Zaher Kharboutly

Numerical and experimental study of blood flow through a patient-specific arteriovenous fistula used for hemodialysis

Arteriovenous fistula (AVF) pathologies related to blood flow necessitate valid calculation tools for local velocity and wall shear stress determination to overcome the clinical diagnostic limits. To illustrate this issue, a reconstructed patient-specific AVF was investigated, using computational fluid dynamics (CFDs) and particle image velocimetry (PIV). The aim of this study was to validate the methodology from medical images to numerical simulations of an AVF by comparing numerical and experimental data. Two numerical grids were presented with a refinement difference of a factor of four. A mold of the same volume was created and mounted on an experimental bench with similar boundary conditions. The patients acquired echo D006Fppler flow waveform was injected at the arterial inlet. Experimental and numerical velocity vector cartography qualitatively produced similar flow fields. Quantification with a point-to-point approach thoroughly investigated the velocity profiles using the mean difference between both results. The finest mesh generated CFD results with a mean percentage of the difference in velocity magnitude, taking the PIV as reference, did not exceed 10%. At specific zones, the coarse mesh required adaptive meshing to improve fitting with experimental data. Meshing refinement was necessary to improve velocity accuracy at wide diameters and wall shear stress at narrow diameters. Provided that these criteria were properly respected, we show through this difficult example the validity of using CFD to properly describe flow patterns in image-based reconstructed blood vessels.

Numerical simulation of the fluid structure interactions in a compliant patient-specific arteriovenous fistula

The objective of the study is to investigate numerically the fluid-structure interactions (FSI) in a patient-specific arteriovenous fistula (AVF) and analyze the degree of complexity that such a numerical simulation requires to provide clinically relevant information. The reference FSI simulation takes into account the non-Newtonian behavior of blood, as well as the variation in mechanical properties of the vascular walls along the AVF. We have explored whether less comprehensive versions of the simulation could still provide relevant results. The non-Newtonian blood model is necessary to predict the hemodynamics in the AVF because of the predominance of low shear rates in the vein. An uncoupled fluid simulation provides informative qualitative maps of the hemodynamic conditions in the AVF; quantitatively, the hemodynamic parameters are accurate within 20% maximum. Conversely, an uncoupled structural simulation with non-uniform wall properties along the vasculature provides the accurate distribution of internal wall stresses, but only at one instant of time within the cardiac cycle. The FSI simulation advantageously provides the time-evolution of both the hemodynamic and structural stresses. However, the higher computational cost renders a clinical use still difficult in routine.

Numerical study of the influence of wall compliance on the haemodynamics in a patient-specific arteriovenous fistula

An arteriovenous fistula (AVF) is a surgical vessel connection between an artery and a vein. It is created in end stage renal disease to provide adequate blood access (300-500 ml.min -1 in the vein) for hemodialysis. In the present study, the local hemodynamics is investigated in a patient-specific AVF using a computational fluid structure interaction (FSI) simulation. We focus on an end-to-side fistula between the end of the cephalic vein and the brachial artery. The geometry of the vessel lumen is obtained by volume reconstruction from CT-scan angiography. The vessel wall is modelled as a monolayer of shell elements of uniform thickness, since the actual wall thickness cannot be obtained from medical images. The fluid and solid governing equations are solved coupling implicitly ANSYS-CFX and ANSYS-Mechanical (ANSYS, Inc.). A physiological time-dependent velocity inlet profile and constant values of outlet resistance are imposed as boundary conditions for the blood flow. The 3 rd order Yeoh constitutive law is used to model the hyperelastic behaviour of the vessel wall. We quantify the timeand space-evolution of wall shear stress (WSS) and oscillatory shear index (OSI) and compare the results with rigid wall simulations to quantify the effect of the vessel wall compliance. The results confirm that AVFs are subjected to complex hemodynamics, the vein being in particular dominated by recirculating flows. Regions of pathological WSS and OSI match regions that are mainly prone to wall lesions in clinical practice. Rigid wall simulations overestimate the WSS by 1013%, the OSI by about 18% and the WSS temporal gradients at the anastomosis by 10%.

Comparison of two endovascular treatments of a stenosed arteriovenous fistula: balloon-angioplasty with and without stenting

Purpose Arteriovenous fistulas (AVFs) are created in patients to enable a permanent vascular access for hemodialysis. The AVF causes changes in the hemodynamic conditions leading to possible complications, stenoses being the most common one. Our objective was to compare the effect of treating the stenosed AVF by balloon-angioplasty, whether followed or not with stenting. Methods We considered an AVF presenting an 60% arterial stenosis and simulated the two endovascular treatments using an implicit approach. We then simulated the fluid-structure interactions (FSI) within (i) the patient-specific stenosed AVF, (ii) the AVF after angioplasty, and (iii) the AVF after angioplasty plus stenting with ANSYS Workbench. Results We show that a self-expandable stent does not modify the curvature of the vessel after angioplasty; it only increases the local Young modulus of the stented wall by an order of magnitude. The results of the FSI simulations indicate that the two treatments induce the same hemodynamic conditions: they both reduce the pressure difference across the stenosis, while maintaining the flow distribution downstream of the stenosis. The venous flow rate that has to be guaranteed for hemodialysis is unaltered. Thanks to its large axial flexibility, the self-expandable stent causes at maximum a three-fold increase in the internal wall stresses at peak systole as compared to angioplasty alone. Conclusions By maintaining the vessel lumen shape over time, the stent is likely to reduce the risk of restenosis that can otherwise occur after balloon-angioplasty because of the viscoelastic recoil of the vessel.

Treatment of a stenosed arteriovenous fistula by balloon angioplasty with or without stenting: comparison of effects on haemodynamics

An arteriovenous fistula (AVF) is a permanent vascular access created by connecting a vein to an artery in patients with end-stage renal failure requiring haemodialysis. The formation of stenosis is one of the major complications that affect the AVF efficiency. The lesion can be treated endovascularly by balloon angioplasty combined or not with stent deployment. The clinical efficacy of stenting has been debated since the late 1980s (Zollikofer et al. 1992). The objective of this study was to model numerically the two treatment options and to estimate their impact on haemodynamics and wall mechanics.

NUMERICAL STUDY OF THE EFFECTS OF ANGIOPLASTY AS TREATMENT OF A STENOSED ARTERIOVENOUS FISTULA

Most patients with end-stage renal disease require hemodialysis, which relies on a permanent vascular access. It is obtained by surgically connecting an artery and a vein, creating an arteriovenous fistula (AVF). The AVF lifespan is limited, the failure being due to insufficient or excessive blood flow inside the vein. Although the reason for failure remains an open question, it has been observed that stenoses are present in about 20% of mature AVF [Van Tricht, 2005]. Numerous studies demonstrated the efficiency of balloon angioplasty in the treatment of stenotic lesions, but this procedure is associated with a high rate of recurrent stenosis. Nevertheless, it continues to be commonly used in clinical practice and the use of stenting procedure remains controversial [Chan, 2008]. Our aim is to investigate the fluid-structure interactions (FSI) in a patient-specific case before and after a numericallysimulated balloon angioplasty treatment.

CFD modeling of unsteady blood flow in a arteriovenous fistula reconstructed from angioscanner

We propose an investigation protocol to understand the correlation between hemodynamic changes in a very specific blood vessel (arteriovenous fistula created in hemodialysis patients) and the associated clinical complications. Realistic 3D geometry of the fistula is the result of the reconstruction of segmented CT angiography images. Then the geometrical model is treated with industrial computation fluid dynamics software for blood flow simulation. Key words: Hemodynamic, ESRD, Recirculation phenomena, Angiography