Hemodialysis vascular access dysfunction: a cellular and molecular viewpoint

Current management of vascular access

Optimizing vascular access outcomes remains an ongoing challenge for clinical nephrologists. All other things being equal, fistulas are preferred over grafts, and grafts are preferred over catheters. Mature fistulas have better longevity and require fewer interventions, as compared with mature grafts. The major hurdle to increasing fistula use is the high rate of failure to mature of newly created fistulas. There is a desperate need for enhanced understanding of the mechanisms of failure to mature and the optimal type and timing of interventions to promote maturity. Grafts are prone to frequent stenosis and thrombosis. Surveillance for graft stenosis with preemptive angioplasty may reduce graft thrombosis, but recent randomized clinical trials have questioned the efficacy of this approach. Graft stenosis results from aggressive neointimal hyperplasia, and pharmacologic approaches to slowing this process are being investigated in clinical trials. Catheters are prone to frequent thrombosis and infection. The optimal management of catheter-related bacteremia is a subject of ongoing debate. Prophylaxis of catheter-related bacteremia continues to generate important clinical research. Close collaboration among nephrologists, surgeons, radiologists, and the dialysis staff is required to optimize vascular access outcomes and can be expedited by having a dedicated access coordinator to streamline the process. The goal of this review is to provide an update on the current status of vascular access management.

Comparison of Survival of Upper Arm Arteriovenous Fistulas and Grafts after Failed Forearm Fistula

Although arteriovenous fistulas are considered superior to grafts, it is unknown whether that is true in the subset of patients with a previous failed fistula. For investigation of this question, a prospective vascular access database was queried retrospectively to compare the outcomes of 59 fistulas and 51 grafts that were placed in the upper arm after primary failure of an initial forearm fistula. Primary access failure was higher for subsequent fistulas than for subsequent grafts (44 versus 20%; P = 0.006). Fistulas required more interventions than grafts before their successful use (0.42 versus 0.16 per patient; P = 0.04). The time to catheter-free dialysis was longer for fistulas than for grafts (131 versus 34 d; P < 0.0001) and was associated with more episodes of bacteremia before permanent access use (1.3 versus 0.4 per patient; P = 0.003). Cumulative survival (from placement to permanent failure) was higher for fistulas than for grafts when primary failures were excluded (hazard ratio 0.51; 95% confidence interval 0.27 to 0.94; P = 0.03), but similar when primary failures were included (hazard ratio 0.99; 95% confidence interval 0.61 to 1.62; P = 0.97). Fistulas required fewer interventions to maintain long-term patency for dialysis after maturation (0.73 versus 2.38 per year; P < 0.001). In conclusion, as compared with grafts, subsequent upper arm fistulas are associated with a higher primary failure rate, more interventions to achieve maturation, longer catheter dependence, and more frequent catheter-related bacteremia. However, once the access is usable for dialysis, fistulas have superior cumulative patency than do grafts and require fewer interventions to maintain patency.

Effects of fluid shear stress on adventitial fibroblast migration: implications for flow-mediated mechanisms of arterialization and intimal hyperplasia

The involvement of vascular fibroblasts (FBs) and smooth muscle (SM)-like cells in physiological and pathological processes in large vessels (intimal hyperplasia) and microvessels (capillary arterialization), and the realization that these cells are exposed to interstitial flow shear stress (SS), motivate this study of SS on FB migratory activity. Rat adventitial FBs were grown to either 30-50% confluence (subconfluent FBs; SFBs) or full confluence (confluent FBs; CFBs) in culture. Immunofluorescence and Western blotting assays were conducted to evaluate the expression of two phenotype markers: SM alpha-actin and SM myosin heavy chain (MHC). Both assays indicated a significant increase in SM alpha-actin expression in CFBs compared with SFBs, suggesting a phenotype difference between the two cell populations. SFBs and CFBs both expressed minimal SM MHC. Both cell populations were seeded on Matrigel-coated cell culture inserts and exposed to 4 h of either 1 or 20 dyn/cm(2) SS via a rotating disk apparatus in the presence of the chemoattractant platelet-derived growth factor-BB to quantify the effect of SS on SFB and CFB migration. Four hours of 20 dyn/cm(2) SS significantly enhanced SFB migration while it suppressed CFB migratory activity. Four hours of 1 dyn/cm(2) SS did not significantly alter either SFB or CFB migration levels. Because of the distinct migratory responses of SFBs and CFBs in response to SS, phenotype modulation appears to be one way to regulate their involvement in both physiological and pathological remodeling processes.

Far-infrared therapy: a novel treatment to improve access blood flow and unassisted patency of arteriovenous fistula in hemodialysis patients

Vascular access malfunction, usually presenting with an inadequate access flow (Qa), is the leading cause of morbidity and hospitalization in hemodialysis (HD) patients. Many methods of thermal therapy have been tried for improving Qa but with limited effects. This randomized trial was designed to evaluate the effect of far-infrared (FIR) therapy on access flow and patency of the native arteriovenous fistula (AVF). A total of 145 HD patients were enrolled with 73 in the control group and 72 in the FIR group. A WS TY101 FIR emitter was used for 40 min, and hemodynamic parameters were measured by the Transonic HD(02) monitor during HD. The Qa(1)/Qa(2) and Qa(3)/Qa(4) were defined as the Qa measured at the beginning/at 40 min later in the HD session before the initiation and at the end of the study, respectively. The incremental change of Qa in the single HD session with FIR therapy was significantly higher than that without FIR therapy (13.2 +/- 114.7 versus -33.4 +/- 132.3 ml/min; P = 0.021). In comparison with control subjects, patients who received FIR therapy for 1 yr had (1) a lower incidence (12.5 versus 30.1%; P < 0.01) and relative incidence (one episode per 67.7 versus one episode per 26.7 patient-months; P = 0.03) of AVF malfunction; (2) higher values of the following parameters, including Delta(Qa(4) - Qa(3)) (36.2 +/- 82.4 versus -12.7 +/- 153.6 ml/min; P = 0.027), Delta(Qa(3) - Qa(1)) (36.3 +/- 166.2 versus -51.7 +/- 283.1 ml/min; P = 0.035), Delta(Qa(4) - Qa(2)) (99.2 +/- 144.4 versus -47.5 +/- 244.5 ml/min; P < 0.001), and Delta(Qa(4) - Qa(2)) - Delta(Qa(3) - Qa(1)) (62.9 +/- 111.6 versus 4.1 +/- 184.5 ml/min; P = 0.032); and (3) a better unassisted patency of AVF (85.9 versus 67.6%; P < 0.01). In conclusion, FIR therapy, a noninvasive and convenient therapeutic modality, can improve Qa and survival of the AVF in HD patients through both its thermal and its nonthermal effects.

In vitro Paclitaxel and Radiation Effects on the Cell Types Responsible for Vascular Stenosis: A Preliminary Analysis

Hemodialysis vascular access dysfunction as a result of venous neointimal hyperplasia in dialysis access grafts and fistulae is currently a huge clinical problem. The aim of this study was to assess the effects of paclitaxel and radiation, both singly and in combination on the proliferation of cell types present within the lesion of venous neointimal hyperplasia (vascular smooth muscle cells, fibroblasts and endothelial cells within the neointimal microvessels). Vascular smooth muscle cells, fibroblasts and endothelial cells were plated onto 96-well plates and exposed to different concentrations and doses of paclitaxel and radiation, respectively (both individually and in combination). Growth inhibition was assessed with an MTT assay. Both paclitaxel and radiation resulted in significant growth inhibition of all three cell types. However, even small doses of paclitaxel appeared to attenuate the antiproliferative effect of radiation on these cell types. Further experiments to elucidate the mechanism behind these findings could result in a better understanding of combination antiproliferative therapies.

Vascular access and cardiac disease: is there a relationship?

PURPOSE OF REVIEW

Despite advances in hemodialysis technology, a stable and well functioning vascular access remains the bane of every hemodialysis patient. It is recognized that vascular access contributes to cardiovascular disease mortality through a number of mechanisms. This review describes the relationship between vascular access and cardiovascular disease by reviewing the relationships between infection risk, inflammation and cardiovascular disease, and the cardiovascular changes that occur as a consequence of vascular access. Improved understanding of these mechanisms and their interrelationship is warranted.

RECENT FINDINGS

The impact of arteriovenous fistula creation on cardiac structural and hemodynamic changes is described, as is vascular remodelling, which occurs in response to alterations in blood-flow properties. The development of central and peripheral vein stenosis is also a type of vascular remodelling and consequences of such events are not yet well understood. In addition, the contribution of vascular access to increased inflammation and atherosclerotic disease is reviewed. Finally, the hypothesis that vascular access dysfunction may be a predictor of vascular disease is explored.

SUMMARY

The relationship between vascular access and cardiac disease exists at different levels, ranging from inflammation promoting atherosclerotic disease to vascular remodelling changes of stenosis formation and left ventricular hypertrophy. Countless research opportunities abound.

BRAVO I: A pilot study of vascular brachytherapy in polytetrafluoroethylene dialysis access grafts

Hemodialysis vascular access dysfunction owing to stenosis and thrombosis in polytetrafluoroethylene dialysis access grafts is a huge clinical problem for which there are currently no long lasting durable therapies. Vascular brachytherapy has been used successfully for the prevention of coronary restenosis following angioplasty and stent placement. The Beta Radiation for Treatment of Arterial-Venous Graft Outflow I study was a pilot study of vascular brachytherapy in hemodialysis patients with patent but dysfunctional grafts. Twenty-five patients were randomized to receive either radiation therapy (a single dose of 18.4 Gy) or sham radiation, following angioplasty. The primary efficacy end point of the study was target lesion primary patency at 6 months. The primary safety end point was a composite of death, emergency surgery on the graft, venous rupture, or aneurysm formation. Forty-two percent of the radiated grafts achieved the target lesion primary patency end point at 6 months as compared to 0% of the control group (P = 0.015), but this did not translate into an improvement in secondary patency at either 6 or 12 months. Radiation therapy was found to be safe in the setting of hemodialysis vascular access dysfunction. Our results suggest that vascular brachytherapy is an intervention that is worthy of further examination in the setting of non-thrombosed dialysis access grafts.

Evaluation of histological techniques for quantifying haemodialysis arteriovenous (AV) graft hyperplasia

BACKGROUND

Assessing treatment efficacies for preventing haemodialysis arteriovenous (AV) graft stenosis requires a reproducible method for quantifying intimal hyperplasia. We identified sources of variability in three histological methods for assessing hyperplasia in a porcine AV graft model.

METHODS

Carotid-jugular synthetic grafts were placed in pigs. After explantation at 3-6 weeks, the tissue was stained with haematoxylin and eosin (H&E), Masson's trichrome or elastic tissue Van Gieson (EVG) stains and examined histologically. Hyperplasia at the anastomosis of 14 grafts was quantified using three different methods, each by four blinded observers. These methods were visual scoring, ratio of intima-to-media surface area (I/M ratio), and ratio of intra-graft hyperplasia to graft surface area (H/G ratio) at the graft-vessel interface.

RESULTS

The EVG stain proved superior in delineation of the elastic lamina yet quantification of the intimal and medial layers was still often difficult. This is illustrated by the greater inter-observer median coefficient of variances (CV) found using the I/M ratio method (intimal area CV=13.7%; medial area CV=32.7%; I/M ratio CV=44.0%) than with the H/G method (intra-graft hyperplasia area CV=7.3%, graft area CV=5.3%; H/G ratio CV=6.9%) or by visual scoring (CV=26.8%). The H/G ratios correlated positively with visual scores (r=0.941; P=0.0007; n=14) and the I/M ratio (r=0.719; P=0.0095; n=14). While hyperplasia was seen in both native vessel and graft lumen, in only one of the 14 anastomoses was the degree of hyperplasia greater in the native vessel than in the graft lumen, suggesting that the degree of hyperplasia occurring within the graft lumen predicted the total hyperplasia around the anastomosis.

CONCLUSIONS

The H/G method for assessing hyperplasia is preferred in a porcine model of AV graft because it is quantitative, less variable and does not require the delineation of the elastic lamina, although it infrequently underestimates the total hyperplasia that occurs.

Why don't fistulas mature?

Fistula maturation requires a compliant and responsive vasculature capable of dilating in response to the increased velocity of blood flowing into the newly created low-resistance circuit. Successful maturation to a high volume flow circuit capable of sustaining hemodialysis typically occurs within the first few weeks after creation. Failure to achieve maturation within 4-8 weeks should prompt a search for reversible etiologies; however, an accepted definition of maturation, particularly for patients not yet on dialysis remains elusive. The most commonly identified etiology is neointimal hyperplasia typically occurring in the juxta-anastomotic vein. However, failed maturation has also been reported secondary to impaired arterial and venous dilation and accessory veins. The exact frequency of each of these etiologies is unclear. Understanding the etiologies of impaired fistula maturation will focus future studies of targeted interventions to improve the rate of fistula maturation and increase the number of dialysis patients with a functioning autogenous fistula.

Perivascular paclitaxel wraps block arteriovenous graft stenosis in a pig model

BACKGROUND

Haemodialysis vascular access dysfunction is currently a huge clinical problem. In an attempt to reduce the morbidity associated with haemodialysis vascular access dysfunction, we have previously developed and validated a local perivascular paclitaxel release system that has been shown to release paclitaxel for at least 3 weeks. The aim of the current study was to evaluate the in vivo use of these perivascular wraps (for both safety and efficacy) at different time points in our pig model of arteriovenous graft stenosis.

METHODS

Paclitaxel-loaded ethylene vinyl acetate wraps were placed around the graft-vein anastomosis on one side, with control polymers being placed on the contralateral side in our pig model of arteriovenous graft stenosis. Animals were sacrificed at early (10-11 days), middle (23-24 days) and late (32-38 days) time points. The entire graft-vein anastomosis was removed at the time of sacrifice and assessed for the extent of luminal stenosis using histomorphometric techniques.

RESULT

Graft-vein anastomoses treated with the paclitaxel-loaded polymers had an almost complete absence of luminal stenosis at the middle (23-24 days) and late (32-38 days) time points (when one would expect the development of neointimal hyperplasia) as compared with the contralateral control graft-vein anastomoses (37.90% luminal stenosis in the controls vs 0.10% in the paclitaxel group). There were minimal local side effects from this procedure.

CONCLUSIONS

Our results demonstrate the safety and efficacy of paclitaxel-loaded perivascular wraps in the setting of a pig model of arteriovenous graft stenosis. We believe that such a local approach which could be easily applied at the time of surgery is ideally suited for use in the clinical setting of haemodialysis vascular access dysfunction. It is likely that this novel approach could result in a significant reduction in the huge economic and health morbidity costs currently associated with this recalcitrant clinical problem.