Hemodynamic Parameters and Early Intimal Thickening in Branching Blood Vessels

Intimal thickening due to atherosclerotic lesions or intimal hyperplasia in medium to large blood vessels is a major contributor to heart disease, the leading cause of death in the Western World. Balloon angioplasty with stenting, bypass surgery, and endarterectomy (with or without patch reconstruction) are some of the techniques currently applied to occluded blood vessels. On the basis of the preponderance of clinical evidence that disturbed flow patterns play a key role in the onset and progression of atherosclerosis and intimal hyperplasia, it is of interest to analyze suitable hemodynamic wall parameters that indicate susceptible sites of intimal thickening and/or favorable conditions for thrombi formation. These parameters, based on the wall shear stress, wall pressure, or particle deposition, are applied to interpret experimental/clinical observations of intimal thickening. Utilizing the parameters as "indicator" functions, internal branching blood vessel geometries are analyzed and possibly altered for different purposes: early detection of possibly highly stenosed vessel segments, prediction of future disease progression, and vessel redesign to potentially improve long-term patency rates. At the present time, the focus is on the identification of susceptible sites in branching blood vessels and their subsequent redesign, employing hemodynamic wall parameters. Specifically, the time-averaged wall shear stress (WSS), its spatial gradient (WSSG), the oscillatory shear index (OSI), and the wall shear stress angle gradient (WSSAG) are compared with experimental data for an aortoceliac junction. Then, the OSI, wall particle density (WPD), and WSSAG are segmentally averaged for different carotid artery bifurcations and compared with clinical data of intimal thickening. The third branching blood vessel under consideration is the graft-to-vein anastomosis of a vascular access graft Suggested redesigns reduce several hemodynamic parameters (i.e., the WSSG, WSSAG, and normal pressure gradient [NPG]), thereby reducing the likelihood of restenosis, especially near the critical toe region.

Particle-Hemodynamics Simulations and Design Options for Surgical Reconstruction of Diseased Carotid Artery Bifurcations

Based on the hypothesis that aggravating hemodynamic factors play a key role in the onset of arterial diseases, the methodology of “virtual prototyping” of branching blood vessels was applied to diseased external carotid artery (ECA) segments. The goals were to understand the underlying particle-hemodynamics and to provide various geometric design options for improved surgical reconstruction based on the minimization of critical hemodynamic wall parameters (HWPs). First, a representative carotid artery bifurcation (CAB) and then CABs with stenosed ECAs, i.e., a distally occluded ECA and an ECA stump, were analyzed based on transient three-dimensional blood flow solutions, employing a user-enhanced commercial finite volume code. Specifically, the HWPs, i.e., oscillatory shear index, wall shear stress angle gradient, near-wall residence time of monocytes, and near-wall helicity angle difference were evaluated to compare the merits of each design option, including a reconstructed near-optimal junction which generates the lowest HWP-values. The results provide physical insight to the biofluid dynamics of branching blood vessels and guide vascular surgeons as well as stent manufacturers towards interventions leading to high sustained patency rates.

Hemodynamic parameters and early intimal thickening in branching blood vessels

Intimal thickening due to atherosclerotic lesions or intimal hyperplasia in medium to large blood vessels is a major contributor to heart disease, the leading cause of death in the Western World. Balloon angioplasty with stenting, bypass surgery, and endarterectomy (with or without patch reconstruction) are some of the techniques currently applied to occluded blood vessels. On the basis of the preponderance of clinical evidence that disturbed flow patterns play a key role in the onset and progression of atherosclerosis and intimal hyperplasia, it is of interest to analyze suitable hemodynamic wall parameters that indicate susceptible sites of intimal thickening and/or favorable conditions for thrombi formation. These parameters, based on the wall shear stress, wall pressure, or particle deposition, are applied to interpret experimental/clinical observations of intimal thickening. Utilizing the parameters as "indicator" functions, internal branching blood vessel geometries are analyzed and possibly altered for different purposes: early detection of possibly highly stenosed vessel segments, prediction of future disease progression, and vessel redesign to potentially improve long-term patency rates. At the present time, the focus is on the identification of susceptible sites in branching blood vessels and their subsequent redesign, employing hemodynamic wall parameters. Specifically, the time-averaged wall shear stress (WSS), its spatial gradient (WSSG), the oscillatory shear index (OSI), and the wall shear stress angle gradient (WSSAG) are compared with experimental data for an aortoceliac junction. Then, the OSI, wall particle density (WPD), and WSSAG are segmentally averaged for different carotid artery bifurcations and compared with clinical data of intimal thickening. The third branching blood vessel under consideration is the graft-to-vein anastomosis of a vascular access graft. Suggested redesigns reduce several hemodynamic parameters (i.e., the WSSG, WSSAG, and normal pressure gradient [NPG]), thereby reducing the likelihood of restenosis, especially near the critical toe region.

Computational analyses and design improvements of graft-to-vein anastomoses

For hemodialysis patients, arteriovenous grafts are omnipresent. Unfortunately, a large percentage of such grafts fail within the first year after surgery because of occlusive lesions mainly at the venous anastomotic site. It is textbook knowledge that critical values of certain hemodynamic parameters, such as low (oscillatory) wall shear stresses, large sustained wall shear stress gradients, significant changes in wall shear stress angles, excessive radial pressure gradients, etc., play significant roles in the onset and/or development of vascular diseases. The idea is to geometrically design graft-to-vein configurations such that aggravating flow patterns are reduced, and hence stenotic developments are minimized. Focusing on a new blood rheological model in conjunction with three graft-to-vein anastomotic configurations, that is, a base case, the Bard-IMPRA Venaflo graft, and a new graft-end design, the corresponding transient laminar 3-D hemodynamics are numerically simulated and compared. The design criterion for the best performance of these junction geometries is the most significant reduction in locally disturbed flow as expressed by equally weighted indicator functions for the onset and progression of stenotic developments. As a result of this comparison study, quantitative recommendations for arteriovenous loop graft designs toward increased patency rates are provided. The resulting improved graft design will be scrutinized in clinical trials.

Computational haemodynamics analysis and comparison study of arterio-venous grafts

Haemodialysis arterio-venous graft failure is related to the development of stenotic lesions most commonly located near the venous anastomosis, especially in the toe region. 'Disturbed flow' interaction with the vessel wall surface, characterized by haemodynamic parameters based on the local wall shear stress or the radial pressure gradient, have been widely recognized as the trigger mechanism of a cascade of abnormal biological events leading to occlusive developments. Assuming incompressible laminar flow and rigid, in-plane vessel walls, validated haemodynamics are numerically simulated for a constant-diameter end-to-side base case, the Venaflo graft, and an improved graft-end configuration. The geometric design of the new graft-end was based on the reduction of three time- and area-averaged haemodynamic parameters, i.e. the wall shear stress gradient, wall shear stress angle gradient, and radial pressure gradient. Considering the critical toe region, the Venaflo graft has demonstrated measurable improvements over the base case configuration in predictive computer simulations as well as in clinical trials. The performance improvement should be further enhanced with the modifications illustrated by the new design.