Numerical analysis of pulsatile blood flow in realistic coronary bypass models

The paper’s objective lies in numerical modelling of pulsatile blood flow in complete aorto-coronary bypass models reconstructed from CT data, especially in models with individual and sequential bypass grafts. Unsteady blood flow is described by the nonlinear system of the incompressible Navier-Stokes equations in 3D, which is numerically solved using developed computational algorithm based on the fully implicit projection method and on the cell-centred finite volume method for hybrid unstructured tetrahedral grids. Obtained numerical results are discussed with regard to distribution of velocity, wall shear stress and oscillatory shear index at proximal and distal anastomoses, i.e., in areas prone to development of intimal hyperplasia

Impact of competitive flow on wall shear stress in coronary surgery: computational fluid dynamics of a LIMA-LAD model

Competitive flow from native coronary vessels is considered a major factor in the failure of coronary bypass grafts. However, the pathophysiological effects are not fully understood. Low and oscillatory wall shear stress (WSS) is known to induce endothelial dysfunction and vascular disease, like atherosclerosis and intimal hyperplasia. The aim was to investigate the impact of competitive flow on WSS in mammary artery bypass grafts. Using computational fluid dynamics, WSS was calculated in a left internal mammary artery (LIMA) graft to the left anterior descending artery in a three-dimensional in vivo porcine coronary artery bypass graft model. The following conditions were investigated: high competitive flow (non-significant coronary lesion), partial competitive flow (significant coronary lesion), and no competitive flow (totally occluded coronary vessel). Time-averaged WSS of LIMA at high, partial, and no competitive flow were 0.3-0.6, 0.6-3.0, and 0.9-3.0 Pa, respectively. Further, oscillatory WSS quantified as the oscillatory shear index (OSI) ranged from (maximum OSI = 0.5 equals zero net WSS) 0.15 to 0.35, < 0.05, and < 0.05, respectively. Thus, high competitive flow resulted in substantial oscillatory and low WSS. Moderate competitive flow resulted in WSS and OSI similar to the no competitive flow condition. Graft flow is highly dependent on the degree of competitive flow. High competitive flow was found to produce unfavourable WSS consistent with endothelial dysfunction and subsequent graft narrowing and failure. Partial competitive flow, however, may be better tolerated as it was found to be similar to the ideal condition of no competitive flow

Flow and wall shear stress in end-to-side and side-to-side anastomosis of venous coronary artery bypass grafts

<p>Abstract</p> <p>Purpose</p> <p>Coronary artery bypass graft (CABG) surgery represents the standard treatment of advanced coronary artery disease. Two major types of anastomosis exist to connect the graft to the coronary artery, i.e., by using an end-to-side or a side-to-side anastomosis. There is still controversy because of the differences in the patency rates of the two types of anastomosis. The purpose of this paper is to non-invasively quantify hemodynamic parameters, such as mass flow and wall shear stress (WSS), in end-to-side and side-to-side anastomoses of patients with CABG using computational fluid dynamics (CFD).</p> <p>Methods</p> <p>One patient with saphenous CABG and end-to-side anastomosis and one patient with saphenous CABG and side-to-side anastomosis underwent 16-detector row computed tomography (CT). Geometric models of coronary arteries and bypasses were reconstructed for CFD analysis. Blood flow was considered pulsatile, laminar, incompressible and Newtonian. Peri-anastomotic mass flow and WSS were quantified and flow patterns visualized.</p> <p>Results</p> <p>CFD analysis based on in-vivo CT coronary angiography data was feasible in both patients. For both types of CABG, flow patterns were characterized by a retrograde flow into the native coronary artery. WSS variations were found in both anastomoses types, with highest WSS values at the heel and lowest WSS values at the floor of the end-to-side anastomosis. In contrast, the highest WSS values of the side-to-side anastomosis configuration were found in stenotic vessel segments and not in the close vicinity of the anastomosis. Flow stagnation zones were found in end-to-side but not in side-to-side anastomosis, the latter also demonstrating a smoother stream division throughout the cardiac cycle.</p> <p>Conclusion</p> <p>CFD analysis of venous CABG based on in-vivo CT datasets in patients was feasible producing qualitative and quantitative information on mass flow and WSS. Differences were found between the two types of anastomosis warranting further systematic application of the presented methodology on multiple patient datasets.</p

Computational modelling of blood flow through sutured and coupled microvascular anastomoses

The research presented in this thesis uses Computational Fluid Dynamics (CFD) to model blood flow through idealised sutured and coupled microvascular Anastomoses to investigate the affect of each surgical technique on the flow within the vessel. Local flow phenomena are examined in detail around suture and coupler sites to study characteristics that could potentially initiate thrombus formation; for example, changes in velocity profile, wall shear stress or recirculating flow (vorticity). Idealised geometries of sutured and coupled blood vessels were created using CFD software with dimensions identical to microvascular suture material and coupling devices. Vessels were modelled as non‐compliant 1mm diameter ducts, and blood was simulated as a Newtonian fluid, in keeping with previous similar studies. All analyses were steady-state and performed on arteries. Comparison of the sutured and coupled techniques in the simulated microarterial anastomoses revealed a reduced boundary velocity profile; high Wall Shear Stress (WSS); high Shear StrainRate(SSR);and elevated vorticity at the suturesites. The coupled anastomosis simulation showed a small increase in maximum WSS at the anastomotic region compared to a pristine vessel. However, this was less than half that of the sutured model. The coupled vessel displayed an average WSS equal to a pristine vessel. Taken together, these observations demonstrate an increased thrombogenic profile in the sutured anastomosis when compared to a pristine, or indeed a coupled vessel. Data from the simulations on a coupled anastomosis reveal a profile that is less thrombogenic than that of the sutured anastomosis, and one that is nearly equivalent to that of a pristine vessel. Overall, it can be concluded that, within the limits of CFD simulations and the assumptions taken in this study, a sutured anastomosis is potentially more likely to generate an intravascular thrombosis than a coupled anastomosis

Constructal design of an arterial bypass graft

Arterial bypass grafts tend to fail after some years due to intimal hyperplasia\u2014an abnormal proliferation of smooth muscle cells that leads to stenosis and graft occlusion. In this regard and on the basis of the constructal design method, this study seeks to investigate the effect of geometric parameters\u2014stenosis degree, junction angle, and diameter ratio\u2014on the flow through a bypass graft circumventing an idealized, partially stenosed coronary artery. The computational model assumes a steady\u2010state Newtonian fluid flow through an artery stenosis degree from 25% to 75%. A computational fluid dynamics model and a response surface methodology were employed to assess the effects of the project parameters on pressure drop. As diameter ratio increases to 1 and the junction angle decreases to 30\ub0, the pressure drop decreases and there is a considerable dependence of pressure drop on the stenosis degree. The effects of the diameter ratio are more pronounced than those of junction angle on the velocity field and wall shear stress. The application of the constructal design method in hemodynamicsmight be a good alternative to provide configurations with enhanced performance and to provide valuable results to the understanding of biological flows

Computational Simulation: Selected Applications In Medicine, Dentistry, And Surgery

This article presents the use of computational modelling software (e.g. ANSYS) for the purposes of simulating, evaluating and developing medical and surgical practice. We provide a summary of computational simulation mo delling that has recently been employed through effective collaborations between the medical, mathematical and engineering research communities. Here, particular attention is being paid to the modelling of medical devices as well as providing an overview o f modelling bone, artificial organs and microvascular blood flows in the machine space of a High Performance Computer (HPC)

3D simulation of a viscous flow past a compliant model of arteriovenous-graft annastomosis

Hemodialysis is a common treatment for end-stage renal-disease patients to manage their renal failure while awaiting kidney transplant. Arteriovenous graft (AVG) is a major vascular access for hemodialysis but often fails due to the thrombosis near the vein-graft anastomosis. Almost all of the existing computational studies involving AVG assume that the vein and graft are rigid. As a first step to include vein/graft flexibility, we consider an ideal vein-AVG anastomosis model and apply the lattice Boltzmann-immersed boundary (LB-IB) framework for fluid-structure-interaction. The framework is extended to the case of non-uniform Lagrangian mesh for complex structure. After verification and validation of the numerical method and its implementation, many simulations are performed to simulate a viscous incompressible flow past the anastomosis model under pulsatile flow condition using various levels of vein elasticity. Our simulation results indicate that vein compliance may lessen flow disturbance and a more compliant vein experiences less wall shear stress (WSS)

Hemodynamics of end-to-end anastomosis bypass: on the specific influnce of a stenosis in the host artery

Local hemodynamics is implicated in the failure of bypass graft surgery. Intimal thickening has been shown to occur at specific sites, namely the floor, the heel, the toe and the suture line. Nevertheless, the recipient artery has often been repre-sented in the literature as a simple flow rate reduction or as a complete obstruc-tion neglecting the perturbing presence of the stenosis. In addition, severe as well as moderate constrictions are sometimes bypassed without taking into account an existing residual flow through the host artery. The purpose of this chapter is therefore to investigate how competitive flows issued from the graft and the dis-eased artery could interact together, and thus to explain the restenosis process at an early stage. The velocity flow rates are issued from in vivo measurements for patients who had undergone coronary bypass surgery approximately three weeks before. 3D unsteady flows through idealized coronary bypass anastomoses are modeled by the finite element method. The influence of the inflows, and of the shape of the stenosis, are notably discussed. The experimental Doppler velocity fields corroborate well the numerical results. The post-stenotic recirculation zone is demonstrated to interact with the graft inflow during the cardiac cycle. These disturbed flow patterns due to the presence of a stenosis may have harmful con-sequences in terms of graft patenc