Patency rate and complications of polytetrafluoroethylene grafts compared with polyurethane grafts for hemodialysis access

Quality assurance in surgical trials of arteriovenous grafts for haemodialysis: A systematic review, a narrative exploration and expert recommendations

BACKGROUND

Introducing new procedures and challenging established paradigms requires well-designed randomised controlled trials (RCT). However, RCT in surgery present unique challenges with much of treatment tailored to the individual patient circumstances, refined by experience and limited by organisational factors. There has been considerable debate over the outcomes of arteriovenous grafts (AVG) compared to AVF, but any differences may reflect differing practice and potential variability. It is essential, therefore, when considering an RCT of a novel surgical procedure or device that quality assurance (QA) is defined for both the new approach and the comparator. The aim of this systematic review was to evaluate the QA standards performed in RCT of AVG using a multi-national, multi-disciplinary approach and propose an approach for future RCT.

METHOD

The methods of this have been previously registered (PROSPERO: CRD420234284280) and published. In summary, a four-stage review was performed: identification of RCT of AVG, initial review, multidisciplinary appraisal of QA methods and reconciliation. QA measures were sought in four areas - generic, credentialing, standardisation and monitoring, with data abstracted by a multi-national, multi-speciality review body.

RESULTS

QA in RCT involving AVG in all four domains is highly variable, often sub-optimally described and has not improved over the past three decades. Few RCT established or defined a pre-RCT level of experience, none documented a pre-trial education programme, or had minimal standards of peri-operative management, no study had a defined pre-trial monitoring programme, and none assessed technical performance.

CONCLUSION

QA in RCT is a relatively new area that is expanding to ensure evidence is reliable and reproducible. This review demonstrates that QA has not previously been detailed, but can be measured in surgical RCT of vascular access, and that a four-domain approach can easily be implemented into future RCT.

Fabrication of Mechanically Enhanced, Suturable, Fibrous Hydrogel Membranes

Poly(vinyl-alcohol) hydrogels have already been successfully utilised as drug carrier systems and tissue engineering scaffolds. However, lacking mechanical strength and suturability hinders any prospects for clinical and surgical applications. The objective of this work was to fabricate mechanically robust PVA membranes, which could also withstand surgical manipulation and suturing. Electrospun membranes and control hydrogels were produced with 61 kDa PVA. Using a high-speed rotating cylindrical collector, we achieved fibre alignment (fibre diameter: 300 ± 50 nm). Subsequently, we created multilayered samples with different orientations to achieve multidirectional reinforcement. Finally, utilising glutaraldehyde as a cross-linker, we created insoluble fibrous-hydrogel membranes. Mechanical studies were performed, confirming a fourfold increase in the specific loading capacities (from 0.21 to 0.84 Nm²/g) in the case of the monolayer samples. The multilayered membranes exhibited increased resistance from both horizontal and vertical directions, which varies according to the specific arrangement. Finally, the cross-linked fibrous hydrogel samples not only exhibited specific loading capacities significantly higher than their counterpart bulk hydrogels but successfully withstood suturing. Although cross-linking optimisation and animal experiments are required, these membranes have great prospects as alternatives to current surgical meshes, while the methodology could also be applied in other systems as well.

Novel electrospun polyurethane grafts for vascular access in rats

OBJECTIVES

The purpose of this study is to develop a new and improved polyurethane (PU) graft using electrospinning and chemical modifications for hemodialysis patients, which will replace the current standard, polytetrafluoroethylene (PTFE) graft. The chemical modifications aim to improve hemocompatibility and reduce thrombogenicity and neointimal hyperplasia.

METHOD

The study population was randomized and divided equally into four groups; one control group received a PTFE graft, and three treatment groups received three different types of polyurethane grafts. Two duplex measurements were performed directly on the graft on the same locations, followed by a histologic examination.

RESULTS

In the first few days after the implantation animals lost some weight, it took a week to recover to pre-surgical weight. Throughout the 28 days, there was no significant difference between animals in wound, activity, and the general appearance. PTFE and PU A groups have lower compliance or reduced graft diameter due to neointimal hyperplasia development on Doppler interrogation. The histological analysis showed limited neointimal hyperplasia development and no excessive inflammatory response to any of the grafts.

CONCLUSION

This study demonstrates that animals with polyurethane grafts show better blood flow because the developed NIH was inconspicuous, as indicated by the different velocity measure than controls on Duplex and minimal NIH development microscopically.

A novel small diameter nanotextile arterial graft is associated with surgical feasibility and safety and increased transmural endothelial ingrowth in pig

Globally, millions of patients are affected by myocardial infarction or lower limb gangrene/amputation due to atherosclerosis. Available surgical treatment based on vein and synthetic grafts provides sub-optimal benefits. We engineered a highly flexible and mechanically robust nanotextile-based vascular graft (NanoGraft) by interweaving nanofibrous threads of poly-L-lactic acid to address the unmet need. The NanoGrafts were rendered impervious with selective fibrin deposition in the micropores by pre-clotting. The pre-clotted NanoGrafts (4 mm diameter) and ePTFE were implanted in a porcine carotid artery replacement model. The fibrin-laden porous milieu facilitated rapid endothelization by the transmural angiogenesis in the NanoGraft. In-vivo patency of NanoGrafts was 100% at 2- and 4-weeks, with no changes over time in lumen size, flow velocities, and minimal foreign-body inflammatory reaction. However, the patency of ePTFE at 2-week was 66% and showed marked infiltration, neointimal thickening, and poor host tissue integration. The study demonstrates the in-vivo feasibility and safety of a thin-layered vascular prosthesis, viz., NanoGraft, and its potential superiority over the commercial ePTFE.

The impact of activated protein C resistance on the patency of arteriovenous grafts for hemodialysis access

INTRODUCTION

Vascular access is required for hemodialysis treatment. An effect of activated protein C resistance on access thrombosis rates has not yet been investigated. The aim of this study is to determine whether an activated protein C resistance is correlated with the patency of polytetrafluoroethylene arteriovenous grafts.

METHODS

The primary endpoint was the impact of activated protein C resistance; secondary endpoints were the influence of Factor V Leiden thrombophilia, homocysteine, ß2-glycoprotein antibodies, and other laboratory values on the assisted primary patency.

RESULTS

Forty-three grafts in 43 patients were included. The overall mean assisted primary patency was 18.4 months (±3.16 SE). Activated protein C resistance (p = 0.01) and ß2-glycoprotein antibodies (p = 0.018) had a significant influence on the assisted primary patency. The assisted primary patency for patients with low (<4) activated protein C resistance was 9.3 months compared to 24.8 of those with a high (≥4) activated protein C resistance. Patients with low (≤2.6) ß2-glycoprotein antibodies presented an assisted primary patency of 31.8 months whereas those with high (>2.6) ß2-glycoprotein antibodies showed 9.3 months. In all patients with a pathologic activated protein C resistance, a heterozygous or homozygous Factor V Leiden thrombophilia was detected.

CONCLUSIONS

This study identified low activated protein C resistance and high ß2-glycoprotein antibodies as risk factors for thrombosis in polytetrafluoroethylene arteriovenous grafts. A prospective study is needed to clarify if oral anticoagulation should be administered to all patients with a pathologic activated protein C resistance blood value and/or factor V Leiden mutation.

Patency of ePTFE Arteriovenous Graft Placements in Hemodialysis Patients: Systematic Literature Review and Meta-Analysis

Arteriovenous grafts (AVGs) are an appropriate option for vascular access in certain hemodialysis patients. Expanded polytetrafluoroethylene (ePTFE) has become the dominant material for such grafts, due in part to innovations in graft design and surgical interventions to reduce complications and improve patency rates. Comprehensive evidence syntheses have not been conducted to update AVG performance in an era in which both access choice and ePTFE graft functioning may have changed. We conducted a systematic review and meta-analysis summarizing outcomes from recent studies of ePTFE AVGs in hemodialysis, following PRISMA standards. Literature searches were conducted in multiple databases to identify observational and interventional studies of AVG patency and infection risk. Primary, primary-assisted, and secondary patency rates were analyzed at 6, 12, 18, and 24 months postplacement. Kaplan-Meier graft survival plots were digitized to recreate individual patient-level data. Patency rates were pooled using a random effects model. We identified 32 studies meeting our selection criteria that were published from 2004 through 2019. A total of 38 study arms of ePTFE grafts were included, representing 3381 AVG accesses placed. The mean primary, primary-assisted, and secondary patency rates at 1 year were 41% (95% CI, 35% to 47%), 46% (95% CI, 41% to 51%), and 70% (95% CI, 64% to 75%), respectively. Mean 24-month patency rates were 28% (95% CI, 22% to 33%), 34% (95% CI, 27% to 41%), and 54% (95% CI, 47% to 61%), respectively. A high degree of heterogeneity across studies was observed. Overall risk of infection was not consistently reported, but among available studies the pooled estimate was 9% per patient-year (95% CI, 6% to 12%). This meta-analysis provides an up-to-date estimate of the performance of ePTFE AVGs, within the context of improved graft designs and improved interventional techniques.

KDOQI Clinical Practice Guideline for Vascular Access: 2019 Update

The National Kidney Foundation's Kidney Disease Outcomes Quality Initiative (KDOQI) has provided evidence-based guidelines for hemodialysis vascular access since 1996. Since the last update in 2006, there has been a great accumulation of new evidence and sophistication in the guidelines process. The 2019 update to the KDOQI Clinical Practice Guideline for Vascular Access is a comprehensive document intended to assist multidisciplinary practitioners care for chronic kidney disease patients and their vascular access. New topics include the end-stage kidney disease "Life-Plan" and related concepts, guidance on vascular access choice, new targets for arteriovenous access (fistulas and grafts) and central venous catheters, management of specific complications, and renewed approaches to some older topics. Appraisal of the quality of the evidence was independently conducted by using a Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach, and interpretation and application followed the GRADE Evidence to Decision frameworks. As applicable, each guideline statement is accompanied by rationale/background information, a detailed justification, monitoring and evaluation guidance, implementation considerations, special discussions, and recommendations for future research.

Hemocompatibility of plasma electrolytic oxidation (PEO) coated Mg-RE and Mg-Zn-Ca alloys for vascular scaffold applications

Percutaneous transluminal coronary angioplasty and subsequent vascular scaffold implantation remains the prevalent invasive treatment of coronary heart disease. In-stent restenosis remained a problem with bare metal stents, until drug-eluting stents were introduced. The inhibition of the healing process by the antimitotic drug coating and the permanent metallic remnant can promote sub-acute and delayed stent thrombosis. Thus, the development of biodegradable stents emerged as a subject of research. Magnesium-based bioabsorbable devices can provide sufficient radial force in the acute phase of vessel-treatment and degrade thoroughly in aqueous environment, making them potential new candidates for vascular scaffold applications. Magnesium alloys tend to degrade very quickly due to their high electrochemical corrosion potential. Plasma Electrolytic Oxidation modification of magnesium alloys improves interface and degradadation properties and may therefore enhance the performance and suitability for vascular scaffold applications of these materials. Assuring the hemocompatibility and foremost assessing the thrombogenicity of new biomaterials prior to their use is essential in order to avoid adverse effects. The goal was to assess thrombocyte adhesion on coated Mg-RE and Mg-Zn-Ca alloys. Static experiments with human blood were carried out on the plasma-electrolytically treated or corresponding untreated Mg alloy in order to assess quantity and quality of thrombocyte adhesion via standardized SEM imaging. In a second step, a parallel plate flow chamber was designed in order to examine thrombocyte adhesion under dynamic flow conditions. During flow chamber experiments the test-materials were exposed to human thrombocyte concentrate and the number of adherent thrombocytes was assessed. The flow chamber was additionally perfused with human blood and thrombocyte adhesion was semiquantitatively and qualitatively assessed via SEM imaging and subsequent scoring. In conclusion, a new parallel plate flow chamber design simulating blood-circulation was successfully established, enabling the further assessment of platelet adhesion on bioabsorbable materials under dynamic flow conditions. Static and dynamic experiments showed, that plasma-electrolytically treated specimens showed low thrombocyte adhesion on both alloys, proposing their potential use in vascular scaffolds. The uncoated magnesium alloys showed rapid degradation along with gas formation due to the chemically active surface and therefore give concern regarding their safety and suitability for vascular applications.

Long term performance evaluation of small-diameter vascular grafts based on polyvinyl alcohol hydrogel and dextran and MSCs-based therapies using the ovine pre-clinical animal model

The functional and structural performance of a 5cm synthetic small diameter vascular graft (SDVG) produced by the copolymerization of polyvinyl alcohol hydrogel with low molecular weight dextran (PVA/Dx graft) associated to mesenchymal stem cells (MSCs)-based therapies and anticoagulant treatment with heparin, clopidogrel and warfarin was tested using the ovine model during the healing period of 24 weeks. The results were compared to the ones obtained with standard expanded polyetetrafluoroethylene grafts (ePTFE graft). Blood flow, vessel and graft diameter measurements, graft appearance and patency rate (PR), thrombus, stenosis and collateral vessel formation were evaluated by B-mode ultrasound, audio and color flow Doppler. Graft and regenerated vessels morphologic evaluation was performed by scanning electronic microscopy (SEM), histopathological and immunohistochemical analysis. All PVA/Dx grafts could maintain a similar or higher PR and systolic/diastolic laminar blood flow velocities were similar to ePTFE grafts. CD14 (macrophages) and α-actin (smooth muscle) staining presented similar results in PVA/Dx/MSCs and ePTFE graft groups. Fibrosis layer was lower and endothelial cells were only detected at graft-artery transitions where it was added the MSCs. In conclusion, PVA/Dx graft can be an excellent scaffold candidate for vascular reconstruction, including clinic mechanically challenging applications, such as SDVGs, especially when associated to MSCs-based therapies to promote higher endothelialization and lower fibrosis of the vascular prosthesis, but also higher PR values.

Alternative grafts for brachioaxillary hemodialysis access: 1-year comparative results

BACKGROUND: Many chronic renal patients lack autologous veins in the upper limbs suitable for construction of arteriovenous fistulas for hemodialysis. Alternative fistula options for these patients should be evaluated and compared.OBJECTIVE: To compare different types of grafts used for brachioaxillary access in hemodialysis patients in terms of their patency and complication rates.METHOD: Forty-nine patients free from arterial system abnormalities and with no venous options for creation of arteriovenous fistulae in the arm and/or forearm underwent brachioaxillary bypass with implantation of autologous saphenous vein, polytetrafluoroethylene (PTFE), or PROPATEN(r) grafts. Patients were assessed by Doppler ultrasonography at 3, 6, and 12 months after surgery,.RESULTS: The four first saphenous vein grafts had failed by 3 or 6 months after surgery. The autologous saphenous vein group was discontinued at the beginning of the study because of extreme difficulty in achieving puncture and hematoma formation. Failure rates of PTFE and PROPATEN(r) grafts did not differ after 3 (p = 0.559), 6 (p = 0.920), or 12 months (p = 0.514). A log-rank test applied to cumulative survival of grafts at 1 year (0.69 for PTFE, 0.79 for PROPATEN(r)) detected no significant differences (p = 0.938). There were no differences in complications resulting in graft failure between the two types of prosthetic graft.CONCLUSION: Autologous saphenous vein grafts do not appear to be a good option for brachioaxillary hemodialysis access because of difficulties with achieving puncture. Brachioaxillary fistulae constructed using PTFE or PROPATEN(r) grafts exhibited similar patency and complication rates. Further studies with large samples size are warranted to confirm our findings.

In vitro and in vivo evaluation of blood coagulation activation of polyvinyl alcohol hydrogel plus dextran-based vascular grafts

Polyvinyl alcohol hydrogel (PVA) is a water-soluble synthetic polymer that is commonly used in biomedical applications including vascular grafting. It was argued that the copolymerization of PVA with dextran (Dx) can result in improvement of blood-biomaterial interactions. The focus of this experimental study was to assess that interaction through an in vivo and in vitro evaluation of the coagulation system activation. The thrombogenicity of the copolymer was determined by quantification of platelet adhesion through the lactate dehydrogenase assay, determination of whole blood clotting time, and by quantification of platelet activation by flow cytometry. The thrombin-antithrombin complex blood levels were also determined. The obtained results for the in vitro assays suggested a non-thrombogenic profile for PVA/Dx. Additionally in vivo coagulation and hematological parameters were determined in an animal model after PVA/Dx vascular graft implantation. For coagulation homeostasis assessment, the intrinsic and extrinsic pathway's activation was determined by measuring prothrombin time (PT) and activated partial thromboplastin time (aPTT). Other markers of coagulation and inflammation activation including d-dimers, interleukin-6, and C-reactive protein were also assessed. The PVA/Dx copolymer tended to inhibit platelet adhesion/activation process and the contact activation process for coagulation. These results were also confirmed with the in vivo experiments where the measurements for APTT, interleukin-6, and C-reactive protein parameters were normal considering the species normal range of values. The response to those events is an indicator of the in vitro and in vivo hemocompatibility of PVA/Dx and it allows us to select this biomaterial for further preclinical trials in vascular reconstruction.

Dialysis access

Although hemodialysis access procedures are considered the most common vascular procedures performed by either general or vascular surgeons, there is a paucity of level-one evidence in the literature. Randomized controlled trials are limited, and most of these studies have small sample sizes compared with other areas of vascular surgery, that is, carotid or aneurysm studies. We summarize the results of the world's literature for arteriovenous access in table format as a tool for those specialists managing patients with arteriovenous access procedures.