Microvascular grafts: effect of diameter discrepancy on patency rates

This study examined the effects of diameter discrepancy between interpositional vein grafts within a range of 0.25 to 2 mm. One hundred rats underwent isolation of a 10 mm segment of the femoral artery, from which a 4 mm segment was removed. Venous grafts measuring 8 +/- 2 mm were interposed. The rats were categorized by their graft to artery diameter ratios. Patency was assessed upon completion and 4-6 days postoperatively. All surgical specimens were submitted to the pathology department for microscopic evaluation. Results showed Group I: graft to artery ratio 1:1, patency rate 90% (18/20); Group II: ratio 0.75:1, patency rate 80% (16/20); Group III: ratio 0.5:1, patency rate 60% (12/20), Group IV: ratio 0.25:1, patency rate 20% (4/20); Group V: ratio 2:1, patency rate 60% (12/20). It is concluded that 1:1 venous graft to artery ratios are optimal. When this is impossible, diameter discrepancies should be at least 0.75:1 for acceptable patency rates to be attained in microvascular grafting.     

Lack of diameter effect on short-term patency of size-matched Dacron aortobifemoral grafts.

This study examined the relationship between graft diameter and subsequent patency in 79 patients who received Dacron aortobifemoral bypass grafts for aortoiliac occlusive disease between 1985 and 1989. Sixty-five percent of these patients were men, 25% were diabetic, and 94% were smokers, with an average age of 62 years. Patients were followed for a mean interval of 24 months. Life-table survival was 92% at 3 years. All surviving patients showed "significant" postoperative improvement by use of Society for Vascular Surgery/International Society for Cardiovascular Surgery combined clinical and vascular laboratory criteria. There were three early and five late graft thromboses. Primary and secondary life-table patencies were 85% and 92%, respectively, at 3 years. Dacron bifurcation grafts were selected to match the size of native arteries. Patients receiving small diameter grafts, defined as 12 mm (n = 9) and 14 mm (n = 39), were compared with patients receiving large diameter grafts of 16 mm (n = 26) and 18 mm (n = 5). Small diameter grafts were more likely to be used in women (p less than 0.01), but patient groups were otherwise comparable with respect to age, smoking history, diabetes, outflow status, operative indications, type of proximal anastomosis (end-to-end or end-to-side), location of distal anastomosis (common femoral vs deep femoral), type of graft construction (knitted vs woven), and functional result. Graft diameter did not influence life-table patency, which was 84% for small and 87% for large diameter grafts at 3 years (p = 0.74). Furthermore, none of the other variables listed above influenced graft patency.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effective hemodynamic diameter: an intrinsic property of vein grafts with predictive value for patency

INTRODUCTION: Conduit size and quality are major determinants of the long-term success of infrainguinal autologous vein grafting. However, accurate measurement of the internal diameter of vein grafts is difficult given their variable wall thickness and taper. The purpose of this study was to define the "effective" internal diameter of a vein graft according to its hemodynamic properties and to determine its significance for graft patency. METHODS: Sixty infrainguinal bypass grafts performed on 57 patients were evaluated intraoperatively. Proximal and distal graft pressure and blood flow (Q(meas)) were measured with fluid-filled catheter transduction and ultrasonic transit-time flowimetry, respectively, after unclamping. Waveforms were recorded digitally at 200 Hz under baseline conditions and after stimulation with 60 mg of papaverine. According to Fourier transformation of the measured pressure gradient (DeltaP), the Womersley solution for fluid flow in a straight rigid tube was used to calculate theoretical flow waveforms (Q(calc)) for a range of graft diameters. The theoretical waveforms were then compared with the measured flow waveforms and the best-fit diameter chosen as the "effective hemodynamic diameter" (EHD). Only grafts in which the correlation coefficient of Q(calc) versus Q(meas) was more than 0.90 were accepted (n = 47) to assure validity of the hemodynamic model. After a mean follow-up of 12.5 months (range, 0.1-43.9 months), patency was determined by the life table method. Hemodynamic and clinical variables were tabulated, and their effect on patency determined the use of univariate and multivariate Cox regression. RESULTS: Mean EHD was 4.1 +/- 0.1 mm with a range of 2.5 to 5.7 mm. Administration of papaverine caused profound changes in DeltaP (+78% +/- 17%) and Q(meas) (+71% +/- 12%) as expected, but had no effect on EHD (+0.05% +/- 0.1%). Univariate regression identified five variables associated with decreased secondary patency (P <.10): low EHD, conduit source other than the greater saphenous vein, high baseline DeltaP(mean), female sex, and redo operation. Of these, only low EHD was significant after multivariate analysis (P =.03). Patency of small diameter grafts (EHD < 3.6 mm; n = 11) was compared with patency of larger grafts (EHD > 3.6 mm; n = 36) to test a frequently espoused clinical guideline. Grafts with an EHD less than 3.6 mm exhibited significantly lower secondary patency compared with larger grafts (P =.0001). The positive and negative predictive values for an EHD less than 3.6 mm for secondary graft failure for grafts with at least 1 year follow-up were 86% and 88%, respectively. CONCLUSION: An EHD is a unique parameter that quantifies conduit size and has a significant impact on vein graft patency. An EHD less than 3.6 mm portends graft failure.     

Factors influencing patency of saphenous vein grafts.

To determine factors affecting saphenous vein graft patency, 218 grafts in 66 unselected patients were studied angiographically 1 year after operation. Fourteen variables were extracted from the angiograms, electrocardiograms, and intraoperative flow measurements to assess their predictive value. Preoperative coronary vessel diameter and degree of proximal stenosis measured angiographically correlated significantly with graft patency. Graft patency for vessels greater than 1.5 mm in diameter with greater than 70% stenosis was 93% (98 out of 105). Vessel size at operation and the presence of reactive hyperemia greater than 20 ml per minute also correlated significantly with graft patency. Reactive hyperemia increased significantly as the severity of the vessel stenosis proximal to the graft increased, thus suggesting a mechanism for the improved patency rate of grafts to more stenotic vessels. The patency rate of the end-to-side component of a continuity graft (left anterior descending coronary artery and diagonal or marginal artery in 1 graft) was 100% and of the side-to-side component, 77%. This study shows that the patency rate for saphenous vein grafts compares favorably with the reported patency for internal mammary grafts when critical factors such as vessel size (greater than 1.5 mm) and degree of stenosis (greater than 70%) are considered in bypass selection.     

Long-term patency of aorto-coronary saphenous vein grafts]

One hundred eighty-two cases with 814 aorto-coronary saphenous vein grafts were studied according to coronary risk factors (smoking, hypertension, hyperlipidemia, diabetes mellitus, obesity and family history). The patency rates of all cases were as follows, early-term (within one year after operation): 92.3%, mid-term (within 5 years after operation) 80.7%, long-term (more than 5 years after operation): 66.0%. Coronary risk factors have great influence upon the mid- and long-term patency, especially upon the latter. The long-term patency rate of the grafts complicated with hyperlipidemia was 57.4% and that without hyperlipidemia was 81.8% (p less than 0.01). Hyperlipidemia, complicating 55.5% of all cases, was one of the most influential factors on the patency and also the most difficult one to be controlled. In the United States and Europe, many cases were complicated with hyperlipidemia, and it was considered that the poor patency of the saphenous vein grafts in those countries was due to this fact. Pathological studies revealed that hyperplasia of intima and media, characteristics of venosclerosis, appeared frequently in the saphenous vein grafts having more than three risk factors, and that the factors had effect not only upon arteries but also upon veins. So we conclude that saphenous vein grafts are the materials of good long-term patency, and that the control of the risk factors, particularly hyperlipidemia, is the key to improve the patency.     

Effect of twist on flow and patency of vein grafts

This article examines the effect of twist on flow through reversed vein segments in vitro and its effect on graft patency in vivo. Excised canine superficial femoral veins were perfused in vitro with normal saline solution or canine blood. Perfusion was carried out at five pressures and against three outflow resistances. Increasing increments of twist were applied to the outflow end of the vein. Flow was measured at each level of twist. With both saline solution and blood, flow was unaltered until twist reached 140 to 180 degrees. Flow then decreased sharply, stopping completely at 175 to 200 degrees of twist. In vivo experiments were then performed in 13 dogs. Reversed superficial femoral veins were used as end-to-end grafts to bypass the iliac arteries. Each graft was deliberately twisted 0, 45, 90, 135, or 200 degrees. All grafts were harvested 6 months after surgery. Eighteen of 20 grafts twisted 135 degrees or less remained patent. However, all five grafts twisted 200 degrees were thrombosed within 4 hours of surgery (p less than 0.05). These data suggest that in patients a slight amount of graft twist probably does not reduce flow; however, more than 135 degrees of twist will greatly reduce flow, leading to early graft thrombosis.     

Influence of vein size (diameter) on infrapopliteal reversed vein graft patency

We reviewed 239 infrapopliteal reversed greater saphenous vein graft bypasses placed for critical ischemia over a 7-year period to determine the influence of vein diameter on graft patency and limb salvage. Grafts were assigned to four groups based on the minimum external diameter measured during operation: less than 3.0 mm, n = 18; 3.0 mm, n = 59; 3.5 mm, n = 67; and greater than or equal to 4.0 mm, n = 145. A pattern of increasing graft patency and limb salvage among the four groups was noted as the minimum external diameter increased from less than 3.0 mm to greater than or equal to 4.0 mm. When compared to the larger grafts greater than or equal to 4.0 mm, primary graft patency was significantly lower both for less than 3.0 mm grafts (0% for less than 3.0 mm vs 65% for greater than or equal to 4.0 mm at 3 years, p less than 0.001) and for long (greater than 45 cm) 3.0 mm grafts (38% for long 3.0 mm vs 75% for greater than or equal to 4.0 mm at 2 years, p less than 0.005). All 3.5 mm and short (less than 45 cm) 3.0 mm grafts had patency rates similar to greater than or equal to 4.0 mm veins. Thus long 3.0 mm and all less than 3.0 mm reversed saphenous vein grafts should be considered at high risk for failure. Veins with fibrotic, thick-walled segments identified during operation (n = 19) had patency rates significantly lower than nonfibrotic veins (n = 270; p less than 0.01), and this may play a role in the failure of some less than 3.0 mm minimum external diameter vein grafts.     

Favorable balance of prostacyclin and thromboxane A2 improves early patency of human in situ vein grafts

Graft thrombosis soon after reconstruction remains a major obstacle to the use of reversed vein grafts in infrapopliteal reconstruction. Our clinical experience with in situ vein grafts corroborates Leather's results by demonstrating an overall graft patency of 95% below the knee at 1 year and 94% in the infrapopliteal group. It has been postulated that this improved early patency rate of in situ vein grafts is the result of more optimal preservation of the endothelium of the vein graft. To investigate this hypothesis, human saphenous veins were handled by an in situ and a reversed technique. The intact vein segments were then tested for luminal production of prostacyclin and thromboxane A₂ and fixed for scanning electron microscopic analysis of the surface morphology. This study demonstrated that endothelial cell prostacyclin release is enhanced in human in situ vein segments but not in reversed vein segments. In addition, luminal production of thromboxane A₂ is significantly greater in the reversed than in the in situ vein segments. These findings are associated with marked endothelial structural damage in the reversed veins and minimal endothelial disruption in the situ veins. Therefore the ratio of the antiaggregatory vasodilator prostacyclin to the proaggregatory vasoconstrictor thromboxane A₂ is significantly more favorable for the in situ vein segment than for the reversed vein segment. The observed excellent early patency of the situ vein grafts in our poor-risk patient population may in part be the result of this favorable balance of prostacyclin and thromboxane A₂ and the more optimally preserved endothelial morphology. (J VASC SURG 1984;1:149-59.)     

Distal in situ saphenous vein grafts for limb salvage. Increased operative blood flow and postoperative patency

Early failure remains a major obstacle to successful distal bypass surgery using vein grafts for limb salvage. Thirty distal bypass graft procedures were performed for limb salvage using the in situ technique. Grafts were anastomosed to the distal popliteal artery in 13 patients and to the infrapopliteal artery in 17 patients. Sixteen patients had inadequate saphenous veins for reversed vein grafts. The mean blood flow measured through these grafts (n = 20) was 164 +/- 22 ml/min and increased to 278 +/- 31 ml/min after administration of 30 mg of papaverine. All grafts were patent at the time of hospital discharge and patients were followed for 1 to 28 months. Life table analysis of the 30 procedures shows a patency of 100 percent at 18 months follow-up. One graft subsequently failed at 22 months. Long-term limb salvage was achieved in 100 percent of the patients in this series. The excellent blood flow through these grafts suggests that the in situ vein graft technique may be more favorable for arterial reconstruction than the reversed vein graft technique. Our preliminary data confirm the observations of Leather et al [3,4], that the rates of vein utilization and graft patency are higher with the in situ technique.     

The effect of wall mechanical properties on patency of arterial grafts

Normal arteries have properties which match the low output impedance of the heart to the high peripheral impedance. These properties can be assessed in terms of compliance (% diameter change per unit pressure change) as well as by other haemodynamic parameters. Experiments were designed using vein, Dacron and expanded polytetrafluoroethylene (PTFE) in a low flow canine femoral artery bypass model. No graft group achieved perfect patency. At twelve weeks 80% of vein grafts, 30% of Dacron grafts, and 15% of PTFE grafts remained patent. The compliance of vein grafts was maintained despite marked thickening of the wall. Patency was correlated at a highly significant level with compliance. The studies demonstrate that the matching of the mechanical properties of grafts to host arteries is important in the design of successful synthetic arterial grafts.     

The effect of a venous anastomosis Tyrell vein collar on the primary patency of arteriovenous grafts in patients undergoing hemodialysis

PURPOSE: Vein collars and patches are used at the distal anastomoses of infrainguinal prosthetic grafts to improve graft patency. We initiated a randomized, prospective study to determine whether a Tyrell vein collar at the venous anastomosis of forearm loop arteriovenous grafts (AVGs) would improve patency. METHODS: Patients who required new forearm AVGs were randomized to (1) a standard end-to-side graft-vein anastomosis (control group) or (2) a Tyrell vein collar between the graft and the vein (study group). End points were (1) graft thrombosis, (2) graft removal and ligation, or (3) inadequate graft function. Randomization of 75 subjects was planned. The study was terminated early for ethical reasons. RESULTS: Seventeen patients (eight men, nine women) with a mean age of 52.8 years (range, 31-79 years) had 17 grafts placed (control group, n = 10; study group, n = 7). Comorbidities were not different between the groups (P>.05). Six (86%) of seven study grafts failed by 9 months (mean, 4.6 months). Four (66%) failed study grafts had venous outflow tract stenosis from intimal hyperplasia. This was confirmed at surgery in three and by angiography in one. The 9-month primary patency was 80% for the control group versus 17% for the study group (P =.015). Smaller outflow vein diameter in the study group (P =. 048) did not account for this inferior graft patency. CONCLUSION: A Tyrell vein collar at the venous anastomosis of a forearm AVG resulted in premature graft failure. The use of a Tyrell vein collar may accelerate venous anastomosis intimal hyperplasia.     

Principles of preparation of vein bypass grafts to maximize patency

Experience in cardiovascular and peripheral vascular surgery with saphenous vein bypass conduits is reviewed. It is clear that meticulous technique and graft preparation are crucial to short-term and long-term patency. The risk of early thrombosis is related to damage to the graft's native intima, graft flow, and coagulability of the patient's blood. Attention to atraumatic harvesting techniques and perfection of anastomoses are crucial to minimizing intimal damage. Graft inflow and outflow are fundamental principles. The use of vitamin K antagonists and platelet inhibitors may improve graft survival. Subacute occlusion is related to structural alterations in the grafts themselves. These include intimal hyperplasia and medial fibrosis as the grafts become "arterialized," valve fibrosis, aneurysmal dilatation, clamp stenosis, and suture stenosis. Long-term patency is threatened primarily by atherosclerosis in the graft itself. There is some evidence that care in vein harvesting and implantation as well as the use of anticoagulant agents affect the development of this complication. A technique for graft preparation is presented that is based on the experience of the authors in harvesting grafts for both cerebral and coronary bypass conduits.     

Analysis of factors influencing early patency of saphenous vein grafts

To determine factors affecting early patency of saphenous vein grafts, 140 grafts in 65 patients were studied angiographically within 2 months after operation. Twenty of the 140 grafts were occluded. Sixteen variables were extracted from the angiographic findings, intraoperative measurements, clinical characteristics, and biochemical data. The univariate and multivariate analyses were performed to assess their predictive value. Of 16 variables, 3 (coronary artery internal diameter, graft flow, and coronary artery resistance) correlated significantly with graft patency in the univariate analysis. The multivariate analysis selected coronary artery resistance, coronary artery internal diameter, and degree of proximal stenosis as predictors of early graft patency. Of these 3 factors, the coronary artery resistance influenced graft patency mostly. Therefore, the coronary artery resistance was considered to be the most reliable predictor of early graft patency.     

The effect of antiaggregating drugs on the patency of grafts in the arterial system

In experiments in dogs we studied the effect of anti-aggregating agents on the patency of prosthetic vascular grafts (4 mm in diameter) and vein grafts, both placed in areas with a rate of flow ranging between 50–60 ml/min. All 6 prosthetic grafts became occluded in the nonmedicated controls, 1 of 5 prosthetic grafts remained patent for a prolonged period of time in dogs receiving 250 mg acetylsalicylic acid 3 times a day, while 5 of 6 prosthetic grafts remained patent in animals receiving an antiaggregating mixture. Follow-up studies showed that antiaggregating medication had no effect on the patency of vein grafts, but it prevented the formation of mural thrombi and subendothelial proliferation.

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Résumé Nous avons étudié chez le chien l'effet d'agents anti-aggrégants sur la perméabilité de greffes vasculaires prothétiques (4 mm de diamètre) et veineuses ayant des débits de 50–60 ml/min. Les greffes prothétiques se sont toutes obstruées chez les 6 témoins non traités; chez 5 animaux recevant 250 m g d'acide acétylsalicylique 3 fois par jour, 1 greffe est restée perméable à long terme; sur les 6 animaux recevant l'agent anti-aggrégant, 5 greffes sont restées perméables. Pour les greffes veineuses, la médication anti-aggrégante n'a aucun effet sur la perméabilité; mais elle prévient la formation de thrombi muraux et la prolifération sous-endothéliale.

     

剪切力对静脉移植后再狭窄的影响

自体静脉移植术是目前医学界普遍采用的治疗血管狭窄、改善组织缺血的重要手段和基本方法,是血管外科最常见的术式之一.然而,由于移植损伤、静脉与动脉的组织结构、血流动力学差异,自体静脉移植术后的通畅率仍不容乐观.本文将从血液流变学的角度对自体静脉移植术后再狭窄的形成过程进行研究.     

Long-term patency of vein grafts preserved in liquid nitrogen in dimethyl sulfoxide.

Autogenous canine jugular veins were stored in 15% dimethyl sulfoxide (DMSO) in liquid nitrogen vapor for one to 28 days and then implanted in the carotid artery as autografts. The patency rate at one year was 62.5-87.5%. The patency rate of fresh jugular vein autografts placed in the carotid artery for one year was 75%. Similar autografts stored in liquid nitrogen vapor for one to 28 days without the cryopreservative DMSO exhibited a zero to 12.5% patency rate at one year. Scanning electron microscope studies revealed preservation of theendothelium in DMSO protected veins and a damaged or sloughed endothelium in veins frozen without DMSO cryopreservation.