Vascular access surgery managed by renal physicians: the choice of native arteriovenous fistulas for hemodialysis

A comparison of arteriovenous fistula failure between Malaysian and Australian and New Zealand participants enrolled in the FAVOURED trial

AIM

To describe and compare de novo arteriovenous fistula (AVF) failure rates between Australia and New Zealand (ANZ), and Malaysia.

BACKGROUND

AVFs are preferred for haemodialysis access but are limited by high rates of early failure.

METHODS

A post hoc analysis of 353 participants from ANZ and Malaysia included in the FAVOURED randomised-controlled trial undergoing de novo AVF surgery was performed. Composite AVF failure (thrombosis, abandonment, cannulation failure) and its individual components were compared between ANZ (n = 209) and Malaysian (n = 144) participants using logistic regression adjusted for patient- and potentially modifiable clinical factors.

RESULTS

Participants' mean age was 55 ± 14.3 years and 64% were male. Compared with ANZ participants, Malaysian participants were younger with lower body mass index, higher prevalence of diabetes mellitus and lower prevalence of cardiovascular disease. AVF failure was less frequent in the Malaysian cohort (38% vs 54%; adjusted odds ratio (OR) 0.53, 95% confidence interval (CI) 0.31-0.93). This difference was driven by lower odds of cannulation failure (29% vs 47%, OR 0.45, 95% CI 0.25-0.80), while the odds of AVF thrombosis (17% vs 20%, OR 1.24, 95% CI 0.62-2.48) and abandonment (25% vs 23%, OR 1.17, 95% CI 0.62-2.16) were similar.

CONCLUSIONS

The risk of AVF failure was significantly lower in Malaysia compared to ANZ and driven by a lower risk of cannulation failure. Differences in practice patterns, including patient selection, surgical techniques, anaesthesia or cannulation techniques may account for regional outcome differences and warrant further investigation.

Intimal CD31-Positive Relative Surfaces Are Associated with Systemic Inflammatory Markers and Maturation of Arteriovenous Fistula in Dialysis Patients

BACKGROUND

Arteriovenous fistula dysfunction is a widely disputed subject in the scientific literature on end-stage kidney disease (ESKD). The main cause of mortality and morbidity in these patients is the non-maturation or dysfunction of the arteriovenous fistula. Despite the many complications, the native arteriovenous fistula remains the gold standard in the treatment of these patients requiring renal replacement. This study aims to discuss the predictive role of some systemic inflammatory biomarkers (NLR, PLR, SII, IL-6), intimal hyperplasia, and neoangiogenesis (characterized by intimal-media CD31-positive relative surface) in arteriovenous fistula maturation failure.

METHODS

The present study was designed as an observational, analytical, and prospective study which included patients diagnosed with ESKD with indications of radio-cephalic arteriovenous fistula (RCAVF). Demographic data, comorbidities, preoperative laboratory data and histological/digital morphometry analysis results were processed. The patients included were divided into two groups based on their AVF maturation status at 8 weeks: "Maturation" (Group 1) and "Failed Maturation" (Group 2).

RESULTS

There was no difference in the demographic data. In terms of comorbidities, the second group had a greater incidence of heart failure (p = 0.03), diabetes (p = 0.04), peripheral artery disease (p = 0.002), and obesity (p = 0.01). Additionally, regarding the laboratory findings, these patients had higher levels of serum uric acid (p = 0.0005), phosphates (p < 0.0001), and creatinine (p = 0.02), as well as lower levels of total calcium (p = 0.0002), monocytes (p = 0.008), and lymphocytes (p < 0.0001). Moreover, all inflammatory markers (p = 0.001; p < 0.0001; p = 0.006, and p = 0.03) and Ca-P product (p < 0.0001) had higher baseline values in Group 2. Upon immunohistochemical analysis, regarding the density of neoformed vessels, there was a higher incidence of CD31-positive surfaces (p = 0.006) and CD31-positive relative surfaces (p = 0.001); the NLR (r = 0.323; p = 0.03), PLR (r = 0.381; p = 0.04), SII (r = 0.376; p = 0.03), and IL-6 (r = 0.611; p < 0.001) are all significantly correlated with vascular density, as evidenced by CD31.

CONCLUSIONS

Heart failure, peripheral artery disease, obesity, and diabetes, as well as the systemic inflammatory markers (NLR, PLR, SII, IL-6), intimal hyperplasia, and CD31-positive relative surfaces are predictors of arteriovenous fistula maturation failures.

Predictors of Arteriovenous Fistula Failure: A Post Hoc Analysis of the FAVOURED Study

Background

An autologous arteriovenous fistula (AVF) is the preferred hemodialysis vascular access, but successful creation is hampered by high rates of AVF failure. This study aimed to evaluate patient and surgical factors associated with AVF failure to improve vascular access selection and outcomes.

Methods

This is a post hoc analysis of all participants of FAVOURED, a multicenter, double-blind, multinational, randomized, placebo-controlled trial evaluating the effect of fish oil and/or aspirin in preventing AVF failure in patients receiving hemodialysis. The primary outcome of AVF failure was a composite of fistula thrombosis and/or abandonment and/or cannulation failure at 12 months post-AVF creation, and secondary outcomes included individual outcome components. Patient data (demographics, comorbidities, medications, and laboratory data) and surgical factors (surgical expertise, anesthetic, intraoperative heparin use) were examined using multivariable logistic regression analyses to evaluate associations with AVF failure.

Results

Of 536 participants, 253 patients (47%) experienced AVF failure during the study period. The mean age was 55±14.4 years, 64% were male, 45% were diabetic, and 4% had peripheral vascular disease. Factors associated with AVF failure included female sex (odds ratio [OR], 1.79; 95% confidence interval [CI], 1.20 to 2.68), lower diastolic BP (OR for higher DBP, 0.85; 95% CI, 0.74 to 0.99), presence of central venous catheter (OR, 1.49; 95% CI, 1.02 to 2.20; P=0.04), and aspirin requirement (OR, 1.60; 95% CI, 1.00 to 2.56).

Conclusions

Female sex, requirement for aspirin therapy, requiring hemodialysis via a central venous catheter, and lower diastolic BP were factors associated with higher odds of AVF failure. These associations have potential implications for vascular access planning and warrant further studies.

Prediction model for successful radiocephalic arteriovenous fistula creation in patients with diabetic nephropathy

Arteriovenous fistula (AVF) is the most appropriate vascular access for all chronic kidney disease patients for hemodialysis. However, patients with diabetic nephropathy are at increased risk for primary failure after AVF creation, mainly due to atherosclerosis and calcification of blood vessels. We conducted this study to find out the risk factors for primary failure of radiocephalic AVF in end-stage renal disease (ESRD) patients due to diabetic nephropathy and develop a risk predicting model. This study was conducted at a tertiary care teaching hospital of South India. Patients with ESRD due to diabetic nephropathy whom underwent left radiocephalic AVF at wrist were enrolled. Risk factors for primary failure were analyzed by univariate and multivariate logistic regression models. Sixty-six patients were included in the study. Thirty-one patients had a primary failure. Independent risk factors for primary failure were palpable vessel wall of the radial artery (P = 0.003, odds ratio [OR] = 15.317), smaller radial artery diameter (P = 0.001, OR = 16.526), radial artery peak systolic velocity (PSV) <45 cm/s (P = 0.005, OR = 8.494), and linear radial artery calcification (P = 0.006, OR = 7.942). The risk predicting model obtained by adding the score given for each risk factors (vessel wall not palpable = 0, palpable = 1, no linear calcification in digital X-ray = 0, linear calcification = 1, PSV ≥45 cm/s = 0, <45 cm/s = 1 and 2.5 - radial artery diameter in mm) had an area under receiver-operating characteristic curve of 0.886. Cutoff score of 1.5 had sensitivity of 83.9% and specificity of 80.0% for primary failure. Risk predicting model for primary failure based on condition of the vessel wall on palpation, radial artery diameter, flow velocity, and calcification may be helpful for suitable patient selection.

Quality assessment of vascular access procedures for hemodialysis: A position paper of the Vascular Access Society based on the analysis of existing guidelines

Quality assessment in vascular access procedures for hemodialysis is not clearly defined. The aim of this article is to compare various guidelines regarding recommendation on quality control in angioaccess surgery. The overall population of end-stage renal disease patients and patients in need for hemodialysis treatment is growing every year. Chronic intermittent hemodialysis is still the main therapy. The formation of a functional angioaccess is the cornerstone in the management of those patients. Native (autologous) arteriovenous fistula is the best vascular access available. A relatively high percentage of primary failure and fistula abandonment increases the need for quality control in this field of surgery. There are very few recommendations of quality assessment on creation of a vascular access for hemodialysis in the searched guidelines. Some guidelines recommend the proportion of native arteriovenous fistula in incident and prevalent patients as well as the maximum tolerable percentage of central venous catheters and complications. According to some guidelines, surgeon’s experience and expertise have a considerable influence on outcomes. There are no specific recommendations regarding surgeon’s specialty, grade, level of skills, and experience. In conclusion, there is a weak recommendation in the guidelines on quality control in vascular access surgery. Quality assessment criteria should be defined in this field of surgery. According to these criteria, patients and nephrologists could choose the best vascular access center or surgeon. Centers with best results should be referral centers, and centers with poorer results should implement quality improvement programs.

Outcomes of Primary Arteriovenous Fistula for Hemodialysis in Elderly Patients (>65 Years) with End Stage Renal Disease: A Study on Indian Population

Introduction:

The outcome of arteriovenous fistula (AVF) for hemodialysis (HD) in elderly population remains an issue. The aim of our study was to evaluate the outcomes of arteriovenous fistulas created at our institute in patients older than 65 years.

Methods:

All chronic HD patients with age >65 years who had an AVF created between January 1, 2010 and January 1, 2017 were included retrospectively. Baseline demographic information including age, gender, etiology of renal failure and comorbidities were recorded. Access characteristics including access type and anatomic location were recorded. The end point of study was primary and secondary patency. Minimum follow up period of study was 1 year.

Results:

A total of 422 AVF were created within the study period. The mean age was 69.3 years. The anatomical site of AVF creation was radiocephalic (RCF) in 74.8% (n = 316), brachiocephalic (BCF) in 18.9% (n = 80) and brachiobasilic (BBF) in 6.1% (n = 26). At one year after creation, cumulative survival of the AVF was 64.7%. At 36 months the primary and secondary patency of RCF, BCF and BBF was 43.6%, 58.6%, 42.6% and 47.3%, 62.5%, 56.9% respectively. The overall median survival did not differ between RCF and BBF fistulas. However, when both were compared with BCF (median survival 1034 days), BBF (median survival 741 days) and RCF (median survival 592 days) had significantly poorer survival (P = 0.004). The most common reason for access failure was thrombosis (28.4%) followed by failure to mature (9%) and aneurysm related complications (9%).

Conclusions:

Age should not be a limiting factor when choosing AVF as the preferred HD access. Brachiocephalic AVF has better primary and secondary patency with higher overall median survival. However RCF also provides reasonably good survival rates with acceptable complications in elderly population. Thrombosis and fistulas that fail to mature present as a primary concern to patients in elderly population, and demand further study.

Effectiveness of a New Single-Needle Single-Pump Dialysis System with Simultaneous Monitoring of Dialysis Dose

To date, single‐needle (SN) hemodialysis (HD) requires a dialysis machine equipped with two blood pumps—one controlling arterial blood flow (Qb) and one controlling venous Qb. B. Braun has developed an innovative single‐pump SN HD system. Therefore, usability is improved by reducing complexity. The aim of this study was to compare dialysis parameters of the new single‐pump SN HD system with a double‐pump SN HD system available on the market (Fresenius Medical Care [FMC] 5008). In this two‐armed crossover study, patients were randomized into two groups (B. Braun ‐ FMC/FMC ‐ B. Braun). Study period was 2 weeks (6 HD sessions) for each SN HD system. Both B. Braun and FMC dialysis machines were operated in the single‐needle auto mode. With the FMC dialysis machines, Qb was optimized manually, whereas for B. Braun machines it was optimized automatically using the auto‐mode functionality. A phase volume of 25 mL, treatment time, needle type and size, and dialyzer type and size were kept constant per patient throughout the study. Due to technical prerequisites in the SN mode, online dialysis adequacy (Kt/V: K ‐ dialyzer clearance of urea; t ‐ dialysis time; V ‐ volume of distribution of urea) monitoring could only be performed in the B. Braun group. Twelve HD patients (5 male/7 female, mean age 75.5 ± 8.8 years, mean time on dialysis 4.97 ± 3.86 years, 3× weekly HD) were enrolled. Total number of treatments performed: n = 132 (65 B. Braun, 67 FMC) and the mean online Kt/V value in the B. Braun group was 1.26 ± 0.29 (n = 63). Mean dialysis time per session: B. Braun 253.4 ± 19.9 min, FMC 251.6 ± 18.8 min. Mean phase volume: B. Braun 25.1 ± 0.2 mL, FMC 25.4 ± 3.1 mL. Mean cumulated blood volume (CBV): B. Braun 55.0 ± 5.5 L, FMC 40.5 ± 5.9 L (P < 0.0001). Mean Qb: B. Braun 217.8 ± 12.9 mL/min, FMC 178.6 ± 14.9 mL/min (effective Qb) (P < 0.0001), which corresponds to a difference of 39.3 mL/min (22.0%). Higher Qb has an influence on the CBV. To evaluate this effect, CBV was corrected for the difference in Qb by calculating the CBV/Qb rate. The mean CBV/Qb rate was 252.2 ± 19.4 min (B. Braun) and 226.8 ± 27.6 min (FMC) (P < 0.0001) per session. This represents a highly significant difference of 11.4%. To support the in vivo data the dead time for opening/closure of the clamps of the FMC 5008 was measured, resulting in 364 milliseconds. Over a 240 min dialysis session, with a blood flow rate of 250 mL/min and a phase volume of 25 mL, it was estimated at about 14.56 min (6.1% of the session). Similarly, it was estimated that the dead time of the pumps of the FMC 5008 during 240 min dialysis session was 4.7 min (1.9% of the session). In case single needle therapy is the only practical option for a patient, the advantages of the new single‐pump single needle system—namely the proven higher cumulative blood volume, the alarm‐free auto‐regulation of the blood flow and the easier handling for the nursing staff—ensure higher treatment efficiency than conventional double‐pump single needle systems.

A Successful Scheme for Reducing Waiting Times for Arteriovenous Fistula Formation

Purpose

In a bid to reduce waiting times for arteriovenous fistula (AVF) formation we introduced a scheme whereby nephrologists were able to place patients directly on the waiting list for surgery. This study evaluated the quality of these direct referrals and assessed the reduction in waiting times.

Methods

Fifty consecutive patients referred directly to the waiting list were compared with 50 patients placed on the waiting list after being assessed in our vascular access clinic.

Results

Forty-nine patients from the direct group and 47 patients from the clinic group underwent surgery. In the direct group 39 patients (80%) underwent the same procedure as they had been originally listed for, compared with 39 patients (83%) in the clinic group (p=0.80). A fistula suitable for needling was created in 37 patients (76%) of the direct group and 38 patients (81%) of the clinic group (p=0.62). The median wait from referral to surgery in the direct group was significantly lower than in the clinic group (62 vs.141 days; p<0.0001).

Conclusion

This study demonstrates that nephrologists are able to effectively assess select patients for AVF formation. This significantly reduces waiting times for surgery, without an increase in cancelled operations, or a reduction in technical success.

Practice Pattern of Hemodialysis Vascular Access in Maiduguri, Nigeria

Background

Despite available evidence based recommendations and guidelines there are differences in the preferred type of vascular access in use in different localities and geographical regions. This one center study on the practice pattern of vascular access was carried out in Maiduguri, Nigeria covering a period of the first 5 years of renal care in the area.

Methods

We analysed the dialysis and hospital records of consecutive patients that received hemodialysis in the renal center during the period between January 2000 and December 2004. Data comprised patients' demographics, vascular access used, complications encountered during the treatment period. Clinical and laboratory features of the patients were also recorded and analysed.

Results

The study involved one hundred and seventy-nine patients (M:F=2:1) who suffered kidney failure whose cause was clinically determined to be mainly due to chronic glomerulonephritis, hypertension, obstructive nephropathy and interstitial nephritis. Majority presented as late referrals to the nephrologist. Overall 9 % of the study population used AVFs while 91 % used catheters inserted into the femoral or internal jugular veins.

Conclusion

The use of catheters for vascular access was the practice in the majority of the patients and this is possibly related to the poor survival of the patients on hemodialysis. Socioeconomic circumstances of the patients in turn largely influenced the rate of AVF use.

Complications of the Arteriovenous Fistula: A Systematic Review

The implementation of patient-centered care requires an individualized approach to hemodialysis vascular access, on the basis of each patient's unique balance of risks and benefits. This systematic review aimed to summarize current literature on fistula risks, including rates of complications, to assist with patient-centered decision making. We searched Medline from 2000 to 2014 for English-language studies with prospectively captured data on ≥100 fistulas. We assessed study quality and extracted data on study design, patient characteristics, and outcomes. After screening 2292 citations, 43 articles met our inclusion criteria (61 unique cohorts; n>11,374 fistulas). Median complication rates per 1000 patient days were as follows: 0.04 aneurysms (14 unique cohorts; n=1827 fistulas), 0.11 infections (16 cohorts; n>6439 fistulas), 0.05 steal events (15 cohorts; n>2543 fistulas), 0.24 thrombotic events (26 cohorts; n=4232 fistulas), and 0.03 venous hypertensive events (1 cohort; n=350 fistulas). Risk of bias was high in many studies and event rates were variable, thus we could not present pooled results. Studies generally did not report variables associated with fistula complications, patient comorbidities, vessel characteristics, surgeon experience, or nursing cannulation skill. Overall, we found marked variability in complication rates, partly due to poor quality studies, significant heterogeneity of study populations, and inconsistent definitions. There is an urgent need to standardize reporting of methods and definitions of vascular access complications in future clinical studies to better inform patient and provider decision making.

Analysis of Vascular Access in Haemodialysis Patients - Single Center Experience

BACKGROUND

Vascular access is the key in successful management of chronic haemodialysis (HD) patients. Though native arteriovenous fistula (AVF) is considered the access of choice, many patients in our country initiate haemodialysis through central venous catheter (CVC). There is paucity of data on vascular access in haemodialysis patients from southern India.

AIM

Aim of the present study was to review our experience of vascular access in Haemodialysis patients (both central venous catheters and arteriovenous fistula) and to assess its success rate and common complications.

MATERIALS AND METHODS

This prospective study was conducted between January 2014 and December 2014 in our institute. A total of 50 patients with Chronic Kidney Disease (CKD) underwent vascular access intervention during the above period.

RESULTS

A temporary venous catheter (96%) in the right internal jugular vein was the most common mode of initiation of haemodialysis with 34.48% incidence of catheter related sepsis. Fifty percent of catheters were removed electively with mean duration of catheter survival of 77.23 ± 14.8 days. Wrist AVF (60%) was the most common site of AVF creation followed by arm (30%), mid-forearm (7.5%) and leg (2.5%). Complications include distal oedema (17.5%) and venous hypertension (2.5%). Primary failure occurred in 25% of patients and was more common in diabetic, elderly (>60 years) and in distal fistulas. Elderly patients (>60 years) starting dialysis with a CVC were more likely to be CVC dependent at 90 days.

CONCLUSION

Late presentation and delayed diagnosis of chronic kidney disease (CKD) necessitates dialysis initiation through temporary catheter. Dialysis catheter with its attendant complications further adds to the morbidity, mortality, health care burden and costs. Early nephrology referral and permanent access creation in the pre dialysis stage could avert the unnecessary complications and costs of catheter.

Preoperative vascular access evaluation for haemodialysis patients

BACKGROUND

Haemodialysis treatment requires reliable vascular access. Optimal access is provided via functional arteriovenous fistula (fistula), which compared with other forms of vascular access, provides superior long-term patency, requires few interventions, has low thrombosis and infection rates and cost. However, it has been estimated that between 20% and 60% of fistulas never mature sufficiently to enable haemodialysis treatment. Mapping blood vessels using imaging technologies before surgery may identify vessels that are most suitable for fistula creation.

OBJECTIVES

We compared the effect of conducting routine radiological imaging evaluation for vascular access creation preoperatively with standard care without routine preoperative vessel imaging on fistula creation and use.

SEARCH METHODS

We searched Cochrane Kidney and Transplant's Specialised Register to 14 April 2015 through contact with the Trials' Search Co-ordinator using search terms relevant to this review.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) that enrolled adult participants (aged ≥ 18 years) with chronic or end-stage kidney disease (ESKD) who needed fistulas (both before dialysis and after dialysis initiation) that compared fistula maturation rates relating to use of imaging technologies to map blood vessels before fistula surgery with standard care (no imaging).

DATA COLLECTION AND ANALYSIS

Two authors assessed study quality and extracted data. Dichotomous outcomes, including fistula creation, maturation and need for catheters at dialysis initiation, were expressed as risk ratios (RR) with 95% confidence intervals (CI). Continuous outcomes, such as numbers of interventions required to maintain patency, were expressed as mean differences (MD). We used the random-effects model to measure mean effects.

MAIN RESULTS

Four studies enrolling 450 participants met our inclusion criteria. Overall risk of bias was judged to be low in one study, unclear in two, and high in one.There was no significant differences in the number of fistulas that were successfully created (4 studies, 433 patients: RR 1.06, 95% CI 0.95 to 1.28; I² = 76%); the number of fistulas that matured at six months (3 studies, 356 participants: RR 1.11, 95% CI 0.98 to 1.25; I² = 0%); number of fistulas that were used successfully for dialysis (2 studies, 286 participants: RR 1.12, 95% CI 0.99 to 1.28; I² = 0%); the number of patients initiating dialysis with a catheter (1 study, 214 patients: RR 0.66, 95% CI 0.42 to 1.04); and in the rate of interventions required to maintain patency (1 study, 70 patients: MD 14.70 interventions/1000 patient-days, 95% CI -7.51 to 36.91) between the use of preoperative imaging technologies compared with standard care (no imaging).

AUTHORS' CONCLUSIONS

Based on four small studies, preoperative vessel imaging did not improve fistula outcomes compared with standard care. Adequately powered prospective studies are required to fully answer this question.

Vascular access Creation and Care Should be Provided by Nephrologists

The long-term survival and quality of life of patients on hemodialysis is dependent on the adequacy of dialysis via an appropriately placed vascular access. Recent clinical practice guidelines recommend the creation of native arteriovenous fistula or synthetic graft before start of chronic hemodialysis therapy to prevent the need for complication-prone dialysis catheters. The direct involvement of nephrologists in the management of referral patterns, predialysis follow-up, policy of venous preservation, pre-operative evaluation, vascular access surgery and vascular access care seems to be important and productive targets for the quality of care delivered to the patients with end-stage renal disease. Early referral to nephrologists is important for delay progression of both kidney disease and its complications by specific and adequate treatment, for education program which should include modification of lifestyle, medication management, selection of treatment modality and instruction for vein preservation and vascular access. Nephrologists are responsible for on-time placement and adequate maturation of vascular access. The number of nephrologists around the world who create their own fistulas and grafts is growing, driven by a need for better patient outcomes on hemodialysis. Nephrologists have also a key role for care of vascular access during hemodialysis treatment by following vascular access function using clinical data, physical examination and additional ultrasound evaluation. Timely detection of malfunctioning vascular access means timely surgical or radiological intervention and increases the survival of vascular access.

Preferred haemodialysis vascular access for diabetic chronic kidney disease patients: a systematic literature review

BACKGROUND

Vascular access problems are one of the main concerns in the diabetic end-stage kidney disease (ESKD) population. However, the optimal strategy for the establishment of vascular access in this population remains to be solved. We performed a systematic review in order to clarify the most advisable approach of vascular access planning in diabetic patients with ESKD.

METHODS

MEDLINE, EMBASE and CENTRAL databases were searched for English-language articles without time restriction through focused, high-sensitive search strategies. We included all studies providing outcome data on diabetics starting chronic haemodialysis treatment on the basis of the type of primary placed vascular access.

RESULTS

A total of 13 studies comprising over 2,800 participants with diabetes were reviewed in detail and included in the review. We found that diabetic patients using a dialysis catheter apparently experience a higher risk of death and infection compared with patients who successfully achieved and maintained an arteriovenous fistula as dialysis access. The comparison between the use of a graft or an autogenous fistula as dialysis access generated conflicting results. Primary patency rates appeared to be lower in diabetics versus non-diabetics.

CONCLUSIONS

Our study suggests that diabetic ESKD patients with dialysis catheters incur a higher risk of death in comparison to those who achieve an arteriovenous access. It is however unclear whether this is caused by residual selection bias or by a true advantage of native vascular access.

Arteriovenous fistula placement in the elderly: when is the optimal time?

Arteriovenous fistula (AVF) is the preferred vascular access for hemodialysis (HD). However, many AVFs fail before starting dialysis. To assess the optimal time for AVF placement in the elderly, we linked data from the US Renal Data System with Medicare claims data to identify 17,511 patients ≥67 years old on incident HD who started dialysis between January 1, 2005, and December 31, 2008, with an AVF placed as the first predialysis access. AVF success was defined as dialysis initiation using the AVF, with time between AVF placement and dialysis start as our primary variable of interest. The mean age was 76.1±6.0 years, and 58.3% of subjects were men. Overall, 54.9% of subjects initiated dialysis using an AVF, and 45.1% of subjects used a catheter or graft. The success rate increased as time from AVF creation to HD initiation increased from 1-3 months (odds ratio [OR], 0.49; 95% confidence interval [95% CI], 0.44 to 0.53) to 3-6 months (OR, 0.93; 95% CI, 0.85 to 1.02) to 6-9 months (OR, 0.99; 95% CI, 0.88 to 1.11) but stabilized after that time. Furthermore, the number of interventional access procedures increased over time starting at 1-3 months, with a mean of 0.64 procedures/patient for AVFs created 6-9 months predialysis compared with 0.72 for AVFs created >12 months predialysis (P<0.001). Although limited by the observational nature of this study, our results suggest that placing an AVF >6-9 months predialysis in the elderly may not associate with a better AVF success rate.

Does the extracorporeal blood flow affect survival of the arteriovenous vascular access?

Hemodiafiltration with high-convective volumes is associated with improved patient survival, whereby practical realization is contingent on high extracorporeal blood flow (Qb) and dialysis treatment time. However, Qb is restricted by vascular access (VA) quality and/or concerns that high Qb could damage the VA. Taking VA quality into consideration, one can investigate the relationship between Qb and VA survival. We analyzed data from 1039 patients treated by hemodiafiltration over a 21-month period where access blood flow (Qa) measurements were also available at baseline. VA failure was defined as a surgical intervention resulting in the generation of a new VA. Qa was included as a stratification variable within a Cox regression model. A second Cox proportional hazard model with a penalized spline was used to describe the association between Qb and VA survival. Compared with Qb in the 350-357 mL/min range, a significantly higher hazard ratio (HR) for VA failure was detected for fistula only, and then only for Qb < 312 mL/min (HR: 2.361, 95% confidence interval [CI]: 1.251-4.453), Qb = 387-397 mL/min (HR: 1.920, 95% CI: 1.007-3.660) and Qb >414 mL/min (HR: 2.207, 95% CI: 1.101-4.424). Age, gender, diabetes, VA vintage, position of the VA, and arterial pressure were not significantly associated with outcome. The form of the penalized spline confirmed higher risk for VA failure for the lowest and the highest values of Qb. Taking Qa into consideration, no association was found between VA failure and Qb up to flows as high as approximately 390 mL/min.

Factors predicting failure of AV "fistula first" policy in the elderly

An arteriovenous fistula (AVF) is the preferential hemodialysis (HD) access. The goal of this study was to identify factors associated with pre-dialysis AVF failure in an elderly HD population. We used United States Renal Data System + Medicare claims data to identify patients ≥ 67 years old who had an AVF as their initial vascular access placed pre-dialysis. Failure of the AVF to be used for initial HD, was used as the outcome. Logistic regression model was used to identify factors associated with AVF failure. The study cohort consisted of 20,360 subjects (76.2 ± 6.02 year old, 58.5% men). Forty-eight percent of patients initiated dialysis using an AVF, while 52% used a catheter or an AVG. The following variables found to be associated with AVF failure when an AVF was created at least 4 months pre-HD initiation: older age (odds ratio [OR] 1.01; 95% confidence interval [CI] 1.00-1.02), female gender (OR 1.69; 95% CI 1.55-1.83), black race (OR 1.41; 95% CI 1.26-1.58), history of diabetes (OR 1.22; 95% CI 1.06-1.39), cardiac failure (OR 1.26; 95% CI 1.15-1.37), and shorter duration of pre-end-stage renal disease (ESRD) nephrology care (OR for a nephrology care of less than 6 months prior to ESRD of 1.22 compared with a pre-ESRD nephrology follow up of more than 12 months; 95% CI 1.07-1.38). OR for AVF failure for the entire cohort showed similar findings. In an elderly HD population, there is an association of older age, female gender, black race, diabetes, cardiac failure and shorter pre-ESRD nephrology care with predialysis AVF failure.

Subclavian vein angioplasty during arteriovenous fistula surgery: case report and literature review

Patients with chronic kidney disease stage 5 are generally treated by hemodialysis, preferentially performed via an arteriovenous fistula (AVF). We report the case of a 58-year-old male patient with diabetes mellitus, hypertension and end-stage renal disease in whom hemodialysis was conducted via a long-term catheter. His medical record described numerous central venous cannulations and several AVF creations. The patient developed subclinical subclavian stenosis that required creation of a new vascular access route. The purpose of this case report is to describe treatment of subclavian vein stenosis during AVF creation.

An experience of arteriovenous fistulas created for hemodialysis in the largest health center in eastern Turkey

BACKGROUND

The aim of the study was to evaluated the primary and secondary (after reoperation) patency rates and some effect factors in fistula patency for hemodialysis patients.

MATERIAL AND METHODS

Over a 10-year period, 1529 arteriovenous fistulas (AVFs) were fashioned in 1003 (611 males, 392 females; median age range 7-72) patients using the native vascular tissue and prosthetic graft material. We also evaluated the effects of various factors in fistula patency and primary and secondary patency rates in AVF patients.

RESULTS

The primary patencies of fistulas in this series were 72%, 64%, 51%, 41%, and 26%, and secondary patencies were 79%, 70%, 56%, 46%, and 33% at 6 months, 1, 2, 4, and 6 years, respectively. There was no statistically significant difference between the primary and secondary patencies (p = 0.082) in the 6-year follow-up. Factors affecting the patency of fistulas were diabetes mellitus (p < 0.005), hypertension (p < 0.005), and smoking habits (p < 0.005).

CONCLUSION

Even if shown to be not statistically significant, successful surgical revision after fistula occlusion improves secondary patency with potential benefits in terms of patient morbidity. Besides, the AVF patency was shortened in chronic renal-insufficiency patients with diabetes mellitus, hypertension, and smoking habits.

Vascular access for hemodialysis in the elderly

OBJECTIVE

The number of elderly patients needing hemodialysis is constantly increasing year by year. Elderly patients with end-stage renal failure represent a challenge for the surgeons who create vascular accesses. The aim of this study was to analyze the outcome of conduit creation in the elderly in our institution and to compare it with the outcome of a cohort of patients aged <65 years.

METHODS

The study was performed retrospectively on prospectively collected data. The study period was between January 1, 2000, and December 31, 2006. We identified first attempts at conduit creations, including arteriovenous fistulas (AVFs) and grafts, in elderly patients (aged ≥65 years) who were allocated to group A, and in nonelderly patients (<65 years) who were allocated to group B. Subsequent attempts at conduit creations in the same patient were omitted from the data set.

RESULTS

There were 246 first AVFs in group A and 89 in group B. At a mean follow-up of 25.46 months (SD, 18.93 months), the primary patency (PP) rate of all AVFs was 70% in group A and 68% in group B (P = .75). The assisted PP rate was 73% in group A and 77% in group B (P = .4). The secondary patency (SP) rate was 73% in group A and 79% in group B (P = .9). Also, the differences in the 12-month cumulative patency rates (including PP, assisted PP, and SP) in the two groups (65% vs 60%) were not significant. At a mean follow-up of 25 months, death with a functioning conduit occurred at the same rate in both groups (56% and 54%), and mean conduit survival did not differ according to age (516 and 511 days). The incidence of failure to mature was higher in group A (6.1% vs 1.1%, P = .03). Patency rates for different types of conduits were similar between the two groups, although polytetrafluoroethylene grafts had a higher cumulative patency in group A (94% vs 69%; P = .05). The rate of procedures to salvage conduits was 2.5% in group A vs 10.1% in group B. Mean hospital stay for group A and group B was 3.2 days.

CONCLUSIONS

In our experience, the creation of permanent hemodialysis access in the elderly with AVF is not only possible but also proved to have a short hospital stay, high patency rates, and an acceptable rate of further intervention.

Successful permanent catheter implantation after percutaneous transluminal angioplasty to the right subclavian and inominate vein stenosis in a hemodialysis patient

Difficulties in gaining vascular access have become one of the major problems in long-term hemodialysis patients. We report a case with an extreme vascular access problem, which was solved by the placement of a permanent central vein catheter after successful angioplasty to the right subclavian and inominate vein stenosis. A 78-year-old woman with end-stage renal disease due to diabetic nephropathy had been on hemodialysis since 1982. She had a history of four procedures that created an arteriovenous fistula during the first four years. In 1986 she was switched to peritoneal dialysis. However, hemodialysis was restarted with a regular arteriovenous fistula in 1991 because of repeated peritonitis, and from 1993 a superficialized brachial artery was used until the artery was exhausted. Since 1997, a permanent central vein catheter was utilized and reinsertion was required four times due to catheter infection during a 6-year period. In 2004, difficulty was experienced in reinserting a new central venous catheter because of multiple stenotic and occlusive lesions of the central veins. Percutaneous transluminal angioplasty was carried out in the stenotic right subclavian and inominate veins, and after successful angioplasty, a permanent central vein catheter could be placed into the right subclavian vein as a vascular access for hemodialysis. Pretreatment with percutaneous transluminal angioplasty of the stenotic central veins before inserting a permanent catheter may be a useful strategy in patients that experience difficulty in the insertion of a catheter.

Modeling survival of arteriovenous accesses for hemodialysis: semiparametric versus parametric methods

BACKGROUND AND OBJECTIVES

Comparing outcomes of arteriovenous grafts and fistulas is challenging because the pathophysiology of access dysfunction and failure rate profiles differ by access type. Studying how risks vary over time may be important.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Longitudinal data from 535 incident hemodialysis patients were used to study the relationship between access type and access survival, without (semiparametric Cox modeling) and with specification of the underlying hazard function (parametric Weibull modeling).

RESULTS

The hazard for failure of fistulas and grafts declined over time, becoming proportional only after 3 months from surgery, with a graft versus fistula hazard ratio of 3.2 (95% confidence interval 1.9 to 5.3; Cox and Weibull estimation) and time ratio of 0.11 (i.e., the estimated access survival time was approximately one tenth shorter in grafts; 95% confidence interval 0.04 to 0.28; Weibull estimation only). Considering the entire observation period, grafts had slower hazard decline (P<0.001) with shorter median survival times than fistulas (8.4 versus 38.3 months; Weibull regression only).

CONCLUSIONS

Parametric models of arteriovenous access survival may provide relevant information about temporal risk profiles and predicted survival times.

Emergence of interventional nephrology at the international level

The development of nephrology has been greatly influenced by the performance of procedures. For example, kidney biopsies have elicited substantial knowledge, thereby linking kidney afflictions with their respective histopathologies, which consequently governed the course of therapy. Further, the growth of nephrology has been marked by the implementations of advanced forms of renal replacement therapy. Moreover, there is now the acute acknowledgement that the success of any modality of renal replacement therapy is contingent upon the integrity of its associated peritoneal or vascular access. Consequently, the concept of nephrologist as interventionalist has garnered broad appeal. In fact, the American Society of Diagnostic and Interventional Nephrology and the International Society of Nephrology promote this concept. Notably, these groups have already demonstrated successful outreach programs that have been replicated at local, regional, and national levels throughout the globe.

Autologous arteriovenous fistula creation by nephrologists

In 1966, physicians from the Bronx Veterans Administration Hospital, New York City described the surgical creation of a distal radial artery to cephalic vein AV fistula. This novel configuration, for the first time, allowed effective and reproducible cannulation for hemodialysis access and has remained the gold standard for hemodialysis access since. More than 40 years later, vascular access remains the Achilles' heel of hemodialysis therapy. In this article, we will review the outcomes and techniques of the pre-eminent nephrologist-surgeons from around the world in an attempt to define those elements that are necessary for successful autologous fistula creation. The hope is that these elements may be employed by others in an effort to increase the numbers and durability of autologous fistulae in incident and prevalent hemodialysis patients, particularly in the United States.

Primary Arteriovenous Fistulas in the Forearm for Hemodialysis: Effect of Miscellaneous Factors in Fistula Patency

BACKGROUND

The provision and maintenance of vascular access remains a major cost to end-stage renal failure programs. In addition, vascular access occlusion, results in significant morbidity in hemodialysis patients. Age, gender, diabetes mellitus, malignancy, smoking habits, administration of heparin per hemodialysis session, previous dialysis catheter insertion, number of hemodialysis sessions and location of the fistula may be associated with survival of the primary arteriovenous fistula. We examined the effects of various factors on fistulas in 412 chronic renal insufficiency patients.

METHODS

From 1995 to 2004, 412 arteriovenous fistulas were created by the Department of Cardiovascular Surgery at the Medical Faculty of Atatürk University for hemodialysis. The mean age of the patients was 45 years (range 6 to 62 years). We evaluated the effects of various factors for patency rates in the patients who had primary arteriovenous fistulas. Primary patency was defined as the duration of fistula patency without revision. Twenty-eight patients (6.7%) with ischemic cardiac disease did not require surgical interference. Analyzed data were age, gender, smoking habits, diabetes mellitus, malignant neoplasm, previous dialysis catheter insertion, number of hemodialysis sessions, and fistula location.

RESULTS

In 298 patients, where lower-arm radiocephalic fistulas were created, the fistula patency was 74.1%, 64.2%, 49.8%, 33.7%, and 4.1% after 1, 2, 3, 4, and 5 years, respectively, in the other 114 patients, where upper-arm fistulas were created, these rates were 84.0%, 72.2%, 53.3%, 39.8%, and 12.3%, respectively. There was no significantly difference between the upper-arm fistulas and the lower-arm fistulas statistically (p = 0.069). Factors affecting the primary patency of arteriovenous fistulas were diabetes mellitus (p = 0.0001), hemodialysis counts > or =3 per week (p < 0.0005), presence of malignancy (p < 0.0005), previous catheter insertion (p < 0.0007), and administration of heparin per hemodialysis session (p = 0.0008).

CONCLUSION

While primary arteriovenous fistula patency was shortened in chronic renal insufficiency patients with diabetes mellitus, presence of malignancy, and previous catheter insertion, patency was longer in patients with heparin used for hemodialysis and hemodialysis count per week (> or =3).

Resolved: Fistulas are preferred to grafts as initial vascular access for dialysis. Pro

There is growing concern that the Fistula First Initiative, KDOQI guidelines, and subsequent pressure from the Centers for Medicare and Medicaid Services lack reasonableness regarding likely success for fistula maturation in a heterogeneous, new-onset dialysis population. Here the various positions are examined from multiple perspectives.

Evaluation of vascular access complications in acute and chronic hemodialysis

PURPOSE

This work aimed to determine the incidence of vascular access (VA)-related complications of temporary venous catheters in acute hemodialysis (HD) prescription, to study the survival rate of chronic HD patients who began HD with a catheter insertion and to evaluate associated complications.

METHODS

The patients were classified into two groups. Group I, patients in whom venous catheters were inserted (652 patients). Group II, 80 patients with end-stage renal disease (ESRD) in whom an arteriovenous fistula (AVF) was established.

RESULTS

Complications in the femoral approach were bleeding from a femoral artery wall laceration in only 6/600 patients (1%), successfully repaired in all patients, bleeding from a punctured femoral artery in 30 patients (5%), the incidence of infection and removal was 40%. Other complications included groin hematoma in 3 patients (0.5%) and catheter thrombosis in 45 patients (7.5%). In the jugular vein approach, infection occurred in 6/30 patients (20%), thrombosis in 9 patients (30%), and accidental withdrawal in 2 patients (6.7%). In the subclavian vein approach, catheter dysfunction was found in 7/22 patients (32%), infection in 10 patients (45.5%), and failure to cannulate the vein in 3 patients (13.6%). In group II, limb edema was found in 14/80 patients (17.5%), hematoma in 5 patients (6.3%), accidental trauma to the fistula in 2 patients (2.5%) and fistula stenosis in 15 patients (18.8%). Patients who began HD with a catheter had higher mortality than those who began with an AVF (12 months survival, respectively, 60 vs. 83%).

CONCLUSION

Patients who began HD with a catheter had more complications and higher mortality. Routine quality assessment of AVF by Doppler ultrasound is recommended.

Arteriovenous fistula surgery: an American perspective from Italy

Hemodialysis depends on durable, long-term vascular access for the patient. In the United States, arteriovenous fistula (AVF) creation rates are lower than they are in Europe. Excess morbidity and mortality has been attributed to reliance on catheters and prosthetic grafts. In Italy, there is a high rate of fistula creation, and they are predominantly created by nephrologists. As American interventional nephrologists become more involved in the procedural aspects of renal care, consideration should be given to replicating the Italian model of dialysis access creation. Specific aspects of the surgical technique, comparison to available literature, and potential reasons for higher fistula creation rates are explored from the vantage point of an American nephrologist visiting Italy. The pertinence to American dialysis access programs is discussed, as is the opportunity for interventional nephrologists in the United States to further expand and improve upon the successes of the Italian experience.

Endovascular Treatment of the “Failing to Mature” Arteriovenous Fistula

In recent literature, surgically created hemodialysis (HD) arteriovenous fistulas (AVF) have high rates of primary failure. Endovascular treatment holds promise to salvage these fistulae. The outcomes of 119 patients who had a "failing to mature" AVF and presented for endovascular management were evaluated prospectively. All patients underwent a fistulogram. Stenotic lesions underwent balloon angioplasty, and accessory veins underwent obliteration. Technical success was determined immediately after the procedure. AVF salvage was determined by successful use during HD. Patients were followed up for 1 yr, during which primary and secondary AVF patency rates were measured. The distribution of stenoses was as follows: Artery, 6 (5.1%); arterial anastomosis, 56 (47.1%); juxta-arterial anastomosis, 76 (63.9%); peripheral vein, 70 (58.8%); and central vein, 10 (8.4%). Significant accessory veins were present in 35 (29.4%). Mixed lesions were found in 85 (71.4%). The technique was successful in 107 (89.9%), and the AVF was salvaged in 99 (83.2%). Follow-up of salvaged fistulae showed a total event rate of 0.38/access-year, thrombosis rate of 0.12/access-year, and loss rate of 0.04/access-year. Endovascular treatment of "failing to mature AVF" is safe and effective when performed in a dedicated center.

Risk Factors for Early Failure of Native Arteriovenous Fistulas

INTRODUCTION

Current guidelines recommend native arteriovenous fistulas (AVF) as the vascular access of choice for hemodialysis on account of the lower incidence of complications. However, this kind of vascular access has a high rate of early failure (early thrombosis or non-maturation). Our aim was to examine whether clear risk factors for early AVF failure could be identified in our patients.

SUBJECTS AND METHODS

Data of all patients who underwent creation of an AVF at the Geneva University Hospital from January 1998 to December 2002 were reviewed. Early failure was defined as a non-functioning fistula (thrombosis or absence of fistula maturation).

RESULTS

119 patients underwent the creation of 148 native AVF, 88 (59.5%) in the forearm and 60 (40.5%) in the upper arm. 48 (32.4%) fistulae were created in diabetic patients. In a multiple logistic regression analysis, significant predictive factors of early failure were a distal location (adjusted odds ratio (aOR) = 8.21, 95% CI = 2.63-25.63, p < 0.001), female gender (aOR = 4.04, 95% CI = 1.44-11.30, p = 0.008), level of surgical expertise (aOR = 3.97, 95% CI = 1.39-11.32, p = 0.010) and diabetes mellitus (aOR = 3.19, 95% CI = 1.17-8.71, p = 0.024).

CONCLUSION

Early failure of AVF occurs mainly in forearm sites among women and diabetic patients. Surgical expertise has also a significant influence. These results suggest that selection of a distal site for a native AVF has to be rigorously made for women and diabetic patients and that surgeon's dedication is of primary importance to avoid early AVF failure occurrence.

Determinants of native arteriovenous fistula blood flow

BACKGROUND

Determinants of native arteriovenous fistula (AVF) placement have been well studied. Little is known on whether these factors impact on subsequent blood flow (Qa) in the mature AVF.

METHODS

Arteriovenous fistula Qa and cardiac index (CI) were determined by ultrasound dilution. Multiple linear regression was used to assess independent predictors of AVF Qa.

RESULTS

Of the 148 patients available for the analysis, 68% were male, with 61% using a radiocephalic AVF. Aetiology of renal disease was: 38% glomerulonephritis (GN), 22% diabetes mellitus (DM), 9% hypertension/ischaemic (HTN) and 31% other. Thirty per cent had coronary artery disease (CAD), 10% cerebrovascular disease and 11% peripheral vascular disease (PVD). Median (iqr) Qa was 1185 mL/min (790-1650) and CI was 3.15 L/min per 1.73 m(2) (2.60-3.93). On univariable analysis, log CI (0.98, P < 0.001), age (-0.1 per 10 years, P = 0.002), access position (upper vs lower 0.26, P = 0.003, PVD (-0.35, P = 0.015), CAD (-0.25, P = 0.008), and primary renal disease (DM vs GN, -0.35, P = 0.003, HTN vs GN, -0.34, P = 0.04) were associated with Qa. On multivariable analysis, CI (0.84, P < 0.001), access position (upper vs lower, 0.17, P = 0.018) and primary renal disease (DM vs GN, -0.26, P = 0.005, and HTN vs GN, -0.26, P = 0.038) remained significant predictors of AVF Qa.

CONCLUSION

Once established, CI, AVF position and primary renal disease (hypertension/ischaemic and diabetes) are the major determinants of AVF Qa while female gender, CAD, PVD and body mass index were not significant determinants of Qa in this cohort.

Medical factors affecting patency of arteriovenous access

Arteriovenous access failure is multifactorial in nature with contributions from both medical and surgical etiologies. Medical causes of arteriovenous access failure are rare, and therefore infrequently identified as a major contributing source of malfunction. Although they account for only 10-15% of all cases of access failure, their importance should not be underestimated, especially in cases where a surgical source cannot be identified. Most medical causes are derived from Virchow's triad of endothelial cell injury, stasis, and hypercoaguability. Endothelial cell injury occurs through oxidative stress, activated platelets, increased levels of circulating tumor necrosis factor-alpha, and preexisting intimal hyperplasia. Stasis can occur through prolonged access compression, hypotension, or hypoalbuminemia. Finally, patients with renal failure requiring hemodialysis are frequently at increased risk for hypercoaguable states, except for situations of platelet dysfunction, and therefore access failure. Potential treatments include identifying and removing the offending source, as well as innovative, new medications to prevent their reoccurrence. Treatment is aimed at improving quality of life, as well as decreasing morbidity and hospital admissions in this difficult patient population.

Failure of arteriovenous fistula maturation: an unintended consequence of exceeding dialysis outcome quality Initiative guidelines for hemodialysis access

PURPOSE

The Dialysis Outcome Quality Initiative (DOQI) guidelines recommend that arteriovenous fistulas (AVF) be constructed in at least 50% of hemodialysis access procedures. Preoperative duplex ultrasound (US) scanning and venography may increase options for AVF with identification of veins that are not clinically evident. However, maturation of autogenous fistulas created on the basis of findings at duplex US scanning and venography has not been carefully examined.

METHODS

From January 1999 to July 2002, 256 new hemodialysis access procedures were performed in 202 patients in an academic tertiary care center. If physical examination failed to disclose adequate vessels for hemodialysis access, patients underwent duplex US scanning mapping. Venography was performed when no usable vein or only a basilic vein was identified at duplex US scanning. Functional maturation rate and mean maturation time (time from fistula creation to initiation of hemodialysis) were determined. This experience was compared with that in a group of 128 patients in whom 148 hemodialysis access fistulas were created before we implemented liberal use of preoperative duplex US scanning and venography (January 1997-December 1998).

RESULTS

From January 1999 to July 2002, preoperative duplex US scanning was performed in 68% of patients, and venography in 32% of patients. Autogenous fistula creation rate increased from 61% to 73% in all patients with hemodialysis access fistulas (P =.15) and from 66% to 83% in patients undergoing a first access procedure (P <.05). The use of basilic vein transposition also increased, from 3% in the earlier period to 13% in the later period (P <.05). Mean maturation time for arteriovenous fistulas was 70 days. Functional maturation rate decreased from 73% to 57% (P <.05) after implementation of preoperative imaging and more aggressive vein use.

CONCLUSION

Implementation of preoperative duplex US scanning and venography as a component of a more aggressive protocol to create native fistulas was pivotal in exceeding DOQI guidelines for hemodialysis access. However, this approach resulted in the unintended sequela of decreased fistula maturation rate. Our experience suggests that improved selection criteria based on findings at preoperative imaging are needed to further refine and optimize arteriovenous access surgery.