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)

Simulation of blood flow past a distal arteriovenous-graft anastomosis at low Reynolds numbers

Patients with end-stage renal disease are usually treated by hemodialysis while waiting for a kidney transplant. A common device for vascular access is an arteriovenous graft (AVG). However, AVG failure induced by thrombosis has been plaguing dialysis practice for decades. Current studies indicate that the thrombosis is caused by intimal hyperplasia, which is triggered by the abnormal flows and forces [e.g., wall shear stress (WSS)] in the vein after AVG implant. Due to the high level of complexity, in almost all of the existing works of modeling and simulation of the blood-flow vessel-AVG system, the graft and blood vessel are assumed to be rigid and immobile. Very recently, we have found that the compliance of graft and vein can reduce flow disturbances and lower WSS [Z. Bai and L. Zhu, “Three-dimensional simulation of a viscous flow past a compliant model of arteriovenous-graft anastomosis,” Comput. Fluids 181, 403–415 (2019)]. In this paper, we apply the compliant model to investigate possible effects of several dimensionless parameters (AVG graft-vein diameter ratio Rgv, AVG attaching angle θ, flow Reynolds numbers Re, and native vein speed Vv) on the flow and force fields near the distal AVG anastomosis at low Reynolds numbers (up to several hundreds). Our computational results indicate that the influences of the parameters Rgv, θ, and Re lie largely on the graft and the influence of Vv lies largely on the vein. In any case, the WSS, wall shear stress gradient, and wall normal stress gradient and their averaged values on the graft are significantly greater than those on the vein. ACKNOWLEDGMENT

Modeling and Simulation of Blood Flow past the Distal Anastomosis of the Arteriovenous Graft for Hemodialysis

Hemodialysis is a common treatment for ESRD patients to manage their chronic renal failure while awaiting kidney transplant. Arteriovenous graft is a major vascular access for hemodialysis but often fails due to the thrombosis close to the anastomosis. Most of existing computational models employ an unrealistic rigid model assuming the vein and graft are rigid. And some of the existing results are inconsistent in characterizing the flow and force fields. We introduce a new three-dimensional computational model that incorporates the vein and graft deformability. The new model is based on the lattice Boltzmann-immersed boundary (LB-IB) framework for handling the fluid-flexible-structure interaction. We extend the framework to the case of non-uniform mesh, including generation of the non-uniform mesh and force computing on the non-uniform mesh. Numerous simulations are designed and conducted with various combinations of different model parameters. Our results suggest that: 1) the deformability has significant influence on the flow and force fields of blood flow through vein-graft anastomosis. The WSS, WSSG, WNSG, and their averaged values are significantly lower than the rigid case; 2) the effects of flow pulsatility, AVG-vein diameter ratio, AVG attaching angle, and Reynolds number are less pronounced in the deformable case than the rigid case; 3) the averaged WSS, WSSG, and WNSG are significantly greater on the graft wall than on the vein wall; 4) the implantation of graft dramatically increases the averaged WSS, WSSG, and WNSG on the vein wall. The major contributions of the dissertation are as follows: 1) introduction of a new 3D computational model for blood flow through the distal AVG anastomosis; 2) non-uniform mesh generation for the vein-AVG anastomosis by elastic fibers and fluid-solid force computation on the non-uniform mesh; 3) identification of the effects of various model parameters on the flow and force fields, in particular, the role of vein and graft deformability on the flow and force fields

Modeling and Simulation of Blood Flow Past the Distal Anastomosis of the Arteriovenous Graft for Hemodialysis

Hemodialysis is a common treatment for ESRD patients to manage their chronic renal failure while awaiting kidney transplant. Arteriovenous graft is a major vascular access for hemodialysis but often fails due to the thrombosis close to the anastomosis. Most of existing computational models employ an unrealistic rigid model assuming the vein and graft are rigid. And some of the existing results are inconsistent in characterizing the flow and force fields. We introduce a new three-dimensional computational model that incorporates the vein and graft deformability. The new model is based on the lattice Boltzmann-immersed boundary (LB-IB) framework for handling the fluid-flexible-structure interaction. We extend the framework to the case of non-uniform mesh, including generation of the non-uniform mesh and force computing on the non-uniform mesh. Numerous simulations are designed and conducted with various combinations of different model parameters. Our results suggest that: 1) the deformability has significant influence on the flow and force fields of blood flow through vein-graft anastomosis. The WSS, WSSG, WNSG, and their averaged values are significantly lower than the rigid case; 2) the effects of flow pulsatility, AVG-vein diameter ratio, AVG attaching angle, and Reynolds number are less pronounced in the deformable case than the rigid case; 3) the averaged WSS, WSSG, and WNSG are significantly greater on the graft wall than on the vein wall; 4) the implantation of graft dramatically increases the averaged WSS, WSSG, and WNSG on the vein wall. The major contributions of the dissertation are as follows: 1) introduction of a new 3D computational model for blood flow through the distal AVG anastomosis; 2) non-uniform mesh generation for the vein-AVG anastomosis by elastic fibers and fluid-solid force computation on the non-uniform mesh; 3) identification of the effects of various model parameters on the flow and force fields, in particular, the role of vein and graft deformability on the flow and force fields