Effects of a venous cuff at the venous anastomosis of polytetrafluoroethylene grafts for hemodialysis vascular 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.

Recruitment into randomised trials of arteriovenous grafts: A systematic review

Although randomised controlled trials (RCT) are considered the optimal form of evidence, there are relatively few in surgery. Surgical RCT are particularly likely to be discontinued with poor recruitment cited as a leading reason. Surgical RCT present challenges over and above those seen in drug trials as the treatment under study may vary between procedures, between surgeons in one unit, and between units in multi-centred RCT. The most contentious and debated area of vascular access remains the role of arteriovenous grafts, and thus the quality of the data that is used to support opinions, guidelines and recommendations is critical. The aim of this review was to determine the extent of variation in the planning and recruitment in all RCT involving AVG. The findings of this are stark: there have been only 31 RCT performed in 31 years, the vast majority of which exhibited major limitations severe enough to undermine the results. This underlines the need for better quality RCT and data, and further inform the design of future studies. Perhaps most fundamental is the planning for a RCT that accounts for the intended population, the uptake of a RCT and the attrition for the significant co-morbidity in this population.

A critical review of surgical strategies to minimise venous stenosis in arteriovenous grafts

It is inevitable that complications arising from surgical procedures are ascribed to surgical technique, and this applies to venous stenosis (VS) in arteriovenous grafts. However, despite a wide range of cellular studies, computer modelling, observational series and clinical trials, there remains uncertainty on whether surgical technique contributes to VS. This article reviews evidence from basic science, fluid dynamics and clinical data to try and rationalise the main surgical options to modify the occurrence of venous stenosis. There is sufficient data from diverse sources to make recommendations on clinical practice (size of target vein, shape of anastomosis, angle of approach, distance from venous needling, trauma to the target vein) whilst at the same time this emphasises the need to carefully report the practical aspects of surgical technique in future clinical trials.

Tailoring of arteriovenous graft-to-vein anastomosis angle to attenuate pathological flow fields

Arteriovenous grafts are routinely placed to facilitate hemodialysis in patients with end stage renal disease. These grafts are conduits between higher pressure arteries and lower pressure veins. The connection on the vein end of the graft, known as the graft-to-vein anastomosis, fails frequently and chronically due to high rates of stenosis and thrombosis. These failures are widely believed to be associated with pathologically high and low flow shear strain rates at the graft-to-vein anastomosis. We hypothesized that consistent with pipe flow dynamics and prior work exploring vein-to-artery anastomosis angles in arteriovenous fistulas, altering the graft-to-vein anastomosis angle can reduce the incidence of pathological shear rate fields. We tested this via computational fluid dynamic simulations of idealized arteriovenous grafts, using the Bird-Carreau constitutive law for blood. We observed that low graft-to-vein anastomosis angles ([Formula: see text]) led to increased incidence of pathologically low shear rates, and that high graft-to-vein anastomosis angles ([Formula: see text]) led to increased incidence of pathologically high shear rates. Optimizations predicted that an intermediate  ([Formula: see text]) graft-to-anastomosis angle was optimal. Our study demonstrates that graft-to-vein anastomosis angles can significantly impact pathological flow fields, and can be optimized to substantially improve arteriovenous graft patency rates.

The Effect of Geometric Graft Modification on Arteriovenous Graft Patency in Haemodialysis Patients: A Systematic Review and Meta-Analysis

OBJECTIVE

Arteriovenous grafts (AVGs) are the second best option for haemodialysis access when native arteriovenous fistulae placement is not possible, because they have a lower patency owing to neointimal hyperplasia at the venous anastomosis. This review aimed to evaluate the effect of geometric graft modification to the graft-vein interface on AVG patency.

DATA SOURCES

The MEDLINE and Embase (OvidSP) databases were systematically searched for relevant studies analysing the effect of geometrically modified AVGs on graft patency and stenosis formation (last search July 2019).

REVIEW METHODS

Data regarding AVG type, patency, and graft outlet stenosis was extracted for further evaluation. Data were pooled in a random effects model to estimate the relative risk of graft occlusion within one year. Follow up, number of patients, and relevant patient characteristics were extracted for the quality assessment of the included studies using Newcastle-Ottawa Scale and Cochrane Risk of Bias Tool. The quality of the evidence was determined according to the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) system.

RESULTS

Search strategies produced 2772 hits, of which eight articles met predetermined inclusion criteria. Overall, the included articles had low to moderate risk of bias. In total, 414 expanded polytetrafluoroethylene AVGs (232 geometrically modified and 182 standard) were analysed, comprising two modified AVG types: a prosthetic cuff design (Venaflo®) and grafts with a Tyrell vein patch. Overall, modified grafts did not show a statistically significantly higher one year primary (relative risk [RR] 0.86, 95% confidence interval [CI] 95% 0.64-1.16; GRADE: "low to very low") or secondary patency (RR 0.57, 95% CI 0.32-1.02; GRADE: "low to very low") when compared with standard AVGs. Analysis of prosthetic cuffed grafts (112 patients) separately demonstrated a statistically significantly higher one year primary (RR 0.75, 95% CI 0.61-0.91) and one year secondary patency (RR 0.47, 95% CI 0.30-0.75) compared with standard grafts (92 patients). The results on stenosis formation were inconclusive and inadmissible to quantitative analyses.

CONCLUSION

The meta-analysis showed that a prosthetic cuff design significantly improves AVG patency, while a venous cuff does not. Although the heterogeneity and low number of available studies limit the strength of the results, this review shows the potential of grafts with geometric modification to the graft-vein anastomosis and should stimulate further clinical and fundamental research on improving graft geometry to improve graft patency.

Anastomotic Strategies to Improve Hemodialysis Access Patency—A Review

The number of patients with end-stage renal disease (ESRD) who require maintenance hemodialysis has risen sharply in the past 2 decades. It is estimated that more than 60% of all patients with ESRD who require chronic hemodialysis are accessed through an arteriovenous fistula (AVF) or graft (AVG), and the incidence is increasing at a rate of 2% to 4% per year. The long-term patency rate of an upper extremity AVF or AVG for hemodialysis access remains suboptimal owing in part to progressive stenosis at the venous anastomosis. This article reviews the causative factors of dialysis access-related anastomotic stenosis, or intimal hyperplasia. This article also reviews the clinical experience of various anastomotic strategies to ameliorate the hemodynamic environment in an effort to improve the clinical outcome of hemodialysis access. These strategies include the use of (1) vein cuff at the expanded polytetrafluoroethylene (ePTFE)-venous anastomosis of AVG, (2) cuffed ePTFE dialysis AVG, and (3) anastomotic devices that create an interrupted anastomosis with staples or clips.

Graft Modification Strategies to Improve Patency of Prosthetic Arteriovenous Grafts for Hemodialysis

Prosthetic arteriovenous grafts (AVGs) are indicated for vascular access for long-term hemodialysis in patients in whom creation or maintenance of an arteriovenous fistula (AVF) has failed or is contraindicated. AVGs have an inferior long-term patency as compared to AVFs. To ameliorate patency rates of prosthetic AVGs, different strategies have emerged to improve graft materials. This review aims to describe current strategies and future perspectives on graft modification, by graft geometry, drug coatings and graft surface technology, to improve AVG patency.

Upper Limb Grafts for Hemodialysis Access

Arteriovenous (AV) grafts are required for hemodialysis access when options for native fistulas have been fully exhausted, where they continue to play an important role in hemodialysis patients, offering a better alternative to central vein catheters. When planning autogenous accesses using Doppler ultrasound, adequate arterial inflow and venous outflow must be consciously preserved for future access creation with grafts. Efforts to improve graft patency include changing graft configuration, graft biology and hemodynamics. Industry offers early cannulation grafts to reduce central catheter use and a bioengineered graft is undergoing clinical studies. Although the outcome of AV grafts is inferior to fistulas, grafts can provide long-term hemodialysis access that is a better alternative to central venous catheters. AV grafts have significant drawbacks, mainly poor patency, infection and cost but also have some advantages: early maturation, ease of creation and needling and widespread availability. The outcome of AV graft surgery is variable from center to center. The primary patency rate for AV grafts is 58% at 6 months and the secondary patency rate is 76% at 6 months and 55% at 18 months. There are centers of excellence that report a 1 year secondary patency rate of up to 91%. In this review of the use of AV grafts for hemodialysis access in the upper extremities, technical issues involved in planning the access and performing the surgery in its different configurations are discussed and the role of surveillance and maintenance with their attendant surgical and radiological interventions is described.

Coronary artery bypass grafting hemodynamics and anastomosis design: a biomedical engineering review

In this paper, coronary arterial bypass grafting hemodynamics and anastomosis designs are reviewed. The paper specifically addresses the biomechanical factors for enhancement of the patency of coronary artery bypass grafts (CABGs). Stenosis of distal anastomosis, caused by thrombosis and intimal hyperplasia (IH), is the major cause of failure of CABGs. Strong correlations have been established between the hemodynamics and vessel wall biomechanical factors and the initiation and development of IH and thrombus formation. Accordingly, several investigations have been conducted and numerous anastomotic geometries and devices have been designed to better regulate the blood flow fields and distribution of hemodynamic parameters and biomechanical factors at the distal anastomosis, in order to enhance the patency of CABGs. Enhancement of longevity and patency rate of CABGs can eliminate the need for re-operation and can significantly lower morbidity, and thereby reduces medical costs for patients suffering from coronary stenosis. This invited review focuses on various endeavors made thus far to design a patency-enhancing optimized anastomotic configuration for the distal junction of CABGs.

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.

The safety and efficacy of a paclitaxel-eluting wrap for preventing peripheral bypass graft stenosis: a 2-year controlled randomized prospective clinical study

OBJECTIVES

To compare the safety and efficacy of a bioresorbable paclitaxel-eluting wrap implanted with a synthetic vascular graft (treatment) versus the graft implanted alone (control).

DESIGN

Prospective, randomized, controlled, multicentre, 2-year clinical study conducted in adults scheduled to undergo femoropopliteal peripheral bypass surgery with a polytetrafluoroethylene (PTFE) graft.

MATERIALS AND METHODS

Hundred and nine subjects were randomized 2:1 to treatment or control. All subjects were implanted with a 6mm expanded PTFE vascular graft; in addition, treated subjects had a 2.5 cm x 4 cm paclitaxel-eluting wrap (1.6 microg/mm(2)) placed around the distal graft anastomosis.

RESULTS

The overall incidence of adverse events was similar in both groups. Treated subjects required fewer limb amputations than controls (15.5% vs 18.4%) and time to amputation for those that required amputation was twice as long (153 days vs 76 days). Among diabetics, this effect was pronounced with 13.8% of treated subjects requiring limb amputations compared with 23.5% of controls. Over the course of study, the diameter at the distal graft anastomosis was greater in treated subjects than in controls (difference of 2.1mm at 2 yr, p=0.03).

CONCLUSIONS

The paclitaxel-eluting wrap maintained graft patency at the distal anastomosis and was safe to use in patients who had received a peripheral bypass PTFE graft.

Results of aggressive graft surveillance and endovascular treatment on secondary patency rates of Vectra Vascular Access Grafts

OBJECTIVE

The aim of the present study was to determine the effect of an aggressive graft surveillance and endovascular treatment protocol on secondary patency rates of a polyetherurethaneurea vascular access graft, specially designed to provide early access and rapid hemostasis.

METHODS

One hundred and ninety Vectra Vascular Access Grafts (C. R. Bard, Inc, Murray Hill, NJ) were placed in 176 patients (78 females and 98 males, mean age 61.7 years). There were 41 forearm grafts, 145 upper arm grafts and four thigh grafts. Graft surveillance was performed by using clinical and hemodialysis parameters to detect a failing/failed graft and followed by endovascular treatment, rheolytic thrombectomy (AngioJet, Possis Medical Inc, Minneapolis, Minn) and/or angioplasty +/- stenting of the anatomical lesion (arterial anastomosis, graft, venous outflow, draining or central veins).

RESULTS

Hemodialysis started after a median of 15.5 days, as soon as from the day of the operation in some cases. Bleeding complications occurred in six patients (3.2%), venous hypertension in seven (3.7%), steal syndrome in two (1.1%), neurological complications in two (1.1%), while late infection (range 2.7-14.6 months) was seen in six patients (3.2%). Thrombectomy and angioplasty (median number of sessions 1, interquartile range 1-2) was performed in 43 grafts. Isolated angioplasty, not associated with thrombosis (median number of sessions 1, interquartile range 1-2), was performed in 50 grafts. These interventions increased primary assisted patency from 69% and 63% at 12 and 18 months, respectively to a secondary patency rate of 86%. Taking into account grafts removed for late infection, functional secondary patency rate dropped to 83% and 81%, at 12 and 18 months, respectively. Arterial anastomosis angioplasty was performed more frequently in thrombosed grafts (28.6%) than failing grafts (6.7%), P < .001 and had a significant negative predictive value on secondary patency rates at 12 and 18 months, which were 60.5% compared with 89% for grafts that had no interventions performed (P = .007) and 90.9% for grafts that had any intra-graft, venous outflow, or draining or central vein stenosis treated with angioplasty at any stage (P = .002). Multivariate analysis identified the presence of arterial anastomosis stenosis as the single predictor of secondary patency (relative risk 0.247, P = .002).

CONCLUSIONS

Aggressive graft surveillance and endovascular treatment increases significantly secondary patency rates of Vectra Vascular Access Grafts. Longer follow-up will determine the effectiveness of this policy. The role of inflow stenosis on graft longevity and alternative treatment options warrant further investigation.

Impact of Cuffed, Expanded Polytetrafluoroethylene Dialysis Grafts on Graft Outlet Stenosis

BACKGROUND

The objective of this study was to determine prospectively the difference between the graft outlet strictures of a polytetrafluoroethylene (ePTFE) graft with a cuff at the graft-vein anastomosis (Venaflo; Bard industries, Tempe, Ariz.) and that of the regular ePTFE graft (Stretch Gore-Tex; Gore, Flagstaff, Ariz.) placed for hemodialysis access.

METHODS

Between January and April 2005, 36 consecutive patients (average age: 63.3 years) underwent ePTFE graft implantation (36 implantations) for hemodialysis at the Vascular Surgery Section of Chang Gung Memorial Hospital. The patients of the study cohort were randomly assigned to two groups based on the graft used: cuffed graft group (Venaflo graft) and non-cuffed standard graft group (Gore-Tex graft). Each patient underwent antegrade venography at the 3-month follow-up to demonstrate the graft outlet stricture. Results of the graft outlet angiography analysis were examined, and all medical records were reviewed at end of the study. The degree of the graft outlet stenosis was compared between the two groups.

RESULTS

Average stenosis of the cuffed graft group and non-cuffed (standard) graft group were 22.76 +/- 26.37%% and 44.95 +/- 27.48%%, respectively; the difference between the two groups was statistically significant (P < 0.05).

CONCLUSIONS

The graft outlet stricture of cuffed ePTFE grafts for hemodialysis 3 months after implantation was less severe than that for the standard ePTFE graft. The correlation between the stricture level and dialysis graft patency requires further clarification.

Primary Vascular Access

Primary vascular access is usually achievable by a distal autogenous arterio-venous fistula (AVF). This article describes the approach to vascular access planning, the usual surgical options and the factors affecting patency.

Randomized comparison of 6-mm straight grafts versus 6- to 8-mm tapered grafts for brachial-axillary dialysis access

OBJECTIVE

This report presents the results of a prospective randomized study that compared 2 grafts of different diameter: 6 mm, and 8 mm tapered to 6 mm at the arterial site, placed in the upper arm for hemodialysis in a selected population of patients younger than 71 years without diabetes.

METHODS

Seventy consecutive patients younger than 71 years without diabetes who required an upper arm graft between January 1997 and January 2002 and without previous access in the same limb were randomly allocated to receive either a 6-mm graft or 6- to 8-mm graft. Graft flow was measured every 3 months with the Doppler dilution technique. When access flow was less than 600 mL/min, fistulography was performed, and any stenosis was surgically treated with venous outflow replacement. Thrombectomy and associated stenosis treatment in the same stage was performed in all cases immediately after detection of thrombosis. Complication rate, and primary, assisted primary, and secondary patency rates were compared between the two groups with the Student t test and life table analysis.

RESULTS

Mean access flow was 975 mL/min for 6-mm grafts (range, 600-1500 mL/min; 95% confidence interval [CI], 889-1070), and for 6- to 8-mm grafts was 1397 mL/min (range, 1122-2700 mL/min; 95% CI, 1122-1672). This difference was significant (P <.01). Complication rate was 0.45 episodes per graft-year in 6-mm grafts, and 0.19 episodes per graft-year in 6- to 8-mm grafts (P <.01). At 1, 2, and 3 years, primary patency rates were 62%, 58%, and 44%, respectively, for 6-mm grafts, and 85%, 78%, and 73% for 6- to 8-mm grafts; log-rank comparison between curves was P =.0259. At 1, 2, and 3 years, secondary patency rates were 85%, 85%, and 85%, respectively, for 6-mm grafts, and 90%, 90%, and 90% for 6- to 8-mm grafts; log-rank comparison between curves was not significant, at P =.0603. At 1, 2, and 3 years, assisted primary patency rates were 84%, 79%, and 76%, respectively, for 6-mm grafts, and 90% for 6- to 8-mm grafts; log-rank comparison was P =.0414.

CONCLUSIONS

The results of this study show an advantage in terms of primary and assisted primary patency rates, and complication rate for upper arm grafts with diameter 6 mm to 8 mm over grafts with 6-mm diameter in a patient population younger than 70 years without diabetes. The finding of a similar secondary patency rate in both groups is probably due to the surveillance program with sequential measurement of access flow and prompt surgical treatment of stenosis. However, we needed twice the number of rescue procedures in 6-mm grafts to achieve a similar patency rate as with large-bore grafts. These study results must be carefully evaluated, taking into consideration the small number of patients and the selected patient population.

Flow pattern and shear stress distribution of distal end-to-side anastomoses. A comparison of the instantaneous velocity fields obtained by particle image velocimetry

OBJECTIVE

To describe the local hemodynamics and pressure losses of crural bypass anastomoses using instantaneous velocity fields acquired by particle image velocimetry (PIV).

METHODS

Silastic models of a Taylor patch, a Miller cuff and a femoro-crural patch prosthesis (FCPP) were attached to a circuit driven by a Berlin Heart, providing a pulsatile flow with an amplitude of 450 to 25 ml/min (mean 200 ml/min). An outflow resistance of 0.5 mmHg/ml/min (peripheral resistance units, PRU) was modeled using small silastic tubes providing a phase shift of -12 degrees between flow and pressure curves. The working fluid consisted of a glycerine/water mixture with a viscosity of 4 mPas. Hollow glass spheres with a mean size of 9-13 microm were used as tracer particles. Instantaneous velocity fields were obtained by means of PIV and shear rates as well as shear stresses were calculated. Triggered by the flowmeter signal, 10 measurements at 100 ms intervals per cardiac cycle were obtained. The pressures were measured on the inflow and at both distal outflows. The resulting mean pressure losses due to flow separation and distal fluid acceleration were calculated.

RESULTS

Inside the Taylor patch anastomosis a large flow separation at the hood containing a clockwise rotating vortex was found. Additionally a smaller flow separation at the heel and a flow stagnation zone on the floor of the recipient artery were observed. Conversely, inside the Miller cuff a counterclockwise rotating vortex was seen inside a large heel flow separation. The FCPP also showed typical separation areas at the hood and heel of the anastomosis, although these were smaller compared to the other anastomoses. Inside the FCPP anastomosis no vortex creation was observed throughout the cardiac cycle. The mainstream velocities at the inlet levels were comparable for the three anastomoses. A significant fluid acceleration was present at the antegrade as well as the retrograde outlets of the Taylor and Miller cuff, while the fluid acceleration at the antegrade outflow of the FCPP was small, which was attributed to the end-to-end configuration of the antegrade FCPP leg. The calculated normalized antegrade and retrograde pressure losses for the Taylor form were 0.90 and 0.88, for the Miller cuff 0.89 and 0.86 and for the FCPP 0.94 and 0.86, respectively. The shear stresses inside the flow separations of the three anastomoses were significantly lower than normal wall shear stresses. High shear stress levels were found inside the transition zones between flow separation and high velocity mainstream.

CONCLUSIONS

The flow pattern inside cuffed or funnel shaped anastomoses consists of large flow separation zones, which are thought to be associated with intimal hyperplasia development. In addition, fluid accelerations at the distal outlets result in pressure losses, which may contribute to impaired crural perfusion.

Evidence-Based Data for the Hemodialysis Access Surgeon

The National Kidney Foundation Dialysis Outcomes Quality Initiative Clinical Practice Guidelines for Vascular Access (DOQI) have defined the access-related care for patients with end-stage renal disease (ESRD). However, the standard of care across the country has fallen short of the DOQI targets. One potential explanation for these shortcomings is the lack of compelling evidence in the literature to support the recommendations. This study was designed to compare the DOQI with the best available evidence in the literature for four clinical questions relevant to the hemodialysis access surgeon: the choice of access type (autogenous versus prosthetic), the type of prosthetic graft, management of the "failing" (nonthrombosed) access, and management of the thrombosed access. The electronic literature databases MEDLINE and Evidence-Based Medicine Reviews were searched and relevant randomized controlled trials or meta-analyses were identified for review. No randomized controlled trials comparing autogenous to prosthetic accesses were identified. However, a recent systematic review reported that the patency rates for upper extremity autogenous accesses were superior to their polytetrafluoroethylene (PTFE) counterparts. The identified randomized trials suggested that the patency rates for the different types of commercially available prosthetic grafts used for access appear comparable. They suggested that standard wall PTFE thickness and prosthetic anastomotic cuffs may be associated with better graft patency, while venous cuffs may be associated with worse patency. Furthermore, the trials suggested percutaneous angioplasty of "failing" prosthetic accesses with greater than 50% stenoses did not appear to improve patency and that routine use of intraluminal stents, as an adjunct to angioplasty, was not beneficial. They did suggest that patency after open surgical revision of "failing" prosthetic accesses was superior to that after percutaneous angioplasty. Lastly, the identified trials suggested that the patency rates after open surgical revision of thrombosed prosthetic accesses was better than after endovascular treatment. Despite the magnitude of hemodialysis-related access problems, the quality of the evidence supporting the clinical decisions relevant to the access surgeon is limited and further clinical trials are justified.

Patency of autogenous and polytetrafluoroethylene upper extremity arteriovenous hemodialysis accesses: a systematic review

OBJECTIVE

Patency rates for autogenous accesses are presumed to be better than for polytetrafluoroethylene (PTFE) accesses, although the strength of the supporting evidence is limited. We undertook this study to test the hypothesis that patency rates for upper extremity autogenous hemodialysis arteriovenous accesses in adults are superior to those for PTFE counterparts.

METHODS

A systematic review of relevant literature and meta-analysis of the patency data were performed. Studies were considered acceptable if patency data were reported by either life table or Kaplan-Meier method, including number of patients at risk.

RESULTS

The thirty-four studies that satisfied the inclusion criteria were composed predominantly of case series or nonrandomized controlled studies; no randomized, controlled studies comparing autogenous and PTFE accesses were included. The primary patency rate for autogenous accesses was 72% (95% confidence interval [CI], 70%-74%) at 6 months and 51% (95% CI, 48%-53%) at 18 months, and the corresponding primary patency rate for PTFE accesses was 58% (95% CI, 56%-61%) and 33% (95% CI, 31%-36%), respectively. The secondary patency rate for autogenous accesses was 86% (95% CI, 84%-88%) at 6 months and 77% (95% CI, 74%-79%) at 18 months, and the corresponding secondary patency rate for PTFE accesses was 76% (95% CI, 73%-79%) and 55% (95% CI, 51%-59%), respectively.

CONCLUSIONS

The patency rate for autogenous upper extremity arteriovenous hemodialysis accesses in adults is superior to that for PTFE counterparts, although the overall quality of the studies in the meta-analysis was less than ideal. Randomized, controlled studies to further examine the differences in outcome between these two access types are necessary.

Hemodynamics of venous cuff interposition in prosthetic arteriovenous fistulas for hemodialysis

PURPOSE

The durability of prosthetic arteriovenous fistulas (AVF) for hemodialysis is jeopardized by thrombotic occlusions due to intimal hyperplastic stenoses. In arterial reconstructive surgery, peripheral arterial bypasses with prosthetic material benefit from a venous cuff at the distal anastomosis. Therefore, a study was performed to assess the effect of a venous cuff at the venous anastomosis of PTFE graft AVFs in terms of stenosis development, hemodynamics and patency rates.

METHODS

A subset of 40 patients from a multicenter study were enrolled into the study, of which 20 patients were randomized for venous cuff interposition. Duplex measurements to detect stenoses and volume flows were performed at 6, 12, 26 and 52 weeks postoperatively. Relative distension (RD) and wall shear rate (WSR) were calculated by means of vessel wall Doppler tracking (VWDT).

RESULTS

The total number of stenoses was significantly less in the cuff group (21 vs. 33; p = 0.045). This feature was found at the site of the venous anastomosis (cuff 5; no cuff 12). Volume flow, graft and efferent vein diameters, RD and WSR in the graft and efferent vein were comparable for both groups. WSR in the venous anastomosis tended to be lower in the cuff group (768 vs. 1,448 s(-1), p = 0.068). Volume flows and WSR were significantly lower in failing grafts. Patency rates were similar in both groups (primary patency 13 vs. 29%; secondary patency 78 vs. 67%).

CONCLUSIONS

A venous cuff at the venous anastomosis of PTFE graft AVFs results in less stenoses, but improved patency rates could not be demonstrated.

Rapid, arteriovenous graft failure due to intimal hyperplasia: a porcine, bilateral, carotid arteriovenous graft model

BACKGROUND

The loss of patency constitutes the major complication of arteriovenous (AV) polytetrafluoroethylene hemodialysis grafts. In most cases, this graft failure is due to intimal hyperplasia at the venous outflow tract, including proliferation of vascular, smooth muscle cells and fibroblasts with deposition of extracellular matrix proteins. Thus far, procedures developed for improving patency have proven unsuccessful, which can be partly explained by the lack of relevant animal models. For this purpose, we developed a porcine model for AV graft failure that will allow the assessment of promising therapeutic strategies in the near future.

MATERIALS AND METHODS

In 14 pigs, AV grafts were created bilaterally between the carotid artery and the jugular vein using expanded polytetrafluoroethylene. Two, 4 or 8 weeks after AV shunting, the grafts and adjacent vessels were excised and underwent histologic analysis.

RESULTS

From 2 weeks onwards, a thick neo-intima developed at the venous anastomosis, predominantly consisting of alpha-actin-positive vascular smooth muscle cells (VSMC). Intimal area increased over time, coinciding with a decreased graft flow. Grafts remained patent for at least 4 weeks. At 8 weeks, patency rates declined to less than 50% due to thrombus formation superimposed on progressive neo-intima formation.

CONCLUSIONS

Implantation of an AV graft between the carotid artery and jugular vein in pigs causes a rapid neo-intimal response, accompanied by a loss of patency of 50% at 8 weeks after surgery. This model offers a suitable tool to study local interventions aimed at the improvement of AV graft patency rates.

Evaluation of 4-mm to 7-mm versus 6-mm prosthetic brachial-antecubital forearm loop access for hemodialysis: results of a randomized multicenter clinical trial

OBJECTIVE

Prosthetic arteriovenous fistulas for hemodialysis vascular access have a high incidence rate of thrombotic occlusions that result in graft failure. This randomized multicenter study was performed to assess the patency rates and the effect of 4-mm to 7-mm grafts on the development of stenoses.

METHODS

A total of 109 patients who needed vascular access for hemodialysis were randomized to receive either 6-mm (n = 57) or 4-mm to 7-mm prosthetic brachial-antecubital forearm loop accesses (polytetrafluoroethylene). Duplex scanning, with measurement of blood flow and peak systolic velocity and detection of stenoses (>50%), was performed at 1, 6, and 12 months after surgery. Clinical data were obtained in a prospective manner and primary, assisted primary, and secondary patency rates were calculated with the Kaplan-Meier life-table analysis. Statistical analysis was performed with the independent samples t test and chi(2) test.

RESULTS

At 1 year, patency rates were similar for both 4-mm to 7-mm and 6-mm prostheses (primary, 46% versus 43%; assisted primary, 62% versus 58%; secondary, 87% versus 91%). The incidence rate of thrombotic occlusion was comparable for both groups (0.74/patient-year versus 0.88/patient-year; P >.05). Mean graft flow at 1, 6, and 12 months was 1416 versus 1415 mL/min, 1345 versus 1319 mL/min, and 1595 versus 1265 mL/min (P >.05) for 4-mm to 7-mm and 6-mm grafts, respectively. Also, no differences in peak systolic velocities in any part of the grafts were observed. The percentage of stenoses detected was equal in both groups at 1 year after surgery (27% versus 20%; P >.05).

CONCLUSION

A 4-mm to 7-mm tapered prosthetic brachial-antecubital forearm loop access did not reduce the incidence rates of stenoses and thrombotic occlusions compared with a 6-mm prosthetic conduit. Moreover, no differences in patency rates were observed. Therefore, we believe that the 4-mm to 7-mm graft should not be used routinely for hemodialysis vascular access.

Computational fluid dynamics and vascular access

Anastomotic intimal hyperplasia caused by unphysiological hemodynamics is generally accepted as a reason for dialysis access graft occlusion. Optimizing the venous anastomosis can improve the patency rate of arteriovenous grafts. The purpose of this study was to examine, evaluate, and characterize the local hemodynamics and, in particular, the wall shear stresses in conventional venous end-to-side anastomosis and in patch form anastomosis (Venaflo) by three-dimensional computational fluid dynamics (CFD). We investigated the conventional form of end-to-side anastomosis and a new patch form by numerical simulation of blood flow. The numerical simulation was done with a finite volume-based algorithm. The anastomotic forms were constructed with usual size and fixed walls. Subdividing the flow domain into multiple control volumes solved the fundamental equations. The boundary conditions were identical for both forms. The velocity profile of the patch form is better than that for the conventional form. The region of high static pressure caused by flow stagnation is reduced on the vein floor. The anastomotic wall shear stress is decreased. The results of this study strongly support patch form use to reduce the incidence of intimal hyperplasia and venous anastomotic stenoses.