Analysis of blood flow behaviour in custom stent grafts

Endovascular aneurysm repair (EVAR) is an attractive alternative to open surgery for treating abdominal aortic aneurysms (AAAs). However, the implantation of stent grafts into AAAs can result in post-operative complications such as stent graft migration, rupture or endoleak. EVAR has therefore been carried out only on selected patients. Stent grafts are usually standard commercial stent grafts (CSGs); however, custom made stent grafts (cmSGs) of various shapes and sizes are sometimes used to fit patients’ anatomies. In the present study, the cmSGs were specially designed and fabricated by the surgeons at the Pitié-Salpétriére hospital in Paris. Two patients carrying cmSGs with unfavourable geometries showing tortuous shapes, angulation, widening, narrowing, curvature and kinking and one patient with a cmSG with a more favourable geometry resembling a straight tube were examined. These three clinical cases were investigated using three-dimensional numerical simulations, and the results showed that even when the cmSG geometries are unfavourable, the drag forces to which they are subjected are of a similar magnitude to those exerted on CSGs, or even smaller. The hemodynamic analysis carried out on the two unfavourable cmSGs showed the occurrence of low velocity values in the main trunk of the cmSGs, high velocities linked to recirculation areas downstream from kinking and strong distal narrowing. These flow patterns are liable to induce thrombus. However, since cmSG implantation can save the lives of patients for whom neither classical stent grafts nor open surgery are indicated, it can be concluded that these devices are useful in some cases.     

Experimental evaluation of flow patterns in interposition vein cuff anastomosis. The influence of non-planarity of the inflow

Clinical evidence suggests that the development of myointimal hyperplasia in prosthetic femorodistal bypass grafts may be reduced by the interposition of a cuff of autologous vein between the graft and the recipient artery. Previous experimental work has shown that some of the benefits may be attributed to the geometry of the cuffed anastomosis. Since the distal anastomosis in vivo is often non-planar we have carried out a preliminary study in a model where the graft is at an angle of 45 degrees to the anterior-posterior plane of the anastomosis. This out-of-plane angulation produces highly asymmetric flow patterns in the anastomosis with significant flow separation on the ipsilateral side of the cuff. In the proximal and distal outflow, however, the velocity vectors show significant helical motion with temporal instability in the distal outflow.     

Gore-Tex small-vessel angioplasty: A suitable substitute for the use of autogenous saphenous vein grafts

Autogenous saphenous vein has been the material of choice for small-vessel angioplasty and for circulatory access graft reconstruction. In an effort to conserve autogenous saphenous vein, we used expanded polytetrafluoroethylene (PTFE) grafts in 45 patients over a 12-month period. We used Gore-Tex(*) to reconstruct 17 circulatory access grafts, 16 carotid arteries, two brachial arteries, seven femoral arteries, and three popliteal anterior or posterior tibial arteries. The indications for reconstruction were chronic occlusion of the access grafts, trauma to the brachial and anterior tibial arteries, and atherosclerotic disease of the carotid, femoral, and popliteal-tibial arteries. Of the reconstructed circulatory access grafts, one failed immediately because of technical problems in the conduit, and one failed 11 months after reconstruction. All other grafts have functioned well and have produced a marked improvement in flow. Of the 28 patients who underwent reconstruction of arteries measuring 3 mm or less, two had patent arteries but died shortly after operation. The remaining 26 have been followed for one to 43 months. All reconstructed arteries are patent, and there have been no instances of distal embolization or false aneurysm formation. From this brief experience, we conclude that Gore-Tex is a suitable short-term alternative to saphenous vein for small vessel arterioplasty; it also may be the material of choice for reconstructing the outflow tract of occluded access grafts.     

静脉桥狭窄家兔动物模型的建立

目的建立冠心病冠状动脉旁路移植术后移植静脉桥狭窄的动物模型。方法取3.0～3.5 kg普通新西兰兔8只,取同侧颈外静脉与颈总动脉进行端端吻合,吻合时采用间断缝合的方法。结果术后2周、4周取下静脉桥及对侧颈外静脉,光镜下见静脉桥新生内膜形成,中膜增厚,弹力纤维减少;胶原纤维不均匀性增厚。结论本模型能反映冠状动脉旁路移植术后静脉桥狭窄的情况,可满意模拟人冠状动脉旁路移植术后大隐静脉桥的病理变化。     

大鼠颈部自体静脉移植模型两种吻合方法的比较

目的比较间断吻合和连续吻合法建立静脉桥狭窄动物模型的优劣。方法SD大鼠20只,分成两组（间断吻合组和连续吻合组）,取颈外静脉与颈总动脉行端端吻合。术后4周取下静脉桥,观察桥管通畅性,分析新生内膜与中膜的厚度、面积比。结果连续组与间断组相比手术时间更短,出血更少,但桥管通畅率低,两组内膜增生程度没有显著差异。结论连续吻合用时短,出血少,对术者要求更高,较易形成吻合口狭窄。两者造模效果一样。     

Distal embolization at the site of a stenotic distal anastomosis of a saphenous vein graft,leading to dye entrapment

This case report describes the entrapment of contrast media after recanalization of a recently occluded saphenous vein graft with balloon predilation, thrombectomy and stent implantation. Recanalization of the respective coronary artery was performed, and the entrapped contrast media within the saphenous vein graft progressed to the left circumflex artery.     

Prolonged systemic delivery of tirofiban in a thrombus-laden saphenous vein graft

Results from previous trials have shown conflicting results from local delivery of thrombolytic agents to diminish thrombus burden before intervention in native coronary arteries and saphenous vein grafts. We described a patient with an acute coronary syndrome who was treated for 24 hours with systemic tirofiban (Aggrastat), a glycoprotein IIb/IIIa inhibitor, for the treatment of a degenerated saphenous vein graft with a TIMI grade 4 thrombus (large-sized thrombus). Angiographic evaluation 48 hours later revealed complete resolution of the thrombus with normal coronary blood flow.     

A periadventitial sirolimus-releasing mesh decreased intimal hyperplasia in a rabbit model

Autologous vein grafts used as aortocoronary bypasses are often prone to intimal hyperplasia, which results in stenosis and occlusion of the vein. The aim of this study was to prevent intimal hyperplasia using a newly developed perivascular system with sustained release of sirolimus. This system of controlled drug release consists of a polyester mesh coated with a copolymer of L-lactic acid and epsilon-caprolactone that releases sirolimus. The mesh is intended for wrapping around the vein graft during surgery. The mesh releasing sirolimus was implanted in periadventitial position onto arteria carotis communis of rabbits, and neointimal hyperplasia was then assessed. We found that implanted sirolimus-releasing meshes reduced intima thickness by 47+/-10 % compared to a vein graft after 3 weeks. The pure polyester mesh decreased vein intima thickness by 35+/-9 %. Thus, our periadventitial system for controlled release of sirolimus prevented the development of intimal hyperplasia in autologous vein grafts in vivo in rabbits. A perivascularly applied mesh releasing sirolimus is a promising device for preventing stenosis of autologous vein grafts.     

ATHEROSCLEROSIS OF AORTOCORONARY ARTERY SAPHENOUS VEIN BYPASS GRAFTS

Among a series of 109 patients who underwent aortocoronary artery bypass reoperation and received a total of 168 grafts, atherosclerosis was found in 29 saphenous vein grafts. Specimens were obtained at reoperation in 25 patients and at postmortem examination in four. Plaques were found from 3 months to 6 years after operation. The plaques often were combined with old or recent mural thrombi, intimal fibroses, or thickened intima associated with intramural hematomas. Clinical findings did not correlate with risk factors such as hypercholesterolemia, hypertension, or diabetes mellitus. In a well-functioning graft with adequate flow, cellular proliferation generally reflects an adjustment of the venous wall to a higher pressure. Such a vessel retains elastic properties of pulsating structures. In the specimens studied, intimal fibrosis associated with plaques was always poorly cellular, causing the grafts to become rigid. This resulted in irregular flow and, in some patients, dissection. The possible pathogenesis of atheromatosis in vein grafts may include the usual mechanisms of atherogenesis and atheromatous transformation of thrombi. It appears that hemodynamically significant stenosis of vein grafts caused by atherosclerosis is an infrequent complication.     

Computational model of blood flow in the aorto-coronary bypass graft

Background

Coronary artery bypass grafting surgery is an effective treatment modality for patients with severe coronary artery disease. The conduits used during the surgery include both the arterial and venous conduits. Long- term graft patency rate for the internal mammary arterial graft is superior, but the same is not true for the saphenous vein grafts. At 10 years, more than 50% of the vein grafts would have occluded and many of them are diseased. Why do the saphenous vein grafts fail the test of time? Many causes have been proposed for saphenous graft failure. Some are non-modifiable and the rest are modifiable. Non-modifiable causes include different histological structure of the vein compared to artery, size disparity between coronary artery and saphenous vein. However, researches are more interested in the modifiable causes, such as graft flow dynamics and wall shear stress distribution at the anastomotic sites. Formation of intimal hyperplasia at the anastomotic junction has been implicated as the root cause of long- term graft failure.Many researchers have analyzed the complex flow patterns in the distal sapheno-coronary anastomotic region, using various simulated model in an attempt to explain the site of preferential intimal hyperplasia based on the flow disturbances and differential wall stress distribution. In this paper, the geometrical bypass models (aorto-left coronary bypass graft model and aorto-right coronary bypass graft model) are based on real-life situations. In our models, the dimensions of the aorta, saphenous vein and the coronary artery simulate the actual dimensions at surgery. Both the proximal and distal anastomoses are considered at the same time, and we also take into the consideration the cross-sectional shape change of the venous conduit from circular to elliptical. Contrary to previous works, we have carried out computational fluid dynamics (CFD) study in the entire aorta-graft-perfused artery domain. The results reported here focus on (i) the complex flow patterns both at the proximal and distal anastomotic sites, and (ii) the wall shear stress distribution, which is an important factor that contributes to graft patency.

Methods

The three-dimensional coronary bypass models of the aorto-right coronary bypass and the aorto-left coronary bypass systems are constructed using computational fluid-dynamics software (Fluent 6.0.1). To have a better understanding of the flow dynamics at specific time instants of the cardiac cycle, quasi-steady flow simulations are performed, using a finite-volume approach. The data input to the models are the physiological measurements of flow-rates at (i) the aortic entrance, (ii) the ascending aorta, (iii) the left coronary artery, and (iv) the right coronary artery.

Results

The flow field and the wall shear stress are calculated throughout the cycle, but reported in this paper at two different instants of the cardiac cycle, one at the onset of ejection and the other during mid-diastole for both the right and left aorto-coronary bypass graft models. Plots of velocity-vector and the wall shear stress distributions are displayed in the aorto-graft-coronary arterial flow-field domain. We have shown (i) how the blocked coronary artery is being perfused in systole and diastole, (ii) the flow patterns at the two anastomotic junctions, proximal and distal anastomotic sites, and (iii) the shear stress distributions and their associations with arterial disease.

Conclusion

The computed results have revealed that (i) maximum perfusion of the occluded artery occurs during mid-diastole, and (ii) the maximum wall shear-stress variation is observed around the distal anastomotic region. These results can enable the clinicians to have a better understanding of vein graft disease, and hopefully we can offer a solution to alleviate or delay the occurrence of vein graft disease.

     

选择性冠状静脉动脉化搭桥术中不同移植物的应用疗效比较

目的:对比选择性冠状静脉动脉化(SCVBG)搭桥治疗弥漫性右冠状动脉狭窄病变中选择乳内动脉和大隐静脉作为桥血管的治疗效果。方法:选择2008年10月到2014年10月在我院行SCVBG搭桥的84例患者资料,其中选择大隐静脉作为桥血管进行冠状静脉动脉化搭桥患者46例(大隐静脉桥组),选择乳内动脉作为桥血管进行冠状静脉动脉化搭桥患者38例(乳内动脉桥组)。随访记录两组患者的生存情况、近期复查超声心动图、冠状动脉CTA及心绞痛复发率。结果:乳内动脉桥组患者总生存率(100%)明显高于大隐静脉桥组(82.6%)(P0.05)。乳内动脉桥组患者桥血管和心中静脉通畅率(100%)明显大于大隐静脉桥组(54.35%)(P0.05)。两组患者左心室射血分数(LVEF)较治疗前明显增加,左心室舒张期末内径(LVEDD)较治疗前明显减小(P0.05)。治疗后,乳内动脉桥组患者心绞痛复发率明显小于大隐静脉桥组(P0.05)。结论:SCVBG搭桥治疗弥漫性右冠状动脉狭窄病变中,选择乳内动脉桥效果优于大隐静脉桥,能明显提高桥血管和心中静脉通畅率,降低心绞痛复发率。     

Characterization of the Inhibition of Vein Graft Intimal Hyperplasia by a Biodegradable Vascular Stent

Characterization of biodegradable stent vein graft thickening. Polydioxanone vascular sutures (PDSs) were used in a biodegradable arteriovenous bypass model. Twenty-four rabbits underwent carotid interposition bypass via ipsilateral jugular vein. One half received the stent (PDS group) and the remaining half a simple vein graft (controls). Group subsets received external stent removal or sham-control exploration at 4 and 12 weeks. At 4 and 12 weeks, the PDS group had significantly less medial and intimal thickening than the control group (P < 0.05), and there were fewer proliferating smooth muscle cells and extra cellular matrix formation than the control group at every interval. At 12 weeks, partial stent degradation occurred without deleterious effects. Furthermore proliferating cell nuclear antigen (PCNA), angiotensin type 1 receptor (AT1R), and transforming growth factor beta 1 (TGF-β1) levels were significantly lower than the control group. The external stent inhibited medial and intimal hyperplasia, an effect that remains after the material has completely degraded. This PDS stent is feasible option for vein grafts.     

Bovine Heterografts for Hemodialysis

Long-term results with 52 bovine, 53 saphenous vein and 78 radial-cephalic arteriovenous fistulas (AVF) were analyzed. Side-to-end radial-cephalic AVF provided the best patency data, and remain the preferred access system for hemodialysis. Bovine AVF were next in ranking with better patency rates than for the saphenous vein AVF studied. Corrected one-year patency rates were 71 percent for bovine, 45 percent for saphenous and 91 percent for radial-cephalic AVF. The incidence of nonthrombotic complications with bovine AVF was higher than with saphenous vein AVF. Distal ischemia due to “steal” and certain bleeding and wound complications were unique to bovine AVF. Excellent dialysis blood flow rates and easy accessibility were provided by bovine grafts. When a satisfactory radial-cephalic AVF cannot be created, bovine graft AVF is an acceptable alternative for hemodialysis access.     

Troglitazone has no effect on K(ATP) channel opener induced-relaxations in rat aorta and in human saphenous veins from patients with type 2 diabetes

Troglitazone, a thiazolidinedione derivative, is an oral antidiabetic agent that enhances insulin sensitivity in insulin-resistant states. K(ATP) channels, on the other hand, have important roles protecting cardiovascular system in ischemic and/or hypoxic states. They are also important in the control of vascular tone, and therefore of blood pressure. We tested whether troglitazone can directly affect vascular K(ATP) channel opener-induced relaxations in vitro. 1, 10 or 100 microM troglitazone incubations for 30 min did not alter cromakalim (a K(ATP) channel opener)--induced relaxations in endothelium-denuded aortas from rat, saphenous veins from type 2 diabetic and nondiabetic patients. In addition, we compared the sensitivity to cromakalim in diabetic saphenous veins with that of nondiabetic veins. The concentration-response curve for cromakalim was shifted to the right in diabetic vein. pD2 values for cromakalim were 6.85+/-0.08 vs. 6.61+/-0.04 (p<0.05) in nondiabetic (n:10) and diabetic (n:7) veins respectively. % maximum response of cromakalim was also significantly decreased by 24+/-3% in diabetic veins. However, responsiveness of veins to phenylephrine or sodium nitroprusside were similar in both groups. The results obtained may be clinically useful 1. suggesting that in ischaemic and/or hypoxic insults troglitazone may not worsen vascular dilatation, through K(ATP) channel, in diabetic patients who are more prone to these conditions than healthy people, 2. providing an evidence that diabetes causes an impaired dilatation of human saphenous vein through K(ATP) channels. This may partly be related with diabetes-induced vascular complications, such as vasospasm and even hypertension. Accordingly, since saphenous veins are used as conduit vessels in coronary by-pass graft surgery, the results also suggest that the defective dilatation through K(ATP) channels may play a role on the performance of saphenous vein grafts in type 2 diabetes.     

To serve you better

Fasting blood lipids were analyzed shortly before revascularization surgery in an attempt to find a possible correlation between the fate of the aortocoronary saphenous vein graft and lipoproteinemia. The patency of the bypass was evaluated by arteriography at two weeks and at one year following operation. Patients with closed grafts at two weeks had an original mean serum triglyceride concentration of 287 mg./100 ml. Patients with grafts which were widely patent after one year had an original triglyceridemia of 224 mg./100 ml. The severely stenosed group had an intermediate average preoperative value of 224 mg./100 ml. The same conclusion was reached with a group of patients with good vessel(s) distal to the graft(s). Cholesterolemia was about the same in all groups. These results suggest the hypothesis that hypertriglyceridemia plays a significant role in the reduction and/or occlusion of the lumen of the vein graft.     

The clopidogrel after surgery for coronary artery disease (CASCADE) randomized controlled trial: clopidogrel and aspirin versus aspirin alone after coronary bypass surgery [NCT00228423]

Background

Saphenous vein graft disease remains a major limitation of coronary artery bypass graft surgery. The process of saphenous vein intimal hyperplasia begins just days after surgical revascularization, setting the stage for graft atherosclerotic disease and its sequalae. Clopidogrel improves outcomes in patients with atherosclerotic disease, and is effective at reducing intimal hyperplasia in animal models of thrombosis. Therefore, the goal of this study will be to evaluate the efficacy of clopidogrel and aspirin therapy versus aspirin alone in the prevention of saphenous vein graft intimal hyperplasia following coronary artery bypass surgery.

Methods

Patients undergoing multi-vessel coronary artery bypass grafting and in whom at least two saphenous vein grafts will be used are eligible for the study. Patients will be randomized to receive daily clopidogrel 75 mg or placebo, in addition to daily aspirin 162 mg, for a one year duration starting on the day of surgery (as soon as postoperative bleeding has been excluded). At the end of one year, all patients will undergo coronary angiography and intravascular ultrasound assessment of one saphenous vein graft as selected by randomization. The trial will be powered to test the hypothesis that clopidogrel and aspirin will reduce vein graft intimal hyperplasia by 20% compared to aspirin alone at one year following bypass surgery.

Discussion

This trial is the first prospective human study that will address the question of whether clopidogrel therapy improves outcomes and reduces saphenous vein graft intimal hyperplasia following cardiac surgery. Should the combination of clopidogrel and aspirin reduce the process of vein graft intimal hyperplasia, the results of this study will help redefine modern antiplatelet management of coronary artery bypass patients.     

EVERSION ENDARTERECTOMY: A NEW APPLICATION OF AN OLD TECHNIQUE

Restoration of arterial flow to a severely ischemic extremity remains a major challenge in vascular surgery. The procedure of choice for limb salvage is a bypass utilizing reversed saphenous vein. When the saphenous vein is unsuitable or unavailable, the surgeon must turn to endarterectomy of the femoral and popliteal systems or synthetic, composite, heterologous, autologous, or homologous grafts. To avoid the problems associated with these techniques and to improve the results of limb salvage, we have revived and modified the technique of superficial femoral artery eversion endarterectomy and combined it with other reconstructive techniques in an effort to salvage the severely ischemic lower extremity. Of 38 patients treated for incapacitating claudication or severe limb ischemia during a one-year period, six patients had an unsuitable saphenous vein for the proposed reconstruction. Five of these patients underwent superficial femoral eversion endarterectomy. These six patients have 100% patency at follow-up seven months to one year postoperatively. Our experience with these six patients, including angiographic follow-up, will be presented.     

"No-reflow" phenomenon during percutaneous carotid angioplasty in a scleroderma patient

Atherosclerosis remains the most common etiology of carotid artery stenosis, but other systemic diseases can also cause this disease. Percutaneous carotid angioplasty and stenting (CAS) with and without distal embolic protection devices is gaining acceptance as the treatment of choice. No-reflow phenomenon due to distal embolization has been previously described mainly during percutaneous intervention of degenerated saphenous vein graft. We describe a patient with systemic sclerosis who underwent CAS with distal embolic protection device with occurrence of no-reflow phenomenon during the procedure that resolved after retrieval of the filter device.     

Manual pressure distension of the human saphenous vein changes its biomechanical properties-implication for coronary artery bypass grafting

Patency rates of saphenous vein grafts following coronary artery bypass grafting (CABG) depend on multiple factors. Information regarding the impact of biomechanical properties of vein grafts on patency rates is not available. The objective of the present study was to evaluate whether uncontrolled manual pressure distension during routine preparation of the saphenous vein in CABG-induced changes in the biomechanical properties of the vein. The morphometric and stress-strain properties were studied in isolated segments of the saphenous vein from 12 patients undergoing elective CABG. Six segments were manually distended without pressure control and six were not distended. The mechanical test was performed as a ramp inflation using syringe pump. The vein dimensions were obtained from digitised images at different pressures as well as at the no-load and zero-stress states. The circumferences, the wall and lumen area, the wall thickness, and the outer diameter as function of the applied pressure were largest in the segments with uncontrolled manual distension compared to those without distension (P<0.05). The opening angle and the absolute value of the residual strains were lower (P<0.01) and the circumferential stress-strain curve shifted to the left, indicating the wall became stiffer with uncontrolled manual distension compared to those without distension (P<0.05). In conclusion, manual pressure distension changed the morphometric and biomechanical properties of the saphenous vein. The perspective is that studies on biomechanical properties on the saphenous vein may guide surgeons how to handle graft material without causing major changes of the biomechanical properties during harvesting and preparation.