Experience with cryopreserved cadaveric femoral vein allografts used 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.

MCP-1-Loaded Poly(l-lactide-co-caprolactone) Fibrous Films Modulate Macrophage Polarization toward an Anti-inflammatory Phenotype and Improve Angiogenesis

Tissue engineering approaches such as the electrospinning technique can fabricate nanofibrous scaffolds which are widely used for small-diameter vascular grafting. However, foreign body reaction (FBR) and lack of endothelial coverage are still the main cause of graft failure after the implantation of nanofibrous scaffolds. Macrophage-targeting therapeutic strategies have the potential to address these issues. Here, we fabricate a monocyte chemotactic protein-1 (MCP-1)-loaded coaxial fibrous film with poly(l-lactide-co-ε-caprolactone) (PLCL/MCP-1). The PLCL/MCP-1 fibrous film can polarize macrophages toward anti-inflammatory M2 macrophages through the sustained release of MCP-1. Meanwhile, these specific functional polarization macrophages can mitigate FBR and promote angiogenesis during the remodeling of implanted fibrous films. These studies indicate that MCP-1-loaded PLCL fibers have a higher potential to modulate macrophage polarity, which provides a new strategy for small-diameter vascular graft designing.

Strategies to counteract adverse remodeling of vascular graft: A 3D view of current graft innovations

Vascular grafts are widely used for vascular surgeries, to bypass a diseased artery or function as a vascular access for hemodialysis. Bioengineered or tissue-engineered vascular grafts have long been envisioned to take the place of bioinert synthetic grafts and even vein grafts under certain clinical circumstances. However, host responses to a graft device induce adverse remodeling, to varied degrees depending on the graft property and host's developmental and health conditions. This in turn leads to invention or failure. Herein, we have mapped out the relationship between the design constraints and outcomes for vascular grafts, by analyzing impairment factors involved in the adverse graft remodeling. Strategies to tackle these impairment factors and counteract adverse healing are then summarized by outlining the research landscape of graft innovations in three dimensions-cell technology, scaffold technology and graft translation. Such a comprehensive view of cell and scaffold technological innovations in the translational context may benefit the future advancements in vascular grafts. From this perspective, we conclude the review with recommendations for future design endeavors.

Aneurysmal degeneration of the hood of a cryopreserved vein allograft two years after thrombosis

Cryopreserved vein allografts are used as alternative conduits for infrainguinal bypass but are prone to aneurysmal degeneration. A 60-year-old man presented with a pulsatile, tender right groin mass 2 years after thrombosis of a cryopreserved vein jump graft emanating from a prosthetic axillary to profunda bypass. Intraoperatively, the aneurysm was consistent with isolated dilatation of the hood of the thrombosed cryopreserved vein graft. This was excised and repaired with bovine pericardial patch angioplasty. The patient recovered with no recurrence for 2 years. Aneurysmal degeneration of the cryopreserved vein allograft can occur even after graft thrombosis.

Bioengineering artificial blood vessels from natural materials

Bioengineering an effective, small diameter (<6 mm) artificial vascular graft for use in bypass surgery when autologous grafts are unavailable remains a persistent challenge. Commercially available grafts are typically made from plastics, which have high strength but lack elasticity and present a foreign surface that triggers undesirable biological responses. Tissue engineered grafts, leveraging decellularized animal vessels or derived de novo from long-term cell culture, have dominated recent research, but failed to meet clinical expectations. More effective constructs that are readily translatable are urgently needed. Recent advances in natural materials have made the production of robust acellular conduits feasible and their use increasingly attractive. Here, we identify a subset of natural materials with potential to generate durable, small diameter vascular grafts.

Individualized tissue-engineered veins as vascular grafts: A proof of concept study in pig

Personalized tissue engineered vascular grafts are a promising advanced therapy medicinal product alternative to autologous or synthetic vascular grafts utilized in blood vessel bypass or replacement surgery. We hypothesized that an individualized tissue engineered vein (P-TEV) would make the body recognize the transplanted blood vessel as autologous, decrease the risk of rejection and thereby avoid lifelong treatment with immune suppressant medication as is standard with allogenic organ transplantation. To individualize blood vessels, we decellularized vena cava from six deceased donor pigs and tested them for cellular removal and histological integrity. A solution with peripheral blood from the recipient pigs was used for individualized reconditioning in a perfusion bioreactor for seven days prior to transplantation. To evaluate safety and functionality of the individualized vascular graft in vivo, we transplanted reconditioned porcine vena cava into six pigs and analyzed histology and patency of the graft at different time points, with three pigs at the final endpoint 4-5 weeks after surgery. Our results showed that the P-TEV was fully patent in all animals, did not induce any occlusion or stenosis formation and we did not find any signs of rejection. The P-TEV showed rapid recellularization in vivo with the luminal surface covered with endothelial cells. In summary, the results indicate that P-TEV is functional and have potential for use as clinical transplant grafts.

In vivo recellularization of xenogeneic vascular grafts decellularized with high hydrostatic pressure method in a porcine carotid arterial interpose model

Autologous vascular grafts are widely used in revascularization surgeries for small caliber targets. However, the availability of autologous conduits might be limited due to prior surgeries or the quality of vessels. Xenogeneic decellularized vascular grafts from animals can potentially be a substitute of autologous vascular grafts. Decellularization with high hydrostatic pressure (HHP) is reported to highly preserve extracellular matrix (ECM), creating feasible conditions for recellularization and vascular remodeling after implantation. In the present study, we conducted xenogeneic implantation of HHP-decellularized bovine vascular grafts from dorsalis pedis arteries to porcine carotid arteries and posteriorly evaluated graft patency, ECM preservation and recellularization. Avoiding damage of the luminal surface of the grafts from drying significantly during the surgical procedure increased the graft patency at 4 weeks after implantation (P = 0.0079). After the technical improvement, all grafts (N = 5) were patent with mild stenosis due to intimal hyperplasia at 4 weeks after implantation. Neither aneurysmal change nor massive thrombosis was observed, even without administration of anticoagulants nor anti-platelet agents. Elastica van Gieson and Sirius-red stainings revealed fair preservation of ECM proteins including elastin and collagen after implantation. The luminal surface of the grafts were thoroughly covered with von Willebrand factor-positive endothelium. Scanning electron microscopy of the luminal surface of implanted grafts exhibited a cobblestone-like endothelial cell layer which is similar to native vascular endothelium. Recellularization of the tunica media with alpha-smooth muscle actin-positive smooth muscle cells was partly observed. Thus, we confirmed that HHP-decellularized grafts are feasible for xenogeneic implantation accompanied by recellularization by recipient cells.

Glycosaminoglycans: From Vascular Physiology to Tissue Engineering Applications

Cardiovascular diseases represent the number one cause of death globally, with atherosclerosis a major contributor. Despite the clinical need for functional arterial substitutes, success has been limited to arterial replacements of large-caliber vessels (diameter > 6 mm), leaving the bulk of demand unmet. In this respect, one of the most challenging goals in tissue engineering is to design a "bioactive" resorbable scaffold, analogous to the natural extracellular matrix (ECM), able to guide the process of vascular tissue regeneration. Besides adequate mechanical properties to sustain the hemodynamic flow forces, scaffold's properties should include biocompatibility, controlled biodegradability with non-toxic products, low inflammatory/thrombotic potential, porosity, and a specific combination of molecular signals allowing vascular cells to attach, proliferate and synthesize their own ECM. Different fabrication methods, such as phase separation, self-assembly and electrospinning are currently used to obtain nanofibrous scaffolds with a well-organized architecture and mechanical properties suitable for vascular tissue regeneration. However, several studies have shown that naked scaffolds, although fabricated with biocompatible polymers, represent a poor substrate to be populated by vascular cells. In this respect, surface functionalization with bioactive natural molecules, such as collagen, elastin, fibrinogen, silk fibroin, alginate, chitosan, dextran, glycosaminoglycans (GAGs), and growth factors has proven to be effective. GAGs are complex anionic unbranched heteropolysaccharides that represent major structural and functional ECM components of connective tissues. GAGs are very heterogeneous in terms of type of repeating disaccharide unit, relative molecular mass, charge density, degree and pattern of sulfation, degree of epimerization and physicochemical properties. These molecules participate in a number of vascular events such as the regulation of vascular permeability, lipid metabolism, hemostasis, and thrombosis, but also interact with vascular cells, growth factors, and cytokines to modulate cell adhesion, migration, and proliferation. The primary goal of this review is to perform a critical analysis of the last twenty-years of literature in which GAGs have been used as molecular cues, able to guide the processes leading to correct endothelialization and neo-artery formation, as well as to provide readers with an overall picture of their potential as functional molecules for small-diameter vascular regeneration.

Considerations in the Development of Small-Diameter Vascular Graft as an Alternative for Bypass and Reconstructive Surgeries: A Review

BACKGROUND

Current design strategies for small diameter vascular grafts (< 6 mm internal diameter; ID) are focused on mimicking native vascular tissue because the commercially available grafts still fail at small diameters, notably due to development of intimal hyperplasia and thrombosis. To overcome these challenges, various design approaches, material selection, and surface modification strategies have been employed to improve the patency of small-diameter grafts.

REVIEW

The purpose of this review is to outline various considerations in the development of small-diameter vascular grafts, including material choice, surface modifications to enhance biocompatibility/endothelialization, and mechanical properties of the graft, that are currently being implanted. Additionally, we have taken into account the general vascular physiology, tissue engineering approaches, and collective achievements of the authors in this area. We reviewed both commercially available synthetic grafts (e-PTFE and PET), elastic polymers such as polyurethane and biodegradable and bioresorbable materials. We included naturally occurring materials by focusing on their potential application in the development of future vascular alternatives.

CONCLUSION

Until now, there are few comprehensive reviews regarding considerations in the design of small-diameter vascular grafts in the literature. Here-in, we have discussed in-depth the various strategies employed to generate engineered vascular graft due to their high demand for vascular surgeries. While some TEVG design strategies have shown greater potential in contrast to autologous or synthetic ePTFE conduits, many are still hindered by high production cost which prevents their widespread adoption. Nonetheless, as tissue engineers continue to develop on their strategies and procedures for improved TEVGs, soon, a reliable engineered graft will be available in the market. Hence, we anticipate a viable TEVG with resorbable property, fabricated via electrospinning approach to hold a greater potential that can overcome the challenges observed in both autologous and allogenic grafts. This is because they can be mechanically tuned, incorporated/surface-functionalized with bioactive molecules and mass-manufactured in a reproducible manner. It is also found that most of the success in engineered vascular graft approaching commercialization is for large vessels rather than small-diameter grafts used as cardiovascular bypass grafts. Consequently, the field of vascular engineering is still available for future innovators that can take up the challenge to create a functional arterial substitute.

From arteries to capillaries: approaches to engineering human vasculature

From micro-scaled capillaries to millimeter-sized arteries and veins, human vasculature spans multiple scales and cell types. The convergence of bioengineering, materials science, and stem cell biology has enabled tissue engineers to recreate the structure and function of different hierarchical levels of the vascular tree. Engineering large-scale vessels has been pursued over the past thirty years to replace or bypass damaged arteries, arterioles, and venules, and their routine application in the clinic may become a reality in the near future. Strategies to engineer meso- and microvasculature have been extensively explored to generate models to study vascular biology, drug transport, and disease progression, as well as for vascularizing engineered tissues for regenerative medicine. However, bioengineering of large-scale tissues and whole organs for transplantation, have failed to result in clinical translation due to the lack of proper integrated vasculature for effective oxygen and nutrient delivery. The development of strategies to generate multi-scale vascular networks and their direct anastomosis to host vasculature would greatly benefit this formidable goal. In this review, we discuss design considerations and technologies for engineering millimeter-, meso-, and micro-scale vessels. We further provide examples of recent state-of-the-art strategies to engineer multi-scale vasculature. Finally, we identify key challenges limiting the translation of vascularized tissues and offer our perspective on future directions for exploration.

Decellularized Carotid Artery Functions as an Arteriovenous Graft

BACKGROUND

Prosthetic arteriovenous grafts (AVG) continue to have a high rate of failure in clinical use, yet there is continued clinical demand for them. However, there is no small animal model of AVG to test novel tissue-engineered vascular grafts. We established a new rat arteriovenous graft model to compare the healing of decellularized carotid artery (CA) to autologous CA.

MATERIALS AND METHODS

The infrarenal vena cava and aorta of Wistar rats were exposed and dissected free below renal artery. A longitudinal 1 mm venotomy and arteriotomy were made on the anterior walls. The conduit was either autologous CA or heterologous decellularized CA; a conduit was sewn to the inferior vena cava and aorta in end-to-side fashion. Rats were sacrificed on postoperative day 21 for examination.

RESULTS

All rats survived without heart failure. Conduits had 100% patency rate (day 21) in both the control and decellularized CA groups (n = 6). Both control and decellularized CA showed similar rates of reendothelialization, inflammatory cell infiltration, and cell turnover. The outflow vein beyond the autologous or decellularized conduits showed similar neointimal thickness and cell turnover.

CONCLUSIONS

Decellularized CA may be a viable tissue engineering graft for use as an arteriovenous graft for dialysis access. The rat aorta-vena cava graft is a useful model to test new materials including tissue-engineered grafts for use as AVG.

Current Strategies for the Manufacture of Small Size Tissue Engineering Vascular Grafts

Occlusive arterial disease, including coronary heart disease (CHD) and peripheral arterial disease (PAD), is the main cause of death, with an annual mortality incidence predicted to rise to 23.3 million worldwide by 2030. Current revascularization techniques consist of angioplasty, placement of a stent, or surgical bypass grafting. Autologous vessels, such as the saphenous vein and internal thoracic artery, represent the gold standard grafts for small-diameter vessels. However, they require invasive harvesting and are often unavailable. Synthetic vascular grafts represent an alternative to autologous vessels. These grafts have shown satisfactory long-term results for replacement of large- and medium-diameter arteries, such as the carotid or common femoral artery, but have poor patency rates when applied to small-diameter vessels, such as coronary arteries and arteries below the knee. Considering the limitations of current vascular bypass conduits, a tissue-engineered vascular graft (TEVG) with the ability to grow, remodel, and repair in vivo presents a potential solution for the future of vascular surgery. Here, we review the different methods that research groups have been investigating to create TEVGs in the last decades. We focus on the techniques employed in the manufacturing process of the grafts and categorize the approaches as scaffold-based (synthetic, natural, or hybrid) or self-assembled (cell-sheet, microtissue aggregation and bioprinting). Moreover, we highlight the attempts made so far to translate this new strategy from the bench to the bedside.

Use of human aortic extracellular matrix as a scaffold for construction of a patient-specific tissue engineered vascular patch

Synthetic or biologic materials are usually used to repair vascular malformation in congenital heart defects; however, non-autologous materials show both mismatch compliance and antigenicity, as well as a lack of recellularization on its surface. Here, we constructed a tissue-engineered vascular patch (TEVP) using decellularized extracellular matrix (ECM) scaffold obtained from excised human aorta during surgery, which was seeded with patient-derived bone marrow CD34-positive (CD34+) progenitor cells. While cellular components were removed, the decellularized ECM scaffold retained native ECM composition, similar mechanical performance to undecellularized aortic tissue, and supported the adhesion, survival and proliferation of CD34+ progenitor cells. Interestingly, after in vitro seeding of decellularized aortic ECM scaffold for 21 d, CD34+ progenitor cells differentiated into mature vascular endothelial cells without addition of any growth factors, as confirmed by the increased levels of endothelial surface markers (CD31, Von Willebrand factor (VWF), VE-cadherin and ICAM-2) and upregulated gene levels (CD31, VWF and eNOS) concurrently with decreased expression of stem cell markers (CD133 and CD34), thus, resulting in surface endothelialization of decellularized ECM scaffold. Consequently, the patient-specific TEVP constructed in this study holds great potential for clinical use in pediatric patients with vascular malformation.

Cryopreserved venous allograft is an acceptable conduit in patients with current or prior angioaccess graft infection

OBJECTIVE

The durability of cryopreserved allograft has been previously demonstrated in the setting of infection. The objective of this study was to examine the safety, efficacy, patency, and cost per day of graft patency associated with using cryopreserved allograft (vein and artery) for hemodialysis access in patients with no autogenous tissue for native fistula creation and with arteriovenous graft infection or in patients at high risk for infection.

METHODS

Patients implanted with cryopreserved allograft for hemodialysis access between January 2004 and January 2014 were reviewed using a standardized, multi-institutional database that evaluated demographic, comorbidity, procedural, and outcomes data.

RESULTS

There were 457 patients who underwent placement of cryopreserved vein (femoral: n = 337, saphenous: n = 11) or artery (femoral: n = 109) for hemodialysis access at 20 hospitals. Primary indications for allograft use included high risk of infection in 191 patients (42%), history of infected prosthetic graft in 169 (37%), and current infection in 97 (21%). Grafts were placed more frequently in the arm (78%) than in the groin, with no difference in allograft conduit used. Mean time from placement to first hemodialysis use was 46 days (median, 34 days). Duration of functional graft use was 40 ± 7 months for cryopreserved vein and 21 ± 8 months for cryopreserved artery (P < .05), and mean number of procedures required to maintain patency at follow-up of 58 ± 21 months was 1.6 for artery and 0.9 for vein (P < .05). Local access complications occurred in 32% of patients and included late thrombosis (14%), graft stenosis (9%), late infection (9%), arteriovenous access malfunction (7%), early thrombosis (3%), and early infection (3%). Early and late infections both occurred more frequently in the groin (P = .030, P = .017, respectively), and late thrombosis occurred more frequently with cryopreserved artery (P < .001). Of the 82 patients (18%) in whom the cryopreserved allograft was placed in the same location as the excised infected prosthetic graft, 13 had infection of the allograft during the study period (early: n = 4; late: n = 9), with no significant difference in infection rate (P = .312) compared with the remainder of the study population. The 1-, 3-, and 5-year primary patency was 58%, 35%, and 17% for cryopreserved femoral vein and 49%, 17%, and 8% for artery, respectively (P < .001). Secondary patency at 1, 3, and 5 years was 90%, 78%, and 58% for cryopreserved femoral vein and 75%, 53%, and 42% for artery, respectively (P < .001). Mean allograft fee per day of graft patency was $4.78 for cryopreserved vein and $6.97 for artery (P < .05), excluding interventional costs to maintain patency.

CONCLUSIONS

Cryopreserved allograft provides an excellent conduit for angioaccess when autogenous tissue is not available in patients with current or past conduit infection. Cryopreserved vein was associated with higher patency and a lower cost per day of graft patency. Cryopreserved allograft allows for immediate reconstruction through areas of infection, reduces the need for staged procedures, and allows early use for dialysis.

Preparation and characterization of small-diameter decellularized scaffolds for vascular tissue engineering in an animal model

BACKGROUND

The development of a suitable extracellular matrix (ECM) scaffold is the first step in vascular tissue engineering (VTE). Synthetic vascular grafts are available as an alternative to autologous vessels in large-diameter arteries (>8 mm) and medium-diameter arteries (6-8 mm). In small-diameter vessels (<6 mm), synthetic vascular grafts are of limited use due to poor patency rates. Compared with a vascular prosthesis, natural tissue ECM has valuable advantages. Despite considerable progress in recent years, identifying an optimal protocol to create a scaffold for use in small-diameter (<6 mm) fully natural tissue-engineered vascular grafts (TEVG), remains elusive. Although reports on different decellularization techniques have been numerous, combination of and comparison between these methods are scarce; therefore, we have compared five different decellularization protocols for making small-diameter (<6 mm) ECM scaffolds and evaluated their characteristics relative to those of fresh vascular controls.

RESULTS

The protocols differed in the choice of enzymatic digestion solvent, the use of non-ionic detergent, the durations of the individual steps, and UV crosslinking. Due to their small diameter and ready availability, rabbit arteria carotis were used as the source of the ECM scaffolds. The scaffolds were subcutaneously implanted in rats and the results were evaluated using various microscopy and immunostaining techniques.

CONCLUSIONS

Our findings showed that a 2 h digestion time with 1× EDTA, replacing non-ionic detergent with double-distilled water for rinsing and the application of UV crosslinking gave rise to an ECM scaffold with the highest biocompatibility, lowest cytotoxicity and best mechanical properties for use in vivo or in situ pre-clinical research in VTE in comparison.

The Tissue-Engineered Vascular Graft-Past, Present, and Future

Cardiovascular disease is the leading cause of death worldwide, with this trend predicted to continue for the foreseeable future. Common disorders are associated with the stenosis or occlusion of blood vessels. The preferred treatment for the long-term revascularization of occluded vessels is surgery utilizing vascular grafts, such as coronary artery bypass grafting and peripheral artery bypass grafting. Currently, autologous vessels such as the saphenous vein and internal thoracic artery represent the gold standard grafts for small-diameter vessels (<6 mm), outperforming synthetic alternatives. However, these vessels are of limited availability, require invasive harvest, and are often unsuitable for use. To address this, the development of a tissue-engineered vascular graft (TEVG) has been rigorously pursued. This article reviews the current state of the art of TEVGs. The various approaches being explored to generate TEVGs are described, including scaffold-based methods (using synthetic and natural polymers), the use of decellularized natural matrices, and tissue self-assembly processes, with the results of various in vivo studies, including clinical trials, highlighted. A discussion of the key areas for further investigation, including graft cell source, mechanical properties, hemodynamics, integration, and assessment in animal models, is then presented.

An Outcomes Comparison of Native Arteriovenous Fistulae, Polytetrafluorethylene Grafts, and Cryopreserved Vein Allografts

BACKGROUND

Despite almost 2 decades of experience with cadaveric vein, there remains a paucity of available data regarding the role of cadaveric vein in hemodialysis, specifically with regard to outcomes and patency. Observations from our own experience have suggested that cadaveric vein grafts (CVGs) provide good outcomes, particularly in patients with recurrent access failure. Accordingly, this study aims to comparatively examine patency, access-related outcomes, and survival in patients undergoing placement of arteriovenous fistulae (AVF), polytetrafluorethylene (PTFE) grafts, and CVGs.

METHODS

This is a single institution 11-year retrospective case series evaluating the outcomes of 210 patients who underwent creation of AVF, PTFE grafts, and CVGs for hemodialysis access. Patients in the AVF (n = 70) and arteriovenous graft (AVG; n = 70) groups were matched to the CVG (n = 70) group by age, gender, and access location. Postoperative end points for all groups included primary and assisted patency, cause of access abandonment, and survival.

RESULTS

Patients were matched for age (P = 0.8707), gender (P = 0.6958), and access location and no significant differences existed between groups. AVF showed superior primary patency at 30 days, 1 year (64.3%, P < 0.0001) and 2 years (54.3%, P = 0.0091) in comparison to both AVG and CVG. AVG had reduced patency at 30 days (84.3%, P = 0.0009), 1 year (50.0%, P < 0.0001), and 2 years (32.9%, P = 0.0001) in comparison to AVF and CVG groups. Overall, AVF had the highest patency at all-time points followed, respectively by CVG and AVG. No significant difference existed between AVF and CVG groups with regard to secondary patency at 30 days (98.6% vs. 97.1%, P = 1.0000), 1 year (81.4% vs. 78.6%, P = 0.6749), and 2 years (68.6% vs. 51.4%, P = 0.0573). AVG patients had decreased survival (years) after access creation in comparison to AVF and CVG groups (P = 0.0003).

CONCLUSIONS

Our findings lend further support to the use of cadaveric vein for hemodialysis access surgery. As demonstrated through this comparative study, CVGs are capable of providing favorable results with regard to patency, access longevity, and patient survival. These current outcomes indicate that cadaveric vein is a sustainable alternative to PTFE for hemodialysis access surgery and should be accordingly considered for difficult access patients.

Long-Term Results of Biological Grafts for Haemodialysis Vascular Access

The quest for suitable conduits for dialysis access has continued since the first patients were dialysed. Whilst synthetic grafts made from expanded polytetrafluoroethylene (ePTFE) have been the main definitive option after autologous arteriovenous fistulas they have a number of drawbacks, which has led to the use and development of biological grafts such as autografts, homografts or xenografts. Technology continues to improve and currently biosynthetic options are available which may combine the benefits of a readily available product without the drawbacks of PTFE. The history and evidence of biological options for haemodialysis access are discussed.

Current Outcomes and Indications for Cryopreserved Vein Allografts in Hemodialysis Access Surgery

Introduction

Cryopreserved vein allografts (cadaveric vein) have emerged as an option for arteriovenous graft reconstruction; however, indications for their use in hemodialysis access remains to be clearly defined. Observations from our own experience have suggested that cadaveric vein grafts (CVGs) provide good outcomes, particularly in patients with a history of infection, recurrent access failure and advanced age.

Methods

This is a 10-year retrospective study. Primary outcomes were ( 1 ) to identify characteristics specific to this patient population and ( 2 ) to better define indications for use of cadaveric vein in hemodialysis access creation.

Results

Indications for creation of CVGs included patient history of either active or recent infection (41.5%), recurrent access failure (43.4%) or surgeon preference secondary to patients’ advanced age (9.4%). Observed primary patency rates were 84.9% (30 days), 22.6% (1 year) and 16.0% (2 years). Secondary patency was 93.4% (30 days), 66.0% (1 year) and 52.8% (2 years). Patient death was the highest cause of graft abandonment (52.9%) followed by thrombosis (19.1%), infection (11.7%) and rupture (11.7%). CVG patency at the time of patient death was 83.7%.

Conclusions

The rates of both primary and secondary patency in CVGs are highly comparable to the reported patency rates of polytetrafluoroethylene (PTFE) grafts and allow for lifelong maintenance of dialysis access. Our observed outcome suggests that CVGs should be considered for patients needing vascular access in the presence of infection. CVGs may likewise be viable alternatives to PTFE grafts in the elderly and patients with limited access options.

Tissue engineering in the vasculature

Tissue engineering holds great promise to address complications and limitations encountered with the use of traditional prosthetic materials, such as thrombogenicity, infection, and future degeneration which represent the major morbidity and mortality after device implant surgery. The general concept of tissue engineering consists of three main components: a scaffold material, a cell type for seeding the scaffold, and biochemical, physio-chemical signaling and remodeling process. This remodeling process is guided by cell signals derived from both seeded cells and host inflammatory cells that infiltrate the scaffold and deposit extracellular matrix, forming the neotissue. Vascular tissue engineering is at the forefront in the translation of this technology to clinical practice, as tissue engineered vascular grafts (TEVGs) have now been successfully implanted in children with congenital heart disease. In this report, we review the history, advances, and state of the art in TEVGs.

Vascular accesses for haemodialysis in the upper arm cause greater reduction in the carotid-brachial stiffness than those in the forearm: study of gender differences

Purpose. To evaluate in chronically haemodialysed patients (CHPs), if: (1) the vascular access (VA) position (upper arm or forearm) is associated with differential changes in upper limb arterial stiffness; (2) differences in arterial stiffness exist between genders associated with the VA; (3) the vascular substitute (VS) of choice, in biomechanical terms, depends on the previous VA location and CHP gender. Methods. 38 CHPs (18 males; VA in upper arm: 18) were studied. Left and right carotid-brachial pulse wave velocity (PWV_c-b) was measured. In in vitro studies, PWV was obtained in ePTFE prostheses and in several arterial and venous homografts obtained from donors. The biomechanical mismatch (BM) between CHP native vessel (NV) and VS was calculated. Results/Conclusions. PWV_c-b in upper limbs with VA was lower than in the intact contralateral limbs (P < 0.05), and differences were higher (P < 0.05) when the VA was performed in the upper arm. Differences between PWV_c-b in upper limbs with VA (in the upper arm) with respect to intact upper limbs were higher (P < 0.05) in males. Independently of the region in which the VA was performed, the homograft that ensured the minimal BM was the brachial artery. The BM was highly dependent on gender and the location in the upper limb in which the VA was performed.

Tissue engineering of small-diameter vascular grafts by endothelial progenitor cells seeding heparin-coated decellularized scaffolds

Successful construction of a small-diameter bioartificial vascular graft remains a great challenge. This study reports on novel tissue engineering vascular grafts (TEVGs) constructed by endothelial progenitor cells and heparin-coated decellularized vessels (DV). The DVs were fabricated from canine carotid arteries with observable depletion of cellular components. After heparin coating, the scaffolds possessed excellent antithrombogeneity. Canine endothelial progenitor cells harvested from peripheral blood were expanded and seeded onto heparin-coated DVs and cocultured in a custom-made bioreactor to construct TEVGs. Thereafter, the TEVGs were implanted into the carotid arteries of cell-donor dogs. After 3 months of implantation, the luminal surfaces of TEVGs exhibited complete endothelium regeneration, however, only a few disorderly cells and thrombosis overlaid the luminal surfaces of control DVs grafts, and immunofluorescent staining showed that the seeded cells existed in the luminal sides and the medial parts of the explanted TEVGs and partially contributed to the endothelium formation. Specifically, TEVGs exhibited significantly smaller hyperplastic neointima area compared with the DVs, not only at midportion (0.64 ± 0.08 vs. 2.13 ± 0.12 mm(2) , p < 0.001), but also at anastomotic sites (proximal sites, 1.03 ± 0.09 vs. 3.02 ± 0.16 mm(2), p < 0.001; distal sites, 1.84 ± 0.15 vs. 3.35 ± 0.21 mm(2), p < 0.001). Moreover, TEVGs had a significantly higher patency rate than the DVs after 3 months of implantation (19/20 vs. 12/20, p < 0.01). Overall, this study provided a new strategy to develop small-diameter TEVGs with excellent biocompatibility and high patency rate.

Vascular tissue engineering: towards the next generation vascular grafts

The application of tissue engineering technology to cardiovascular surgery holds great promise for improving outcomes in patients with cardiovascular diseases. Currently used synthetic vascular grafts have several limitations including thrombogenicity, increased risk of infection, and lack of growth potential. We have completed the first clinical trial evaluating the feasibility of using tissue engineered vascular grafts (TEVG) created by seeding autologous bone marrow-derived mononuclear cells (BM-MNC) onto biodegradable tubular scaffolds. Despite an excellent safety profile, data from the clinical trial suggest that the primary graft related complication of the TEVG is stenosis, affecting approximately 16% of grafts within the first seven years after implantation. Continued investigation into the cellular and molecular mechanisms underlying vascular neotissue formation will improve our basic understanding and provide insights that will enable the rationale design of second generation TEVG.

Vascular cryografts offer better biomechanical properties in chronically hemodialyzed patients: role of cryograft type, arterial pathway, and diabetic nephropathy as matching determinants

This study aimed to characterize the following: (i) in chronically hemodialyzed subjects (CHDSs), with and without diabetic nephropathy (DN), and in healthy subjects (non-CHDSs) different arterial pathways stiffness to determine potential pathology-dependent, etiology- and/or pathway-dependent differences; and (ii) the biomechanical mismatch (BM) between arteries from non-CHDSs or CHDSs (with and without DN) and arterial cryografts, venous cryografts, and synthetic prosthesis to determine arterial pathway, pathology, and/or etiology-related differences in the substitute of election in terms of BM. Carotid-femoral and carotid-brachial pulse wave velocity (PWV) were measured in 30 non-CHDSs and 71 CHDSs (11 with DN). In addition, PWV was measured in arterial (elastic and muscular) and venous cryografts and in expanded polytetrafluorethylene prosthesis. The arterial pathways regional differences and the subjects' arterial pathways-substitutes BM were calculated. Arterial stiffness levels and regional differences were higher in CHDS than in non-CHDS. Among CHDS, those with DN showed higher stiffness in the aorto-femoral pathway and larger regional differences. Cryografts showed always the least BM. Non-CHDS and CHDS differed in the cryograft of election. In CHDS, the BM was related with the cryograft type, arterial pathway, and renal disease etiology. The BM could be minimized, selecting the most adequate cryograft type, taking into account the recipient specific characteristic (i.e., arterial pathway and renal disease etiology).

Development and validation of small-diameter vascular tissue from a decellularized scaffold coated with heparin and vascular endothelial growth factor

To overcome shortcomings of current small-diameter vascular prostheses, we developed a novel allogenic vascular graft from a decellularized scaffold modified through heparin immobilization and vascular endothelial growth factor (VEGF) coating. The VEGF coating and release profiles were assayed by enzyme-linked immunosorbent assay, the biological activity of modified surface was validated by human umbilical vein endothelial cells seeding and proliferation for 10 days in vitro. In vivo, we implanted either a modified or a nonmodified scaffold as bilateral carotid allogenic graft in canines (n = 15). The morphological examination of decellularized scaffolds showed complete removal of cellular components while the extracellular matrix structure remained intact. After modification, the scaffolds possessed local sustained release of VEGF up to 20 days, on which the cells cultured showed significantly higher proliferation rate throughout the time after incubation compared with the cells cultured on nonmodified scaffolds (P < 0.0001). After 6 months of implantation, the luminal surfaces of modified scaffolds exhibited complete endothelium regeneration, however, only a few disorderly cells and thrombosis overlay the luminal surfaces of nonmodified scaffolds. Specifically, the modified scaffolds exhibited significantly smaller hyperplastic neointima area compared with the nonmodified, not only at midportion (0.56 +/- 0.07 vs. 2.04 +/- 0.12 mm(2), P < 0.0001), but also at anastomotic sites (1.76 +/- 0.12 vs. 3.67 +/- 0.20 mm(2), P < 0.0001). Moreover, modified scaffolds had a significantly higher patency rate than the nonmodified after 6 months of implantation (14/15 vs. 7/15, P = 0.005). Overall, this modified decellularized scaffold provides a promising direction for fabrication of small-diameter vascular grafts.

Clinical studies of hemodialysis access through formaldehyde-fixed arterial allografts

Efficient hemodialysis requires establishing a permanent stable vascular access. Our study was designed to evaluate formaldehyde-fixed arterial allografts as hemodialysis access for end-stage renal disease. Various parameters were determined for 68 formaldehyde-fixed, cadaver-derived allografts transplanted into 43 hemodialysis patients. The sources of the allografts were determined to be free of cytomegalovirus, hepatitis B and hepatitis C, and HIV infections. These allografts were monitored for rejection, blood flow, patency rates, and complications. Overall, antigenicity of the allografts was reduced after formaldehyde fixation with no acute rejection. The mean access blood flow was 696+/-282 ml with reasonable primary and secondary patency rates even after 3 years. Allograft intimal hyperplasia, determined by immunohistochemistry, was evident as the proliferation of smooth muscle-like cells expressing actin but cells not expressing the endothelial markers von Willebrand factor or CD34. The incidence of thrombus formation was about 37% after allograft transplant with other limited complications of pseudoaneurysms and local infection. Our results support the clinical use of formaldehyde-fixed arterial allografts for hemodialysis access.

A Comparison of Cryopreserved Vein Allografts and Prosthetic Grafts for Hemodialysis Access

In hemodialysis patients with insufficient vasculature for creation of a native arteriovenous fistula (AVF), a polytetrafluoroethylene (PTFE) graft is commonly utilized. Because of PTFE complications, our group and others have used cryopreserved cadaver femoral vein allografts (Synergraft [SYN], CryoLife, Marietta, GA) in selected patients. Based on our experience with these allografts, we hypothesized that they were more resistant to thrombosis than PTFE grafts. The purpose of this study was to compare the thrombosis rates of SYN and PTFE grafts in a prospective, randomized fashion. Our study was interrupted when the FDA ordered CryoLife, Inc. to retain certain vascular tissue products, and patient accrual stopped in 2003. Most patients referred for hemodialysis access are evaluated with bilateral, upper extremity Doppler ultrasound. Starting in 2001, those with insufficient vasculature for native AVF were offered randomization into the PTFE or SYN groups. All accesses were placed in the upper extremity, above the elbow. Access patency and complications were recorded, and failure was defined as access removal, abandonment, or replacement of > 50% with a new conduit. Prior to FDA interruption of the study, 27 patients were randomized into each group. Patient characteristics were similar, but there were significantly more males and African-Americans in the SYN group. No significant differences were seen in primary or secondary patency, number of thrombectomies, revisions, or total interventions. Significantly more fistulagrams were performed in the SYN group (p < 0.05). No infections were seen in either group, but 2 aneurysms occurred in the SYN group. Nine (33%) patients in each group died with functioning access. Access failures: In the SYN group, 8 of 27 (30%) failed, with 5 failing from multiple access stenoses unresponsive to balloon angioplasty; in the PTFE group 4 of 27 (18%) failed, with 2 failing from multiple stenoses. In conclusion, for initial hemodialysis access in patients without sufficient vasculature for native AVF, our results do not support the routine use of SYN allografts in the general dialysis population.