Computational model of airflow in upper 17 generations of human respiratory tract

https://doi.org/10.1016/j.jbiomech.2007.12.019Get rights and content

Abstract

Computational fluid dynamics (CFD) studies of airflow in a digital reference model of the 17-generation airway (bronchial tree) were accomplished using the FLUENT® computational code, based on the anatomical model by Schmidt et al. [2004. A digital reference model of the human bronchial tree. Computerized Medical Imaging and Graphics 28, 203–211]. The lung model consists of 6.744×106 unstructured tetrahedral computational cells. A steady-state airflow rate of 28.3 L/min was used to simulate the transient turbulent flow regime using a large eddy simulation (LES) turbulence model. This CFD mesh represents the anatomically realistic asymmetrical branching pattern of the larger airways. It is demonstrated that the nature of the secondary vortical flows, which develop in such asymmetric airways, varies with the specific anatomical characteristics of the branching conduits.

Introduction

A widely used model for CFD simulations and predictions of gas transport, particle deposition and dosimetry in the human lung is that characterized by a regular, dichotomous branching pattern (Snyder et al., 1981; Zhao and Lieber, 1994; Calay et al., 2002; Lee and Lee, 2002; Liu et al., 2003; Kleinstreuer and Zhang, 2003; Nowak et al., 2003; Shi et al., 2004; Zhang and Kleinstreuer, 2004). Nowak et al., (2003) and Cebral and Summer (2004) used computed tomography (CT) scans to characterize the tracheobronchial airways more realistically, albeit only down to the fourth generation subunit (G0–G4). Kriete et al. (2004) reported the oxygen gas transport and particle deposition in a digital reference model based on data by Schmidt et al. (2004) with a limited 29-terminal bronchi outlet; whereas, our study includes all 720 bronchi outlets. The final mesh of Kriete et al. (2004) contained a total of 456,463 prism and tetrahedral elements.

Calay et al. (2002) studied the respiratory flow patterns in a single first-generation bifurcation distal to the trachea and in a multiple-bifurcation model, with their three generations based on the anatomy given by Horsfield et al. (1971). They used four different grid densities comprising, respectively, 31,104, 79,820, 159,872 and 320,980 nodes. Lee and Lee (2002) generated a 3D conduit network model with four generations which conformed closely to Weibel (1963) model in order to study aerosol bolus dispersion. The total number of cells in their 90° out-of-plane model ranged from 40,000 to 60,000. Liu et al. (2003) studied the 3D inspiratory flow in a third-generation asymmetric model from the 5th to the11th branches of Weibel's B model. Kleinstreuer and Zhang (2003) analyzed targeted aerosol drug deposition analysis in a rigid triple-bifurcation tracheobronchial airway model. Their model represents the symmetrically bifurcating generations G3–G6 as in Weibel's lung model with three different hemispherical tumor models placed along the side wall of the G5 airway. Their final mesh comprised about 360,000 cells for the triple-bifurcation configuration. Nowak et al. (2003) demonstrated a four-subunit CFD simulation method for the human tracheobronchial tree. They economized on computational effort by segmenting the first 12 generations into four 3.5-generation “tranches” (G0–G3, G3–G6, G6–G9, and G9–G12), without adequately accounting for the fluctuation in boundary conditions between each tranche. Their results indicate dramatic differences in the predicted particle deposition patterns between the two models. The absence of airway curvature and surface irregularities in a Weibel-based model renders the flow fields very different from those in a real human lung (CT-based model). Cebral and Summer (2004) studied the central tracheal and bronchial airways down to four generations by using a virtual bronchoscopy reconstruction method. Airflow patterns resulting from airway stenoses in generations from G0 to G4 were simulated computationally. van Ertbruggen et al. (2005) studied the gas flow and particle deposition in a realistic 3D model of the bronchial tree, extending from the trachea to the segmental bronchi (seventh airway generation for the most distal airways), based on the morphometrical data of Horsfield et al. (1971). Considering symmetric double-bifurcation models, Longest and Vinchurkar (2007b) have recently assessed the effects of upstream transition to turbulence on the flow field and particle deposition in the generations G3–G5 of the respiratory tract. Turbulence was shown experimentally to influence the local deposition of 10-mm-diameter particles, primarily by influencing the initial velocity and particle profiles. Their results underline the importance of correct inlet conditions and the need to consider upstream effects in experimental and computational studies of the respiratory tract. In addition, Longest et al. (2006) compared flow patterns and particle deposition in both normal and childhood asthma-induced constricted models of pulmonary tree generations G3–G5 and G7–G9. Both laminar solutions and the low Reynolds number (LRN) κ–ω turbulence model were employed using a Fluent-6 software package along with their own Fortran and C programs. The authors concluded that airway constriction caused local cellular-level deposition rates to increase by one to two orders of magnitude. Asthma constriction may significantly increase branch-averaged particle deposition with larger increases in local cellular-level deposition, resulting in an aggravated health risk.

More realistic anatomical models have been published by Kitaoka et al. (1999), Tawhai et al., 2000, Tawhai et al., 2004, Spencer et al. (2001), Tgavalekos et al. (2003), Sera et al. (2003), and Schmidt et al. (2004), using both mathematical algorithms and new experimental imaging techniques. Kitaoka et al. (1999) introduced a 3D model of the human airway tree down to the terminal bronchioles, which by a deterministic algorithm that incorporated duct branching and space division. Tawhai et al., 2000, Tawhai et al., 2004 developed a 3D tree-growing algorithm specific to a given host geometry derived from magnetic resonance imaging (MRI). Spencer et al. (2001) developed a dynamic surface modeling technique based on data from idealized models to construct 3D computer simulations of tubular pulmonary airway structures within lungs extending from the trachea (G0) to the alveoli (G23). Sera et al. (2003) developed a two-step method to visualize small airways in detail by staining excised rodent pulmonary airways with a radio-opaque solution and then visualizing the tissue with a cone-beam microfocal X-ray CT system. Tgavalekos et al. (2003) advanced the 3D airway tree model of Kitaoka et al. (1999) to predict pulmonary function on the basis of airway structure, particularly when constriction patterns are imposed heterogeneously on the pulmonary tree in specific anatomic locations and compared their model predictions with ventilation images obtained from positron emission tomography (PET) and measurements of dynamic mechanical pulmonary function. The current study includes these upper generations as well as both central and lower tracheobronchial airways within the new 17-generation model.

Section snippets

Current 17-generation anatomical model

We have based our present CFD analysis on the best published anatomically explicit human lung model available to date; i.e. the 17-generation anatomical model developed by Schmidt et al. (2004). Fig. 1 represents the anatomy of the human lung of an adult male, free of pathological alterations. Fig. 1a portrays a realistic 3D surface representation of the segmented volume of the bronchial tree. The conduit model reconstructed from an abstracted and adapted topological graph, as shown in Fig. 1b,

CFD simulation

Our characterization of the anatomy of airways must be sufficiently realistic as we intend to extend the present model to design more efficient delivery of inhaled medications and to understand better the health effects of inhaled pollutants. Particle deposition patterns in the branching tracheobronchial network are highly influenced by the detailed branching structures of the human lung, which are important for optimizing aerosol therapy protocols.

In this study, the incompressible airflow in a

Tracheobronchial model up to a maximum 17 generations

During inhalation, the flow transitions from laminar to turbulent flow in the larynx and in the tracheal section, and continues into the first main generation. In the conducting zone, containing the trachea, bronchi, bronchioles, and the terminal bronchioles, there is no gas-blood exchange. The downstream increase in cross-sectional area causes a drop in airway pressure and flow rate, such that final air-blood exchange occurs by diffusion at the alveolar level. Our CFD calculations indicate

Conclusions

CFD evaluations of the quasi-steady incompressible airflow in the human respiratory tract have been presented here with a view to their subsequent extension to the study of the uptake of particulate matter by the tracheobronchial airways in flows varying periodically in time.

We have adapted the anatomical graph data of Schmidt et al. (2004), based on measurements derived from high-resolution computer tomography (HRCT). Assumed pressure drops across successive bifurcations agree reasonably well

Conflict of interest

We do not have any conflict of interest.

Acknowledgments

We express our thanks to Dr. Andres Kriete at the Coriell Institute for Medical Research, Camden, NJ for providing us with the abstracted topological graph data. The access to the supercomputers at NSCEE of UNLV and Pittsburgh Supercomputing Center (Grant #CTS010015P) is also gratefully acknowledged.

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