Hemodialysis Vascular Access Dysfunction

A successful functioning vascular access is the “lifeline” for a hemodialysis patient. Hemodialysis vascular access dysfunction is a major cause of morbidity and mortality in hemodialysis patients1-3. Improving vascular access outcomes remains an ongoing challenge for nephrologists, vascular access surgeons, and interventionists. In arteriovenous fistulas (AVF) and grafts (AVG), the most common cause of this vascular access dysfunction is venous stenosis as a result of neointimal hyperplasia within the peri-anastomotic region (AVF) or at the graft-vein anastomosis (AVG) 4,5. There have been few effective treatments to-date for venous neointimal hyperplasia in part because of the poor understanding of the pathogenesis of venous neointimal hyperplasia. Central venous catheters (CVC) are prone to frequent thrombosis and infection and the treatment of catheter-related bacteremia (CRB) remains on ongoing debate 6-8. Therefore, this review will: (1) describe the pathology and pathophysiology of hemodialysis access stenosis in AVFs and AVGs, (2) discuss the pathogenesis of CRB and catheter thrombosis (3) discuss current and future novel therapies for treating venous neointimal hyperplasia, (4) discuss current strategies to treat CRB and catheter thrombosis, and (5) suggest future research areas in the field of hemodialysis vascular access dysfunction.


Pathophysiologic mechanisms of neointimal hyperplasia formation in hemodialysis access dysfunction
The pathogenesis of venous neointimal hyperplasia in AVG stenosis and late AVF stenosis has been well described and is commonly divided into upstream and downstream events 4 . Upstream events are characterized as the initial events and insults that are responsible for endothelial and smooth muscle cell injury, which leads to a cascade of mediators (downstream events) that regulate oxidative stress, endothelial dysfunction, and inflammation (eventually resulting in venous neointimal hyperplasia). Upstream events that are believed to contribute to the pathogenesis of neointimal hyperplasia include 4,62,67-70 : (1) surgical trauma at the time of AV surgery, (2) hemodynamic shear stress at the veinartery or vein-graft anastomosis, (3) bioincompatability of the AVG, (4) vessel injury due to dialysis needle punctures, (5) uremia resulting in endothelial dysfunction, and (6) repeated angioplasties causing further endothelial injury. Downstream events represent the response to endothelial (vascular) injury from the upstream events, resulting in the migration of smooth muscle cells from the media to the intima and eventually the development of neointimal hyperplasia 65 . The pathogenesis in AVFs that fail to mature (early failure) for dialysis, in contrast to AVG and late AVF failure, remains poorly understood. At a histological level early AVF failure is also characterized by aggressive neointimal hyperplasia in both animal and human models, seen as early as 1 month in animals 63,71 and 3 months in humans 64,66 . The underlying factors (upstream events) which may contribute to early AVF failure, include 4,72-81 : (1) small diameter sizes in the vein and artery, (2) surgical injury at the time AV fistula placement, (3) previous venipunctures, (4) development of accessory veins after surgery, (5) hemodynamic shear stress at the AV anastomosis, (6) a genetic predisposition to vascular constriction and neointimal hyperplasia, and (7) pre-existing venous neointimal hyperplasia. The subsequent sections will focus on the downstream events and three main mechanisms responsible for neointimal hyperplasia such as oxidative stress, inflammation, endothelial dysfunction, and alternative origins of neointimal-derived cells.

Oxidative stress
Many of the upstream mechanisms above (particularly hemodynamic shear stress and angioplasty injury) have been documented to result in an increase in the production of free radicals and its downstream products nitrotyrosine and latter (peroxynitrate). The latter is a potent upregulator of the matrix metalloproteinases (MMPs) 82,83 . MMPs are key enzymes that cause breakdown of extracellular matrix proteins such as collagen and elastin which facilitate the migration of vascular smooth muscle cells (VSMCs) in neointimal hyperplasia formation 84 . MMPs, paradoxically, have also been shown to facilitate a beneficial dilatation of the feeding artery (through degradation of the internal elastic laminae) in both rabbit and mouse AVF models 82,85 . Experimental studies of AVGs have demonstrated a differential upregulation of MMP-2 at the graft-vein anastomosis, with early expression (9 days) in the adventitia and a later expression (19 days) within the intima, supporting the concept of cellular migration from the adventitia to the intima 86 . Furthermore, linkages between hemodynamic shear stress and the expression of oxidative stress markers and cytokines have also been described in a porcine model of AVG stenosis 87 . Clinical studies of stenotic and thrombotic AVGs and AVFs have also demonstrated an upregulation of MMPs 88 , and have documented the co-localization of oxidative stress markers with inflammatory cytokines such as transforming growth factor-beta (TGF-), and platelet-derived growth factor (PDGF), within the neointima of both stenotic AVGs and AVFs 60 . Heme-oxygenase-1 (HO-1) is an important enzyme pathway which has been shown to confer protective effects in the vascular endothelium and other organ systems through its anti-inflammatory, antioxidant, or antiproliferative actions and properties 89 . Experimental studies in AVFs have described an increase in both the magnitude of arteriovenous stenosis and the frequency of thrombosis following the creation of AVFs in HO-1 knock out mice (increased baseline oxidative stress) as compared to wild type animals 90 . Furthermore, in the HO-1 knockout mice, there was significant induction of MMP-9 expression in the vein at 1 week compared to wild type mice, suggesting that MMP expression in vascular tissue and its deleterious effects with regard to promoting cellular migration may in part be inhibited by HO-1. Clinical studies have demonstrated a higher frequency of AVF failure in patients with heme-oxygenase-1 (HO-1) gene polymorphisms with long GT repeats (resulting in increased oxidative stress) 73 .

Inflammation
ESRD is associated with a chronic inflammatory state, characterized by the elevation of circulating cytokines and chemokines 91 . This inflammation has been proposed to play an important role in the initiation and progression of atherosclerosis in ESRD, but may also play a significant role in vascular access stenosis. Support for this paradigm comes from www.intechopen.com recent work in which uremic mice developed a 2-3 fold greater magnitude of neointimal hyperplasia at the arteriovenous anastomosis as compared to non-uremic animals in a mouse model of AVF stenosis 92 , and a recent study which showed marked upregulation of monocyte chemoattractant protein-1 (MCP-1) in the venous segment of AVF compared to rats deficient in the MCP-1 gene 93 .
In clinical studies, possible linkages have described the presence of inflammatory cells (macrophages and lymphocytes), cytokines such as TGF-ß and insulin-like growth factor-1 (IGF-1) and the magnitude of neointimal hyperplasia and venous stenosis within stenotic AVFs 94 . Local bioincompatability to synthetic polytetrafluoroethylene (PTFE) material in AVGs could also result in local inflammation 95 . In vitro studies have demonstrated that conditioned media obtained after the interaction of peripheral blood mononuclear cells (PBMCs) with PTFE graft material resulted in a significant upregulation of smooth muscle cell proliferation as compared to control media 96 . This proliferative response has been shown to be attenuated by tumor necrosis-alpha (TNF-) inhibitors 96 . Furthermore, the presence of macrophages that line PTFE graft material has been described in both experimental and clinical AVG stenosis with co-expression of inflammatory cytokines such as basic fibroblast growth factor (bFGF) 61,97 .

Endothelial dysfunction
An intact and functional endothelium is essential for the vein to properly respond to acute changes in blood flow that occurs after creation of AVFs and AVGs 98 . Nitric oxide (NO) is an important mediator responsible for these transformations 99,100 . The presence of uremia in hemodialysis patients has been shown to exacerbate endothelial dysfunction, possibly through the pathways of inflammation and oxidative stress described above 101,102 . In the specific context of vascular access stenosis, endothelial dysfunction is likely to be responsible for the development of pre-existing venous neointimal hyperplasia [77][78][79][80][81] , medial hypertrophy 77,81 and radial artery intima-media thickening 103-105 that is present even before the creation of AVFs in uremic patients. Pre-existing arterial intima-media thickness has been correlated with future AVF dysfunction 103 . Recently, pre-existing venous neointimal hyperplasia has been linked to poor AVF maturation in a small clinical study 77 . Asymmetrical dimethylarginine (ADMA) is an endogenous inhibitor of NO synthase and has been implicated as an important contributor to endothelial dysfunction in ESRD patients 106 . ADMA is not excreted in ESRD patients and its levels have been reported to be two to six times higher in this patient population as compared to non-uremic individuals 107 . In a recent clinical study in AVFs, patients with elevated ADMA levels at the time of percutaneous transluminal angioplasty of an initial AVF stenosis had a significantly increased risk of a recurrent AVF stenosis 108 .

Alternative origins of neointimal cells
Although the traditional paradigm for the pathogenesis of neointimal hyperplasia has emphasized the migration of smooth muscle cells from the media to the intima, where they proliferate and contribute to the final volume of neointimal hyperplasia, a number of studies have reported that following coronary angioplasty or saphenous vein bypass grafting there is also a migration of cells (fibroblasts) from the adventitia, through the media, and into the intima, where these cells transform into "myofibroblasts" [109][110][111] . In dialysis access, a number of recent studies in AVGs have supported the concept of a migration of adventitial cells into the intima where they contribute to final neointimal volume 59,112 . In addition, recent data from several experimental AVF stenosis models have shown that smooth muscle cells in the neointima, may in part, originate from bone-marrow-derived cells that bind to the site of vascular injury and later differentiate into a smooth muscle cell phenotype in the neointima 82,113,114 . From a therapeutic standpoint, it is likely that better information about the true source of neointimal cells will allow for the development of novel therapeutic interventions targeting specific cell types.

Hemodynamic and vascular remodeling in hemodialysis access dysfunction
A number of experimental studies have shown that turbulent, low flow, low fluid sheer stress are involved in neointimal hyperplasia development [115][116][117][118][119] . High sheer stress has been associated with vascular dilatation through inhibition of smooth muscle cell proliferation and high levels of nitric oxide release, whereas low sheer stress has been associated with smooth muscle cell proliferation and lack of vasodilatation [120][121][122][123] . Poor hemodynamic profiles could be a risk factor for neointimal hyperplasia development and poor venous dilatation, and the degree of luminal stenosis is dependent upon both the magnitude of neointimal hyperplasia and the capacity for vasodilatation or vasocontriction. Therefore, a significant amount of neointimal hyperplasia and medial hypertrophy may not result in luminal stenosis in the presence of adequate vasodilatation, while a small amount of neointimal hyperplasia, but with poor vasodilatation, may result in severe venous stenosis 4,124 . Unfortunately, the factors that are responsible for vascular remodeling are unknown, but adventitial angiogenesis and scar formation are hypothesized to play a significant role 125,126 . Thus, the ideal therapy for vascular stenosis would be an intervention that would prevent vascular constriction (adverse remodeling) and neointimal hyperplasia 4

Central venous dialysis catheters
CVC dysfunction and related-infection remains a common cause of morbidity, mortality, and high economic costs in treating chronic hemodialysis patients. This section will provide a brief overview of catheter dysfunction and catheter-related infections.

Catheter dysfunction
Catheter dysfunction can occur immediately after placement or in a catheter which has been previously functioning without difficulties, and most commonly manifests with low catheter blood flows during dialysis or negative arterial pressures on the dialysis machine 6 . In more severe cases catheter thrombosis is characterized by the inability to aspirate blood from the dialysis port 6 . Catheter dysfunction which occurs immediately after placement is most likely due to placement problems 6 . Installation of a thrombolytic agent for 30 to 60 minutes is a treatment for catheter dysfunction, followed by a second installation if necessary 6 . Recently published studies have reported varied success rate when treating catheter dysfunction with thrombolytics, ranging anywhere between 60-95% 26,28,35,127,128 . When thrombolytic therapy is unsuccessful in providing adequate blood flow and adequate dialysis, despite repeated installations, then catheter exchange needs to be performed. The current K/DOQI guidelines recommends treatment with thrombolytic agents in all catheters with a persistently low blood flow rate (<300 ml/min) 19 .

www.intechopen.com
The current standard of care to prevent catheter thrombosis is installation of an anticoagulant in both dialysis ports at the completion of each dialysis session. In the United States, heparin is most commonly used, while in Europe citrate is the more common anticoagulant 6 . The studies to-date have shown similar efficacy when comparing citrate to heparin for prophylaxis of catheter thrombosis, but with fewer complications of systemic bleeding with citrate [129][130][131][132] . A recent multicenter, randomized-controlled trial has reported that use of a thrombolytic, tissue plasminogen activator as a locking solution compared to heparin had reduced incidence of catheter dysfunction 34 .

Catheter-related bacteremia
Currently, a precise definition for diagnosis catheter-related bacteremia is lacking. More rigorous definitions require a positive blood culture obtained from the catheter and a peripheral vein with the quantitative colony count being at minimum four-fold higher from the catheter sample 133 . However, recently, the Infectious Disease Society of America has recognized the challenges in obtaining peripheral blood cultures from hemodialysis patients (e.g. priority for preserving veins and difficult cannulations) and has considered a definition of "possible" catheter-related bacteremia as positive blood culture obtained from the catheter in a symptomatic patient 134 . The two main pathways where organisms can gain entry into the blood stream to initiate catheter-related bacteremia are intraluminal and extraluminal 135 . Organisms gain entrance through the bloodstream extraluminally through contact between the skin surface organisms and the external surface of the catheter at the time of catheter placement or following catheter placement before healing of the exit site or endothelialization of the subcutaneous tunnel 7 . Subsequently, the organisms colonize or migrate through the intracutaneous exterior tract of the catheter to the tip, allowing for hematagenous dispersion of the organisms and leading to catheter-related bacteremia 7 . Intraluminal-derived infections results from the transfer of organisms from hand contact with the catheter, leading to contamination of the internal catheter surfaces 7 . Infection from the extraluminal pathway most commonly occurs immediately after catheter insertion, while infections from the intraluminal pathway occurs throughout the life of the catheter 7 . Irrespective of the route of bacterial entry, the bacteria will either adhere to the CVC or become incorporated into a fibrin sheath. Adherence of the bacterial organisms to the CVC initiates a common pathway of biofilm production. A mature biofilm is a self-sustaining colony of microorganism, guarded by an exopolysaccharide matrix, that is stimulated and secreted by the organism and very difficult to eradicate 7,136-140 . Catheter-related bacteremia can result in devastating complications such as endocarditis, osteomyelitis, thrombophlebitis, septic arthritis, spinal epidural abscess, and large atrial thrombi 30,31,[141][142][143][144][145][146][147][148][149] . The majority of isolated organisms from catheter-related bacteremia are gram-positive organisms (52-84%) with Staph Aureus responsible for the majority of these organisms 7,30,31,143,150,151 . Gram-negative are isolated in 27-36% of episodes and fungal isolated are relatively uncommon (<10%) [141][142][143]149,152 . Therefore, it is important to identify catheter-related bacteremia early so treatment can be initiated immediately.

Treatment of catheter-related bacteremia
Initial empiric antibiotic treatment should include broad-spectrum coverage for grampositive and gram-negative organisms using knowledge of the common organisms and www.intechopen.com sensitivity patterns that are grown at the dialysis center. Due to the high prevalence of methicillin-resistant Staph Aureus (MRSA), empiric therapy should include coverage for MRSA. When the specific organism and antibiotic sensitivities are identified, it is important to narrow the antibiotic therapy to prevent the development of drug resistant organisms. While the exact duration of antibiotic treatment for catheter-related bacteremia is uncertain, the Infectious Disease society of America recommends a 2 week course of antibiotics 153 , while the K/DOQI guidelines recommends a 3 week course of antibiotics 19 . Other therapies, which have been used in conjunction with systemic antibiotics, to treat catheterrelated bacteremia are antibiotic catheter locks. A number of studies have shown that antibiotic locks (which may treat the biofilm layer) used in conjunction with systemic antibiotics, in tunneled dialysis catheters, have documented a 70% cure rate 30,145,[154][155][156] . Recent studies have evaluated pharmacologic therapies to prevent catheter-related bacteremia. Routine application of topical antibiotic ointments at the CVC exit such as mupirocin, povidine-iodine, and polysporin triple ointment has been associated with a 73-93% reduction in the risk of catheter-related bacteremia 7,151,157-159 . Prophylactic antibiotic catheter locks have also recently been evaluated. A marked reduction in catheter-related bacteremia has been reported, ranging from 51-99%, with use of a prophylactic antibiotic catheter locking solution 7,160-164 . However, of concern, a recent study has shown emergence of gentamicin-resistant organisms after 6 months when using a gentamicin-heparin prophylactic catheter lock 165 . The above strategies for treatment of catheter-related bacteremia apply to patients who are clinically stable. However, catheter removal, in addition to antibiotic therapy, should be the treatment of choice when patients: (1) are clinically unstable, (2) have persistent fever for 48 hours, (3) have evidence of tunnel infection, or (4) develop metastatic infectious complications 7 .

Translating science to therapies in hemodialysis vascular access dysfunction: from the bench to bedside
There are currently few if any effective therapies to treat hemodialysis vascular access stenosis and neointimal hyperplasia. However, the knowledge obtained in recent years regarding the pathology and pathogenesis of vascular access access dysfunction has provided a framework for development of therapies that target neointimal hyperplasia and vascular stenosis. The purpose of the next section is to (1) describe current therapies for AVF and AVG stenosis and (2) novel therapies using localized delivery systems for AVF and AVG.

Systemic therapies
Systemic therapies, such as dipyridamole, angiotensin-converting enzyme inhibitors, aspirin, and fish oil, from small clinical trials and observational studies have been shown to have the potential to block smooth muscle cell proliferation and migration and to prevent thrombosis in AVFs and AVGs [166][167][168][169][170] . Most recently, two large randomized controlled trials, sponsored by the National Institutes of Health, evaluating anti-platelet agents in AVG and AVF to prevent neointimal hyperplasia were published 16,171 . In the AVG study, dipyridamole and aspirin, modestly reduced the risk of stenosis and improved primary unassisted patency 171 . In the AVF study clopidogrel reduced frequency of early thrombosis but did not improve AVF suitability defined as cannulation with two needles, minimum www.intechopen.com dialysis blood flow of 300ml/min, successful use 8/12 dialysis sessions, and use after 120 days from creation 16 . While these two studies have shown some promising results, the clinical significance of these drugs used as standard treatment for hemodialysis access stenosis remains questionable. Fish oil has been shown to prevent AVG stenosis and thrombosis in one randomized, controlled trial 172 . Currently, another study evaluating fish oil and AVG stenosis and thrombosis is ongoing 173 . Other systemic agents, though not tested in randomized clinical trials, which have shown potential anti-proliferative effects targeting neointimal hyperplasia in CVD or PVD models, include peroxisome proliferation-activated receptor  agonist 174-176 , sirolimus 177 , and imatinib mesylate 176,178,179 .

Radiation therapy
Radiation therapy has been hypothesized to be a potential therapy to treat vascular stenosis due to its antiproliferative effects and potential beneficial effects of vascular remodeling [180][181][182][183] . In experimental models, both external beam and endovascular radiation therapy has proven effective to reduce neointimal hyperplasia in AVF and AVG 184,185 . However, in clinical studies, a recent randomized-controlled trial of in AVGs 25 patients showed that 42% of the radiated AVGs 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 186 .

Far infrared therapy
Infrared radiation is an invisible electromagnetic wave with a longer wavelength than that of visible light. In experimental models, far infrared therapy has been shown to improve skin blood flow and endothelial function in cardiovascular disease [187][188][189] . The rationale for far infrared therapy to treat dialysis vascular access stenosis is that the dialysis vascular access in patients are located at a superficial site and improving access flow may improve vascular access performance. In the lone clinical study of far infrared in dialysis access in AVFs, patients who received far infrared therapy had improved access flows and longer unassisted patencies 190 .

Local drug delivery systems for hemodialysis access
The rationale behind local delivery of drugs treat hemodialysis vascular access stenosis is that (1) AVFs and AVGs could be the ideal clinical model for the use of perivascular therapies since these can be easily applied at the time of surgery, (2) perivascular therapies preferentially target the "active" adventitia, (3) studies have demonstrated that lipophilic molecules when placed over the adventitia rapidly diffuse through all the layers of the vessel wall, and (4) small amounts of otherwise toxic drugs can be safely delivered to the site of stenosis using the perivascular approach resulting in high local concentrations with minimal systemic toxicity 4 . The subsequent section will discuss local therapies to treat hemodialysis vascular access stenosis from experimental models and clinical studies.

Drug eluting paclitaxel perivascular wraps
Experimental studies have previously demonstrated the efficacy of paclitaxel eluting wraps in AVG stenosis likely due to anti-proliferative effects [191][192][193] . In 2007, a large multi-center randomized-controlled study, evaluating the use of paclitaxel-eluting mesh wraps, Vascular Wrap TM , (Angiotech Pharmaceuticals, Inc.; Vancouver, British Columbia, Canada), was initiated to study the effectiveness and safety of this therapy on primary AVG patency compared to a standard AVG. However, this study was recently suspended in 2009 following a data safety monitoring review, due to an imbalance in the incidence of infections in one of the arms (either control or treatment). An alternative approach is the use of sirolimus eluting COLL-R® wraps (Covalon Technologies Ltd: Mississauga, Ontario, Canada). An initial Phase II study demonstrated primary unassisted AVG patency of 75% and 38% at 1 and 2 years respectively with these wraps 194 .

Endothelial cell loaded gel foam wraps
The rationale behind the use of these wraps is that the endothelial cell (in addition to lining blood vessels) is also a "bioreactor" which produces a large number of beneficial mediators that reduces thrombosis, inflammation, stenosis, and increases lumen diameter. Initial experimental studies have documented a beneficial effect of endothelial cell loaded gel-foam wraps in porcine models of AV fistula and graft stenosis [195][196][197][198] . A recent Phase II study ("V-HEALTH") was able to demonstrate technical feasibility and safety in hemodialysis patients who received a "Vascugel®" wrap loaded with treated human aortic endothelial cells at the time of AVF or AVG placement 97 . A phase III multi-center randomized-controlled study using the Vascugel® (Pervasis Therapeutics, Inc., Cambridge, MA) wraps in human AVGs is currently being designed.

Vascular Endothelial Growth Factor D (VEGF-D) gene therapy
In animal models of angioplasty induced restenosis, the delivery of adenoviral particles encoding for vascular-endothelial growth factor C to the site of vascular injury has been shown to trigger the release of nitric oxide and prostacyclin and reduce neointimal hyperplasia 199 . Preliminary studies on the use of VEGF-D gene therapy (using a packaged adenoviral vector and a biodegradable local delivery device (collar) made of collagen wrapped at the venous anastomosis at the time of surgery), "Trinam ® " (Ark Therapeutics; London, UK), in patients receiving AVGs, have been able to document technical feasibility and safety. A phase III study using this technology was initiated in 2009 but terminated in 2010 due to poor enrollment.

Recombinant elastase PRT-201
PRT-201 (Proteon Therapeutics; Waltham, MA) is a recombinant pancreatic elastase topically applied at the outflow vein at the time of surgery access creation which has been shown to result in both arterial and venous dilation and an increase in AVF blood flow in experimental models 200 . The clinical benefit of this approach is the potential ability to enhance AVF maturation (through rapid vascular dilation) and prevent venous stenosis in AVGs. A phase II study using this novel technology is ongoing in the United States evaluating this therapy and whether or not it improves primary patency and cumulative survival in AVG and AVF, as well as safety.

Endovascular stent therapy
Endovascular vascular therapies (angioplasty or angioplasty with stent placement) remains the only true intervention available to treat vascular stenosis. The main advantage of stent therapy after angioplasty is a reduction in adverse remodeling. In dialysis access, placement www.intechopen.com of bare metal stents after angioplasty compared to angioplasty 201 alone has been shown to improve primary patency 202,203 . However, bare-metal stents have yielded poor results due to aggressive development of in-stent restenosis. In experimental models of dialysis access in AVGs, drug-eluting stents have shown to reduce neointimal hyperplasia and improve luminal stenosis compared to bare-metal stents 204 . However, there are no clinical studies evaluating drug-eluting stents in dialysis access to date. Stent grafts (covered stents constructed from the same material of AVGs) have received recent attention as a therapy for prevention of restenosis due to its ability to prevent elastic recoil and inability of the neointimal cells to penetrate the covered barrier. A recently published multicenter, randomized controlled, clinical trial showed stent grafts (Bard Peripheral Vascular, Tempe, AZ), placed after angioplasty, to treat venous stenosis had better primary unassisted patency compared to angioplasty alone 205 . This is the only treatment to date that has shown to be effective to treat vascular access stenosis in a large, randomized, clinical trial.

Improving hemodynamics
Hemodynamic sheer stresses play a significant role in development of neointimal hyperplasia 87,112,206,207 . Therefore, altering the sheer stress pattern to prevent turbulent, lowflow, and low-sheer stresses could reduce the development of neointimal hyperplasia. Previous clinical data to date to support such an intervention comes from several studies evaluating cuffed AVG grafts ("Venaflo"; Bard Vascular, Tempe Arizona) [208][209][210] . In a recent randomized control trial evaluating cuffed vs non-cuffed AVG, cuffed AVGs showed better primary patency and cumulative survival 211 . Finally, results from a newly developed anastomotic implant device, "OptiflowTM" (Bioconnect Systems; Ambler, PA), to connect the artery and vein in AVFs and improve hemodynamics by providing a symmetric flow pattern, have shown a primary patency of 83% at 90 days 212 . This primary patency rate was higher compared to other similarly published studies 213 .

Future perspectives: new frontiers in research
In the last decade our knowledge of vascular access dysfunction has significantly evolved. We now understand that the most common pathologic lesion seen in AVF and AVG dysfunction is aggressive venous neointimal hyperplasia, and biofilms and fibrin sheaths play a major role in CVC infection and dysfunction. In order to advance the field further, we need to further our current understanding of both the clinical and experimental pathways that result in venous neointimal hyperplasia and mechanisms that lead to biofilm and fibrin sheath production in C V C s b y u s i n g t h e a d v a n c e d technologies and tools in cellular and molecular biology, bioengineering, genomics, proteomics, and vascular imaging (ultrasound, computed tomography, and magnetic resonance imaging) 65,124,214 . Finally, small and large animal models of AVF and AVG, which a number of investigators in this field have already developed 61,93,207,[215][216][217] , will play an essential role in "translating" our knowledge of pathophysiologic mechanisms in vascular access dysfunction to novel therapies for patients.

Conclusion
The magnitude and costs of dialysis access dysfunction is clearly evident, and will only become magnified in the coming years as the prevalent dialysis population continues to www.intechopen.com increase. Only by launching a "translational" research initiative ("from animal to human") can recent advances in the understanding of the mechanisms of neointimal hyperplasia formation and vascular stenosis and catheter dysfunction be translated to the development of novel effective therapies for patients.