5-year-old child under sitting breathing conditions by a stochastic lung deposition model. The highest deposition values were observed in the extrathoracic region regardless of subjects age and gender, and chemical species and size distributions. Deposition in the tracheobronchial tree and acinar region was much smaller than that in the extrathoracic region. Variations in the deposition fractions due to differences in the size distributions were really significant only in the extrathoracic region. Surprisingly, the different size distributions yielded similar depositions in the thoracic region for a given gender as far as the shape of the deposition curve and the total amount are concerned. Regional deposition fractions were compared for the male, female and child, and for various size distributions (sampling location) and elements.

Detailed mathematical description of the geometry of airway bifurcations.

Health effects related to the deposition of inhaled aerosol particles in the respiratory system strongly depend on the local deposition patterns. These patterns are highly sensitive to the shape of the airway geometry. The current study presents an exact mathematical description of a morphologically realistic airway bifurcation by further developing an earlier study of the published literature. In addition, numerical methods are elaborated to solve some important tasks, which are necessary for the development of computational fluid dynamics (CFD) techniques in the area of aerosol deposition calculations in the airways. Finally, single and multiple airway geometries and computational grids are generated and analysed.

Inspiratory Deposition Efficiency of Ultrafine Particles in a Human Airway Bifurcation Model

The inspiratory deposition efficiency of ultrafine particles in a physiologically realistic bronchial airway bifurcation model, approximating the airway generation 3-4 juncture, was computed for different particle sizes, ranging from 1 to 500 nm, under three different flow conditions, representing resting to heavy exercise breathing conditions. For the smallest particle sizes, say between 1 and 10 nm, molecular diffusion is the primary deposition mechanism, as indicated by the inverse relationship with flow rate, except for the highest flow rate where the additional effect of convective diffusion has to be considered as well. For the larger particle sizes, say above 20 nm, the independence from particle size and dependence on flow rate suggests that convective diffusion plays the major role for ultrafine particle deposition in bifurcations. A semiempirical equation for the inspiratory deposition efficiency, m (D, Q), as a function of diffusion coefficient D and flow rate Q, due to the combined effect of molecular and convective diffusion was derived by fitting the numerical data. The very existence of a mixed term demonstrates that molecular and convective diffusion are not statistically independent from each other.

Quantification of particle deposition in asymmetrical tracheobronchial model geometry

The primary objective of this study was to quantify the local inspiratory and expiratory aerosol deposition in a highly asymmetric five-generation tracheobronchial tree. User-enhanced commercial codes and self-developed software was used to compute the air velocity field as well as particle deposition distributions for a large size range of inhalable particles. The numerical model was validated by comparison of our results with experimental flow measurements and particle deposition data available in the open literature. Our simulations show highly localised deposition patterns for all particle sizes, but mainly for the larger particles. As expected, deposition efficiencies and deposition fractions proved to be very sensitive to the particle size. The deposition density in the hot spots can be hundreds and even thousand times higher than the mean deposition density. Present results can be of interest to researchers involved in the assessment of adverse health effects of inhaled aerosols or optimising the drug aerosol delivery into the lung.

Effect of physical exertion on the deposition of urban aerosols in the human respiratory system

Abstract Deposition of element-specific particulate matter in the respiratory system of a Caucasian-type healthy adult male and female was computed by a stochastic lung deposition model for different reference levels of physical exertion using mass size distributions in the aerodynamic diameter interval of 0.125– 16 μm experimentally determined in urban environments. Particles with an aerodynamic diameter smaller than about 0.3 μm are deposited in the whole respiratory system decreasingly with rising physical exertion, while the opposite is observed for particles with an aerodynamic diameter larger than about 0.7 μm (except for the highest physical activity). It is the light exercise that causes the largest extrathoracic (ET) deposition efficiency of the particles in this last diameter range, and, consequently, the smallest tracheobronchial and acinar depositions. The results obtained indicate that ET deposition depends primarily on the size distribution of the inhaled particles, while physical exertion plays a minor role. In contrast, deposition fractions of different aerosol species in the lungs are very similar to each other for a given physical exertion, despite the rather diverse size distributions of some species, but depend significantly on the subjects physical exertion. The differential deposition curves generally exhibit two peaks, one in the tracheobronchial and one in the acinar region. Both differential and regional deposition fractions do not change in a monotonical fashion with physical exertion but display maximum values approximately at the exertion level corresponding to the switching point between the nose-breathing mode and the combined nose- and mouth-breathing mode. Deposition rates (mass doses), however, increase monotonically with physical exertion due to increased ventilation rates, and more particles reach the deeper parts of the lung.

Three-Dimensional Model for Aerosol Transport and Deposition in Expanding and Contracting Alveoli

Particle transport and deposition within a model alveolus, represented by a rhythmically expanding and contracting hemisphere, was modeled by a three-dimensional analytical model for the time-dependent air velocity field as a superposition of uniform and radial flow components, satisfying both the mass and momentum conservation equations. Trajectories of particles entrained in the airflow were calculated by a numerical particle trajectory code to compute simultaneously deposition by inertial impaction, gravitational sedimentation, Brownian diffusion, and interception. Five different orientations of the orifice of the alveolus relative to the direction of gravity were selected. Deposition was calculated for particles from 1 nm to 10 μm, for 3 breathing conditions, and for 5 different entrance times relative to the onset of inspiration. For the analyzed cases, the spatial orientation of the orifice of an alveolus has practically no effect on deposition for particles below about 0.1 μm, where deposition is dominated by Brownian motion. Above about 1 μm, where deposition is governed primarily by gravitational settling, deposition can vary from 0 to 100%, depending on the spatial orientation, while deposition of particles 0.1–1 μm falls between these two extreme cases. Due to the isotropic nature of Brownian motion, deposition of the 10-nm particles is practically uniform for all spatial orientations. However, for larger particles, deposition can be quite inhomogeneous, consistent with the direction of gravity. While nearly all particles are exhaled during the successive expiration phase, there are a few cases where particles still leave the alveolus even after many breathing cycles.

Simulation of fiber deposition in bronchial airways

Penetration probabilities of inhaled man-made mineral fibers to reach central human airways were computed by a stochastic lung deposition model for different flow rates and equivalent diameters. Results indicate that even thick and long fibers can penetrate into the central airways at low flow rates. Deposition efficiencies and localized deposition patterns were then computed for man-made fibers with variable lengths in a three-dimensional physiologically realistic bifurcation model of the central human airways by computational fluid dynamics (CFD) techniques for characteristic breathing patterns. The results obtained for inspiratory flow conditions indicate that deposition efficiencies were highest for parallel orientation of the fibers, increasing with rising flow rate, branching angle, and fiber length at all orientations. Furthermore, deposition patterns were highly inhomogeneous and their localized distributions showed hot spots in the vicinity of the carinal ridge and at the inner sides of the daughter airways. Comparisons with other theoretical results demonstrate that the equivalent diameter concept, if including interception, presents a reasonable approximation for the parameter ranges employed in the present study.

The effect of morphological variability on surface deposition densities of inhaled particles in human bronchial and acinar airways.

Deposition fractions in human airway generations were computed with a stochastic deposition model, which is based on a randomly, asymmetrically dividing lung morphology, applying Monte Carlo techniques. Corresponding uncorrelated surface deposition densities were obtained by dividing the average deposition fraction in a given generation by the average total surface area of that generation. In order to consider the statistical correlation between deposition probability and linear airway dimensions in each airway, correlated surface deposition densities were calculated by dividing the deposition fraction in a randomly selected bronchial or acinar airway by the surface area of that airway and by the total number of bronchial or acinar airways in that generation. Average surface deposition densities are relatively constant throughout bronchial airway generations, while average acinar surface deposition densities exhibit a distinct decrease with rising penetration into the acinar region. Due to the correlation between deposition fraction and surface area in a given airway generation, average correlated surface deposition densities are consistently higher than average uncorrelated densities, particularly in the acinar region, where differences can be as high as a few orders of magnitude. Already significant statistical fluctuations of the deposition fractions in individual airway generations are even exacerbated for surface deposition densities, with coefficients of variation about twice as high as for correlated deposition fractions.

The relationship between secondary flows and particle deposition patterns in airway bifurcations

The relationship between localized fluid dynamics and localized particle deposition patterns within bronchial airway bifurcations upon inspiration and expiration was analyzed for different bifurcation geometries, flow conditions, and particle sizes. For the simulation of three-dimensional airflow patterns in airway bifurcation models, the Navier-Stokes and continuity equations were solved numerically by the finite volume Computational Fluid Dynamics (CFD) program package FIRE. Spatial particle deposition patterns were determined by the intersection of randomly selected particle trajectories with the surrounding wall surfaces. While three-dimensional flow patterns were characterized by their corresponding two-dimensional secondary flow fields, three-dimensional deposition patterns were represented by their related two-dimensional deposition density plots. Two particle sizes were selected to explore the relationship between secondary flows and localized particle deposition patterns: 0.01 w m, to illustrate the effects of Brownian motion, and 10 w m, to display the effects of impaction and sedimentation. Changes in bifurcation geometry (shape of bifurcation zone, branching angle) and flow conditions (flow rate, inlet flow profile, direction of flow) lead to variations in resulting secondary flow patterns, which were reflected by corresponding differences in related particle deposition patterns. In conclusion, a distinct relationship could be observed between secondary flow patterns and deposition density plots, demonstrating that particle deposition patterns in airway bifurcations are not only determined by physical forces acting upon individual particles, but also by convective transport processes of the carrier fluid.