The Effect of Arterial Curvature on Blood Flow in Arterio-Venous Fistulae: Realistic Geometries and Pulsatile Flow

Optimal Renal Artery-Aorta Angulation Revealed by Flow Simulation

INTRODUCTION

In the circulatory system, the vessel branching angle may have hemodynamic consequences. We hypothesized that there is a hemodynamically optimal range for the renal artery's branching angle.

METHODS

Data on the posttransplant kinetics of estimated glomerular filtration rate (eGFR) were analyzed according to the donor and implant sides (right-to-right and left-to-right position; n = 46). The renal artery branching angle from the aorta of a randomly selected population was measured using an X-ray angiogram (n = 44). Computational fluid dynamics simulation was used to elucidate the hemodynamic effects of angulation.

RESULTS AND DISCUSSION

Renal transplant patients receiving a right donor kidney to the right side showed faster adaptation and higher eGFR values than those receiving a left donor kidney to the right side (eGFR: 65 ± 7 vs. 56 ± 6 mL/min/1.73 m2; p < 0.01). The average branching angle on the left side was 78° and that on the right side was 66°. Simulation results showed that the pressure, volume flow, and velocity were relatively constant between 58° and 88°, indicating that this range is optimal for the kidneys. The turbulent kinetic energy does not change significantly between 58° and 78°.

CONCLUSION

The results suggest that there is an optimal range for the renal artery's branching angle from the aorta where hemodynamic vulnerability caused by the degree of angulation is the lowest, which should be considered during kidney transplantations.

A realistic arteriovenous dialysis graft model for hemodynamic simulations

Objective

The hemodynamic benefit of novel arteriovenous graft (AVG) designs is typically assessed using computational models that assume highly idealized graft configurations and/or simplified boundary conditions representing the peripheral vasculature. The objective of this study is to evaluate whether idealized AVG models are suitable for hemodynamic evaluation of new graft designs, or whether more realistic models are required.

Methods

An idealized and a realistic, clinical imaging based, parametrized AVG geometry were created. Furthermore, two physiological boundary condition models were developed to represent the peripheral vasculature. We assessed how graft geometry (idealized or realistic) and applied boundary condition models of the peripheral vasculature (physiological or distal zero-flow) impacted hemodynamic metrics related to AVG dysfunction.

Results

Anastomotic regions exposed to high WSS (>7, ≤40 Pa), very high WSS (>40 Pa) and highly oscillatory WSS were larger in the simulations using the realistic AVG geometry. The magnitude of velocity perturbations in the venous segment was up to 1.7 times larger in the realistic AVG geometry compared to the idealized one. When applying a (non-physiological zero-flow) boundary condition that neglected blood flow to and from the peripheral vasculature, we observed large regions exposed to highly oscillatory WSS. These regions could not be observed when using either of the newly developed distal boundary condition models.

Conclusion

Hemodynamic metrics related to AVG dysfunction are highly dependent on the geometry and the distal boundary condition model used. Consequently, the hemodynamic benefit of a novel graft design can be misrepresented when using idealized AVG modelling setups.

Prediction of the anastomosis angle of arteriovenous fistula in hemodialysis to standardize the surgical technique

BACKGROUND: Arteriovenous fistula (AVF) is the preferred route of vascular access in hemodialysis. The primary reason of fistula failure is Intimal hyperplasia (IH), which leads to stenosis. Wall shear stress (WSS) and disturbed flow are the critical parameters in the formation of IH. OBJECTIVE: The primary goal of this study is to explore the influence of anastomosis angle on WSS and venous outflow rate, as well as to find the ideal angle of anastomosis for AVF to standardize surgical technique. METHODS: Three-dimensional idealized geometries of end-to-side type AVF for the five various angles of anastomosis are considered in this study. The WSS, blood flow rate at the venous outlet for non-Newtonian, pulsatile blood flow are calculated using a numerical simulation technique. RESULTS: The WSS is higher at 75° compared to other angles and least at 45° for pulsating arterial inflows. The WSS is moderate at 30°, 60° and 90°. On the arterial bed and outer wall of the vein, immediately after the anastomosis, the recirculation zone is observed. At an angle of 45° and 90° anastomosis, the outflow rate is greater at distal venous end. CONCLUSIONS: If one believes that high wall shear stress causes IH within the AVF, the results suggest that the AVF should be formed at a 45° angle to avoid IH. However, if one believes that low wall shear stress causes IH within the AVF, the results suggest that AVF should be formed at either 30° or 75° to avoid IH. The findings spotlight the importance of anastomosis angle in determining AVF hemodynamics.

Hydrodynamic Approach for Revealing Venous Anastomotic Stenosis Formation Within a Dialysis Arteriovenous Graft

A conventional arteriovenous graft in patients on dialysis often leads to anastomotic stenosis, which decreases the blood flow rate and increases the risk of complications. In this study, based on hydrodynamics, the pulsatile pressure at the blood vessel graft-vein junction was investigated experimentally and numerically for revealing the causes of stenosis formation and inward remodeling. In the experiments, the pulsatile pressure and displacement at the anastomotic connection were measured at a branched collapsible tube. It was revealed that the pressure becomes negative between pressure peaks of the pulsatile flow; furthermore, tube diameter changes in accordance with the pressure pulsation. Subsequently, numerical simulations revealed that a relatively large pressure difference occurs at the anastomotic connection because of flow collision and separation as compared with the other part, and the pulsatile pressure. Therefore, it is possible that vein at an anastomotic connection may change its shape under pulsating flow. Furthermore, it was found that the pressure difference slightly increased with the anastomosis angle, but the anastomosis angle did not affect the flow rate. Clinical trials in the next step are required to reveal the causal relationship between stenosis and the pulsatile pressure, but the pulsatile flow and its pressure are likely to be one factor in stenosis and inward remodeling.

The Story of Wall Shear Stress in Coronary Artery Atherosclerosis: Biochemical Transport and Mechanotransduction

Coronary artery atherosclerosis is a local, multifactorial, complex disease, and the leading cause of death in the US. Complex interactions between biochemical transport and biomechanical forces influence disease growth. Wall shear stress (WSS) affects coronary artery atherosclerosis by inducing endothelial cell mechanotransduction and by controlling the near-wall transport processes involved in atherosclerosis. Each of these processes is controlled by WSS differently and therefore has complicated the interpretation of WSS in atherosclerosis. In this paper, we present a comprehensive theory for WSS in atherosclerosis. First, a short review of shear stress-mediated mechanotransduction in atherosclerosis was presented. Next, subject-specific computational fluid dynamics (CFD) simulations were performed in ten coronary artery models of diseased and healthy subjects. Biochemical-specific mass transport models were developed to study low-density lipoprotein, nitric oxide, adenosine triphosphate, oxygen, monocyte chemoattractant protein-1, and monocyte transport. The transport results were compared with WSS vectors and WSS Lagrangian coherent structures (WSS LCS). High WSS magnitude protected against atherosclerosis by increasing the production or flux of atheroprotective biochemicals and decreasing the near-wall localization of atherogenic biochemicals. Low WSS magnitude promoted atherosclerosis by increasing atherogenic biochemical localization. Finally, the attracting WSS LCS's role was more complex where it promoted or prevented atherosclerosis based on different biochemicals. We present a summary of the different pathways by which WSS influences coronary artery atherosclerosis and compare different mechanotransduction and biotransport mechanisms.

Assessment of the Hemodynamics of Autogenous Arteriovenous Fistulas With 4D Phase Contrast-Based Flow Quantification MRI in Dialysis Patients

BACKGROUND

Regular monitoring of autogenous arteriovenous fistulas (AVFs) for hemodialysis patients has importance. Hence, 4D flow MRI may be an alternative for assessing the hemodynamics of AVFs.

PURPOSE

To compare the hemodynamics of AVFs using Doppler ultrasound (DUS) and 4D-MRI in renal dialysis patients.

STUDY TYPE

Case-control study from October 2017 to April 2018.

POPULATION

Fifty patients (age [range] = 59.52 [39-71] years) with AVFs were included.

FIELD STRENGTH/SEQUENCE

Black-blood MRI and 4D flow MRI at 3.0T and AVF ultrasonography were also performed.

ASSESSMENT

The hemodynamics acquired from 4D flow MRI and ultrasonography by two radiologists were compared. The AVF anatomy was described through an examination of the black-blood MRI.

STATISTICAL TESTS

The consistency of AVF anatomy and hemodynamics and the consistency of the hemodynamics of AVFs from 4D flow MRI and ultrasound were analyzed by paired t-tests. The morphological parameters of AVFs acquired from black-blood MRI were used for a Pearson correlation analysis with the hemodynamic parameters obtained from 4D flow MRI data.

RESULTS

The consistency of the morphological and hemodynamic parameters measured from MRI by the two radiologists was good (all P < 0.01). The velocities and flow volumes from the 4D flow MRI and vascular ultrasound of AVFs were in moderate agreement (all P < 0.05, r = 0.292-0.569), except for the peak flow velocity at the anastomosis (P = 0.366, r = -0.078). The flow volume and WSS near the anastomotic site were closely related to the morphology of the AVFs (all P < 0.05). The hemodynamics of the complications group were significantly different from those of patients without any complications (normal patients group) (all P < 0.01).

DATA CONCLUSION

Compared with ultrasonography, 4D flow MRI is a promising technique to noninvasively estimate the AVF hemodynamics of renal dialysis patients.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2020;51:1272-1280.

Towards Additive Manufacture of Functional, Spline-Based Morphometric Models of Healthy and Diseased Coronary Arteries: In Vitro Proof-of-Concept Using a Porcine Template

The aim of this study is to assess the additive manufacture of morphometric models of healthy and diseased coronary arteries. Using a dissected porcine coronary artery, a model was developed with the use of computer aided engineering, with splines used to design arteries in health and disease. The model was altered to demonstrate four cases of stenosis displaying varying severity, based on published morphometric data available. Both an Objet Eden 250 printer and a Solidscape 3Z Pro printer were used in this analysis. A wax printed model was set into a flexible thermoplastic and was valuable for experimental testing with helical flow patterns observed in healthy models, dominating the distal LAD (left anterior descending) and left circumflex arteries. Recirculation zones were detected in all models, but were visibly larger in the stenosed cases. Resin models provide useful analytical tools for understanding the spatial relationships of blood vessels, and could be applied to preoperative planning techniques, but were not suitable for physical testing. In conclusion, it is feasible to develop blood vessel models enabling experimental work; further, through additive manufacture of bio-compatible materials, there is the possibility of manufacturing customized replacement arteries.