Hypothesis: a potential role for the vasa vasorum in the maintenance of vein graft patency

An Obligatory Role of Perivascular Adipose Tissue in Improved Saphenous Vein Graft Patency in Coronary Artery Bypass Grafting

The gold standard graft for coronary artery bypass grafting (CABG) is the internal thoracic artery (ITA), and the second recommendation is the radial artery. However, complete revascularization with arterial grafts alone is often difficult, and the saphenous vein (SV) is the most commonly used autologous graft for CABG, because it is easier to use without restriction for the length of the graft. On the other hand, the patency of SV grafts (SVGs) is poor compared with that of arterial grafts. The SVG is conventionally harvested as a distended conduit with surrounding tissue removed, a procedure that may cause vascular damage. A no-touch technique of SVG harvesting has been reported to result in improved long-term patency in CABG comparable to that when using the ITA for grafting. Possible reasons for the excellent long-term patency of no-touch SVGs are the physical support provided by preserved surrounding perivascular adipose tissue, preservation of the vascular wall structure including the vasa vasorum, and production of adipocyte-derived factors. In this review, we discuss recent strategies aimed at improving the performance of SVGs, including no-touch harvesting, minimally invasive harvesting and mechanical support using external stents.

Progression of Arterial Vasa Vasorum from Regulator of Arterial Homeostasis to Promoter of Atherogenesis

The vasa vasorum (vessels of vessels) are a dynamic microvascular system uniquely distributed to maintain physiological homeostasis of the artery wall by supplying nutrients and oxygen to the outer layers of the artery wall, adventitia, and perivascular adipose tissue, and in large arteries, to the outer portion of the medial layer. Vasa vasorum endothelium and contractile mural cells regulate direct access of bioactive cells and factors present in both the systemic circulation and the arterial perivascular adipose tissue and adventitia to the artery wall. Experimental and human data show that proatherogenic factors and cells gain direct access to the artery wall via the vasa vasorum and may initiate, promote, and destabilize the plaque. Activation and growth of vasa vasorum occur in all blood vessel layers primarily by angiogenesis, producing fragile and permeable new microvessels that may cause plaque hemorrhage and fibrous cap rupture. Ironically, invasive therapies, such as angioplasty and coronary artery bypass grafting, injure the vasa vasorum, leading to treatment failures. The vasa vasorum function both as a master integrator of arterial homeostasis and, once perturbed or injured, as a promotor of atherogenesis. Future studies need to be directed at establishing reliable in vivo and in vitro models to investigate the cellular and molecular regulation of the function and dysfunction of the arterial vasa vasorum.

Building a Scaffold for Arteriovenous Fistula Maturation: Unravelling the Role of the Extracellular Matrix

Vascular access is the lifeline for patients receiving haemodialysis as kidney replacement therapy. As a surgically created arteriovenous fistula (AVF) provides a high-flow conduit suitable for cannulation, it remains the vascular access of choice. In order to use an AVF successfully, the luminal diameter and the vessel wall of the venous outflow tract have to increase. This process is referred to as AVF maturation. AVF non-maturation is an important limitation of AVFs that contributes to their poor primary patency rates. To date, there is no clear overview of the overall role of the extracellular matrix (ECM) in AVF maturation. The ECM is essential for vascular functioning, as it provides structural and mechanical strength and communicates with vascular cells to regulate their differentiation and proliferation. Thus, the ECM is involved in multiple processes that regulate AVF maturation, and it is essential to study its anatomy and vascular response to AVF surgery to define therapeutic targets to improve AVF maturation. In this review, we discuss the composition of both the arterial and venous ECM and its incorporation in the three vessel layers: the tunica intima, media, and adventitia. Furthermore, we examine the effect of chronic kidney failure on the vasculature, the timing of ECM remodelling post-AVF surgery, and current ECM interventions to improve AVF maturation. Lastly, the suitability of ECM interventions as a therapeutic target for AVF maturation will be discussed.

On vasa vasorum: A history of advances in understanding the vessels of vessels

The vasa vasorum are a vital microvascular network supporting the outer wall of larger blood vessels. Although these dynamic microvessels have been studied for centuries, the importance and impact of their functions in vascular health and disease are not yet fully realized. There is now rich knowledge regarding what local progenitor cell populations comprise and cohabitate with the vasa vasorum and how they might contribute to physiological and pathological changes in the network or its expansion via angiogenesis or vasculogenesis. Evidence of whether vasa vasorum remodeling incites or governs disease progression or is a consequence of cardiovascular pathologies remains limited. Recent advances in vasa vasorum imaging for understanding cardiovascular disease severity and pathophysiology open the door for theranostic opportunities. Approaches that strive to control angiogenesis and vasculogenesis potentiate mitigation of vasa vasorum-mediated contributions to cardiovascular diseases and emerging diseases involving the microcirculation.

What is the impact of preserving the endothelium on saphenous vein graft performance? Comments on the 'NO' touch harvesting technique

Saphenous veins used for coronary artery bypass surgery are subjected to considerable vascular trauma when harvested by conventional methods. This vascular damage is responsible, at least in part, for the inferior patency of the saphenous vein when compared with the internal thoracic artery. The performance of saphenous vein grafts is improved when this conduit is harvested atraumatically using the no-touch technique. There is growing evidence that the success of the no-touch technique is due to the preservation of a number of vascular structures including the endothelium, vasa vasorum and perivascular fat. There is conflicting evidence regarding the degree of endothelial damage to the endothelium of conventional versus no-touch saphenous vein grafts. In general, it has been shown that this single layer of cells lining the lumen exhibits considerable damage associated with a combination of vascular trauma and high pressure intraluminal distension. Increased platelet aggregation and thrombus formation at the exposed subendothelial membrane is due to a local reduction of endothelium-derived factors including nitric oxide. In addition, damage to the vasa vasorum of conventionally-harvested veins will reduce transmural blood flow, a condition shown to promote neointimal hyperplasia and atheroma formation. By stripping off the perivascular fat during conventional harvesting, mechanical support of the graft is reduced and the source of adipocyte-derived factors potentially beneficial for graft patency removed. While most agree that endothelial damage to the saphenous vein affects graft patency, the contribution of other tissue-derived factors affected by vascular damage at harvesting need to be considered.

Comparison of alternate preparative techniques on wall thickness in coronary artery bypass grafts: The HArVeST randomized controlled trial

BACKGROUND

The success of coronary artery bypass grafting surgery (CABG) is dependent on long-term graft patency, which is negatively related to early wall thickening. Avoiding high-pressure distension testing for leaks and preserving the surrounding pedicle of fat and adventitia during vein harvesting may reduce wall thickening.

METHODS

A single-centre, factorial randomized controlled trial was carried out to compare the impact of testing for leaks under high versus low pressure and harvesting the vein with versus without the pedicle in patients undergoing CABG. The primary outcomes were graft wall thickness, as indicator of medial-intimal hyperplasia, and lumen diameter assessed using intravascular ultrasound after 12 months.

RESULTS

Ninety-six eligible participants were recruited. With conventional harvest, low-pressure testing tended to yield a thinner vessel wall compared with high-pressure (mean difference [MD; low minus high] -0.059 mm, 95% confidence interval (CI) -0.12, +0.0039, p = .066). With high pressure testing, veins harvested with the pedicle fat tended to have a thinner vessel wall than those harvested conventionally (MD [pedicle minus conventional] -0.057 mm, 95% CI: -0.12, +0.0037, p = .066, test for interaction p = .07). Lumen diameter was similar across groups (harvest comparison p = .81; pressure comparison p = .24). Low-pressure testing was associated with fewer hospital admissions in the 12 months following surgery (p = .0008). Harvesting the vein with the pedicle fat was associated with more complications during the index admission (p = .0041).

CONCLUSIONS

Conventional saphenous vein graft preparation with low-pressure distension and harvesting the vein with a surrounding pedicle yielded similar graft wall thickness after 12 months, but low pressure was associated with fewer adverse events.

A Brief Comment on Vasa Vasorum of Human Saphenous Vein: relevance for Coronary Artery Bypass Surgery

The importance of the vasa vasorum and blood supply to the wall of human saphenous vein (hSV) used for coronary artery bypass grafting (CABG) is briefly discussed. This is in the context of the possible physical link of the vasa vasorum connecting with the lumen of hSV and the anti-ischaemic impact of this microvessel network in the hSV used for CABG.

Saphenous Vein Vasa Vasorum as a Potential Target for Perivascular Fat-Derived Factors

Perivascular adipose tissue (PVAT) is a source of factors affecting vasomotor tone with the potential to play a role in the performance of saphenous vein (SV) bypass grafts. As these factors have been described as having constrictor or relaxant effects, they may be considered either beneficial or detrimental. The close proximity of PVAT to the adventitia provides an environment whereby adipose tissue-derived factors may affect the vasa vasorum, a microvascular network providing the vessel wall with oxygen and nutrients. Since medial ischaemia promotes aspects of graft occlusion the involvement of the PVAT/vasa vasorum axis in vein graft patency should be considered.

COVID-19 - Endothelial Axis and Coronary Artery Bypass Graft Patency: a Target for Therapeutic Intervention?

It has been reported that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection induces endothelial inflammation, therefore facilitating the progression of endothelial and vascular dysfunction in coronavirus disease 2019 (COVID-19) patients. Coronary artery bypass grafting (CABG) involves mainly the use of the saphenous vein (SV) and internal mammary artery as graft material in the stenosed coronary arteries. Unfortunately, graft patency of the SV is low due to endothelial dysfunction and inflammation. We propose that SARS-CoV-2 might cause vascular inflammation, endothelial dysfunction, and thrombosis in coronary artery bypass graft vessels by binding angiotensin-converting enzyme 2 receptor. Therefore, in this Special Article, we consider the potential influence of COVID-19 on the patency rates of coronary artery bypass graft vessels, mainly with reference to the SV. Moreover, we discuss the technique of SV graft harvesting and the therapeutic potential of focusing on endothelial dysfunction, vascular inflammation, and thrombosis for protecting coronary artery bypass grafts in COVID-19 infected CABG patients.

The Role of Immunomodulation in Vein Graft Remodeling and Failure

Obstructive arterial disease is a major cause of morbidity and mortality in the developed world. Venous bypass graft surgery is one of the most frequently used revascularization strategies despite its considerable short and long time failure rate. Due to vessel wall remodeling, inflammation, intimal hyperplasia, and accelerated atherosclerosis, vein grafts may (ultimately) fail to revascularize tissues downstream to occlusive atherosclerotic lesions. In the past decades, little has changed in the prevention of vein graft failure (VGF) although new insights in the role of innate and adaptive immunity in VGF have emerged. In this review, we discuss the pathophysiological mechanisms underlying the development of VGF, emphasizing the role of immune response and associated factors related to VG remodeling and failure. Moreover, we discuss potential therapeutic options that can improve patency based on data from both preclinical studies and the latest clinical trials. This review contributes to the insights in the role of immunomodulation in vein graft failure in humans. We describe the effects of immune cells and related factors in early (thrombosis), intermediate (inward remodeling and intimal hyperplasia), and late (intimal hyperplasia and accelerated atherosclerosis) failure based on both preclinical (mouse) models and clinical data.

Vascularization of the arteriovenous fistula wall and association with maturation outcomes

BACKGROUND AND OBJECTIVES

The venous vasa vasorum is the mesh of microvessels that provide oxygen and nutrients to the walls of large veins. Whether changes to the vasa vasorum have any effects on human arteriovenous fistula outcomes remains undetermined. In this study, we challenged the hypothesis that inadequate vascularization of the arteriovenous fistula wall is associated with maturation failure.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

This case-control pilot study includes pre-access veins and arteriovenous fistula venous samples (i.e. tissue pairs) from 30 patients undergoing two-stage arteriovenous fistula creation (15 matured and 15 failed to mature). Using anti-CD31 immunohistochemistry, we quantified vasa vasorum density and luminal area (vasa vasorum area) in the intima, media, and adventitia of pre-access veins and fistulas. We evaluated the association of pre-existing and postoperative arteriovenous fistula vascularization with maturation failure and with postoperative morphometry.

RESULTS

Vascularization of veins and arteriovenous fistulas was predominantly observed in the outer media and adventitia. Only the size of the microvasculature (vasa vasorum area), but not the number of vessels (vasa vasorum density), increased after arteriovenous fistula creation in the adventitia (median vasa vasorum area 1366 µm2/mm2 (interquartile range 495-2582) in veins versus 3077 µm2/mm2 (1812-5323) in arteriovenous fistulas, p < 0.001), while no changes were observed in the intima and media. Postoperative intimal thickness correlated with lower vascularization of the media (r 0.53, p = 0.003 for vasa vasorum density and r 0.37, p = 0.045 for vasa vasorum area). However, there were no significant differences in pre-existing, postoperative, or longitudinal change in vascularization between arteriovenous fistulas with distinct maturation outcomes.

CONCLUSION

The lack of change in intimal and medial vascularization after arteriovenous fistula creation argues against higher oxygen demand in the inner walls of the fistula during the vein to arteriovenous fistula transformation. Postoperative intimal hyperplasia in the arteriovenous fistula wall appears to thrive under hypoxic conditions. Vasa vasorum density and area by themselves are not predictive of maturation outcomes.

Twenty-Five Years of No-Touch Saphenous Vein Harvesting for Coronary Artery Bypass Grafting: Structural Observations and Impact on Graft Performance

The saphenous vein is the most common conduit used in coronary artery bypass grafting (CABG) yet its failure rate is higher compared to arterial grafts. An improvement in saphenous vein graft performance is therefore a major priority in CABG. No-touch harvesting of the saphenous vein is one of the few interventions that has shown improved patency rates, comparable to that of the left internal thoracic artery. After more than two decades of no-touch research, this technique is now recognized as a Class IIa recommendation in the 2018 European Society of Cardiology and the European Association for Cardio-Thoracic Surgery guidelines on myocardial revascularization. In this review, we describe the structural alterations that occur in conventional versus no-touch saphenous vein grafts and how these changes affect graft patency. In addition, we discuss various strategies aimed at repairing saphenous vein grafts prepared at conventional CABG.

Biomechanics and Mechanobiology of Saphenous Vein Grafts

Within several weeks of use as coronary artery bypass grafts (CABG), saphenous veins (SV) exhibit significant intimal hyperplasia (IH). IH predisposes vessels to thrombosis and atherosclerosis, the two major modes of vein graft failure. The fact that SV do not develop significant IH in their native venous environment coupled with the rapidity with which they develop IH following grafting into the arterial circulation suggests that factors associated with the isolation and preparation of SV and/or differences between the venous and arterial environments contribute to disease progression. There is strong evidence suggesting that mechanical trauma associated with traditional techniques of SV preparation can significantly damage the vessel and might potentially reduce graft patency though modern surgical techniques reduces these injuries. In contrast, it seems possible that modern surgical technique, specifically endoscopic vein harvest, might introduce other mechanical trauma that could subtly injure the vein and perhaps contribute to the reduced patency observed in veins harvested using endoscopic techniques. Aspects of the arterial mechanical environment influence remodeling of SV grafted into the arterial circulation. Increased pressure likely leads to thickening of the medial wall but its role in IH is less clear. Changes in fluid flow, including increased average wall shear stress, may reduce IH while disturbed flow likely increase IH. Nonmechanical stimuli, such as exposure to arterial levels of oxygen, may also have a significant but not widely recognized role in IH. Several potentially promising approaches to alter the mechanical environment to improve graft patency are including extravascular supports or altered graft geometries are covered.

Vasa vasorum inside out/outside in communication: a potential role in the patency of saphenous vein coronary artery bypass grafts

The saphenous vein (SV) is the most commonly used conduit for revascularization in patients undergoing coronary artery bypass surgery (CABG). The patency rate of this vessel is inferior to the internal thoracic artery (ITA). In the majority of CABG procedures the ITA is removed with its outer pedicle intact whereas the (human) SV (hSV) is harvested with pedicle removed. The vasa vasorum, a microvessel network providing the adventitia and media with oxygen and nutrients, is more pronounced and penetrates deeper towards the lumen in veins than in arteries. When prepared in conventional CABG the vascular trauma caused when removing the hSV pedicle damages the vasa vasorum, a situation affecting transmural flow potentially impacting on graft performance. In patients, where the hSV is harvested with pedicle intact, the vasa vasorum is preserved and transmural blood flow restored at graft insertion and completion of CABG. By maintaining blood supply to the hSV wall, apart from oxygen and nutrients, the vasa vasorum may also transport factors potentially beneficial to graft performance. Studies, using either corrosion casts or India ink, have shown the course of vasa vasorum in animal SV as well as in hSV. In addition, there is some evidence that vasa vasorum of hSV terminate in the vessel lumen based on ex vivo perfusion, histological and ultrastructural studies. This review describes the preparation of the hSV as a bypass conduit in CABG and its performance compared with the ITA as well as how and why its patency might be improved by harvesting with minimal trauma in a way that preserves an intact vasa vasorum.

Vasa vasorum: an old term with new problems

PURPOSE

The aim of the study was to describe the origin of the Latin anatomical term vasa vasorum and its role in current medical research and to present examples of grammatical errors in its use.

METHODS

Literary searches oriented on the term vasa vasorum were used to identify publications using it in the medical literature from the seventeenth century up to the present.

RESULTS

The Latin term vasa vasorum was introduced by Ludwig in 1739. The vasa vasorum became an important topic in clinical research around the middle of the twentieth century, with implications in angiology, cardiology and cardiosurgery. We report 18 grammatical errors concerning the use of the term vasa vasorum, starting from the year 1959. A similar decline in the correct use of Latin terminology is also evident in other medical research disciplines.

CONCLUSIONS

The numerous errors found in the use of Latin terminology in recent medical literature have occurred as a consequence of decreased use of Latin in the medical community. The only way to improve this situation is by improving awareness of international standard anatomical terminology, which is available worldwide in both Latin and English.

Silk fibroin-based woven endovascular prosthesis with heparin surface modification

A novel seamless silk fibroin-based endovascular prosthesis (SFEPs) with bifurcated woven structure and anticoagulant function for the improvement of patency is described. The SFEPs were prepared from silk fibroin (SF) and polyester filaments using an installed weaving machine. The production processing parameters were optimized using orthogonal design methods. The inner surface of SFEPs was modified with polyethylenimine (PEI) and EDC/NHS-activated low-molecular-weight heparin (LMWH) to enhance anticoagulant function. The surface morphology and mechanical properties of the SFEPs were evaluated according to standard protocols. The thickness of modified SFEPs was lower than 0.085 ± 0.004 mm and water permeability was lower than 5.19 ± 0.30 mL/(cm² × min). The results of mechanical properties showed that the diametral tensile strength and burst strength reached 61.6 ± 1.8 and 23.7 ± 2.2 MPa, respectively. Automatic coagulometer and energy-dispersive X-ray (EDX) confirmed LMWH immobilization on the surface of the SFEPs and the blood compatibility was improved with the heparin modification with PEI polymerization. In conclusion, the new prosthesis has potential applications in the blood vessel repairs where minimal thickness but superior mechanical strength and biocompatibility are important.

Exercise-induced saphenous vein graft spasm prevented by stenting

Recurrence of anginal symptoms following coronary artery bypass surgery is usually secondary to graft closure or progression of native vessel disease. The present case demonstrates severe exercise-induced saphenous vein graft (SVG) spasm associated with transmural ischemia refractory to maximal vasodilator therapy. Symptoms resolved and exercise electrocardiography normalized following stenting of SVG regions demonstrating spasm. © 2017 Wiley Periodicals, Inc.

No-Touch Technique for Radiocephalic Arteriovenous Fistula – Surgical Technique and Preliminary Results

Purpose

The radiocephalic arteriovenous fistula (RC-AVF) has significant failure rates due to occlusions and failure to mature. The size and quality of the veins are considerable limiting factors for the procedure. The aim of this pilot study was to describe the No-Touch technique (NTT) to create RC-AVF and present the results up to 1 year of follow-up.

Methods

Thirty-one consecutive patients who were referred for surgery for a RC-AVF were included (17 men, mean age 63 years, range 35-84) and operated by NTT where the vein and artery were dissected with a tissue cushion around it. Twenty-two patients had small veins or arteries (≤2 mm), 12 patients had a small cephalic vein (≤2 mm), and the mean distal cephalic vein diameter was 2.4 mm (range 1.0-4.1 mm).

Results

Technical surgical success and immediate patency were obtained in all patients. Clinical success was achieved in 23 of the 27 (85%) patients who required hemodialysis. The proportion of primary patency at 30 days and 6 months was 84% and 64%, respectively. Secondary patency at 30 days and 6 months was 97% and 83%, respectively. At 1-year follow-up, primary patency was 54% and secondary patency was 80%. There was no major difference in patency due to preoperative vein diameter.

Conclusions

The results of this study indicate that NTT can be used for primary radio-cephalic fistula surgery with very good results. This method offers the potential to create a RC-AVF in patients who are not usually considered appropriate for a distal arm fistula due to a small cephalic vein.

Characterization of the vasa vasorum in the human great saphenous vein: a scanning electron microscopy and 3D-morphometry study using vascular corrosion casts

The vasa vasorum (VV) of explanted segments of the human great saphenous vein (Vena saphena magna; HGSV), harvested during dissection for coronary bypass grafts or diseased vein segments from the "Salzburger Landesklinikum," were studied by scanning electron microscopy and three-dimensional morphometry of microvascular corrosion casts. The main objective of this study was to examine the VV's structural arrangement in order to find the most vital segments of the HGSV and in turn to improve the results of coronary bypass surgeries. The study presents a meticulous analysis of the whole microvascular system of the VV of the HGSV and its three-dimensional arrangement. It is one of the first studies yielding detailed quantitative data on geometry of the VV of the HGSV. A detailed insight into different vascular parameters such as vessel diameter, interbranching, intervascular distances, and branching angles at different levels of the VV's angioarchitecture and in different parts of the HGSV in health and disease is given. Further, the geometry of bifurcations was examined in order to compute the physiological optimality principles of this delicate vascular system based on its construction, maintenance, and function.

Vein graft failure

After the creation of an autogenous lower extremity bypass graft, the vein must undergo a series of dynamic structural changes to stabilize the arterial hemodynamic forces. These changes, which are commonly referred to as remodeling, include an inflammatory response, the development of a neointima, matrix turnover, and cellular proliferation and apoptosis. The sum total of these processes results in dramatic alterations in the physical and biomechanical attributes of the arterialized vein. The most clinically obvious and easily measured of these is lumen remodeling of the graft. However, although somewhat less precise, wall thickness, matrix composition, and endothelial changes can be measured in vivo within the healing vein graft. Recent translational work has demonstrated the clinical relevance of remodeling as it relates to vein graft patency and the systemic factors influencing it. By correlating histologic and molecular changes in the vein, insights into potential therapeutic strategies to prevent bypass failure and areas for future investigation are explored.

'No-touch' saphenous vein harvesting improves graft performance in patients undergoing coronary artery bypass surgery: a journey from bedside to bench

The saphenous vein is the most commonly used conduit in patients undergoing coronary artery bypass surgery yet its patency is inferior to the internal thoracic artery. Vascular damage inflicted to the vein when using conventional harvesting techniques affects its structure. Endothelial denudation is associated with early vein graft failure while damage of the outermost vessel layers has adverse long-term effects on graft performance. While many in vitro and in vivo experimental studies aimed at improving vein graft patency have been performed to date no significant 'bench to bedside' advances have been made. Among experimental strategies employed is the use of pharmacological agents, gene targeting and external stents. A 'no-touch' technique, where the saphenous vein is removed with minimal trauma and normal architecture preserved, produces a superior graft with long term patency comparable to the internal thoracic artery. Interestingly, many experimental studies are aimed at repairing or replacing those regions of the saphenous vein damaged when harvesting conventionally. 'No-touch' harvesting is superior in coronary artery bypass patients with long-term data published 5years ago. Here we describe a 'bedside to bench' situation where the mechanisms underlying the improved performance of 'no touch' saphenous vein grafts in patients have been studied in the laboratory.

The vasa vasorum and associated endothelial nitric oxide synthase is more important for saphenous vein than arterial bypass grafts

No-touch (NT) saphenous vein (SV) grafts are superior to SVs harvested by the conventional technique (CT), with a patency comparable with the internal thoracic artery (ITA). Preservation of the vasa vasorum is implicated in the success of NT harvesting. We compared the vasa vasorum and endothelial nitric oxide synthase (eNOS) in NT SV with ITA and radial artery (RA) grafts. Skeletonized SV (SSV) was also analyzed. The NT SV had a higher number and larger vasa vasorum compared with ITA (P = .0001) and RA (P = .0004) that correlated with eNOS protein. Activity of eNOS in SSV grafts was significantly lower than NT SV grafts (P = 004). Since a high proportion of the vasa vasorum are removed in SSV using the CT, we suggest that preservation of the vasa vasorum and eNOS-derived NO contributes to the high patency for NT as compared with SSV grafts.

Vasa vasorum quantification in human varicose great and small saphenous veins

Recent research regarding saphenous vasa vasorum (VV) has focused on two main topics: the VV during varicogenesis in chronic venous insufficiency and the VV in saphenous grafts used in reconstructive vascular surgery. Our aim has been (i) to establish a technique for the histological quantification of the VV in human varicose great and small saphenous veins and (ii) to describe the density and distribution of the vasa vasorum within varicose veins. Great (n=11) and small (n=5) saphenous veins (length, 15-40cm) were collected from 12 patients who were undergoing venous stripping due to chronic venous insufficiency (Clinical-Etiology-Anatomy-Pathophysiology class 2-3). The veins were divided into 5-cm long segments. In total, 92 tissue blocks were collected to trace the variability of the density and distribution of the vasa vasorum in the proximo-distal direction. The endothelium was detected by immunohistochemistry using the von Willebrand factor. We quantified the number of microvessel profiles per section area and the relative distance of the microvessels from the outer border of the adventitia. The VV did not exhibit a preferential orientation in the varicose veins. VV density profiles were highest in the middle third of the venous wall and lowest in the inner third of the venous wall. Both the density and distribution of VV were uniform along the veins, and no differences were observed between the great and small saphenous veins. The VV density was statistically independent of the relative distance from the adventitia. The usability of this technique for perioperative frozen sections remains to be tested.

Vein graft failure: current clinical practice and potential for gene therapeutics

Autologous saphenous vein is commonly used as a conduit to bypass atherosclerotic lesions in coronary and femoral arteries. Despite the wide use of arterial conduits, which are less susceptible to complications and failure, as alternative conduits, the saphenous vein will continue to be used in coronary artery bypass grafting until acceptable alternative approaches are evaluated. Hence, preservation of vein graft patency is essential for the long-term success. Gene therapy is attractive in this setting as an ex-vivo technology to genetically manipulate the conduit before grafting. The use of safe and efficient vectors for delivery is a necessity as well as a strategy to improve patency in the long term. Here, we review the current clinical practice, the pathogenesis of bypass graft failure and adenovirus-mediated gene therapy strategies designed to improve late vein graft failure by modulation of smooth muscle cells in the vein wall.

Tissue-engineered vascular adventitia with vasa vasorum improves graft integration and vascularization through inosculation

Tissue-engineered blood vessel is one of the most promising living substitutes for coronary and peripheral artery bypass graft surgery. However, one of the main limitations in tissue engineering is vascularization of the construct before implantation. Such a vascularization could play an important role in graft perfusion and host integration of tissue-engineered vascular adventitia. Using our self-assembly approach, we developed a method to vascularize tissue-engineered blood vessel constructs by coculturing endothelial cells in a fibroblast-laden tissue sheet. After subcutaneous implantation, enhancement of graft integration within the surrounding environment was noted after 48 h and an important improvement in blood circulation of the grafted tissue at 1 week postimplantation. The distinctive branching structure of end arteries characterizing the in vivo adventitial vasa vasorum has also been observed in long-term postimplantation follow-up. After a 90-day implantation period, hybrid vessels containing human and mouse endothelial cells were still perfused. Characterization of the mechanical properties of both control and vascularized adventitia demonstrated that the ultimate tensile strength, modulus, and failure strain were in the same order of magnitude of a pig coronary artery. The addition of a vasa vasorum to the tissue-engineered adventitia did not influence the burst pressure of these constructs. Hence, the present results indicate a promising answer to the many challenges associated with the in vitro vascularization and in vivo integration of many different tissue-engineered substitutes.

Arterial pO₂ stimulates intimal hyperplasia and serum stimulates inward eutrophic remodeling in porcine saphenous veins cultured ex vivo

Ex vivo culture of arteries and veins is an established tool for investigating mechanically induced remodeling. Porcine saphenous veins (PSV) cultured ex vivo with a venous mechanical environment, serum-supplemented cell-culture medium and standard cell-culture conditions (5% CO₂ and 95% balance air ~140 mmHg pO₂) develop intimal hyperplasia (IH), increased cellular proliferation, decreased compliance and exhibit inward eutrophic remodeling thereby suggesting that nonmechanical factors stimulate some changes observed ex vivo. Herein we explore the contribution of exposure to greater than venous pO₂ and serum to these changes in cultured veins. Removing serum from culture medium did not inhibit development of IH, but did reduce cellular proliferation and inward eutrophic remodeling. In contrast, veins perfused using reduced pO₂ (75 mmHg) showed reduced IH. Among the statically cultured vessels, veins cultured at arterial pO₂ (95 mmHg) and above showed IH as well as increase in proliferation and vessel weight compared to fresh veins; veins cultured at venous pO₂ did not. Taken together, these data suggest that exposure of SV to arterial pO₂ stimulates IH and cellular proliferation independent of changes in the mechanical environment, which might provide insight into the etiology of IH in SV used as arterial grafts.

The "no-touch" harvesting technique for vein grafts in coronary artery bypass surgery preserves an intact vasa vasorum

OBJECTIVES

Our objective was to evaluate the impact of vein graft harvesting technique on structure and function of vasa vasorum.

METHODS

Paired segments of great saphenous veins harvested either with conventional harvesting technique or no-touch technique were obtained from 9 consecutive patients undergoing coronary artery bypass grafting. Quantitative measurements, using immunohistochemistry and morphometry, were performed. Ultrastructural analyses of vasa vasorum were performed with electron microscopy. Video footage of superficial vasa vasorum in an implanted saphenous vein graft harvested with the no-touch technique was captured during a coronary bypass operation and is presented for online viewing.

RESULTS

The total area of vasa vasorum in vein grafts harvested with the conventional technique was significantly reduced both in the media (P = .007) and in the adventitia (P = .014) compared with vein grafts harvested with the no-touch technique. Ultrastructural findings indicated that the no-touch technique preserved an intact vasa vasorum whereas the conventional technique did not. Video footage showed retrograde flow in the vasa vasorum in vein graft harvested with the no-touch technique.

CONCLUSIONS

These findings show that the no-touch technique for saphenous vein graft harvesting for coronary bypass grafting preserves an intact vasa vasorum. This could represent one of the mechanisms underlying the improved patency of saphenous vein grafts harvested with this technique.

Endothelin-1 and vein graft occlusion in patients undergoing bypass surgery

The saphenous vein is the most commonly used graft for revascularization procedures in patients with coronary artery disease and critical limb ischaemia. However, the patency rate of this vessel is poor, with a high proportion of patients requiring further surgery. Early graft occlusion is caused by vasoconstriction or thrombus formation, with later stages of graft failure being due to neointimal formation or atherosclerosis. Apart from its potent constrictor action, endothelin-1 is also a potent proliferative and proinflammatory peptide that is implicated in a number of vascular diseases. The surgical trauma caused during preparation of the saphenous vein as a bypass graft stimulates the release of a number of factors affecting vascular reactivity and structure, including endothelin-1. Endothelin-1 not only constricts animal and human isolated saphenous vein segments but also causes vascular smooth muscle proliferation and neointimal thickening in vitro, actions that are mediated via endothelin (A and B) receptors. Experimentally, the effects of subtype-selective and dual receptor antagonists have been shown to inhibit endothelin-1-mediated constriction and cell proliferation of the saphenous vein. In this review, data supporting a role of endothelin-1 in vein graft occlusion are presented, and the therapeutic potential of endothelin receptor antagonists in improving graft performance is discussed.

Retaining perivascular tissue of human saphenous vein grafts protects against surgical and distension-induced damage and preserves endothelial nitric oxide synthase and nitric oxide synthase activity

OBJECTIVE

Conventional harvesting of saphenous vein used for coronary artery bypass surgery induces a vasospasm that is overcome by high-pressure distension. Saphenous vein harvested with its cushion of perivascular tissue by a "no touch" technique does not undergo vasospasm and distension is not required, leading to an improved graft patency. The aim of this study is to investigate the effect of surgical damage and high-pressure distension on endothelial integrity and endothelial nitric oxide synthase expression and activity in saphenous vein harvested with and without perivascular tissue.

METHODS

Saphenous veins from patients (n = 26) undergoing coronary artery bypass surgery were prepared with and without perivascular tissue. We analyzed the effect of 300 mm Hg distension on morphology and endothelial nitric oxide synthase/nitric oxide synthase activity using a combination of immunohistochemistry, Western blot analysis, reverse transcriptase polymerase chain reaction, and enzyme assay in distended (with and without perivascular tissue) compared with nondistended (with and without perivascular tissue) segments.

RESULTS

Distension induced substantial damage to the luminal endothelium (assessed by CD31 staining) and vessel wall. Endothelial nitric oxide synthase expression and activity were significantly reduced by high-pressure distension and removal of, or damage to, perivascular tissue. The effect of distension was significantly less for those with perivascular tissue than for those without perivascular tissue in most cases.

CONCLUSION

The success of the saphenous vein used as a bypass graft is affected by surgical trauma and distension. Veins removed with minimal damage exhibit increased patency rates. We show that retention of perivascular tissue on saphenous vein prepared for coronary artery bypass surgery by the "no touch" technique protects against distension-induced damage, preserves vessel morphology, and maintains endothelial nitric oxide synthase/nitric oxide synthase activity.

The dynamic vasa vasorum

The function of vasa vasorum is both to deliver nutrients and oxygen to arterial and venous walls and to remove "waste" products, either produced by cells in the wall or introduced by diffusional transport through the endothelium of the artery or vein. Although the relationship between changes in vasa vasorum characteristics and the development of atheromatous plaques is well documented, the role of vasa vasorum, especially in terms of their appearance and disappearance in disease processes such as atherosclerosis, are still not clearly understood in terms of their being causative or merely reactive. However, even if their proliferation is merely reactive, these new microvessels may be a source of disease progression by virtue of endothelial impairment and as a pathway for monocytic cells to migrate to sites of early disease. As both these features are aspects of the vasa vasorum function, this Review focuses on the following issues: 1) acute modulation of vasa vasorum patency due to surrounding compressive forces within vessel wall and due to variable tone in the smooth muscle within proximal vasa vasorum and 2) chronic angiogenic responses due to local cytokine accumulations such as occur in the wall of arteries in the presence of hypertension, hypercholesterolemia, accumulation of lipids, extravasated blood products (e.g., red blood cells, macrophages, inflammatory products) which attract monocytes, and response of vasa vasorum to pharmacological stimuli.

Yin Yang-1 inhibits vascular smooth muscle cell growth and intimal thickening by repressing p21WAF1/Cip1 transcription and p21WAF1/Cip1-Cdk4-cyclin D1 assembly

Vascular injury initiates a cascade of phenotype-altering molecular events. Transcription factor function in this process, particularly that of negative regulators, is poorly understood. We demonstrate here that the forced expression of the injury-inducible GLI-Krüppel zinc finger protein Yin Yang-1 (YY1) inhibits neointima formation in human, rabbit and rat blood vessels. YY1 inhibits p21(WAF1/Cip1) transcription, prevents assembly of a p21(WAF1/Cip1)-cdk4-cyclin D1 complex, and blocks downstream pRb(Ser249/Thr252) phosphorylation and expression of PCNA and TK-1. Conversely, suppression of endogenous YY1 elevates levels of p21(WAF1/Cip1), PCNA, pRb(Ser249/Thr252) and TK-1, and increases intimal thickening. YY1 binds Sp1 and prevents its occupancy of a distinct element in the p21(WAF1/Cip1) promoter without YY1 itself binding the promoter. Additionally, YY1 induces ubiquitination and proteasome-dependent degradation of p53, decreasing p53 immunoreactivity in the artery wall. These findings define a new role for YY1 as both an inducer of p53 instability in smooth muscle cells, and an indirect repressor of p21(WAF1/Cip1) transcription, p21(WAF1/Cip1)-cdk4-cyclin D1 assembly and intimal thickening.

Does periadventitial fat-derived nitric oxide play a role in improved saphenous vein graft patency in patients undergoing coronary artery bypass surgery?

BACKGROUND/AIMS

The saphenous vein is commonly used for coronary artery bypass surgery but its patency is poor. Vascular damage occurs during conventional surgery. However, patency improves when the graft is harvested with minimal surgical trauma, partly due to preservation of vascular endothelial nitric oxide synthase (eNOS) and tissue sources of nitric oxide (NO), a factor possessing both dilatory and anti-proliferative properties. Apart from these grafts exhibiting an intact luminal endothelium they are harvested complete with a surrounding cushion of tissue, much of which is fat.

METHODS

Immunostaining for eNOS was performed on vein graft sections and reverse-transcriptase polymerase chain reaction and Western blotting were used to identify eNOS mRNA and protein. NO synthase activity was measured using the citrulline assay.

RESULTS

Immunohistochemistry identified eNOS staining of vein graft segments, including dense staining of the cushion of perivascular fat and associated structures surrounding the vein. eNOS protein was confirmed in both the vein and surrounding fat by Western blot analysis. Using the citrulline assay, the perivascular fat and underlying vein possessed comparable NO synthase activity.

CONCLUSIONS

Our observations suggest that perivascular fat-derived NO plays a beneficial role in saphenous veins harvested atraumatically and used as grafts in patients undergoing coronary artery bypass surgery.

Architectonic arrangement of the vasa vasorum of the human great saphenous vein

OBJECTIVE

The detailed spatial arrangement of the vasa vasorum (VV) of the human great saphenous vein (HGSV) was demonstrated in qualitative and quantitative terms.

MATERIALS AND METHODS

Segments of the HGSV taken from cadavers 12-24 h post mortem and from patients undergoing aortocoronary bypassing were studied by light microscopy of India-ink-injected specimens and by scanning electron microscopy of vascular corrosion casts.

RESULTS

Arterial feeders were found to approach the HGSV from nearby arteries every 15 mm forming a rich capillary network within the adventitia and the outer two thirds of the media in normal HGSV, while in HGSV with intimal hyperplasia capillary meshes extended into the inner layers of the media. Within the media, capillary meshes ran circularly. Postcapillary venules drained centrifugally towards the adventitial venous vessels which finally formed venous drainers running adjacent to the arterial feeders. Three-dimensional morphometry of vascular corrosion casts of VV revealed that diameters of (i) arterial VV ranged from 11.6 to 36.6 microm, (ii) capillary VV from 4.7 to 11.6 microm and (iii) venous VV ranged from 11.6 to 200.3 microm.

CONCLUSIONS

The 3D network of VV suggests these layers are metabolically highly active and therefore require a continuous blood supply. We conclude, therefore, that the VV network must be preserved during in situ bypassing.

Development of a local vasodilator delivery system using fibrin glue to prevent arterial graft from spasm

The clinical benefits of coronary artery bypass graft operations can be compromised by postoperative vasospasm. Traditionally, local papaverine (PPV) has been employed during the procedure to prevent and counteract vasospasm. But the relatively short action period limited its application. Fibrin glue (FG) might be a potential carrier of PPV for counteracting vasospasm in a longer action period than PPV solution. After FG incorporated with PPV (PPV-FG) was locally administrated in axillary and femoral arteries of dogs, PPV concentrations in artery vessels surrounding the administration sites were compared with the concentrations at the same sites in dogs given PPV solution. The properties of PPV's release in vitro and maintenance in vessel as well as the influence on the mean peripheral blood pressure and drug concentration in peripheral vein after the introduction PPV-FG on the surface of artery in dogs were evaluated. FG was considered to provide a sustained release of PPV and could maintain a high PPV concentration in artery vessel around the administration site. The results suggested that FG was an effective substrate for reserving PPV in the administrated site in a defined period.

On the biology of saphenous vein grafts fitted with external synthetic sheaths and stents

Autologous saphenous vein is used as a conduit to bypass atherosclerotic lesions in both the coronary artery (coronary artery bypass graft surgery [CABG]) and in femoral arteries (infrainguinal bypass graft surgery [IIBS]). Despite the undoubted success and benefits of the procedures, graft failure occurs in 50% of cases within 10 years after surgery. A principal cause of late vein graft failure is intimal and medial hyperplasia and superimposed atherogenesis. Apart from lipid lowering therapy, no intervention has hitherto proved clinically effective in preventing late vein graft failure which clearly constitutes a major clinical and economic problem that needs to be urgently resolved. However, we have studied the effect of external synthetic stents and sheaths in pig models of vein into artery interposition grafting and found them to have a profound effect on vein graft remodelling and thickening. In this review, therefore, we will summarise the mechanisms underlying vein graft failure and how these stents influence these processes and the possible mechanisms involved as well as the application of these devices in preventing vein graft failure clinically.

Effect of vein graft harvesting on endothelial nitric oxide synthase and nitric oxide production

BACKGROUND

Although the saphenous vein is the most commonly used conduit for coronary artery bypass surgery occlusion rates are high, with more than 50% grafts failing within 10 years. Nitric oxide, a potent vasodilator, also inhibits platelet aggregation, thrombus formation and vascular smooth muscle cell proliferation, is implicated in various vascular pathologies, including graft failure.

METHODS

Saphenous veins were obtained from patients undergoing bypass surgery harvested by conventional methods and with minimal handling, using a "no-touch" technique. Tissue distribution and protein expression of endothelial nitric oxide synthase was compared using immunohistochemistry and Western blot analysis. Nitric oxide generation was assessed using the citrulline assay.

RESULTS

There was injury to conventional compared with no-touch vein segments, in particular to the lumenal endothelium and tunica adventitia. This injury was accompanied by an absence of endothelial nitric oxide synthase immunostaining at regions of endothelial denudation and damaged adventitial layer of conventional veins and a significant reduction (p < 0.05) in endothelial nitric oxide synthase protein expression. Furthermore, nitric oxide release from conventional tissue extracts was significantly (p < 0.05) lower than no-touch vein segments.

CONCLUSIONS

Our results show that there is a reduction in endothelial nitric oxide synthase and nitric oxide release in saphenous veins harvested by conventional surgical methods compared with those prepared atraumatically. These observations may influence graft performance.

Decellularized Xenogenic Heart Valves Reveal Remodeling and Growth Potentialin Vivo

We have developed an advanced tissue processing technique on porcine pulmonary heart valves for pulmonary valve replacement and its initial clinical application during the autograft operation according to Ross. The novel concept consists of a cell-free matrix achieved by deoxycholic acid treatment that is repopulated by host cells in vivo. Molecular biology, radioligand binding, and electron microscopy consistently showed that these valves are almost free of cellular components. Animal experiments and clinical investigations revealed excellent hemodynamic properties of the valves, no need for antithrombotic therapy, and repopulation by host cells without any signs of calcification. In juvenile sheep the internal diameter of the implanted valves significantly increased in growing animals by approximately 10 mm. The repopulation of the decellularized heart valves was found not only in sheep but also in humans, which indicates that the underlying mechanisms, presumably repair mechanisms, might be common in mammals. If these findings can be confirmed by others, they will lead to new concepts in the field of cardiovascular tissue engineering that will eliminate the need for in vitro construction of autologous heart valves.

Does the method of harvesting the saphenous vein for coronary artery bypass surgery affect venous smooth muscle cells? iNOS immunolabelling and ultrastructural findings

OBJECTIVES

Coronary artery bypass surgery (CABG) using conventionally harvested saphenous vein (SV) as a graft is characterised by a high graft failure rate. It is believed that vein handling during harvesting is responsible for this, as damage to the vein's structure including its sources of endothelial nitric oxide synthase (eNOS) is apparent. Here we investigated the distribution of inducible nitric oxide synthase (iNOS) in conventionally harvested SV grafts and grafts prepared by a less-invasive 'no-touch' technique for CABG. Focus was on vascular smooth muscle cells (VSMCs).

PATIENTS AND METHODS

Six patients undergoing CABG were selected for the study; their SVs were harvested as grafts by conventional and 'no-touch' techniques and subsequently examined using fluorescent and transmission electron microscopy combined with immunolabelling to detect iNOS and structural changes in the grafts at the time of implantation.

RESULTS

The following were observed in conventionally harvested grafts: (i) damage to the VSMCs and (ii) induction of iNOS in these cells, (iii) heterogeneity of VSMCs-the presence of iNOS-positive and iNOS-negative VSMCs which were also (iv) in close contact with each other. In contrast, no damage to VSMCs and no expression of iNOS in these cells were observed in the 'no-touch' SV preparations.

CONCLUSIONS

The harvesting procedure influences the structure of VSMCs and the expression of iNOS in SV graft at the time of implantation. Whether rapid stimulation of iNOS in VSMCs in conventional SV grafts has a protective or harmful effect on the graft patency remains to be determined.