Association of Shear Stress with Subsequent Lumen Remodeling in Hemodialysis Arteriovenous Fistulas

Vascular remodeling in arteriovenous fistula treated with PDE5A inhibitors

The arteriovenous fistula (AVF) is the lifeline for hemodialysis patients. However, there are currently no effective therapies promoting AVF maturation. AVF dilation by smooth muscle cell relaxation, through increased cyclic guanosine monophosphate (cGMP), is one potential mechanism to improve AVF remodeling. In this study, we examined the cGMP pathway and its inhibitor phosphodiesterase 5A (PDE5A) in rat, pig, and human AVF. We administered the PDE5A inhibitor, sildenafil, to rats with femoral AVFs and analyzed AVF histological and hemodynamic parameters. We observed that AVF creation increases PDE5A expression in rodent and porcine AVF models. Similarly, we observed an increase in PDE5A expression in the anastomotic regions of AVFs from hemodialysis patients when compared to pre-AVF placement. Sildenafil-treated rats showed significantly increased ultrasound-derived AVF volumetric blood flow and increased MRI-derived 3-dimensional lumen diameter when compared to controls. MRI-based computational fluid dynamics showed that sildenafil-treated rats had increased anastomotic hemodynamics compared to control rats. Histology showed similar intimal hyperplasia in sildenafil-treated and control rats. In conclusion, sildenafil treatment increases AVF vein outward expansion and blood flow without affecting the level of intimal hyperplasia. PDE5A inhibitors serve as a potential therapeutic approach to promote AVF maturation by enhancing outward vascular remodeling.

Temporal evolution of hemodynamics in murine arteriovenous fistula: a micro-CT based computational fluid dynamics study

In this study, we investigated the hemodynamic characteristics of arteriovenous fistulae (AVF) in murine models using micro-CT based computational fluid dynamics (CFD). By combining high-resolution micro-CT imaging with ultrasound flow measurements, our methodology offers a cost-effective and efficient alternative to traditional MRI-based approaches. CFD simulations performed at 7 and 21 days post-surgery revealed significant temporal changes in both geometry and hemodynamics. Geometric analysis showed that: the proximal artery diameter increased from 0.29 mm to 0.38 mm, while the initial 2 mm fistula segment showed a 21.6% decrease (0.74 mm to 0.58 mm). Blood flow through the AVF nearly doubled from 1.33 mL/min to 2.57 mL/min. Time-averaged wall shear stress (TAWSS) peak values increased from 142 Pa (day 7) within the proximal artery to 200 Pa (day 21), in the stenotic region. The oscillatory shear index (OSI) showed marked elevation at the anastomosis (increasing from 0.22 to 0.48), indicating disturbed flow development. An inverse relationship between TAWSS and OSI was identified consistent with previous studies. Our methodology demonstrates the capability to analyze relationships between early hemodynamics and subsequent geometric changes. This approach could enable identification of regions susceptible to stenosis development and monitoring of AVF maturation, which could ultimately lead to quantitative metrics to evaluate surgical outcomes and early therapeutic interventions.

The role of cellular senescence in endothelial dysfunction and vascular remodelling in arteriovenous fistula maturation

Haemodialysis (HD) is often required for patients with end-stage renal disease. Arteriovenous fistulas (AVFs), a surgical procedure connecting an artery to a vein, are the preferred vascular access for HD due to their durability and lower complication rates. The aim of AVFs is to promote vein remodelling to accommodate increased blood flow needed for dialysis. However, many AVFs fail to mature properly, making them unsuitable for dialysis. Successful maturation requires remodelling, resulting in an increased luminal diameter and thickened walls to support the increased blood flow. After AVF creation, haemodynamic changes due to increased blood flow on the venous side of the AVF initiate a cascade of events that, when successful, lead to the proper maturation of the AVF, making it suitable for cannulation. In this process, endothelial cells play a crucial role since they are in direct contact with the frictional forces exerted by the blood, known as shear stress. Patients requiring HD often have other conditions that increase the burden of senescent cells, such as ageing, diabetes and hypertension. These senescent cells are characterized by irreversible growth arrest and the secretion of pro-inflammatory and pro-thrombotic factors, collectively known as the senescence-associated secretory phenotype (SASP). This accumulation can impair vascular function by promoting inflammation, reducing vasodilatation, and increasing thrombosis risk, thus hindering proper AVF maturation and function. This review explores the contribution of senescent endothelial cells to AVF maturation and explores potential therapeutic strategies to alleviate the effects of senescent cell accumulation, aiming to improve AVF maturation rates.

Hemodynamics are associated with subsequent lumen remodeling and clinical maturation of hemodialysis arteriovenous fistula

The pathogenesis of arteriovenous fistula (AVF) maturation failure is unclear. We evaluated the associations of wall shear stress (WSS) with subsequent AVF remodeling and clinical maturation using regression models in this prospective cohort study. Participants underwent duplex ultrasound at postoperative Day 1, Week 2, and Week 6 to measure AVF blood flow rate and diameter of the draining vein and proximal artery. The median vein WSS of 602 participants decreased from Day 1 to Week 6 (from 33.4 to 21.6 dyne/cm2) but did not change noticeably for the artery (from 58.4 to 55.1 dyne/cm2). WSS was positively associated with subsequent lumen expansion, with doubling of Day-1 WSS presaging a 9% (95% confidence interval (CI) 5%-14%; P < 0.001) greater Day 1-to-Week 6 increase in vein lumen cross-sectional area and a 5% (95% CI: 1%-10%; P = 0.020) greater increase in artery lumen area. The odds of unassisted clinical maturation increased by 45% (95% CI: 11%-89%; P = 0.006) with each doubling of Day-1 vein WSS, and by 82% (95% CI: 39%-250%; P < 0.001) with each doubling of Day-1 artery WSS. These findings show that WSS was positively associated with subsequent lumen expansion and AVF unassisted clinical maturation.

The Potential of Sound Analysis to Reveal Hemodynamic Conditions of Arteriovenous Fistulae for Hemodialysis

PURPOSE

Arteriovenous fistula (AVF), the preferred vascular access for hemodialysis, is associated with high failure rate. The aim of this study was to investigate the potential of AVF sound auscultation in providing quantitative information on AVF hemodynamic conditions.

METHODS

This single-center prospective study involved six patients with native radio-cephalic AVFs who underwent multiple follow-up visits. Doppler Ultrasound blood flow volume (BFV) assessment and electronic stethoscope-based sound recordings were performed during each visit, whereas MRIs were acquired 3 days, 3 weeks and 1 year after surgery. Computational fluid dynamic (CFD) simulations were performed on patient-specific MRI-derived geometrical models.

RESULTS

Higher values of median peak amplitudes ratios (high-low peak ratio-HLPR) were found to be associated with complex blood flow and velocity streamlines recirculation at systolic peak, and corresponding extended regions of high oscillatory shear index (OSI). On the contrary, lower values of HLPR were associated with laminar flow pattern and low values of OSI. Significant differences were observed in HLPR between subgroups with extended or limited areas with OSI > 0.1 (0.67 vs 0.31, respectively). Significant relationships were found between AVF sound intensity and brachial BFV (slope = 0.103, p < 0.01) as well as between longitudinal changes in brachial BFV and HLPR (slope = - 0.001, p < 0.01).

CONCLUSION

Our results show that AVF sound can be exploited to extract fundamental information on AVF hemodynamic conditions, providing indication of the presence of complex hemodynamic and adequate BFV to perform hemodialysis. Sound analysis has therefore the potential to improve clinical AVF surveillance and to ameliorate outcome.

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.

Effect of early cannulation with metal needles on arteriovenous fistula patency in patients undergoing hemodialysis

BACKGROUND

Autogenous arteriovenous fistula (AVF) is the preferred vascular access mode. However, the earliest possible time for AVF puncture and whether premature puncture affects the AVF patency rate remain unclear.

METHODS

In this multicenter retrospective cohort study, adult uremic patients who underwent AVF surgery for the first time at Taizhou Hospital or Enze Hospital of Zhejiang Province between September 1, 2018 and August 31, 2021 were enrolled. All patients were followed up for 1 year after puncture, and the status of fistula establishment and puncture, subsequent patency, loss to follow-up, renal transplantation, conversion to peritoneal dialysis, abandonment of the fistula, and death, were recorded.

RESULTS

A total of 465 patients with AVFs were included in this study, including 59 (12.7%) patients with fistulas that were cannulated within 30 days. In the early puncture group, the levels of serum creatinine and urea nitrogen were higher, while the levels of hemoglobin and albumin were lower, suggesting that these patients needed urgent dialysis. Furthermore, the rate of non-cuffed catheter use was higher, while the rate of cuffed catheter use was lower, and femoral vein puncture was preferred over internal jugular vein puncture. The mean duration of catheter indwelling was shorter in the early puncture group (19 vs 70 days, p < 0.001). The estimated AVF primary and cumulative functional patency at 12 months was 81.1% versus 82.3% and 98.3% versus 98.7% in the early puncture and control groups, respectively. Kaplan-Meier analysis revealed no significant difference in AVF primary and cumulative functional patency between the two groups.

CONCLUSIONS

In patients with an established fistula in urgent need of hemodialysis, to avoid new catheterization, a puncture can be performed within 30 days in those with well-developed blood vessels after adequate ultrasound and clinical evaluation without affecting the patency of the fistula.

Arteriovenous Fistula Histology, Hemodynamics, and Wall Mechanics: A Case Report of Successful and Failed Access in a Single Patient

The low 1-year patency rate of mature arteriovenous fistulas (AVFs) remains a significant clinical problem. Although vessel properties and biomechanics have been suggested to affect AVF function, understanding their roles in AVF patency failure is challenging owing to the heterogeneity within the patient population, including demographics and comorbid conditions. In this study, we present a case of a patient with 2 upper-arm AVFs with different 1-year patency outcomes and investigate whether they had different histologic features before the AVF creation surgery and biomechanics at 1 day and 6 weeks after the AVF creation surgery using magnetic resonance imaging-based fluid structure interaction simulations. Despite both AVFs being in the upper arm, created <1 year apart by the same surgeon, and having similar preoperative vessel diameters, the 1-year patent AVF had less preoperative intimal collagen and higher wall shear stress 1 day after AVF creation, when compared with the AVF that failed by 1 year. Thus, a low intimal collagen content before the AVF surgery and higher wall shear stress immediately after the AVF creation surgery may be important for long-term AVF patency and should be investigated with larger cohorts.

Effects of endothelial nitric oxide synthase on mouse arteriovenous fistula hemodynamics

Newly created arteriovenous fistulas (AVFs) often fail to mature for dialysis use due to disturbed blood flow at and near the AVF anastomosis. The disturbed flow inhibits the endothelial nitric oxide synthase (NOS3) pathway, thus decreasing the production of nitric oxide, a vasodilator. Previously, our group reported that NOS3 expression levels affect AVF lumen size in a mouse model. In this study, we performed MRI-based computational fluid dynamics simulations to investigate the hemodynamical parameters (velocity, wall shear stress (WSS), and vorticity) in a mouse AVF model at day 7 and day 21 post-AVF creation using three NOS3 strains: overexpression (OE), knockout (KO), and wild-type (WT) control. This study is the first to reveal hemodynamics over time in mouse AVFs, consider spatial heterogeneity along the vein, and reveal the effect of NOS3 on the natural history of mouse AVF hemodynamics. From day 7 to day 21, OE has smoother streamlines and had significantly lower vorticity and WSS than WT and KO, suggesting that WSS was attempting to return to pre-surgery baseline, respectively. Our results conclude that the overexpression of NOS3 leads to desired optimal hemodynamics during AVF remodeling. Future studies can investigate enhancing the NOS3 pathway to improve AVF development.

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.