Assessment of the pulsatile wall shear stress in the stenosed and recanalized carotid bifurcations by the lattice Boltzmann method

摘要

Objective: Carotid atherosclerosis is one of the most common cardiovascular diseases. It is now widely accepted that the local wall shear stress (WSS) plays a key role in the development and progression of atherosclerosis. Although the WSS has been a focus of many hemodynamic studies, few comprehensive analyses of the segmental distributions of the physiological WSS in the entire carotid bifurcation have been presented in detail. In this paper we comprehensively investigated the WSS distributions along the outer and inner walls of the internal carotid arteries (ICA) in two-dimensional severe stenosed and recanalized carotid bifurcations to explore the risk of atherosclerosis and carotid plaque build-up. Method: The lattice Boltzmann method (LBM), a tool for the numerical simulation of fluid flows, is considered a suitable approach for hemodynamics studies. The shear stress in the LBM can directly be assessed locally and independently of the velocity and its gradient. In this study, we applied the LBM to simulate the pulsatile blood flow in carotid artery bifurcations. The stenosed carotid geometry was digitally reconstructed from a two-dimensional real human angiography. In addition, three virtually surgically recanalized cases were analyzed with changing sizes of the ICA bulbs and bifurcation angles between the common carotid artery and the ICA based on the normal carotid bifurcation geometry parameters. Results: First we investigated the accuracy of measuring the WSS and the 'staircase' effect of the bound-back boundary condition in the LBM based on a two-dimensional inclined Poiseuille flow. The results show that the WSS need be evaluated neither too close nor too far away from the wall. Next, comparing the instantaneous vorticity distributions of the flow fields for the stenosed and recanalized carotid bifurcations during a blood cycle, we can observe that severe stenosis strongly affects the normal flow and negatively increases the blood resistance in the ICA. In particular, we investigated the spatiotemporal and average distributions of the wall shear stress along the outer and inner walls of the ICA in severely stenosed and recanalized carotid bifurcations for different sizes of carotid bulbs and bifurcation angles between the common and internal carotid arteries. The results show that the WSS distributions in the ICA depend on the carotid sinus size, bifurcation angle and stenosis. Conclusions: The spatiotemporal distributions of the WSS can exactly reveal the imperceptible changes of the near-boundary layer flows that strongly depend on the vessel geometries. Simultaneously, the variation curves of the absolute time-average wall shear stress, oscillatory shear index (OSI) and their ratio along the walls clearly indicated the presence of atherosclerosis-prone sites of low WSS and high OSI in the ICA. Thus, the results of the wall shear stress distributions could be a valuable reference to assess the risks of atherosclerosis and carotid plaque initiation, progression and rupture.