Inhibition of neointimal hyperplasia in a sheep model of dialysis access failure with the bioabsorbable Vascular Wrap paclitaxel-eluting mesh

OBJECTIVE: This study evaluated the effect of a bioabsorbable mesh containing paclitaxel on neointimal hyperplasia in a sheep model of dialysis access failure. METHODS: Forty neutered male sheep were randomized to one of five parallel groups: no mesh; or a 3-cm x 6-cm mesh with 0.0, 0.3, 0.7, or 1.2 microg/mm(2) of paclitaxel for a total dose of 0.0, 0.6, 1.3, or 2.2 mg, respectively. Commercially available 6-mm internal diameter expanded polytetrafluoroethylene grafts were surgically placed between the left common carotid artery and the right external jugular vein. For those animals randomized to one of the mesh groups, the mesh was placed around the distal end of the graft and venous anastomosis. Patency was assessed at weekly intervals throughout the study. Animals were euthanized 8 weeks after implantation, and grafts and veins were harvested. After histologic processing, six cross sections were cut at the venous end of the graft and vessel. The primary and secondary efficacy outcome measures, respectively, were the area and capillary density of the neointima at the graft-vein anastomosis. Histologic analyses were also performed to investigate the effects of the paclitaxel-eluting mesh on the anastomotic site. RESULTS: Grafts occluded before the scheduled sacrifice in five animals, and they were excluded from the study and not replaced. Control animals developed significant neointimal hyperplasia at the cross section taken perpendicular to the graft at its most distal end: the neointimal area measured 10.5 +/- 6.8 mm(2) in the no mesh group and 6.4 +/- 3.2 mm(2) in the zero-dose mesh group (P = .28). In contrast, neointimal area was significantly reduced in the paclitaxel mesh groups: 0.9 +/- 1.4 mm(2) in the 0.3 microg/mm(2) group (P = .008 vs zero-dose mesh), 1.3 +/- 1.5 mm(2) in the 0.7 microg/mm(2) group (P = .004 vs zero-dose mesh), and 1.2 +/- 1.4 mm(2) in the 1.2 microg/mm(2) group (P = .008 vs zero-dose mesh). Capillary density in the neointima at the graft-vein anastomosis decreased with paclitaxel and was significantly reduced in the paclitaxel mesh groups with 0.3 and 1.2 mug/mm(2) compared with the zero-dose mesh control (3.6 +/- 2.9 vs 8.9 +/- 5.6 per mm(2) [P = .022] and 1.1 +/- 1.7 vs 8.9 +/- 5.6 per mm(2) [P = .001] respectively). The paclitaxel mesh had no significant effect on healing of the anastomosis or on the thickness of the adjacent vein. CONCLUSIONS: In this model, the paclitaxel-eluting mesh significantly reduced neointimal hyperplasia and neointimal capillary density without apparent toxicity to the adjacent vein.     

Venous neointimal hyperplasia in polytetrafluoroethylene dialysis grafts

BACKGROUND: Vascular access dysfunction is the most important cause of morbidity and hospitalization in the hemodialysis population in the United States at a cost of $1 billion per annum. Venous neointimal hyperplasia (VNH) characterized by stenosis and subsequent thrombosis accounts for the overwhelming majority of pathology resulting in polytetrafluoroethylene (PTFE) dialysis graft failure. Despite the magnitude of the problem and the enormity of the cost ($1 billion), there are currently no effective therapies for the prevention or treatment of venous neointimal hyperplasia in PTFE dialysis grafts. METHODS: Tissue samples were collected from the graft-vein anastomosis of stenotic PTFE grafts during surgical revision. Specimens were graded using standard light microscopy and immunohistochemistry for the magnitude of neointimal hyperplasia and for the expression of specific cell types, cytokines, and matrix proteins. RESULTS: VNH was characterized by the (1) presence of smooth muscle cells/myofibroblasts, (2) accumulation of extracellular matrix components, (3) angiogenesis within the neointima and adventitia, and (4) presence of an active macrophage cell layer lining the PTFE graft material. Platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) were expressed by smooth muscle cells/myofibroblasts within the venous neointima, by macrophages lining both sides of the PTFE graft, and by vessels within the neointima and adventitia. CONCLUSIONS: Our results suggest that macrophages, specific cytokines (bFGF, PDGF, and VEGF), and angiogenesis within the neointima and adventitia are likely to contribute to the pathogenesis of VNH in PTFE dialysis grafts. Interventions aimed at these specific mediators and processes may be successful in reducing the very significant human and economic costs of vascular access dysfunction.     

Aggressive venous neointimal hyperplasia in a pig model of arteriovenous graft stenosis

BACKGROUND: Vascular access dysfunction is the most important cause of morbidity and hospitalization in the hemodialysis population in the United States at a cost of well over one billion dollars per annum. Venous neointimal hyperplasia characterized by stenosis and subsequent thrombosis, is the major cause of polytetrafluoroethylene (PTFE) dialysis graft failure. Despite the magnitude of the problem, there are currently no effective therapies for the prevention or treatment of venous neointimal hyperplasia in PTFE dialysis grafts. We believe that this is partly due to the lack of a validated large animal model of arteriovenous stenosis that could be used to test out novel interventions. METHODS: Seven-centimeter PTFE loop grafts were placed between the femoral artery and vein of domestic pigs. The grafts were removed at 2, 4, 7, 14 and 28 days after surgery and subjected to a detailed histological and immunohistochemical examination. RESULTS: Significant neointimal hyperplasia and venous stenosis developed by 28 days at the graft-vein anastomosis. There was minimal neointimal hyperplasia at the graft-artery anastomosis. Venous neointimal hyperplasia (VNH) was characterized by (a) the presence of smooth muscle cells/myofibroblasts; (b) angiogenesis within both the neointima and adventitia; and (c) the presence of an active macrophage cell layer lining the PTFE graft material. These results are very similar to the human lesion previously described by us in dialysis patients. CONCLUSIONS: We have developed and validated a pig model of venous neointimal hyperplasia that is very similar to the human lesion. We believe that this is an ideal model in which to test out novel interventions for the prevention and treatment of clinical hemodialysis vascular access dysfunction.     

Saratin,an inhibitor of collagen-platelet interaction,decreases venous anastomotic intimal hyperplasia in a canine dialysis access model

Prosthetic dialysis access thrombosis and/or stenosis is the most common cause of graft impairment or loss and is primarily attributed to venous outflow stenosis due to intimal hyperplasia. Intimal hyperplasia is thought to result from interactions between areas of exposed subendothelial collagen in an injured vessel and platelets, resulting in platelet adhesion. Saratin, an inhibitor of the vWF-dependent binding of platelet to collagen interaction, has been shown in vitro to reduce the adhesion of platelets to collagen. In the current study, the authors investigated the effects of topical saratin administration in a canine dialysis access model in regard to intimal hyperplasia development at the venous anastomosis. Fourteen female mongrel dogs underwent placement of a femoral polytetrafluoroethylene (PTFE) dialysis access graft and were placed into 1 of 2 groups: 1) control or 2) experimental with topical saratin application. The experimental group had 600 microg of saratin (1 microg/microL) applied for 5 minutes directly onto the venous anastomosis before restoration of blood flow;control groups received vehicle control. At 4 weeks postoperative, a portion of the graft was removed along with a segment of the outflow vein. Veins were subsequently processed, sectioned, and analyzed along the length of the excised segment and divided into blocks that included the area of the graft toe, midanastomotic region and heel, and blocks A-E. Intimal hyperplasia was assessed by a computer-assisted morphometric analysis. Platelet counts and bleeding times were also measured. Vein segments in the control group (n=7) showed pronounced intimal hyperplasia in blocks B, C, and D as compared to the saratin group (n=6). Distribution of intimal hyperplasia by blocks between control and saratin groups were as follows: block [A] 8.6 +/- 1.9 vs 9.7 +/- 3.0% (p=NS), [B] 32.7 +/- 6.3 vs 10.7 +/- 3.5% (p=0.01), [C] 44.8 +/- 6.2% vs 10.3 +/- 1.5% (p=0.0004), [D] 40.8 +/- 11.0 vs 9.1 +/- 4.2% (p=0.02), [E] 7.5 +/- 5.5 vs 2.7 +/- 0.4% (p=NS). Intimal hyperplasia normalized to vein wall thickness also showed a significant reduction with saratin application. Bleeding times and platelet counts obtained at different time points during the experiment showed no difference between control and saratin groups. In a canine dialysis access model using PTFE grafts, topical application of saratin at the venous anastomosis decreased intimal hyperplasia development by as much as 77% when compared with control animals. Saratin provides for a method of substantially reducing intimal hyperplasia by direct local application without systemic side effects.     

Neointimal hyperplasia on a cell-seeded polytetrafluoroethylene graft is promoted by transfer of tissue plasminogen activator gene and inhibited by transfer of nitric oxide synthase gene

OBJECTIVE: The objective of this study was to examine the effect of tissue plasminogen activator (tPA) and endothelial nitric oxide synthase (eNOS) on thrombosis and neointimal hyperplasia on a polytetrafluoroethylene (PTFE) graft seeded with smooth muscle cells (SMCs). METHODS: SMCs retrovirally transduced with tPA and eNOS genes were seeded on PTFE grafts and then implanted into the infrarenal rabbit aorta. Thrombosis and neointimal hyperplasia on the grafts were examined after 30 and 100 days of implantation. RESULTS: At 30 days of implantation, thrombus was observed on the luminal surface of both unseeded and SMC seeded control grafts, whereas grafts seeded with SMCs secreting tPA were nearly free of thrombus. At 100 days, the neointima on grafts seeded with tPA transduced SMCs was significantly thicker (925 +/- 150 microm, n = 5) than neointima on the other grafts (range, 132 to 374 microm; P < .001). Neointima thickness on grafts seeded with eNOS transduced SMCs (154 +/- 27 microm) was similar to that of unseeded grafts (132 +/- 16 microm, P > .05); both were thinner than those on grafts seeded with SMCs transduced with only lacZ gene (287 +/- 35 microm). The ratio of seeded cells in the neointima was significantly higher on SMC/tPA grafts (46% +/- 8%) than SMC/NOS grafts (21% +/- 6%, P < .05), indicating tPA transduced cells proliferated more than eNOS transduced cells. CONCLUSIONS: Engineered tPA expression in seeded SMCs causes significantly more neointimal hyperplasia, despite the favorable inhibition of luminal thrombus. eNOS expression in the seeded cells inhibits neointimal hyperplasia.     

Coating with paclitaxel improves graft survival in a porcine model of haemodialysis graft stenosis.

BACKGROUND: Most commonly resulting from intimal hyperplasia at the venous anastomosis, stenosis leading to thrombosis is a major cause of failure of polytetrafluoroethylene (PTFE) dialysis grafts. We recently reported that coating haemodialysis grafts with paclitaxel could reduce neointimal hyperplasia. This study tested whether paclitaxel-coating could prolong graft survival in a porcine model. METHODS: PTFE grafts were double-coated with paclitaxel. Bilateral grafts were created between the carotid arteries and the external jugular veins, and we evaluated graft survival by weekly measurements of blood flow for 12 weeks. RESULTS: We successfully implanted four pairs of paclitaxel-coated grafts and four pairs of control grafts in eight Landrace pigs. One control pig had to be euthanized at 4 weeks after graft placement. The grafts in the other three controls and four paclitaxel pigs survived until harvesting of the grafts. All paclitaxel-coated grafts remained patent for 12 weeks without decrease of blood flow. Median blood flow was 702 ml/min at three weeks and 818 ml/min at 12 weeks after placement. In contrast, the four control grafts lost luminal patency at 5, 6, 6 and 8 weeks, respectively. In Kaplan-Meier analysis, paclitaxel-coated grafts showed better survival than uncoated grafts (P = 0.011). CONCLUSIONS: Double-coating with paclitaxel improved graft survival. Coated PTFE grafts may be effective for the prevention of graft failure in patients on haemodialysis.     

Vein cuff interposition prevents juxta-anastomotic neointimal hyperplasia.

This study sought to minimize juxta-anastomotic neointimal hyperplasia (JNIH) following the use of polytetrafluorethylene (PTFE) conduits. PTFE anastomoses to canine carotid arteries (noncuff grafts) were compared with grafts with vein cuffs interposed proximally and distally between the graft and native artery. This technique has been suggested clinically for below-knee PTFE femoropopliteal reconstruction. Twelve dogs received aspirin for 1 week before operation, which was continued after each animal received bilateral cuff and noncuff 4-mm PTFE grafts. At sacrifice, after 3-12 weeks, graft patency was assessed and luminal diameters measured with ophthalmic calipers at three sites along the anastomoses and 1 mm proximal or distal to graft toe (A' diameter). Specimens were perfusion fixed at arterial pressure for gross and histologic study; selected arteries were additionally fixed with 4% buffered glutaraldehyde, stored at 4 C, and examined immunochemically using antimyosin antibody immunopurified for smooth muscle. Overall patency of noncuff grafts in 11 long-term surviving dogs was 4 of 11; patency of the cuff grafts was 7 of 11. Regardless of graft thrombosis, antibody positive cellular proliferation occurred mainly at noncuffed PTFE anastomoses. Luminal encroachment was predominantly due to subintimal proliferation of cells highly reactive to smooth muscle derived antibody. JNIH was most prominent 1 mm distal to the graft toe (A' distal diameter). Average A' for noncuff grafts was 1.82 mm +/- 0.97 SEM; average A' diameter for cuff grafts was 3.41 mm +/- 0.74 SEM (p less than 0.001). Vein cuff inhibition of proliferation of smooth muscle or cells derived from smooth muscle possibly relates to wider distribution of kinetic energy (less compliance mismatch) or to interposition of venous endothelium.     

Small-caliber heparin-coated ePTFE grafts reduce platelet deposition and neointimal hyperplasia in a baboon model

PURPOSE: Intimal hyperplasia and graft thrombosis are major causes of graft failure. Heparin prolongs graft patency and inhibits neointimal hyperplasia in animal models. The purpose of this study was to evaluate the effect of a heparin-coated expanded polytetrafluoroethylene (ePTFE) graft on platelet deposition and anastomotic neointimal hyperplasia after aortoiliac bypass grafting in a baboon model. METHODS: Heparin-coated ePTFE grafts (4-mm diameter) were incorporated into exteriorized femoral arteriovenous shunts placed in five baboons. Platelet deposition was analyzed by measuring the accumulation of indium 111-labeled platelets on the grafts, with dynamic scintillation camera imaging. Eight adult male baboons (mean weight, 9.3 kg) underwent bilateral aortoiliac bypass grafting with ePTFE grafts (4-mm internal diameter). In each animal a heparin-coated ePTFE graft was placed in one aortoiliac artery, and an uncoated graft, which served as the control, was placed in the contralateral aortoiliac artery. All grafts were harvested at 4 weeks, and were analyzed quantitatively for neointimal hyperplasia at graft-vessel anastomoses. RESULTS: Early platelet deposition on heparin-coated grafts after 1 to 4 hours of ex vivo circuitry was significantly reduced. All the harvested aortoiliac grafts were patent at 4 weeks. There was a significant reduction in neointimal area at both proximal (0.26 +/- 0.11 mm(2)) and distal (0.29 +/- 0.14 mm(2)) anastomoses in the heparin-coated grafts, compared with proximal (0.56 +/- 0.18 mm(2)) and distal (0.63 +/- 0.21 mm(2)) anastomoses in the untreated control grafts (P <.05). In addition, neointimal cell proliferation assayed with bromodeoxyuridine (BrdU) incorporation was reduced in the graft neointima (3.47% +/- 0.43%) in heparin-coated grafts compared with the graft neointima (6.21% +/- 0.59%) in untreated control grafts (P <.05). CONCLUSIONS: Small-caliber heparin-coated ePTFE grafts significantly reduce platelet deposition and anastomotic neointimal hyperplasia and cell proliferation, without measurable side effects, in baboons. Surface coating with heparin in small-caliber ePTFE grafts is useful for improving prosthetic bypass graft patency. Clinical relevance: An autologous vein graft is the ideal bypass conduit in peripheral arterial reconstruction; however, many patients who undergo bypass grafting do not have adequate or available autologous vein graft. As a result surgeons often must rely on prosthetic grafts as an alternative conduit in arterial bypass procedures. Clinical outcomes with prosthetic grafts in peripheral arterial reconstruction are generally inferior to those with autologous vein bypass grafts, in part because of anastomotic neointimal hyperplasia. This study evaluated the effect of small-caliber heparin-coated expandable polytetrafluoroethylene (ePTFE) grafts in aortoiliac reconstruction in a baboon model. The study found that heparin-coated ePTFE grafts resulted in less intimal hyperplasia and less platelet deposition after implantation, compared with noncoated control ePTFE grafts.     

Improved spontaneous endothelialization by postfixation treatment of bovine pericardium

The longstanding release of locally cytotoxic glutaraldehyde concentrations from fixed biological materials is one reason for the lack of spontaneous endothelialization in vivo and in vitro of biological grafts (and valves). Preliminary studies have shown that bovine pericardium postfixation-treated with aminoacids may be endothelialized in vitro. To test whether such treatment improves spontaneous endothelialization in vivo 8 cm long segments grafts (6 mm I.D.) were interposed into the carotid arteries of 10 sheep. Ten grafts were sewn from postfixation treated pericardium (PTP), 4 from standard glutaraldehyde fixed pericardium (GAP) and 6 polytetrafluoroethylane (PTFE) grafts were implanted to study the degree of spontaneous endothelialization. In two pregnant animals, all grafts occluded (2 PTP, 1 GAP, 1 PTFE). In the remaining animals 1 additional PTFE graft occluded and 2 PTFE and 1 GA grafts showed significant thrombotic obstruction. The patent grafts were harvested at 3 months and the surface covered with red thrombus was determined by planimetry. PTP grafts had significantly less (12.2% +/- 15%) thrombotic appositions than glutaraldehyde treated grafts (49% +/- 20%; P = 0.01) and PTFE grafts (40.5% +/- 13%; P = 0.01). In the central areas of the PTP grafts, endothelial cells spread directly on the collagenous matrix and produced a basal lamina. In GAP, endothelial cells spread on amorphous material or macrophages and in PTFE grafts, only a neointima composed of myofibroblasts was endothelialized. Neutralization of glutaraldehyde concentrations by postfixation with aminoacids improves spontaneous endothelialization in vivo in biological materials.     

Mechanisms of healing in synthetic grafts

In previous baboon studies we have shown that porous (60 micron mean internodal distance) polytetrafluoroethylene (PTFE) grafts heal by ingrowth of endothelium and smooth muscle cells from the adjacent artery and from capillaries penetrating through the interstices of the graft. However, porous grafts (principally made of Dacron) in humans do not heal. This has been attributed to a wound healing deficiency in humans; however, it might be due to an inhibitory effect of the Dacron itself. To examine the latter possibility, we undertook this study to compare the healing of 4 mm internal diameter porous Dacron grafts (USCI, Sauvage Filamentous Knitted) with that of Gore-Tex 60 micron PTFE grafts in baboons (the latter graft not available for clinical use). The grafts were harvested at 2, 4, and 12 weeks and assessed for (1) percentage of endothelial coverage, (2) endothelial cell (EC) proliferation (thymidine labeling index), (3) intimal area, and (4) smooth muscle cell (SMC) proliferation (thymidine labeling index). The PTFE grafts at all three time points were fully covered, whereas only one of five Dacron grafts was completely covered at 12 weeks. The intima of the PTFE grafts consisted of ECs and SMCs, whereas that of the Dacron grafts contained ECs and SMCs as well as focal accumulations of thrombus. The intimal cross-sectional areas in the Dacron grafts (3.0 +/- 1.2 mm2) were significantly greater than in the PTFE grafts (0.8 +/- 0.6 mm2) at 4 weeks; there was no difference at 12 weeks (Dacron, 2.6 +/- 2.3 mm2 and PTFE, 3.0 +/- 2.5 mm2).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of platelet deposition and neointimal hyperplasia of heparin-coated small-caliber ePTFE grafts in a canine femoral artery bypass model

INTRODUCTION: Bypass graft failure due to acute thrombosis and intimal hyperplasia remains a major challenge in small-diameter vascular prosthetic graft reconstruction. Heparin has been shown to prevent thrombus formation and inhibit intimal antithrombotic in animal studies. In this study, we evaluated the effect of small-caliber heparin-coated expanded polytetrafluoroethylene (ePTFE) grafts on platelet deposition and intimal hyperplasia in a canine model of femoral artery bypass grafting. METHODS: Nine adult greyhound dogs underwent placement of bilateral femorofemoral artery bypass grafts with ePTFE grafts (4 mm diameter and 7 cm long). In each animal, a heparin-coated ePTFE graft was placed on one side while a noncoated graft was placed on the contralateral side which served as the control. Platelet deposition was measured by autologous (111)indium-labeling and scintillation camera imaging analysis in 24 h. The graft patency was assessed at 4 weeks following the bypass. The effect of intimal hyperplasia was assessed with histological and morphometric analysis. RESULTS: Platelet deposition on the heparin-coated grafts at 24 h was significantly reduced by 72% as compared to controls (P = 0.001). The patency rate was 44% in control grafts and 89% in heparin-coated grafts. There was a significant reduction of graft intimal hyperplasia at both proximal (0.38 +/- 0.21 mm(2)) and distal (0.19 +/- 0.06 mm(2)) anastomoses in the heparin-coated grafts as compared with proximal (1.01 +/- 0.28 mm(2)) and distal (0.42 +/- 0.01 mm(2)) anastomoses in the untreated control grafts, respectively (P < 0.05). Heparin coating significantly reduced graft neointimal hyperplasia at patent graft anastomoses by 55-72% as compared to controls. CONCLUSIONS: These data demonstrate that heparin coating of ePTFE significantly reduced early platelet deposition and inhibited anastomotic neointimal hyperplasia. Moreover, small-caliber heparin-coated ePTFE graft significantly increased graft patency in a canine femoral artery bypass model. This may represent a promising treatment strategy for improving the clinical performance of small-caliber prosthetic vascular grafts.     

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

INTRODUCTION AND METHODS: The most frequent complication of polytetrafluoroethylene (PTFE) arteriovenous grafts for hemodialysis is thrombotic occlusion due to stenosis caused by intimal hyperplasia. This complication is also known for peripheral bypass grafts. Because the use of a venous cuff at the distal anastomosis improves the patency of peripheral bypass grafts, we considered that it might also improve the patency of PTFE arteriovenous grafts. Therefore, a randomized multicenter trial was carried out to study the effect of a venous cuff at the venous anastomosis of PTFE arteriovenous grafts on the development of stenoses and the patency rates. RESULTS: Of the 120 included patients, 59 were randomized for a venous cuff. The incidence of thrombotic occlusion was lower in the cuff group (0.68 per patient-year) than in the no-cuff group (0. 88 per patient-year; P =.0007). However, the primary and secondary patency rates were comparable. The cuff group tended to have fewer stenoses at the venous and arterial anastomoses when examined with duplex scan. Graft failure was higher in patients with an initial anastomosing vein diameter smaller than 4 mm (7 of 18 [39%]) than in those with a vein diameter of 4 mm or larger (16 of 88 [18%]; P =. 052). Local edema, skin atrophy, and obesity yielded a higher risk on graft failure (23% vs 11%). CONCLUSION: A venous cuff at the venous anastomosis of PTFE arteriovenous grafts for hemodialysis reduced the incidence of thrombotic occlusions; stenosis at the venous anastomosis was reduced. However, this did not result in a better patency rate. Therefore, the venous cuff should not be used routinely. Initial vein diameter and local problems (edema, obesity, or skin atrophy) appear to be the most important risk factors for graft failure.     

Beneficial effects of banding on venous intimal-medial hyperplasia in arteriovenous loop grafts

Local hemodynamics were modified in a canine arteriovenous loop graft model by placing a flow-limiting band on femoral polytetrafluoroethylene (PTFE) grafts. Banded and unbanded grafts were implanted in a paired fashion. Hemodynamic studies included Reynolds number and phonoangiography as measures of turbulence. Intimal-medial thickness was measured 8 weeks after implantation. Reduction of the volumetric flow rate by 50 percent resulted in significant changes in flow velocity, flow pulsatility, pressure, and turbulence at the venous anastomosis. Hyperplastic lesions developed in a reproducible manner at the venous anastomosis of the unbanded but not the banded grafts, as evidenced by combined intimal-medial thickness measurements: unbanded grafts 0.68 +/- 0.13 mm (p less than 0.01 versus control), banded grafts 0.25 +/- 0.03 mm (p greater than 0.05 versus control). Stepwise regression analysis indicated Reynolds number had the best correlation with the development of hyperplasia (r = 0.915, p less than 0.005), this being the first time this correlation has been quantitatively determined. We conclude that flow disturbance or turbulence is a major factor in the development of venous intimal-medial hyperplasia in arteriovenous loop grafts.     

Neointimal hyperplasia rapidly reaches steady state in a novel murine vein graft model

OBJECTIVE: Neointimal hyperplasia remains a principal cause of vein graft failure. Genetic contributions to vein graft neointimal hyperplasia could be well studied in the mouse; however, surgical approaches to vein bypass surgery in the mouse have yet to replicate approaches commonly employed in human patients. Consequently, the goal of this study was to develop a murine interposition vein graft model that reproduces characteristics of human vein graft disease. METHOD: Using C57BL/6J mice, we excised inferior venae cavae (IVCs) from donor mice and grafted them, with end-to-side anastomosis, into the carotid circulation of recipients. IVC grafts were harvested from 3 to 56 days postoperatively, and analyzed for the development of neointima and media. RESULTS: Thickening of both the vein graft neointima and media progressed rapidly between postoperative weeks 1 and 4, and reached steady state levels by approximately week four, with a graft-wall thickness of 91 +/- 4 microm (14 cell layers), a lumen area of 0.56 mm(2), an average neointima-media ratio of 0.4 to 0.6, and a predominance of alpha-smooth muscle actin-staining cells. Comprising predominately smooth muscle actin-expressing cells, the neointima was 50% thicker in the proximal than in the distal third of the grafts (P <.001), but proximal and distal vein graft anastomoses were widely patent. CONCLUSIONS: In syngeneic murine carotid interposition IVC grafts implanted with end-to-side anastomoses, moderate, nonocclusive neointimal hyperplasia reaches steady state after the fourth postoperative week. This neointimal hyperplasia, like that of human grafts, predominates near vein graft anastomoses. This vein graft model should facilitate genetic analyses of the pathogenesis of neointimal hyperplasia.     

Neoendothelialization of small-diameter polytetrafluoroethylene arterial grafts is not delayed by aspirin and dipyridamole

The effect of aspirin and dipyridamole on neoendothelialization of polytetrafluoroethylene (PTFE) was studied in the rabbit aortic graft model. Forty-three New Zealand white rabbits were allocated to receive a combination of aspirin 10 mg/kg/day and dipyridamole 10 mg/kg/day (n = 23) or placebo (n = 20). Both regimens began 3 days before insertion of PTFE aortic grafts (10 mm long and 3 mm internal diameter). Serum thromboxane B2 concentration in the control group averaged 254 +/- 22 ng/ml (+/- standard error of the mean) and 40 +/- 23 ng/ml in the treatment group (p less than 0.001). Grafts and adjacent aorta were harvested at 2 weeks (n = 4), 4 weeks (n = 9), 8 weeks (n = 13), and 12 weeks (n = 17) after implantation. Morphologic techniques, including conventional light microscopy, immunoperoxidase staining for endothelial factor VIII-related antigen, and scanning electron microscopy (SEM) demonstrated that neointima was composed of endothelial cells arising by ingrowth at anastomotic site and as islands in the center of the graft. The percentage of graft neoendothelialization was measured by SEM. At 2 weeks 18% +/- 2% of the PTFE surface was covered with endothelium in the aspirin/dipyridamole group. The percentages of graft neoendothelialization for the treatment and control groups at 4 weeks were 44% +/- 13% (n = 5) and 46% +/- 10% (n = 4) (p = NS), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Transplantation of Autologous Tissue Fragments into an e-PTFE Graft with Long Fibrils

Abstract: An expanded polytetrafluoroethylene (e-PTFE) graft with long fibrils transplanted with bone marrow showed rapid and uniform neointima formation in a dog study. The e-PTFE grafts (fibril length, 90 μm; 6 mm internal diameter; length, 6–8 cm) transplanted with autologous bone marrow were implanted in the abdominal aortae of 10 dogs and retrieved at 3 weeks and 3 months after implantation. Control e-PTFE grafts without bone marrow treatment were also implanted in the same manner in 8 dogs. Macroscopically the treated graft wall appeared red in color; however, there was no thrombus deposition on the surface. Light microscopic observation revealed that the treated grafts were completely lined with endothelial cells at 3 weeks. The neointima was uniform without intimai hyperplasia at the anastomotic sites.
Inside the graft wall many capillary blood vessels were observed. At 3 months moderate intimai hyperplasia throughout the graft with complete endothelialization was observed. In the control grafts, endothelialization was observed at the anastomotic sites; however, half of the other areas were covered with a fibrin layer devoid of endothelial cells even in the 3-month grafts. These results indicated that neointima formation was effectively accelerated with the autologous bone marrow transplantation, but moderate intimai hyperplasia throughout the graft was inevitable in e-PTFE grafts even after complete endothelialization.     

Expression of C-reactive protein in myointimal hyperplasia in a porcine arteriovenous graft model.

BACKGROUND: The migration and proliferation of myofibroblasts are prominent features of myointimal hyperplasia associated with haemodialysis polytetrafluoroethylene (PTFE) grafts. Since C-reactive protein (CRP) possesses functional activities on vascular smooth muscle cells (SMCs), we examined the expression of this protein in PTFE grafts early in the course of myointimal hyperplasia development in a porcine model. METHOD: Bilateral carotid-jugular PTFE loop grafts were placed in pigs. After euthanasia at 2-4 weeks, the graft-venous and graft-arterial anastomoses and the adjacent blood vessels were excised en bloc and subjected to immunohistochemical analyses and in situ hybridization for CRP. The ability of CRP to stimulate proliferation was examined in cultured porcine venous SMCs using the bromodeoxyuridine assay after incubation for 48 h. RESULTS: Severe myointimal hyperplasia was found at 3 weeks after graft placement at both graft-venous and graft-arterial anastomoses. Compared to normal tissues, staining for CRP was far more intense in cells in the hyperplastic lesions at both anastomoses, which also stained positive for smooth muscle alpha-actin. In situ hybridization showed that these cells also expressed mRNA for CRP. At 1 microg/ml, CRP increased the proliferation of cultured porcine venous SMCs by 45.9+/-5.8%. CONCLUSION: CRP was produced in venous and arterial SMCs and its expression was enhanced in the hyperplastic lesions associated with arteriovenous PTFE grafts in a porcine model. Together with the ability of CRP to promote SMC proliferation, these data suggest that CRP might play a pathogenic role in the development of myointimal hyperplasia in PTFE grafts.     

Impact of endothelial cell seeding on long-term patency and subendothelial proliferation in a small-caliber highly porous polytetrafluoroethylene graft

Previous reports have demonstrated that endothelial cell seeding of polytetrafluoroethylene (PTFE) grafts enhances short-term patency. This experiment was undertaken to study its impact on the long-term patency of a highly porous, experimental PTFE graft and to determine whether increasing the internodal distance of the graft material resulted in increased proliferation of the subendothelium. Ten centimeter long, 4 mm internal diameter segments of an unreinforced, experimental PTFE graft were implanted into 36 mongrel dogs as carotid interpositions. In each animal, one graft was seeded with autologous endothelial cells, enzymatically derived from the external jugular veins, whereas the contralateral graft was treated in identical fashion except that endothelial cells were not added to the preclot mixture. Nineteen animals were killed at 12 weeks; six at 22 weeks; eight at 26 weeks; and three at 52 weeks. The mean follow-up period was 20.1 weeks. The overall patency rate was 58.3% (21 of 36 grafts) for seeded grafts vs. 27.8% (10 of 36 grafts) for control grafts (p less than 0.01). The thrombus-free area was planimetrically measured at 83.4% +/- 4.5% in seeded grafts vs. 55.1% +/- 9.7% in control grafts (p less than 0.05). Scanning electron microscopy confirmed the presence of a confluent cellular monolayer in seeded grafts, whereas control grafts exhibited a variable coagulum of fibrin, platelets, and endothelial cells. The thickness of the subendothelial layer varied from 56 to 95 micron with no progressive increase in thickness between 12 and 52 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased risk of infection associated with polyurethane dialysis grafts

Conventional polytetrafluoroethylene (PTFE) dialysis grafts cannot be cannulated for 2-3 weeks following their creation. Polyurethane grafts, made of a self-sealing material, can be cannulated within 24 hours of implantation, representing a potential advantage in patients with limited catheter options. However, early cannulation may increase the risk of graft infection. We retrospectively queried a prospective, computerized access database to identify 31 patients receiving a polyurethane graft, and 56 date-matched controls with a PTFE graft. Survival techniques were used to plot graft survival. Thrombosis-free graft survival (from creation to first thrombosis or failure) was similar for polyurethane and PTFE grafts (1-year survival, 28%, vs. 32%, p = 0.98). Cumulative graft survival (from creation to permanent failure) was also similar (1-year survival 42% vs. 52%, p = 0.40). Finally, the cumulative risk of graft infection was 37.5% for polyurethane thigh grafts, 23% for polyurethane upper extremity grafts, 21% for PTFE thigh grafts, and 5% for PTFE upper extremity grafts (p = 0.06 for polyurethane vs. PTFE grafts). The likelihood of thrombosis and failure is similar for polyurethane and PTFE grafts. However, polyurethane grafts may have a higher risk of infection, particularly when they are placed in the thigh. In patients with an access emergency, implantation of a polyurethane graft incurs a tradeoff between earlier cannulation and a higher risk of infection.     

Management of tunnel infections of dialysis polytetrafluoroethylene grafts

Extensive tunnel infections of polytetrafluoroethylene (PTFE) dialysis grafts have traditionally been managed by total excision, including venous and arterial anastomoses. This article describes an alternative vascular approach to excision of infected dialysis PTFE grafts; success occurred in 14 of 15 excised grafts in 12 patients. The surgical procedure consisted of a 2 to 3 mm oversewn cuff of PTFE left at the arterial anastomosis, with excision of the remaining graft including the venous anastomosis. The entire tunnel tract was opened, debrided, and irrigated extensively with cefazolin solution. Wounds were closed over drains. Fourteen PTFE graft excisions healed without sequelae. This small but successful series of excised, infected PTFE dialysis grafts supports the procedure to leave a small cuff of oversewn PTFE on the artery and to close the wounds primarily over drains.