Reduction of intimal and medial thickening in sheathed vein grafts

Advances in preclinical surgical therapy of cardiovascular diseases

Cardiovascular disease is the most common cause of death worldwide, resulting in millions of deaths annually. Currently, there are still some deficiencies in the treatment of cardiovascular diseases. Innovative surgical treatments are currently being developed and tested in response to this situation. Large animal models, which are similar to humans in terms of anatomy, physiology, and genetics, play a crucial role in connecting basic research and clinical applications. This article reviews recent preclinical studies and the latest clinical advancements in cardiovascular disease based on large animal models, with a focus on targeted delivery, neural regulation, cardiac remodeling, and hemodynamic regulation. It provides new perspectives and ideas for clinical translation and offers new methods for clinical treatment.

1-Year Patency of Biorestorative Polymeric Coronary Artery Bypass Grafts in an Ovine Model

Many attempts have been made to inhibit or counteract saphenous vein graft (SVG) failure modes; however, only external support for SVGs has gained momentum in clinical utility. This study revealed the feasibility of implantation, and showed good patency out to 12 months of the novel biorestorative graft, in a challenging ovine coronary artery bypass graft model. This finding could trigger the first-in-man trial of using the novel material instead of SVG. We believe that, eventually, this novel biorestorative bypass graft can be one of the options for coronary artery bypass graft patients who have difficulty harvesting SVG.

Large animal model of vein grafts intimal hyperplasia: A systematic review

Coronary artery bypass grafting remains the treatment of choice for a large cohort of patients with significant coronary disease. Despite the increased use of arterial grafts, the long saphenous vein remains the most commonly used conduit. Long-term graft patency continues to be the Achilles heel of saphenous vein grafts. This is due to the development of intimal hyperplasia, a chronic inflammatory disease that results in the narrowing and occlusion of a significant number of vein grafts. Research models for intimal hyperplasia are essential for a better understanding of pathophysiological processes of this condition. Large animal models resemble human anatomical structures and have been used as a surrogate to study disease development and prevention over the years. In this paper, we systematically review all published studies that utilized large animal models of vein graft disease with a focus on the type of model and any therapeutic intervention, specifically the use of external stents/mesh.

Device Profile of the VEST for External Support of SVG Coronary Artery Bypass Grafting: Historical Development, Current Status and Future Directions

A search for strategies to address saphenous vein graft (SVG) failure - the main factor limiting the long-term success of coronary bypass grafting - has led to trialing of external stenting technologies.

AREAS COVERED

The manuscript covers historical development and current status of external scaffolding for the treatment of SVG intimal hyperplasia. Comprehensive literature review and personal communication with VGS leadership, the developer of the VEST device, served as the sources.

EXPERT OPINION

If the external scaffolding concept proves to be successful in mitigating the intimal hyperplasia inherent to arterialized saphenous vein conduits, it could have a dramatic impact on the recurrence of anginal symptoms, the need for repeat revascularization, and the incidence of myocardial infarction following CABG surgery. These laudable sequelae could ultimately convey significant public health repercussions by reducing healthcare resource use and improving the long-term survival and quality of life of CABG recipients.

Functional remodeling of an electrospun polydimethylsiloxane-based polyether urethane external vein graft support device in an ovine model

Saphenous vein graft (SVG) failure rates are unacceptably high, and external mechanical support may improve patency. We studied the histologic remodeling of a conformal, electrospun, polydimethylsiloxane-based polyether urethane external support device for SVGs and evaluated graft structural evolution in adult sheep to 2 years. All sheep (N = 19) survived to their intended timepoints, and angiography showed device-treated SVG geometric stability over time (30, 90, 180, 365, or 730 days), with an aggregated graft patency rate of 92%. There was minimal inflammation associated with the device material at all timepoints. By 180 days, treated SVG remodeling was characterized by minimal/nonprogressive intimal hyperplasia; polymer fragmentation and integration; as well as the development of a neointima, and a confluent endothelium. By 1-year, the graft developed a media-like layer by remodeling the neointima, and elastic fibers formed well-defined structures that subtended the neo-medial layer of the remodeled SVG. Immunohistochemistry showed that this neo-media was populated with smooth muscle cells, and the intima was lined with endothelial cells. These data suggest that treated SVGs were structurally remodeled by 180 days, and developed arterial-like features by 1 year, which continued to mature to 2 years. Device-treated SVGs remodeled into arterial-like conduits with stable long-term performance as arterial grafts in adult sheep.

Review of risk factors, treatment, and prevention of saphenous vein graft disease after coronary artery bypass grafting

Saphenous vein graft disease (SVGD) is a type of vascular disease that may develop after coronary artery bypass grafting (CABG). SVGD seriously affects the short-term and long-term effects of CABG and increases the incidence of major adverse cardiovascular events. It is very important to identify patients at greatest risk and carry out prevention and treatment measures to determine the risk factors for SVGD. Many factors contribute to SVGD when the vein is grafted into an arterial environment, such as surgery-related factors, smoking, diabetes mellitus, hyperlipidemia, and others. In this review, we discuss the risk factors for SVGD, current surgical and pharmacologic therapies with which to manage SVGD, and the prevention of SVGD.

Biomaterial-Based Approaches to Address Vein Graft and Hemodialysis Access Failures

Veins used as grafts in heart bypass or as access points in hemodialysis exhibit high failure rates, thereby causing significant morbidity and mortality for patients. Interventional or revisional surgeries required to correct these failures have been met with limited success and exorbitant costs, particularly for the US Centers for Medicare & Medicaid Services. Vein stenosis or occlusion leading to failure is primarily the result of neointimal hyperplasia. Systemic therapies have achieved little long-term success, indicating the need for more localized, sustained, biomaterial-based solutions. Numerous studies have demonstrated the ability of external stents to reduce neointimal hyperplasia. However, successful results from animal models have failed to translate to the clinic thus far, and no external stent is currently approved for use in the US to prevent vein graft or hemodialysis access failures. This review discusses current progress in the field, design considerations, and future perspectives for biomaterial-based external stents. More comparative studies iteratively modulating biomaterial and biomaterial-drug approaches are critical in addressing mechanistic knowledge gaps associated with external stent application to the arteriovenous environment. Addressing these gaps will ultimately lead to more viable solutions that prevent vein graft and hemodialysis access failures.

Novel no touch technique of saphenous vein harvesting: Is great graft patency rate provided?

Coronary artery bypass grafting surgery effectively relieves signs and symptoms of myocardial ischemia. The left internal thoracic artery (LITA) graft is the gold standard having 90-95% patency rate at 10 years, whereas only 50% of saphenous vein (SV) grafts are patent at 10 years. However, there is a novel "no touch" technique in order to harvest an SV complete with its cushion of surrounding tissue, thus maintaining its endothelium-intact. Significantly superior short- and long-term graft patency rates comparable to LITA grafts can be achieved. Consequently, the SV may be revived as an important conduit in coronary artery bypass surgery.

Reinforced Long Saphenous Vein Bypass Graft for Infrainguinal Reconstruction Procedures: Case Series and Literature Review

Poor rehabilitation rates and the high-cost of managing postamputation patients justify an aggressive revascularization policy in critical lower limb ischemia. Endovascular therapy is our first choice for limb salvage in these patients. However there are patients for whom endovascular therapy is not feasible. When bypass is necessary, autologous vein is a superior conduit to synthetic material. However, varicosities usually contraindicate autologous vein bypass because of the risk of aneurysm formation, rupture and increased intimal hyperplasia compared with nonvaricose venous grafts. We report the use of varicosed long saphenous vein (LSV) with external Dacron support in infrainguinal bypass procedures for limb salvage, where endovascular therapy was not feasible. The external Dacron tube was not brought close to the distal anastomotic area itself. With a mean follow-up of 18 months, duplex ultrasonography and computed tomography angiography showed no evidence of stenosis of the reinforced vein segments or aneurysmal degeneration of the residual vein. External reinforcement with Dacron prosthesis allows the use of autogenous greater saphenous veins with varicose dilatation without compromising graft patency and limb salvage.

Numerical analysis of Venous External Scaffolding Technology for Saphenous Vein Grafts

This paper presents a method for analyzing and comparing numerically Saphenous Vein Grafts (SVGs) following Coronary Artery Bypass Graft surgery (CABG). The method analyses the flow dynamics inside vein grafts with and without supporting using Venous External Scaffolding Technology (VEST). The numerical method uses patients׳ specific computational fluid dynamics (CFD) methods to characterize the relevant hemodynamic parameters of patients׳ SVGs. The method was used to compare the hemodynamics of six patient׳s specific model and flow conditions of stented and non-stented SVGs, 12 months post-transplantation. The flow parameters used to characterize the grafts׳ hemodynamics include Time Averaged Wall Shear Stress (TAWSS), Oscillatory Shear Index (OSI) and Relative Residence Time (RRT). The effect of stenting was clearly demonstrated by the chosen parameters. SVGs under constriction of VEST were associated with similar spatial average of TAWSS (10.73 vs 10.29 dyn/cm(2)), yet had fewer lesions with low TAWSS, lower OSI (0.041 vs 0.08) and RRT (0.12 vs 0.24), and more uniform flow with less flow discrepancies. In conclusion, the suggested method and parameters well demonstrated the advantage of VEST support. Stenting vein grafts with VEST improved hemodynamic factors which are correlated to graft failure following CABG procedure.

Inhibition of neointimal formation and hyperplasia in vein grafts by external stent/sheath

Synthetic and to a lesser extent vein graft failure is still a major problem in the treatment of peripheral arterial disease, with neointimal hyperplasia being the main cause for graft occlusion in the medium and long term. This review aims to establish the current status of external stents or sheaths in the prevention of intimal hyperplasia in small diameter (< 6 mm) vein grafts.

Experimental comparison of four methods of end-to-side anastomosis with expanded polytetrafluoroethylene

Background

Four established techniques of distal end-to-side anastomosis (direct anastomosis, Linton patch, Taylor patch and Miller cuff) were compared to investigate the local distribution of anastomotic intimal hyperplasia. The study aimed to elucidate whether mechanical factors or flow alterations are mainly responsible for the improved patency rates reported for vein cuff interposition techniques in infrainguinal arterial reconstructions using prosthetic graft material.

Methods

Thirty-two expanded polytetrafluoroethylene (ePTFE) femoropopliteal bypass grafts were implanted in 16 sheep using the four anastomotic techniques. After 6 months the grafts were explanted and examined histologically. The local distribution of intimal hyperplasia was determined, particularly for areas of material transition and of high and low shear stress.

Results

The mean amount and distribution of intimal hyperplasia were similar for all anastomotic types. Intimal hyperplasia was greatest along all transitions between ePTFE and venous patches, and between ePTFE and recipient artery. It was lower along the transitions between venous patches and artery, and was lowest at the host artery floor.

Conclusion

Vein interposition did not reduce anastomotic intimal hyperplasia and did not change the distribution patterns of hyperplasia, which were influenced mainly by mechanical factors. The effect of vein interposition is to move areas of maximum intimal hyperplasia away from the small recipient artery up to the more capacious graft-patch anastomosis.

External supports and the prevention of neointima formation in vein grafts

AIMS AND METHODOLOGY

the aim of this review is to provide an overview of the aetiology of neointima formation in vein grafts and to highlight the use of an external support to modulate this phenomenon. A systematic literature review was performed via computerised search on MEDLINE, OVID and the Cochrane Library. The search terms initially employed were broad-based; "vein graft", "neointima" and "external stent". Subsequently, more specific search terms were utilised; "perivenous mesh", "external prosthesis" and "varicose vein". Articles from indexed journals relevant to the objective, external venous supports, from the earliest reports in the 1960's to the latest in 2001 were included to obtain an exhaustive list. Reviews, abstracts and proceedings of scientific meetings, case reports and the results of both animal model investigations and human clinical trials in all languages were included. Articles describing an external support employed in both peripheral and aortocoronary bypass investigations were included.

Localization of nitric oxide synthase in saphenous vein grafts harvested with a novel "no-touch" technique: potential role of nitric oxide contribution to improved early graft patency rates

OBJECTIVE

The use of the saphenous vein in coronary artery bypass graft surgery is associated with high 1-year occlusion rates of as much as 30%. A new "no-touch" technique of saphenous vein harvesting in which the vein is harvested with a pedicle of surrounding tissue and not distended may result in improved early patency rates. We hypothesize that nitric oxide synthase is better preserved with the no-touch technique, and the aim of this study was the investigation of whether nitric oxide synthase distribution and quantity in saphenous veins harvested with the no-touch technique differ from those veins harvested with the conventional technique. The separate contribution of perivascular tissue removal and distension to alterations in nitric oxide synthase was also studied.

METHODS

Segments of 10 saphenous veins were harvested from 10 patients who underwent coronary artery bypass grafting surgery with the no-touch and conventional techniques. Samples were also taken from segments that were stripped of surrounding tissue but not distended. Nitric oxide synthase distribution was studied with reduced nicotinamide adenine dinucleotide phosphate--diaphorase histochemistry, and staining was quantified with image analysis. Immunohistochemistry was used for the identification of specific nitric oxide synthase isoforms, and immunomarkers were used for the identification of associated cell types.

RESULTS

Nitric oxide synthase content was higher in no-touch vessels as compared with conventionally harvested vessels (35.5%; P <.05, with analysis of variance). This content was associated with endothelial nitric oxide synthase on the lumen while all three isoforms were present in the media. In the intact adventitia of no-touch vessels, all three isoforms of nitric oxide synthase were also present, associated with microvessels and perivascular nerves. Perivascular tissue stripping and venous distension both contribute to the reduced nitric oxide synthase in conventionally harvested veins.

CONCLUSION

The new no-touch technique of saphenous vein harvesting preserves nitric oxide synthase, which suggests that improved nitric oxide availability may be an important mechanism in the success of this technique.

Perivenous application of fibrin glue reduces early injury to the human saphenous vein graft wall in an ex vivo model

OBJECTIVES

From animal and clinical studies it is known that prevention of 'over-distention' of vein grafts by using extravascular support ameliorates the arterialization process in vein grafts with subsequent more favorable patency. The most ideal support is a biodegradable, porous, elastic graft (Biomaterials, 15 (1994) 83). However, a specific graft meeting these criteria is not available yet. Fibrin glue on the other hand, although used for other purposes in cardiac surgery, theoretically meets the criteria for ideal extravascular support. In this ex vivo study, we evaluated the possible beneficial effect of perivenous application of fibrin glue.

METHODS

Segments of human vein graft obtained during CABG procedures in 14 consecutive patients were placed in a side loop of the extracorporeal perfusion circuit. In this way the study vein grafts did meet identical circumstances as the vein grafts implanted. Perfusion in the loop was started with a flow just enough to counteract the collapse of the vein, usually about 8 mm Hg, and alternately around the segments fibrin glue was applied or no perivenous support was administered as control. After 1 min of soldification, perfusion was started with a pressure of about 60 mm Hg (non-pulsatile flow). Perfusion was maintained for 60 min, after which the grafts were collected for light microscopic and electron microscopic assessment.

RESULTS

Light microscopy and electron microscopy showed remarkable attenuation of endothelial cell loss and less injury of smooth muscle cells of the circular muscle layer of the media in the fibrin glue supported vein grafts compared to the non-supported group.

CONCLUSION

Fibrin glue is able to accomplish adequate external vein graft support, preventing overdistention, in an ex vivo model. This provides a basis for clinical application. Further investigation is necessary to evaluate long-term effects.

Impaired contractility of left-over vein grafts used for CABG and the possible trauma caused by air exposure

OBJECTIVE

The patency of vein grafts limits the long-term results after bypass surgery. Vein graft trauma may contribute to the clinical results.

DESIGN

Left-over pieces of vein after routine coronary bypass surgery were compared with references of the same vein that were stored with maximum care. Blood perfusion (20 h, 37 degrees C) was also tested to evaluate possible recovery. Vessel rings were measured by isometric tone recordings.

RESULTS

Left-over veins showed a contractile dysfunction compared to reference (p < 0.01-0.001, n = 13), and that did not recover after blood perfusion. Four trauma models were tested, using saphenous vein graft (SVG)-references (n = 5). Chemical exposures (0-90 min) to NaCl-papaverin or cardioplegic medium affected the baseline (p < 0.05-0.01). With mechanical distension, up to 450 mmHg leak testing, an increased norepinephrine (NE) reactivity appeared. Air contact, 20 min, reduced the NE reactivity to half maximum (p < 0.05). Longer exposures (40-60 min) completely abolished the contractility.

CONCLUSIONS

The saphenous vein smooth muscle function appeared permanently traumatized following routine harvesting and handling. Air exposure gave dramatic deterioration and could in theory contribute to hamper the long-term results after bypass surgery.

Prevention of venous graft sclerosis with clopidogrel and aspirin combined with a mesh tubing in a dog model of arteriovenous bypass grafting

OBJECTIVE

to investigate the effect of clopidogrel combined with aspirin or aspirin alone on fibromyointimal hyperplasia (FIMH) in a bypass model with native vein grafts (NVG) and biocompound grafts (BCG).

DESIGN

twelve Beagle dogs were randomised into two equal groups. In each animal NVG and a BCG were interposed in the common carotid arteries. Postoperatively, Group 1 received clopidogrel (200 mg/d) and aspirin (100 mg/d) and Group 2 received aspirin (100 mg/d) alone.

METHODS

the BCG was constructed by sheathing the ipsilateral jugular vein with highly flexible metal mesh tubing. After 30 days the grafts were harvested and pressure fixed. FIMH was determined by morphometry.

RESULTS

the average wall thickness of the BCG was significantly lower than that of the NVG in both groups (0.26 (SD)0.02 mm vs 0.47 (SD)0.15 mm, p = 0.04 and 0.28 (SD)0.05 mm vs 0.70 (SD)0.29 mm, p = 0.01, respectively). For BCG treated with aspirin, the wall cross section area was lower (5.0 (SD)0.6 mm(2)vs 9.1 (SD)3.3 mm(2), p = 0.02) and the lumen larger (25.2 (SD)5.9 mm(2)vs 9.7 (SD)3.4 mm(2), p < 0.01) than for the NVG. There was also a difference in the lumen cross section area of the NVG, which was larger after combined therapy with clopidogrel and aspirin (17.9 (SD)7.8 mm(2)vs 9.7 (SD)3.4 mm(2), p = 0.04).

CONCLUSIONS

in this dog model the sheathing of vein grafts effectively prevents FIMH following bypass surgery. Clopidogrel is effective in NVG.