Toward a physiological model of vascular wall vibrations in the arteriovenous fistula

The mechanism behind hemodialysis arteriovenous fistula (AVF) failure remains poorly understood, despite previous efforts to correlate altered hemodynamics with vascular remodeling. We have recently demonstrated that transitional flow induces high-frequency vibrations in the AVF wall, albeit with a simplified model. This study addresses the key limitations of our original fluid-structure interaction (FSI) approach, aiming to evaluate the vibration response using a more realistic model. A 3D AVF geometry was generated from contrast-free MRI and high-fidelity FSI simulations were performed. Patient-specific inflow and pressure were incorporated, and a three-term Mooney-Rivlin model was fitted using experimental data. The viscoelastic effect of perivascular tissue was modeled with Robin boundary conditions. Prescribing pulsatile inflow and pressure resulted in a substantial increase in vein displacement ( + 400 %) and strain ( + 317 %), with a higher maximum spectral frequency becoming visible above -42 dB (from 200 to 500 Hz). Transitioning from Saint Venant-Kirchhoff to Mooney-Rivlin model led to displacement amplitudes exceeding 10 micrometers and had a substantial impact on strain ( + 116 %). Robin boundary conditions significantly damped high-frequency displacement ( - 60 %). Incorporating venous tissue properties increased vibrations by 91%, extending up to 700 Hz, with a maximum strain of 0.158. Notably, our results show localized, high levels of vibration at the inner curvature of the vein, a site known for experiencing pronounced remodeling. Our findings, consistent with experimental and clinical reports of bruits and thrills, underscore the significance of incorporating physiologically plausible modeling approaches to investigate the role of wall vibrations in AVF remodeling and failure.

Computational fluid-structure interaction analysis of the end-to-side radio-cephalic arteriovenous fistula

BACKGROUND AND OBJECTIVE

In the current work, we present a descriptive fluid-structure interaction computational study of the end-to-side radio-cephalic arteriovenous fistula. This allows us to account for the different thicknesses and elastic properties of the radial artery and cephalic vein.

METHODS

The core of the work consists in simulating different arteriovenous fistula configurations obtained by virtually varying the anastomosis angle, i.e. the angle between the end of the cephalic vein and the side of the radial artery. Since the aim of the work is to understand the blood dynamics in the very first days after the surgical intervention, the radial artery is considered stiffer and thicker than the cephalic vein.

RESULTS

Our results demonstrate that both the diameter of the cephalic vein and the anastomosis angle play a crucial role to obtain a blood dynamics without re-circulation regions that could prevent fistula failure.

CONCLUSIONS

When an anastomosis angle close to the perpendicular direction with respect to the radial artery is combined with a large diameter of the cephalic vein, the recirculation regions and the low Wall Shear Stress (WSS) zones are reduced. Conversely, from a structural point of view, a low anastomosis angle with a large diameter of the cephalic vein reduces the mechanical stress acting on the vessel walls.

Percutaneous ecoguided endovascular lithotripsy to recruit failed-to-mature arteriovenous fistula due to completely calcified radial artery

The first-choice vascular access for starting dialysis is autogenous distal forearm arteriovenous-fistula (AVF); the increasing demand to create more fistulas may lead to their creation in borderline vessels and, in this setting, the early failure (EF) and failure of maturation (FTM) remain the main issues. The size of vessels or preexisting stenotic lesions of artery or vein are no longer considered absolute exclusion criteria for the creation of distal AVF, but huge arterial calcification still represents an indication to create upper arm AVF. A novel approach to treat arterial calcifications is represented by intravascular lithotripsy (IVL). This technique could represent a valid option to save failed to mature AVF due to extended calcified artery. We describe a case of a male patient, 43 years old with middle forearm AVF failed to mature with a completely calcified radial artery, low brachial flow (Qa) and small efferent vein. We treated the patient AVF with less invasive, percutaneous, endovascular, eco-guided IVL on the entire radial artery. After the procedure was observed a rapid increase of Qa, with reduction of calcification in the arterial wall, increase of arterial caliper and flow. This procedure could represent a valid alternative to surgical upper-arm AVF creation in patient with functioning but failed to mature fistula due to spread artery calcification, with a rapid, less invasive procedure.

Pentoxifylline decreases the activity of the nucleotide-binding oligomerization domain-like receptor protein 3 pathway: potential role for preventing arteriovenous fistula stenosis

PURPOSE

This study aimed to determine the effect of pentoxifylline (PTX) on the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome pathway and its role in preventing arteriovenous fistula (AVF) failure.

METHODS

Vein samples were collected from AVF failure patients and from patients who underwent surgical AVF as a control. The expressions of CD34 and NLRP3 in AVF tissues were detected by immunohistochemistry and Western blotting. Arteriovenous fistula rat models were established by the end-to-end anastomosis of the common carotid artery and external jugular vein. The AVF models were divided into the following groups: AVF, AVF + PTX, AVF + uraemia and AVF + uraemia + PTX. Six weeks after surgery, the AVF tissues in each group were collected to detect the expressions of CD34, NLRP3, caspase-1 and interleukin (IL)-1β by immunohistochemistry, Western blotting and real-time polymerase chain reaction.

RESULTS

The expressions of NLRP3 and CD34 in human AVF failure tissues were significantly higher than those in normal veins (p < 0.001), indicating that NLRP3 was upregulated in patients with AVF failure. In our animal study, the veins in the AVF + uraemia group exhibited heavy hyperplasia, and the boundary between the media and the adventitia was not clear. However, PTX alleviated this hyperplasia. Compared with the AVF models, the AVF + uraemia models had much higher expressions of NLRP3, caspase-1, IL-1β and CD34 (p < 0.001). However, PTX had the opposite effect against uraemia on the NLRP3 inflammasome pathway at both the gene and protein levels.

CONCLUSIONS

Our findings provide new insights that show that PTX can decrease the activity of the NLRP3 inflammasome pathway in AVF models. Pentoxifylline has the potential as a drug for preventing intimal hyperplasia and AVF failure.

A "Snake-Like" Vessel in the Thorax

We report a congenital extracardiac arteriovenous fistula revealed incidentally in a patient undergoing computed tomographic coronary angiography for angina. This clinical vignette panel describes the origin and the trajectory of this rare vascular lesion.

Hemodynamics in AVF over time: A protective role of vascular remodeling toward flow stabilization

The mechanisms underlying vascular stenosis formation in the arteriovenous fistula (AVF) for hemodialysis (HD) remain mostly unknown. Several computational fluid dynamics (CFD) studies have suggested a potential role for unsteady flow in inducing intimal hyperplasia and AVF stenosis, but the majority of these observations have been limited to a single time point after surgical creation. The aim of the present study was to investigate the relation between hemodynamic conditions and AVF vascular remodeling through a CFD longitudinal study. Non contrast-enhanced MR images and Doppler Ultrasound (US) examinations were acquired at 3 days, 40 days, 6 months, 1 year, and 1.5 years after surgery in a 72-year male referred for native radio-cephalic AVF. Three-dimensional AVF models were generated and high fidelity CFD simulations were performed using pimpleFoam, setting patient-specific boundary conditions derived from US. Morphological and hemodynamic changes over time were then analyzed. Analysis of vessel morphology and hemodynamics during follow-up showed that the AVF had a successful maturation process, characterized by a massive arterial and venous dilatation within the 6 months after surgery, a corresponding increase in blood flow volume and important flow instabilities. Between 6 months and 1 year, a stenosis developed in the juxta-anastomotic vein and caused AVF failure at 1.5 years. The development of stenosis was paralleled by the regularization of blood flow velocity pattern and consequent decrease in the near-wall disturbed flow metrics. These results suggest that development of intimal hyperplasia and vessel stenosis, triggered by unsteady flow, could be the result of vascular inward remodeling toward regularization of turbulent-like flow.

A longitudinal study of the arterio-venous fistula maturation of a single patient over 15 weeks

Arterio-venous fistula creation is the preferred vascular access for haemodialysis, but has a large failure rate in the maturation period. Previous research, considering the remodelling mechanisms for failure-to-mature patients, has been limited by obtaining the patient-specific boundary conditions at only a few points in the patient history. Here, a non-invasive imaging system was used to reconstruct the three-dimensional vasculature, and computational fluid dynamics was used to analyse the haemodynamics for one patient over 15 weeks. The analysis suggested evidence of a control mechanism, which adjusts the lumen diameter to keep the wall shear stress near constant in the proximal regions of the vein and artery. Additionally, the vein and artery were shown to remodel at different growth rates, and the blood flow rate also saw the largest increase within the first week. Wall shear stress at time of creation may be a useful indicator for successful AVF maturation.

Expert consensus on the establishment and maintenance of native arteriovenous fistula

Vascular access is the lifeline of hemodialysis patients. There are great differences in the establishment and use of vascular access in different countries and regions around the world. We believe that on the basis of good evaluation and planning, it is recommended that hemodialysis patients choose native arteriovenous fistula first. In view of the new progress of vascular access views domestic and international at home and abroad in recent years, we organized experts to recommend the establishment and maintenance of arteriovenous fistula (AVF) for the Chinese population, including preoperative evaluation and planning of the establishment of AVF, AVF surgery, perioperative drug intervention measures and postoperative maintenance, and put forward suggestions for future research directions. The recommendations in this consensus are general and clinicians need to make treatment decisions based on the actual situation.