Airway Wall Stiffening Increases Peak Wall Shear Stress: A Fluid–Structure Interaction Study in Rigid and Compliant Airways

Abstract

The airflow characteristics in a computed tomography (CT) based human airway bifurcation model with rigid and compliant walls are investigated numerically. An in-house three-dimensional (3D) fluid-structure interaction method is applied to simulate the flow at different Reynolds numbers and airway wall stiffness. As the Reynolds number increases, the airway wall deformation increases and the secondary flow becomes more prominent. It is found that the peak wall shear stress on the rigid airway wall can be five times stronger than that on the compliant airway wall. When adding tethering forces to the model, we find that these forces, which produce larger airway deformation than without tethering, lead to more skewed velocity profiles in the lower branches and further reduced wall shear stresses via a larger airway lumen. This implies that pathologic changes in the lung such as fibrosis or remodeling of the airway wall -both of which can serve to restrain airway wall motion -have the potential to increase wall shear stress and thus can form a positive feed-back loop for the development of altered flow profiles and airway remodeling. These observations are particularly interesting as we try to understand flow and structural changes seen in, for instance, asthma, emphysema, cystic fibrosis, and interstitial lung disease.

Airway Gas Flow

“…Three cases were studied: a rigid airway, a flexible airway without tethering, and a flexible airway tethered to the parenchyma. For the rigid airway, Xia et al (109) predicted shear stress of approximately 0.05 Pa, which was similar to the level estimated by Tarran et al (87) for the same generations of airway. When allowing the (flexible) airway diameter to increase by 25% so as to be consistent with FSI simulations of airway wall motion during breathing, then equation (1) would predict wall shear stress to reduce by about 50%.…”

“…When allowing the (flexible) airway diameter to increase by 25% so as to be consistent with FSI simulations of airway wall motion during breathing, then equation (1) would predict wall shear stress to reduce by about 50%. However, for the FSI simulations by Xia et al (109), the peak wall shear stress decreased by about 80% (Fig. 11).…”

“…For example, in a study of asthmatic subjects via the Severe Asthma Research Program (SARP), Fain et al (28) analyzed MDCT images of 118 human subjects with asthma (58 severe, 35 nonsevere, and 25 normal controls) and found that across all airway segments, there was significant increase in the airway wall thickness and the lumen eccentricity in subjects with severe asthma compared with nonsevere cases. The remodeling of the airway in asthmatics, or the stiffening of lung tissue in pulmonary fibrosis, serves to restrain the motion of the airway-similar to the effect of the rigid airway in Xia et al (109)-and leads to a higher wall shear stress on the surface. The study of Xia et al (109) implies that tissue degeneration in the lungs has the potential to increase wall shear stress and contribute to airway rigidity.…”

“…To study the effect of airway-parenchymal tethering and airway-wall motion on shear stress, Xia et al (109) simulated airflow in an elastic generation 3-4; MDCT-defined airway tethered to the parenchymal tissue. A 3D time-accurate fluid-structure interaction (FSI) technique was used, based upon the arbitrary Lagrangian-Eulerian (ALE) methodology, and composed of a structural dynamics solver and a fluid dynamics solver, which are coupled through a dynamic mesh algorithm (108).…”

Steady streaming: A key mixing mechanism in low-Reynolds-number acinar flows

“…The incompressible Navier-Stokes equations are solved in an Arbitrary Lagrangian-Eulerian framework. 24,25 The equations are normalized using the alveolus mouth dimension of L = 416 m and peak velocity of U 0 = 0.032 m / s. A sinusoidal flow rate is specified at one end of the duct while a Neumann boundary condition is employed at the other. Homothetic wall motion, where corresponding sides of the duct and al-veolar wall remain parallel in a geometric expansion or contraction is prescribed.…”