Effects of venous needle turbulence during ex vivo hemodialysis on endothelial morphology and nitric oxide formation

The Role of Cardio-Renal Inflammation in Deciding the Fate of the Arteriovenous Fistula in Haemodialysis Therapy

Vascular access is an indispensable component of haemodialysis therapy for end-stage kidney disease patients. The arteriovenous fistula (AVF) is most common, but importantly, two-year failure rates are greater than fifty percent. AVF failure can occur due to a lack of suitable vascular remodelling, and inappropriate inflammation preventing maturation, or alternatively neointimal hyperplasia and vascular stenosis preventing long-term use. A comprehensive mechanistic understanding of these processes is still lacking, but recent studies highlight an essential role for inflammation from uraemia and the AVF itself. Inflammation affects each cell in the cascade of AVF failure, the endothelium, the infiltrating immune cells, and the vascular smooth muscle cells. This review examines the role of inflammation in each cell step by step and the influence on AVF failure. Inflammation resulting in AVF failure occurs initially via changes in endothelial cell activation, permeability, and vasoprotective chemokine secretion. Resultingly, immune cells can extravasate into the subendothelial space to release inflammatory cytokines and cause other deleterious changes to the microenvironment. Finally, all these changes modify vascular smooth muscle cell function, resulting in excessive and unchecked hyperplasia and proliferation, eventually leading to stenosis and the failure of the AVF. Finally, the emerging therapeutic options based off these findings are discussed, including mesenchymal stem cells, small-molecule inhibitors, and far-infrared therapies. Recent years have clearly demonstrated a vital role for inflammation in deciding the fate of the AVF, and future works must be centred on this to develop therapies for a hitherto unacceptably underserved patient population.

Ultrasound for the Clinical Management of Vascular Access Cannulation and Needle Position in Hemodialysis Patients

A native arteriovenous fistula (AVF) is the vascular access of choice for hemodialysis (HD) treatment. Compared with other types of vascular access such as grafts and central venous catheters, it functions longer and is associated with a lower risk of complications. The aim of the study described here was to assess, in an HD population, the position of the fistula needles during an HD session and evaluate the role of ultrasound in the management of AVF puncture. Forty-five consecutive chronic HD patients with an AVF or an arteriovenous vascular graft were included in the study for ultrasound evaluation. Each patient underwent an ultrasound evaluation during HD treatment to assess the position of the needles inside the vascular access. The ultrasound evaluation revealed that 81.8% of the traditional needles were incorrectly adjacent to the vessel walls, in the absence of clinical symptoms or hemodynamic alterations detectable on the dialysis monitor. A greater frequency of malpositioning has been observed for needles in the arterial portion of the vascular access, closer to the anastomosis. The absence of clinically detectable signs of venipuncture-related complications does not ensure correct positioning of the needles within the AVF. Ultrasound evaluation may not only resolve suboptimal cannulation problems of new or complicated vascular accesses but may also be useful in the prevention of acute and chronic damage to the AVF.

Factors affecting arteriovenous fistula dysfunction: A narrative review

Vascular access dysfunction is one of the most important causes of morbidity and mortality in haemodialysis patients, contributing to up to one third of hospitalisations and accounting for a significant amount of the health care costs of these patients. In the past decades, significant scientific advances in understanding mechanisms of arteriovenous fistula maturation and failure have contributed to an increase in the amount of research into techniques for creation and strategies for arteriovenous fistula dysfunction prevention and treatment, in order to improve patient care and outcomes. The aim of this review is to describe the pathogenesis of vascular access failure and provide a comprehensive analysis of the associated risk factors and causes of vascular access failure, in order to interpret possible future therapeutic approaches. Arteriovenous fistula failure is a multifactorial process resulting from the combination of upstream and downstream events with consequent venous neo-intimal hyperplasia and/or inadequate outward remodelling. Inflammation appears to be central in the biology of arteriovenous fistula dysfunction but important triggers still need to be revealed. Given the significant association of arteriovenous fistula failure and patient's prognosis, it is therefore imperative to further research in this area in order to improve prevention, surveillance and treatment, and ultimately patient care and outcomes.

Near-Infrared Fluorescence Imaging of Matrix Metalloproteinase 2 Activity as a Biomarker of Vascular Remodeling in Hemodialysis Access

PURPOSE

To establish the capability of near-infrared fluorescence (NIRF) imaging for the detection of matrix metalloproteinase 2 (MMP-2) activity as a biomarker of vascular remodeling (VR) in arteriovenous fistulae (AVFs) in vivo.

MATERIALS AND METHODS

AVFs were created in the right groins of Wistar rats (n = 10), and sham procedures were performed in the contralateral groins. Fistulography via a left common carotid artery approach confirmed stenosis (> 50%) in a subset of animals (n = 5) 4 weeks after AVF creation. After administration of MMP-2-activated NIRF probe, near-infrared imaging was performed in vivo and ex vivo of both the AVF and the sham-treated vessels to measure radiant efficiency of MMP-2-activated NIRF signal over background. Histologic analyses of AVF and sham-treated vessels were performed to measure VR defined as inward growth of the vessel caused by intimal thickening.

RESULTS

AVFs demonstrated a significantly higher percentage increase in radiant efficiency over background compared with sham vessels (45.5 ± 56% vs 16.1 ± 17.8%; P = .008). VR in AVFs was associated with increased thickness of neointima staining positively for MMP-2 (161.8 ± 45.5 μm vs 73.2 ± 36.7 μm; P = .01). A significant correlation was observed between MMP-2 activity as measured by relative increase in radiant efficiency for AVFs and thickness of neointima staining positively for MMP-2 (P = .039).

CONCLUSIONS

NIRF imaging can detect increased MMP activity in remodeled AVFs compared with contralateral sham vessels. MMP-2-activated NIRF signal correlates with the severity of intimal thickening. These findings suggest NIRF imaging of MMP-2 may be used as a biomarker of the vascular remodeling underlying stenosis.

Dialysis Needle Hemodynamics in Arterio-Venous Fistulae: A Technical Report

Background

Vascular access is the lifeline for end-stage renal disease patients needing hemodialysis treatment. For dialysis treatment two needles are placed into the vascular access. Few studies on needle hemodynamics have been published.

Methods

We investigated needle hemodynamics by means of Doppler ultrasonography, both in B-mode and in pulsed Doppler mode.

Results

Direct visualization of the needles, turbulence around the arterial needle and blood flow jet from the venous needle are presented.

Conclusions

Ultrasound investigation of the hemodynamics of the dialysis needles is feasible. The extreme blood flow jet through the arterial needle was directed in all patients to the anterior vessel wall.

High-Resolution Computational Fluid Dynamic Simulation of Haemodialysis Cannulation in a Patient-Specific Arteriovenous Fistula

Arteriovenous fistulae (AVF) are the preferred choice of vascular access in hemodialysis patients; however, complications such as stenosis can lead to access failure or recirculation, which reduces dialysis efficiency. This study utilized computational fluid dynamics on a patient-specific radiocephalic fistula under hemodialysis treatment to determine the dynamics of access recirculation and identify the presence of disturbed flow. Metrics of transverse wall shear stress (transWSS) and oscillatory shear index (OSI) were used to characterize the disturbed flow acting on the blood vessel wall, while a power spectral density (PSD) analysis was used to calculate the any turbulence within the access. Results showed that turbulence is generated at the anastomosis and continues through the swing segment. The arterial needle dampens the flow as blood is extracted to the dialyzer, while the venous needle reintroduces turbulence due to the presence of jet flows. Adverse shear stresses are present throughout the vascular access and coincide with these complex flow fields. The position of the needles had no effect in minimizing these forces. However, improved blood extraction may occur when the arterial needle is placed further from the anastomosis, minimizing the effects of residual turbulent structures generated at the anastomosis. Furthermore, the arterial and venous needle may be placed in close proximity to each other without increasing the risk of access recirculation, in a healthy mature fistula, due to the relatively stable blood flow in this region. This may negate the need for a long cannulation segment and aid clinicians in optimizing needle placement for hemodialysis.

Blood Flow in Idealized Vascular Access for Hemodialysis: A Review of Computational Studies

Although our understanding of the failure mechanism of vascular access for hemodialysis has increased substantially, this knowledge has not translated into successful therapies. Despite advances in technology, it is recognized that vascular access is difficult to maintain, due to complications such as intimal hyperplasia. Computational studies have been used to estimate hemodynamic changes induced by vascular access creation. Due to the heterogeneity of patient-specific geometries, and difficulties with obtaining reliable models of access vessels, idealized models were often employed. In this review we analyze the knowledge gained with the use of computational such simplified models. A review of the literature was conducted, considering studies employing a computational fluid dynamics approach to gain insights into the flow field phenotype that develops in idealized models of vascular access. Several important discoveries have originated from idealized model studies, including the detrimental role of disturbed flow and turbulent flow, and the beneficial role of spiral flow in intimal hyperplasia. The general flow phenotype was consistent among studies, but findings were not treated homogeneously since they paralleled achievements in cardiovascular biomechanics which spanned over the last two decades. Computational studies in idealized models are important for studying local blood flow features and evaluating new concepts that may improve the patency of vascular access for hemodialysis. For future studies we strongly recommend numerical modelling targeted at accurately characterizing turbulent flows and multidirectional wall shear disturbances.

The clinical characteristics of posterior reversible encephalopathy syndrome in patients with chronic renal failure

Few studies have investigated posterior reversible encephalopathy syndrome (PRES) in patients with chronic renal failure (CRF). The present study analyzed the clinical manifestations, laboratory examinations and imaging features of PRES in patients with CRF. A total of 42 patients with CRF with or without PRES were recruited in the current retrospective case-control study. Patient data taken prior to the onset of PRES in patients with CRF and PRES (n=21) were collected and analyzed. At the same time, data from patients with CRF but without PRES (n=21) were also analyzed. Brain magnetic resonance imaging (MRI) scans were collected from patients in the PRES group. The mean blood pressure of patients in the PRES group was significantly higher than that of the control group (systolic blood pressure: 172±15 mmHg vs. 135±14 mmHg, P<0.01; diastolic blood pressure: 95±16 mmHg vs. 64±13 mmHg, P<0.01). Furthermore, compared with the control group, mean serum albumin (Alb) and hemoglobin (Hb) concentrations in the PRES group were significantly lower (Alb: 29.1±5.3 g/l vs. 34.6±6.1 g/l, P=0.001; Hb: 74±16 g/l vs. 89±28 g/l, P=0.037). By contrast, mean LDH concentration was significantly higher in the PRES group (LDH: 336±141 U/l vs. 235±89 U/l, P=0.004). In the PRES group, 24 h urine volume was significantly lower in the PRES group than in the control group (24 h urine volume: 651±520 ml vs. 982±518 ml, P=0.046). No significant differences in levels of serum potassium (4.5±0.6 mmol/l vs. 4.4±0.5 mmol/l, P=0.377), sodium (138.3±4.9 mmol/l vs. 139.0±6.8 mmol/l, P=0.325), calcium (2.0±0.24 mmol/l vs. 1.9±0.24 mmol/l, P=0.673), alanine aminotransferase; (24±14 U/l vs. 18±8 U/l, P=0.975); aspartate aminotransferase (29±11 U/l vs. 24±9 U/l, P=0.619) and uric acid (448±148 µmol/l vs. 378±116 µmol/l, P=0.599) were found between the two groups. PRES is a relatively common nervous system complication arising in patients with CRF. Certain biochemical markers, including Hb and Alb, may be associated with PRES. Diagnosing PRES is difficult as computed tomography (CT) brain scans may be normal and MRI scans, which are more sensitive than CT scans at diagnosing PRES, are not always performed in patients with CRF. Thus, brain MRI scans should be taken first in such patients when PRES is suspected.

Variation of flow rate and angle of injected venous needle on influencing intimal hyperplasia at the venous anastomosis of the hemodialysis graft

During hemodialysis, arteriovenous (AV) grafts tends to result in intimal hyperplasia (IH) at the venous anastomosis which leads to graft failure. It is well documented that hemodynamic factors have been implicated in IH, as well as pathogenesis of graft stenosis. In this paper, we investigate the flow rate and angle of injection of a venous needle on damaging the hemodialysis graft. Such damage is mainly caused by hemodynamics rather than the actual physical puncture of the needle. By computational fluid dynamic analysis of flow through the AV grafts, we demonstrate that slower flow rate of the needle preserve a larger region of low wall shear stress (WSS). High needle flow angle and fast flow rate tends to induce high shearing of blood against the graft wall, and therefore resulting in a concentrated region of high WSS. Despite that, the increased flow rate causes more significant change to wall shear stress gradient than the flow angle. Obviously, it is important to optimize the injection rate since a high angle can reduce the size of the injection puncture and have smaller injury for the vessel walls; but a slower injection rate may delay hemodialysis. Therefore, the ideal angle and flow rate of needle is sought in this study.

Computational Model of the Arterial and Venous Needle During Hemodialysis

Arteriovenous fistulae (AVF) are the favored choice of vascular access but still have poor long-term success. Hemodynamic parameters play an important role in vascular health and have been linked to the development of intimal hyperplasia (IH), a pathological growth of the blood vessel initiated by injury. This study aimed to investigate the hemodynamics surrounding the arterial needle (AN) and venous needle (VN), using computational fluid dynamics. A range of blood flow rates, needle positions, and needle orientations were examined. Disturbed flows were found around AN tip in both antegrade and retrograde orientations, which result in regions of high residency time on the surface of the vein and may disrupt endothelial function. Conversely, a high speed jet exits the VN, which produced high wall shear stresses (WSSs) at the point of impingement which can damage the endothelium. The secondary flows produced by jet dissipation also resulted in regions of high residency time, which may influence endothelial structure, leading to IH. The use of shallow needle angles, a blood flow rate of approximately 300 ml/min, and placement of the needle tip away from the walls of the vein mitigates this risk.

Arteriovenous Vascular Access Selection and Evaluation

When making decisions regarding vascular access creation, the clinician and vascular access team must evaluate each patient individually with consideration of life expectancy, timelines for dialysis start, risks and benefits of access creation, referral wait times, as well as the risk for access complications. The role of the multidisciplinary team in facilitating access choice is reviewed, as well as the clinical evaluation of the patient.

Impact of Needles in Vascular Access for Hemodialysis

This article reviews pragmatic aspects of cannulation practice and types of cannulation devices, as well as their impact in vascular access for hemodialysis. Hemodialysis treatment requires successful insertion of two needles for each dialysis treatment. The first needle is the arterial needle; it removes blood with toxin accumulation from the patient and delivers it to the dialysis machine. The second needle, called the venous needle, returns the purified blood from the dialyzer to the patient. Mechanical and hemodynamic trauma related to needle insertions will be discussed.

The care and keeping of vascular access for home hemodialysis patients

Creating and maintaining a healthy vascular access is a critical factor in successful home hemodialysis (HD). This article aims to serve as a "how-to manual" regarding vascular access issues for both patients and health-care providers in a home HD program. This document outlines cannulation options for patients with arteriovenous access and describes troubleshooting techniques for potential complications; strategies are suggested to help patients overcome fear of cannulation and address problems associated with difficult cannulation. Technical aspects of central venous catheter care, as well as a guide to troubleshooting catheter complications, are covered in detail. Monitoring for access-related complications of stenosis, infection, and thrombosis is a key part of every home HD program. Key performance and quality indicators are important mechanisms to ensure patient safety in home HD and should be used during routine clinic visits.

The Role of Shear Stress in Arteriovenous Fistula Maturation and Failure: A Systematic Review

INTRODUCTION

Non-maturation and post-maturation venous stenosis are the primary causes of failure within arteriovenous fistulae (AVFs). Although the exact mechanisms triggering failure remain unclear, abnormal hemodynamic profiles are thought to mediate vascular remodelling and can adversely impact on fistula patency.

AIM

The review aims to clarify the role of shear stress on outward remodelling during maturation and evaluate the evidence supporting theories related to the localisation and development of intimal hyperplasia within AVFs.

METHODS

A systematic review of studies comparing remodelling data with hemodynamic data obtained from computational fluid dynamics of AVFs during and after maturation was conducted.

RESULTS

Outward remodelling occurred to reduce or normalise the level of shear stress over time in fistulae with a large radius of curvature (curved) whereas shear stress was found to augment over time in fistulae with a small radius of curvature (straight) coinciding with minimal to no increases in lumen area. Although this review highlighted that there is a growing body of evidence suggesting low and oscillating shear stress may stimulate the initiation and development of intimal medial thickening within AVFs. Further lines of evidence are needed to support the disturbed flow theory and outward remodelling findings before surgical configurations and treatment strategies are optimised to conform to them. This review highlighted that variation between the time of analysis, classification of IH, resolution of simulations, data processing techniques and omission of various shear stress metrics prevented forming pooling of data amongst studies.

CONCLUSION

Standardised measurements and data processing techniques are needed to comprehensively evaluate the relationship between shear stress and intimal medial thickening. Advances in image acquisition and flow quantifications coupled with the increasing prevalence of longitudinal studies commencing from fistula creation offer viable techniques and strategies to robustly evaluate the relationship between shear stress and remodelling during maturation and thereafter.

New Developments in Our Understanding of Neointimal Hyperplasia

The vascular access remains the lifeline for the hemodialysis patient. The most common etiology of vascular access dysfunction is venous stenosis at the vein-artery anastomosis in arteriovenous fistula and at the vein-graft anastomosis in arteriovenous grafts (AVG). This stenotic lesion is typically characterized on histology as aggressive venous neointimal hyperplasia in both arteriovenous fistula and AVG. In recent years, we have advanced our knowledge and understanding of neointimal hyperplasia in vascular access and begun testing several novel therapies. This article will (1) review recent developments in our understanding of the pathophysiology of neointimal hyperplasia development in AVG and fistula failure, (2) discuss atypical factors leading to neointimal hyperplasia development, (3) highlight key novel therapies that have been evaluated in clinical trials, and (4) discuss future opportunities and challenges to improve our understanding of vascular access dysfunction and translate this knowledge into novel and innovative therapies.

Hemodynamic and biologic determinates of arteriovenous fistula outcomes in renal failure patients

The outcome of patients with end-stage renal disease on hemodialysis depends on a functioning vascular access. Although a variety of access options are available, the arteriovenous fistula remains the best vascular access. Unfortunately the success rate of mature fistula use remains poor. The creation of an arteriovenous fistula is followed by altered hemodynamic and biological changes that may result in neointimal hyperplasia and eventual venous stenosis. This review provides an overview of these changes and the needed research to provide a long lasting vascular access and hence improve outcomes for patients with end-stage renal disease.

THE INFLUENCE OF USING THE NEEDLE ADAPTER TO REDUCE THE BIOMECHANICAL RISK FACTORS WITHIN HEMODIALYSIS ARTERIOVENOUS GRAFTS

Hemodialysis vascular access failure is related to increased morbidity and mortality in hemodialysis patients, representing a challenging clinical problem which results in a high percentage of hospital entrance and an important economic burden on government's disbursement. In this paper, the feasibility of using the needle adapter to reduce the biomechanical risk factors within arteriovenous grafts is considered. The three-dimensional (3D) tapered 6 to 8 mm loop graft in the presence of venous and arterial needles with and without adapter was numerically simulated. Navier–Stokes equations for incompressible Newtonian fluid are the governing equation of this problem. k – ω two equations turbulence modeling were applied to capture flow features of low Reynolds turbulent flow regions in this simulation. The physiological velocity waveform was used as an arterial inlet boundary condition. The venose outlet boundary condition was a time dependent physiological pressure waveform. The results for the dialysis without the adapter demonstrated that the graft wall experiences increased hemodynamic stresses as a result of the hitting needle jet flow. The dialysis with the adapter demonstrated that the venous anastomosis experiences lower biomechanical risk factors in comparison to the dialysis without the adapter and it reduced the vascular access failure. Using adapter caused less damage to endothelial cells during hemodialysis.

Incomplete restoration of homeostatic shear stress within arteriovenous fistulae

Arteriovenous fistulae are surgically created to provide adequate access for dialysis patients suffering from end-stage renal disease. It has long been hypothesized that the rapid blood vessel remodeling occurring after fistula creation is, in part, a process to restore the mechanical stresses to some preferred level, i.e., mechanical homeostasis. We present computational hemodynamic simulations in four patient-specific models of mature arteriovenous fistulae reconstructed from 3D ultrasound scans. Our results suggest that these mature fistulae have remodeled to return to ''normal'' shear stresses away from the anastomoses: about 1.0 Pa in the outflow veins and about 2.5 Pa in the inflow arteries. Large parts of the anastomoses were found to be under very high shear stresses >15 Pa, over most of the cardiac cycle. These results suggest that the remodeling process works toward restoring mechanical homeostasis in the fistulae, but that the process is limited or incomplete, even in mature fistulae, as evidenced by the elevated shear at or near the anastomoses. Based on the long term clinical viability of these dialysis accesses, we hypothesize that the elevated nonhomeostatic shear stresses in some portions of the vessels were not detrimental to fistula patency.

The flow field near a venous needle in hemodialysis: a computational study

The vascular access used in hemodialysis can suffer from numerous complications, which may lead to failure of the access, patient morbidity, and significant costs. The flow field in the region of the venous needle may be a source of damaging hemodynamics and hence adverse effects on the fistula. In this study, the venous needle flow has been considered, using three-dimensional computational methods. Four scenarios where the venous needle flow could potentially influence dialysis treatment outcome were identified and examined: Variation of the needle placement angle (10°, 20°, 30°), variation of the blood flow rate settings (200, 300, 400 mL/min), variation of the needle depth (top, middle, bottom), and the inclusion of a back eye in the needle design. The presence of the needle has significant effect on the flow field, with different scenarios having varying influence. In general, wall shear stresses were elevated above normal physiological values, and increased presence of areas of low velocity and recirculation-indicating increased likelihood of intimal hyperplasia development-were found. Computational results showed that the presence of the venous needle in a hemodialysis fistula leads to abnormal and potentially damaging flow conditions and that optimization of needle parameters could aid in the reduction of vascular access complications. Results indicate shallow needle angles and lower blood flow rates may minimize vessel damage.

"Venopathy" at work: recasting neointimal hyperplasia in a new light

Hemodialysis vascular access is a unique form of vascular anastomosis. Although it is created in a unique disease state, it has much to offer in terms of insights into venous endothelial and anastomotic biology. The development of neointimal hyperplasia (NH) has been identified as a pathologic entity, decreasing the lifespan and effectiveness of hemodialysis vascular access. Subtle hints and new data suggest a contrary idea-that NH, to some extent an expected response, if controlled properly, may play a beneficial role in the promotion of maturation to a functional access. This review attempts to recast our understanding of NH and redefine research goals for an evolving discipline that focuses on a life-sustaining connection between an artery and vein.