Arteriovenous fistula in the rat tail: a new model of hemodialysis access dysfunction

The rodent models of arteriovenous fistula

Arteriovenous fistulas (AVFs) have long been used as dialysis access in patients with end-stage renal disease; however, their maturation and long-term patency still fall short of clinical needs. Rodent models are irreplaceable to facilitate the study of mechanisms and provide reliable insights into clinical problems. The ideal rodent AVF model recapitulates the major features and pathology of human disease as closely as possible, and pre-induction of the uremic milieu is an important addition to AVF failure studies. Herein, we review different surgical methods used so far to create AVF in rodents, including surgical suturing, needle puncture, and the cuff technique. We also summarize commonly used evaluations after AVF placement. The aim was to provide recent advances and ideas for better selection and induction of rodent AVF models. At the same time, further improvements in the models and a deeper understanding of AVF failure mechanisms are expected.

Experimental murine arteriovenous fistula model to study restenosis after transluminal angioplasty

Percutaneous transluminal angioplasty (PTA) is a very common interventional treatment for treating stenosis in arteriovenous fistula (AVF) used for hemodialysis vascular access. Restenosis occurs after PTA, resulting in vascular lumen loss and a decrease in blood flow. Experimental animal models have been developed to study the pathogenesis of stenosis, but there is no restenosis model after PTA of stenotic AVF in mice. Here, we describe the creation of a murine model of restenosis after angioplasty of a stenosis in an AVF. The murine restenosis model has several advantages, including the rapid development of restenotic lesions in the vessel after angioplasty and the potential to evaluate endovascular and perivascular therapeutics for treating restenosis. The protocol includes a detailed description of the partial nephrectomy procedure to induce chronic kidney disease, the AVF procedure for development of de novo stenosis and the angioplasty treatment associated with progression of restenosis. We monitored the angioplasty-treated vessel for vascular patency and hemodynamic changes for a period of 28 d using ultrasound. Vessels were collected at different time points and processed for histological analysis and immunostaining. This angioplasty model, which can be performed with basic microvascular surgery skills, could be used to identify potential endovascular and perivascular therapies to reduce restenosis after angioplasty procedures.

Decellularized Carotid Artery Functions as an Arteriovenous Graft

BACKGROUND

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

C-reactive protein variability is associated with vascular access outcome in hemodialysis patients

BACKGROUND

Hemodialysis (HD) vascular access failure is one of the most important causes of morbidity and contributes to the cost of dialysis care. There is paucity of data evaluating long-term monitoring of C-reactive protein (CRP) on outcome of HD vascular access.

METHODS

We conducted a retrospective study to investigate whether variability of serum CRP level was associated with vascular access failure rate over a 7-year period. A total of 318 HD patients were included. Their demographic data, co-morbidities and biochemical data were reviewed and collected. Serum high-sensitivity CRP (hs-CRP) level was measured every 6 months. Patients were divided into three groups according to their serial hs-CRP levels. Patients with their hs-CRP below 2 mg/L were defined as low group (n=65, 20.4%) and those with higher than 4 mg/L were defined as high (n=39, 12.3%). The rest were classified as fluctuated hs-CRP group (n=214, 67.3%). Treatment of vascular access failure includes angioplasty and access re-creation.

RESULTS

Their body mass index, indicators of dialysis adequacy and serum albumin and hs-CRP levels differed significantly among three groups. The annual vascular access failure rate was significantly higher in fluctuated hs-CRP group than in high hs-CRP group (0.41 vs 0.36, P=.037). Serum albumin was a significant associate of vascular access failure. Kaplan-Meier survival analysis indicated patients with high or fluctuated hs-CRP had shorter free interval of vascular access failure than low hs-CRP group.

CONCLUSIONS

HD patients with fluctuated hs-CRP levels were associated with increased vascular access failure.

Animal Models for Studying Pathophysiology of Hemodialysis Access

Despite extensive efforts, most approaches to reduce arteriovenous (AV) access-related complications did not results in substantial improvement of AV access patency thus far. Part of this disappointing progress relates to incomplete understanding of the underlying pathophysiology of hemodialysis access failure. In order to unravel the pathophysiology of hemodialysis access failure, animal models that closely mimic human pathology are of utmost importance. Indeed, it is impossible to study the extremely complex response of the AV access at a molecular and cellular level in great detail in dialysis patients. Over the past decades, numerous animal models have been developed in an attempt to unravel the vascular pathology of AV access failure and to design new therapeutic strategies aimed to improve durability of these vascular conduits. While large animals such as pigs are suitable for intervention studies, murine models have the greatest potential to gain more insight in the molecular mechanisms underlying AV access failure due to the availability of transgenic mice. In the present review, we describe several existing models of AV access failure and discuss the advantages and limitations of these models.

Novel therapies for hemodialysis vascular access dysfunction: myth or reality?

Hemodialysis vascular access dysfunction is a major source of morbidity for patients with ESRD. Development of effective approaches to prevent and treat vascular access failure requires an understanding of the underlying mechanisms, suitable models for preclinical testing, systems for targeted delivery of interventions to maximize efficacy and minimize toxicity, and rigorous clinical trials that use appropriate outcome measures. This article reviews the substantial progress and ongoing challenges in developing novel treatments for arteriovenous vascular access failure and focuses on localized rather than systemic interventions.

Association of vascular access type with inflammatory marker levels in maintenance hemodialysis patients

Aggressive NIH is a common histopathological lesion found at the sites of venous stenosis in arteriovenous fistula (AVF) and arteriovenous grafts (AVG). Inflammatory mediators have been proposed to play a pathogenic role in NIH, but there is paucity of data evaluating this hypothesis in clinical studies or in animal models. Serum levels of inflammatory mediators can potentially identify patients at high risk of AVF and AVG dysfunction. In a cross-sectional cohort study of 754 HD patients who were part of the NIED study cohort, we examined the associations between inflammatory markers including serum interleukin (IL) 1β, IL-6, C-reactive protein (CRP), and tumor necrosis factor-α (TNF-α) and type of vascular access. Unadjusted and multivariate-adjusted linear regression models were used. In addition, time-dependent regression model was used to assess the association between inflammatory markers and mortality. We observed that in the multivariate-adjusted model, inflammatory mediators interleukin-6 (IL-6), interleukin-1L-ß (IL-1ß), and C-reactive protein (CRP), the predicted value in hemodialysis patients, are lowest in patients with AVF and highest in central venous catheter (CVC) and AVG even in case-mix and malnutrition-inflammation complex syndrome (MICS)-adjusted models. IL-6 and CRP levels fall consistently in the same patients when AVG or CVC is changed to AVF and increase if the same patient changes access from AVF to AVG or CVC. Obesity is a risk factor for fistula failure and fistulas are associated with the lowest mortality compared with CVC and AVG. We did not find any statistically significant association between tumor necrosis factor-α (TNF- α) and vascular access outcomes. Higher levels of inflammatory mediators seen in CVC and AVG compared with AVF could potentially explain the higher mortality seen in patients with CVC and AVG compared with AVF.

Vascular remodeling and intimal hyperplasia in a novel murine model of arteriovenous fistula failure

OBJECTIVE

The arteriovenous fistula (AVF) still suffers from a high number of failures caused by insufficient outward remodeling and intimal hyperplasia (IH) formation from which the exact mechanism is largely unknown. A suitable animal model is of vital importance in the unraveling of the underlying pathophysiology. However, current murine models of AVF failure do not incorporate the surgical configuration that is commonly used in humans. Because the hemodynamic profile is one of the key determinants that play a role in vascular remodeling in the AVF, it is preferable to use this same configuration in an animal model. Here we describe a novel murine model of AVF failure in which the configuration (end-to-side) is similar to what is most frequently performed in humans.

METHODS

An AVF was created in 45 C57BL/6 mice by anastomosing the end of a branch of the external jugular vein to the side of the common carotid artery with interrupted sutures. The AVFs were harvested and analyzed histologically at days 7, 14, and 28. Identical veins of unoperated-on mice served as controls. Intravenous near-infrared fluorescent fluorophores were used to assess the patency of the fistula.

RESULTS

The patency rates at days 7, 14, and 28 days were 88%, 90%, and 50%, respectively. The mean circumference increased up to day 14, with a maximum 1.4-fold increase at day 7 compared with the control group (1.82 ± 0.7 vs 1.33 ± 0.3 mm; P = .443). Between days 14 and 28, the circumference remained constant (2.36 ± 0.2 vs 2.45 ± 0.2 mm; P = .996). At 7 days after surgery, the intimal area consisted mainly of an acellular layer that was structurally analogous to a focal adherent thrombus. Starting at 14 days after surgery, venous IH increased significantly compared with the unoperated-on group (14 days: 115,090 ± 22,594 μm(2), 28 days: 234,619 ± 47,828 μm(2), unoperated group: 2368 ± 1056 μm(2); P = .001 and P < .001, respectively) and was mainly composed of cells positive for α-smooth muscle actin. We observed leukocytes in the adventitial side of the vein at all time points.

CONCLUSIONS

Our novel murine AVF model, which incorporates a clinically relevant configuration of the anastomosis, displays similar features that are characteristic of failing human AVFs. Moreover, our findings suggest that coagulation and inflammation could both potentially play an important role in the formation of IH and subsequent AVF failure. Near-infrared fluoroscopy was a suitable alternative for conventional imaging techniques. This murine AVF-model is a valuable addition to the AVF animal model arsenal.

Comparison of invasive blood pressure measurements from the caudal ventral artery and the femoral artery in male adult SD and Wistar rats

Background and Purpose

Studies have suggested that the caudal ventral artery is a potential site for continuous arterial blood pressure monitoring in rats. However, the agreement of mean arterial pressure values between the femoral artery and the caudal ventral artery has not been investigated. This study was performed to identify whether the caudal ventral artery could be safely used for continuous blood pressure monitoring as an alternative site to the femoral artery.

Methods

Rats were randomized into four groups: Sprague Dawley rats under normothermia; Wistar rats under normothermia; Sprague Dawley rats under hypothermia; Wistar rats under hypothermia. Each rat underwent simultaneous monitoring of blood pressure using femoral artery and caudal ventral artery catheterization during a stable hemodynamic state and three periods of acute severe hemodynamic changes. The effects of rat strain, rectal temperature, experimental time course and hemodynamic factors on pressure gradients, the concordance of mean arterial pressure values between the femoral artery and the caudal ventral artery, and the rates of distal ischemia after surgery were determined.

Results

There was a significant difference in the rate of distal ischemia between femoral and caudal ventral arteries after catheterization (25% vs 5%, P<0.05). The overall mean gradient and the mean gradient under a steady hemodynamic state were 4.9±3.7 mm Hg and 5.5±2.5 mm Hg, respectively. The limits of agreement (bias±1.96 SD) were (−2.5 mm Hg, 12.3 mm Hg) and (-0.5 mm Hg, 10.5 mm Hg), respectively. Although the concordance decreased during the first 30 sec of each period of severe hemodynamic changes, the degree of agreement was acceptable regardless of the effects of rat strain and rectal temperature.

Conclusions

Based on the degree of agreement and the safety of catheterization, the caudal ventral artery may be a preferred site for continuous arterial blood pressure monitoring without acute severe hemodynamic changes.

Sulodexide may alleviate neointimal hyperplasia by inhibiting angiopoietin‑2 in an arteriovenous fistula model

The present study was undertaken to confirm whether sulodexide aleviates neointimal hyperplasia by regulating angiopoietin/Tie in a rat femoral arteriovenous fistula (AVF) model. Sprague Dawley rats were divided into four groups: sham, model, treatment and treatment control. An arteriovenous shunt model was created in the model and treatment groups. Sulodexide was subcutaneously administered (10 mg/kg/day) 6 times per week for 8 weeks in the treatment and treatment control groups. Histology and immunofluorescence were analyzed and the protein expression of angiopoietin‑1, angiopoietin‑2, Tie‑2, p‑ERK and total‑ERK were tested by ELISA and/or western blotting after 8 weeks. HE staining revealed that sulodexide was able to partially alleviate intimal hyperplasia of remodeled veins in the AVF model. Additionally, sulodexide was able to decrease angiopoietin‑2 and Tie‑2 expression while increasing angiopoietin‑1 expression in AVF tissue. Sulodexide was also able to decrease ERK phosphorylation which was increased in the model. Serum levels of soluble Tie-2 (sTie‑2) were also significantly decreased by sulodexide compared with the model. Immunofluorescent analysis also confirmed that sulodexide was able to decrease angiopoietin‑2 expression, possibly partially by inhibiting endothelial cell proliferation. Sulodexide may alleviate venous intimal hyperplasia by regulating the angiopoietin/Tie system, which may play a significant role in assisting remodeled veins to cope with their new biomechanical environment, but whether the angiopoietin/Tie system is beneficial or not requires further study.

A new arteriovenous fistula model to study the development of neointimal hyperplasia

This study describes an alternative arteriovenous fistula (AVF) model in the rat in which the animals develop significant neointimal hyperplasia (NIH) not only at the distal anastomotic site, but also throughout the fistula body. This aortocaval fistula was established by anastomosing the distal end of the renal vein to the abdominal aorta after unilateral nephrectomy. The increased hemodynamic stress resulting from exposing the renal vein to the arterial circulation induced venous NIH as early as 7 days after surgery. This experimental AVF was characterized by the early lack of endothelium, the accumulation of proliferating vascular smooth muscle cells and the neovascularization of the fistula adventitia. In summary, we have described an informative animal model to study the pathobiology of NIH in native AVF.

Cardiovascular remodeling during arteriovenous fistula maturation in a rodent uremia model

PURPOSE

The aim of this study was to evaluate cardiovascular remodeling after arteriovenous fistula (AVF) surgery and to characterize the effect of chronic kidney disease (CKD) in a rodent femoral AVF model.

METHODS

Sixteen rats (8 healthy; 8 CKD) underwent femoral AVF surgery; 4 animals served as controls. AVF and cardiac morphology as well as function were assessed during the fistula maturation process (until day 84 after surgery) using magnetic resonance imaging and histopathological analyses.

RESULTS

Histopathological analysis revealed that a glomerular and interstitial nephropathy caused CKD. In healthy and CKD animals, AVF surgery resulted in progressive downstream vein dilation and a subsequent cardiac adaptation. This vein dilation during maturation was less in CKD rats during the early postoperative course (day 21: p=0.0475) and similar thereafter until day 84. The dilation was accompanied by an aggravation of neointimal hyperplasia (NIH) and calcification in AVFs of CKD rats. The chronic volume overload resulted in both groups in a significantly increased end-diastolic volume (healthy rats: p=0.0087; CKD rats: p=0.0333). Simultaneously, cardiac output increased 195% in healthy and 244% in uremic rats, which was caused by both a significantly increased stroke volume and heart rate. The left ventricular mass rose in AVF animals and was increased at the end of the study period, indicating a distinct cardiac hypertrophy.

CONCLUSION

Our rat model showed typical cardiovascular features of the AVF maturation process, which strongly resemble clinical findings in patients. Uremia caused inferior dilation in the early phase after surgery and an exacerbation of NIH. This model should help to identify the cellular and molecular mechanisms that contribute to AVF failure.

Characterization of a model of an arteriovenous fistula in the rat: the effect of L-NAME

Vascular access dysfunction contributes to the mortality of patients undergoing chronic hemodialysis. The present study analyzed the changes that evolve in a femoral arteriovenous fistula in the rat. The venous segment of this model exhibited, at 1 week, activation of pro-inflammatory transcription factors and up-regulation of pro-inflammatory, proliferative, procoagulant, and profibrotic genes; and at 4 weeks, the venous segment displayed neointimal hyperplasia, smooth muscle proliferation, and thrombus formation. These changes were accompanied by endothelial (e) nitric oxide synthase (NOS) and inducible (i) NOS up-regulation. The administration of NG-nitro-L-arginine methyl ester, an inhibitor of NOS activity, increased venous neointimal hyperplasia and pro-inflammatory gene expression (monocyte chemoattractant protein-1 and cytokine-induced neutrophil chemoattractant-1), increased systolic blood pressure, and decreased blood flow through the fistula. In another hypertensive model, the rat subtotal nephrectomy model, venous neointimal hyperplasia in the arteriovenous fistula was also exacerbated. We conclude that this arteriovenous fistula model recapitulates the salient features observed in dysfunctional, hemodialysis arteriovenous fistulas, and that venous neointimal hyperplasia is exacerbated when this model is superimposed in two different models of systemic hypertension. Since the uremic milieu contains increased amounts of asymmetric dimethylarginine, we speculate that such accumulation of this endogenous inhibitor of NOS, by virtue of its pressor or nitric oxide-depleting effects, or a combination thereof, may contribute to the limited longevity of arteriovenous fistulas used for hemodialysis.

Advances and new frontiers in the pathophysiology of venous neointimal hyperplasia and dialysis access stenosis

Hemodialysis vascular access dysfunction is a major cause of morbidity and mortality in hemodialysis patients. The most common cause of this vascular access dysfunction is venous stenosis as a result of venous neointimal hyperplasia within the perianastomotic region (arteriovenous fistula) or at the graft-vein anastomosis (polytetrafluoroethylene, or PTFE, grafts). 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. Therefore, this article will (1) describe the pathology of hemodialysis access stenosis in arteriovenous fistulas and grafts, (2) review and describe both current and novel concepts in the pathogenesis of neointimal hyperplasia formation, (3) discuss current and future novel therapies for treating venous neointimal hyperplasia, and (4) suggest future research areas in the field of hemodialysis vascular access dysfunction.