Current arterial prostheses. Experimental evaluation by implantation in the carotid and circumflex coronary arteries of the dog

Impaired endothelial cell proliferative, migratory, and adhesive abilities are associated with the slow endothelialization of polycaprolactone vascular grafts implanted into a hypercholesterolemia rat model

Most small diameter vascular grafts (inner diameter<6 mm) evaluation studies are performed in healthy animals that cannot represent the clinical situation. Herein, an hypercholesterolemia (HC) rat model with thickened intima and elevated expression of pro-inflammatory intercellular adhesion molecular-1 (ICAM-1) in the carotid branch is established. Electrospun polycaprolactone (PCL) vascular grafts (length: 1 cm; inner diameter: 2 mm) are implanted into the HC rat abdominal aortas in an end to end fashion and followed up to 43 days, showing a relative lower patency accompanied by significant neointima hyperplasia, abundant collagen deposition, and slower endothelialization than those implanted into healthy ones. Moreover, the proliferation, migration, and adhesion behavior of endothelial cells (ECs) isolated from the HC aortas are impaired as evaluated under both static and pulsatile flow conditions. DNA microarray studies of the HC aortic endothelium suggest genes involved in EC proliferation (Egr2), apoptosis (Zbtb16 and Mt1), and metabolism (Slc7a11 and Hamp) are down regulated. These results suggest the impaired proliferative, migratory, and adhesive abilities of ECs are associated with the bad performances of grafts in HC rat. Future pre-clinical evaluation of small diameter vascular grafts may concern more disease animal models with clinical complications. STATEMENT OF SIGNIFICANCE: During the development of small diameter vascular grafts (D<6 mm), young and healthy animal models from pigs, sheep, dogs, to rabbits and rats are preferred. However, it cannot represent the clinic situation, where most cardiovascular grafting procedures are performed in the elderly and age is the primary risk factor for disease development or death. Herein, the performance of electrospun polycaprolactone (PCL) vascular grafts implanted into hypercholesterolemia (HC) or healthy rats were evaluated. Results suggest the proliferative, migratory, and adhesive abilities of endothelial cells (ECs) are already impaired in HC rats, which contributes to the observed slower endothelialization of implanted PCL grafts. Future pre-clinical evaluation of small diameter vascular grafts may concern more disease animal models with clinical complications.

Fístula arteriovenosa como tratamento adjunto na revascularização arterial do membro em risco

Acute arterial occlusion remains a major challenge for the vascular surgeon. The therapeutic approach depends mostly upon the severity of the tissue injury and the duration of symptoms. Several techniques are available in the current therapeutic arsenal, however, regardless of the technique chosen, postoperative factors frequently observed, such as poor outflow status, or even low graft flow, can contribute negatively to the outcome of revascularization. We describe a case of acute limb ischemia, in the postoperative period of a femoral-tibial bypass, which was occluded due to outflow limitation and high peripheral vascular resistance. The patient underwent a second tibial revascularization combined with construction of an arteriovenous fistula, followed by forefoot amputation and partial skin graft. An energetic approach to the at-risk limb makes it possible to reduce unfavorable outcomes, such as amputation and death, and accelerates recovery of tissues affected by acute ischemia.

Numerical Simulation of Hemodynamic Changes in Central Veins after Tunneled Cuffed Central Venous Catheter Placement in Patients under Hemodialysis

The tunneled central venous catheter (CVC) plays an important role for hemodialysis patients, but CVC-related thrombosis in the central veins remain problematic. This study is the first try to numerically find out what hemodynamic parameters are predisposed to the initiation and formation of thrombus after CVC insertion. And the potential relationship between hemodynamic parameters and the incidence rates of thrombosis occurrence was explored. The results revealed that the CVC insertion led to a significant increase of hydraulic resistance, wide-ranging abnormally high wall shear stress (WSS), and a great loss of flow rotation in the vein. Moreover, the clinical data showed that thrombosis mainly occurred at sections where most blood flow lost spiral rotation after the CVC insertion, but no corresponding match was observed between the occurrence of thrombosis and the flow velocity or WSS. We speculate that the destruction of the flow rotation in the central vein is a precursor to the thrombus formation around CVC, and an introduction of spiral flow with the CVC insertion may possibly help to protect the central vein from thrombosis. Further animal and clinical experiments should be carried out to test and verify this speculation.

The Use of Arteriovenous Fistulae as an Adjunct to Peripheral Arterial Bypass: A Systematic Review and Meta-analysis

BACKGROUND

Peripheral arterial bypass is an effective procedure for the management of patients with critical limb ischaemia. However, it is commonly associated with high rates of graft occlusion and subsequent limb loss. This is particularly apparent when the distal anastomosis is to the below-knee arterial segment. A number of studies have suggested that an arteriovenous fistula (AVF) sited at the distal anastomosis may reduce afterload, improve graft patency, and boost subsequent limb salvage. The aim of this study was to assess the effects of adjuvant AVF on the outcomes of peripheral arterial bypass.

METHODS

The following databases were searched up to May 2015: Medline through Pubmed; the Cochrane Library; EMBASE; and reference lists of articles.

STUDY ELIGIBILITY

All randomised controlled and observational studies that assessed the role of AVF as an adjunct to peripheral arterial bypass were included. Studies were required to include at least one pre-defined outcome. Data were extracted and assessed by two reviewers with any disagreements adjudicated on by the senior author. Pooled risk ratios were calculated using a random effects model. Additional subgroup analyses were performed.

RESULTS

Two randomised controlled trials and seven retrospective cohort studies comprising 966 participants were included. Pooled standardized data showed no difference in primary graft patency (pooled RR = 1.25, 95% CI 0.73-2.16), secondary patency (pooled RR = 1.16, 95% CI 0.82-1.66), or limb salvage at 12-months (pooled RR = 1.13, 95% CI 0.80-1.60) for the peripheral bypass with AVF group compared with peripheral bypass alone. Subgroup analysis indicated a reduction in reintervention rates associated with AVF when performed in conjunction with a synthetic graft (pooled RR = 0.55, 95% CI 0.30-0.98).

CONCLUSION

Although adjuvant AVF is not associated with additional operative complication there is little evidence to support its use. The evidence assessing its merits is weakened by small, retrospective studies with heterogeneous cohorts.

Interposition vein cuff for infragenicular prosthetic bypass graft

BACKGROUND

The use of prosthetic grafts such as polytetrafluorethylene (PTFE) or Dacron to bypass occluded arteries in the lower leg is an accepted practice in the absence of suitable autologous vein. The aim is limb salvage or functional improvement in critical limb ischaemia, but patency rates for below knee prosthetic bypasses are low. Creating a vein cuff at the distal anastomosis is thought to improve outcomes. Other techniques including the use of pre-cuffed synthetic grafts, spliced segments of vein and the creation of an arterio-venous fistula (AVF) are also used to improve patency.

OBJECTIVES

To compare the beneficial effects of using vein cuffed prosthetic grafts for below knee bypass in critical limb ischaemia with other types of reconstruction.

SEARCH METHODS

The Cochrane Peripheral Vascular Diseases Group Trials Search Co-ordinator searched the Specialised Register (last searched May 2012) and CENTRAL (2012, Issue 5) for publications comparing prosthetic infragenicular bypass using vein cuffs with other bypass techniques.

SELECTION CRITERIA

Randomised controlled trials comparing interposition vein cuff prosthetic graft with autologous vein graft and non-cuffed prosthetic graft for infragenicular bypass in patients with critical limb ischaemia were included. Trials comparing vein cuff prosthetic grafts with or without AVF and vein cuff prosthetic grafts with pre-cuffed prosthetic grafts were also included.

DATA COLLECTION AND ANALYSIS

The trials were selected and assessed independently by two review authors.

MAIN RESULTS

Six trials with a combined total of 885 patients were included in this review. Only studies using prosthetic PTFE grafts were identified.Two trials compared PTFE graft with or without a vein cuff. In one underpowered trial for below knee bypass the cumulative primary patency rate was statistically significantly higher in the vein cuff group (80.3% versus 65.3% at 12 months and 51.8% versus 29.1% at 24 months, P = 0.03). There was no statistically significant difference in secondary patency (82.9% versus 72.5% and 58.6% versus 34.9%, P = 0.14) and limb salvage rates (86.3% versus 71.8% and 82.6% versus 62.2%, P = 0.08) at 12 and 24 months respectively. The other trial showed no statistically significant difference between the groups at three years in the below knee femoro-popliteal bypasses (primary patency rate 26% (95% confidence interval (CI) 18 to 38) and 43% (95% CI 33 to 58), secondary patency rate 32% (95% CI 23 to 44) and 42% (95% CI 31 to 56) and limb salvage rate 64% (95% CI 54 to 75) and 61% (95% CI 50 to 74) in the collar and no collar groups respectively). In the femoro-distal bypass group, the differences in primary patency, secondary patency and limb salvage rates were also not statistically significant at three years (primary patency rate 20% (95% CI 11 to 38) and 17% (95% CI 9 to 33), secondary patency rate 22% (95% CI 12 to 39) and 20% (95% CI 11 to 35) and limb salvage rate 59% (95% CI 46 to 76) and 44% (95% CI 32 to 61) in the collar and no collar groups respectively).One trial compared pre-cuffed PTFE grafts with vein cuffed grafts. There was no statistically significant difference in primary patency rate (62% pre-cuffed PTFE versus 52% vein cuff PTFE and 49% versus 44%, P = 0.53), secondary patency rate (66% pre-cuffed PTFE versus 53% vein cuff PTFE and 55% versus 50%, P = 0.30) or limb salvage rate (75% pre-cuffed PTFE versus 72% vein cuff PTFE and 62% versus 65%, P = 0.88) at 12 and 24 months respectively.One trial compared spliced vein grafts with vein cuffed PTFE grafts. At 24 months, the secondary patency rate was statistically significantly higher in the spliced vein group (86% in the spliced vein and 52% in the vein cuff group, P < 0.05). There was no statistical significant difference in primary patency rate (44% versus 50%, P > 0.05) and limb salvage rate (94% versus 85%, P > 0.05).Two trials compared vein cuff PTFE grafts with and without AVF. There was no statistical significant difference at 24 months in primary patency rate (29% versus 36%, P = 0.77; 32% versus 28%, P = 0.2), secondary patency rate (40% versus 40%, P = 0.89; 28% versus 24%, P = 0.2) and limb salvage rate (65% versus 70%, P = 0.97; 62% versus 71%, P = 0.3).

AUTHORS' CONCLUSIONS

There is evidence that a vein cuff at the distal anastomosis site improves primary graft patency rates for below knee PTFE graft, but this does not reduce the risk of limb loss. Pre-cuffed PTFE grafts have comparable patency and limb salvage rates to vein cuff PTFE grafts. The use of spliced veins improved secondary patency but this did not translate into improved limb salvage. The use of an AVF alone showed no added benefits. Evidence for a beneficial effect of vein cuffed PTFE grafts is weak and based on underpowered trials. A large study with a specific focus on below knee vein cuff prosthetic grafts, including PTFE, is required.

Spiral laminar flow prosthetic bypass graft: medium-term results from a first-in-man structured registry study

BACKGROUND

A number of surgical strategies and graft enhancements have been trialled to improve the performance of prosthetic grafts. Neointimal hyperplasia may, in part, be a normal cellular response to an abnormal (turbulent) flow environment. This first-in-many study assesses the safety and medium-term patency performance of a new graft designed to induce stable laminar flow through the distal anastomosis.

METHOD

Forty patients who required an infrainguinal bypass graft were recruited/registered from a number of centers in Belgium and The Netherlands. Thirty-nine received a Spiral Laminar Flow graft as part of a standard treatment protocol (23 above-the-knee and 16 below-the-knee bypasses). Kaplan-Meier analyses were used to calculate primary and secondary patency rates.

RESULTS

The 12-, 24-, and 30-month primary patency rates were 86%, 81%, and 81% for above-the-knee bypasses and 73%, 57%, and 57% for below-the-knee bypasses, respectively. In the case of secondary patency rates, numbers were unchanged for above-the-knee bypasses and were 86%, 64%, and 64%, respectively, for below-the-knee bypasses. There were no amputations in the study population.

CONCLUSION

This first-in-man series shows potential for the idea of spiral flow-enhanced prosthetic grafts. As always, randomized studies are required to explore the role of different enhanced prosthetic grafts.

The use of antithrombotic therapies in reducing synthetic small-diameter vascular graft thrombosis

BACKGROUND

Thrombosis of synthetic small-diameter bypass grafts remains a major problem. The aim of this article is to review the antithrombotic strategies that have been used in an attempt to reduce graft thrombogenicity.

METHODS

A PubMed/MEDLINE search was performed using the search terms "vascular graft thrombosis," "small-diameter graft thrombosis," "synthetic graft thrombosis" combined with "antithrombotic," "antiplatelet," "anticoagulant," "Dacron," "PTFE," and "polyurethane."

RESULTS

The majority of studies on antithrombotic therapies have used either in vitro models or in vivo animal experiments. Many of the therapies used in these settings do show antithrombotic efficacy against synthetic graft materials. There is however, a distinct lack of human in vivo studies to further delineate the performance and limitations of therapies displaying good antithrombotic characteristics.

CONCLUSION

Very few antithrombotic therapies have translated into clinical use. More human in vivo studies are required to assess the efficacy and safety of such therapies.

Numerical Model Study of Flow Dynamics through an End-to-Side Anastomosis: Choice of Anastomosis Angle and Prosthesis Diameter

The purpose of this numerical model study was to determine the angle of anastomosis and prosthesis-to-artery diameter ratio that theoretically limits development of myointimal hyperplasia (MIH). Blood flow patterns were investigated in a model simulating a distal end-to-side anastomosis on a 2-mm-diameter artery. Tests were carried out under steady and pulsatile flow conditions with and without taking into account the non-Newtonian behavior of blood and compliance. The wall shear stress gradient (WSSG), a potential factor for development of MIH, was analyzed as a function of the angle of anastomosis (18 degrees , 25 degrees , 35 degrees , and 45 degrees ) and prosthesis diameter (4, 5, and 6 mm). The angle of anastomosis that minimized WSSG was 18 degrees . Prosthesis diameter had no effect on WSSG, with similar results for all three diameters. These findings suggest that surgeons should choose as acute an angle of anastomosis as possible. Prosthesis diameter played no role in reducing WSSG values.

Thrombogenicity and Related Biological Properties of Heparin Bonded Collagen Coated Polyester and Human Umbilical Vein Prosthetic Vascular Grafts

BACKGROUND

Multiple factors contribute to the process of prosthetic graft failure. Some of them are specifically related to the biological behavior of the used materials. To pursue the ideal substitute for the autologous vein graft, many materials have been taken into consideration. Of these, polyester (Dacron) and human umbilical vein (HUV, Dardik) bypass grafts have gained much attention in vascular surgical practice over the years. This study compares the results of both in vivo and in vitro investigations on graft thrombogenicity and neo-intimal formation in collagen-coated heparin bonded Dacron and in HUV bypass grafts. It is an adjunct to our clinical comparison of graft materials in infrainguinal arterial reconstruction.

METHODS

In 12 adult Beagle dogs, a patch was sewn onto the abdominal aorta (Dacron, n = 6; HUV, n = 6). At defined interval times, thrombocyte aggregation was measured with nuclear imaging of 99mTechnetium labeled platelets. Post-mortem histological analysis of the interface between the native vessel wall and the patch was performed in all animals.

RESULTS

At 4 h (2.67, SD = 0.77) and after 2 weeks (2.21, SD = 0.28) after implantation, significantly higher thrombogenicity was measured in the HUV grafts compared to Dacron grafts (1.98, SD = 0.10 and 1.98, SD = 0.11, P = 0.02 and 0.025, respectively). At 4 weeks, no significant difference could be found (HUV, 2.26; SD = 0.29; Dacron, 2.11; SD = 0.16; P = 0.23). Measurement of 'neo-intimal' thickness after explantation of the patch at 28 days after the initial procedure showed a significant difference: in HUV grafts the mean thickness of the inner lining was 0.76 mm (SD = 0.50), compared to 0.16 mm (SD = 0.10) in the Dacron grafts (P = 0.013).

CONCLUSION

HUV grafts showed a higher thrombogenicity at 4 h and 2 weeks after insertion of the graft compared to Dacron grafts. At 4 weeks this difference is not present. After 28 days the inner ('neo-intimal') lining is significantly more pronounced in HUV grafts than in Dacron grafts.

A new device for in vitro evaluation of thrombogenicity

A device to measure the time of coagulation of whole blood has been designed in order to facilitate testing of thrombogenicity of biomaterials. The principle of operation of the apparatus is to record the time taken for a sphere to fall through a sample of blood. The coagulation time is defined here as the time from collection of the blood sample to coagulation, coagulation having deemed to have occurred when the sphere is prevented from falling by the presence of the fibrin-cell network. The device was tested with homogeneous fluids of different viscosity, milk containing different amounts of rennin and calcium chloride and non anti-coagulated whole blood obtained from five volunteers. Repeat measurements made with each homogeneous fluid show that the variance in the transit time of the sphere is small and consistent with small dispersion. In contrast, the onset of coagulation in milk and blood samples was readily detected. The clotting of milk was highly dependant on the concentration of rennin and calcium chloride. The coagulation time of blood samples from five individuals ranged from 23 to 33 min but simultaneous measurements of samples from the same individual in two identical devices agreed to within +/-1 min in all cases. This device may be easily adapted for use in studies to determine the thromboresistance of biomaterials where the onset of coagulation of whole blood in contact with different materials may be readily compared.

Internal Arteriovenous Fistula Within a Radial Forearm Flap – A Novel Technique to Increase Femorodistal Bypass Graft Flow to the Diabetic Foot and Flap Covering Ischaemic Tissue Loss

BACKGROUND

The fascio-cutaneous radial forearm flap is especially suitable to rebuild the contour of the foot, but because of low natural low flow this flap lacks the beneficial effect of large muscle flaps on bypass graft flow. The aim of this study was to introduce a novel technique of flap coverage combined to vascular bypass: an internal av-fistula was created within a radial forearm flap.

METHODS

Nine critically ischaemic limbs were treated with a modified radial forearm flap in the Department of Plastic and Vascular Surgery, Helsinki University Central Hospital 1998-2003. All the patients were candidates for a major amputation unless this combined operation was attempted. A two-team approach was used: the vascular surgeon performed the distal bypass and the radial forearm flap was raised by the plastic surgeon. In eight cases a femorodistal bypass was performed and in the ninth the vein graft supplied the flap directly. The internal fistula within the flap was created between the distal end of the radial artery and either the cephalic vein or the concomitant vein of the radial artery. Flow was measured during surgery.

RESULTS

Vein graft flow increased significantly after the radial forearm flap anastomosis (76 vs 44 ml/min, p=0.016). The flow of both the bypass graft and the flap artery were higher with the av-fistula patent (p=0.016 and p=0.004). Graft patency was 89% at 2 years. Infection was a major cause of amputation, 1- and 2-year limb salvages being 67 and 53%.

CONCLUSION

In a group of diabetic patients increased flow in a vascular bypass graft was achieved by an internal av-fistula within a radial forearm flap. This method is useful in selected cases with poor run off and large ischaemic lesions.

The Impact of Adjuvant Av-fistula on Cuffed Femorocrural PTFE Bypass Grafting: Flow and Pressure Response

OBJECTIVE

The aim of the present study was to assess the effect of an adjuvant av-fistula on prosthetic bypass grafting and whether intraoperative flow measurements could predict patency and adverse events of cuffed femorocrural PTFE bypass with or without an av-fistula.

METHODS

A total of 50 patients in need of vascular reconstruction for critical limb ischaemia (CLI) but with no suitable venous conduit were included.

RESULTS

The flow values in patients with av-fistula were significantly higher (p=0.009) than in the group without the fistula but the higher flow values did not result in improved patency. The maximum flow velocity (Vmax) in the av-fistula group was significantly higher in the immediate postoperative period (p=0.04), but there was no difference in patency. When a flow value of 50 ml/min was used as a cut-off point, patients with a higher flow had significantly better immediate patency (p=0.025).

CONCLUSION

The adjuvant av-fistula neither caused any adverse effects nor had any effect on patency.

Haemodynamic effect of intermittent pneumatic compression of the leg after infrainguinal arterial bypass grafting

Background

Intermittent pneumatic compression (IPC) may increase blood flow through infrainguinal arterial grafts, and has potential clinical application as blood flow velocity attenuation often precedes graft failure. The present study examined the immediate effects of IPC applied to the foot (IPCfoot), the calf (IPCcalf) and to both simultaneously (IPCfoot+calf) on the haemodynamics of infrainguinal bypass grafts.

Methods

Eighteen femoropopliteal and 18 femorodistal autologous vein grafts were studied; all had a resting ankle : brachial pressure index of 0·9 or more. Clinical examination, graft surveillance and measurement of graft haemodynamics were conducted at rest and within 5 s of IPC in each mode using duplex imaging. Outcome measures included peak systolic (PSV), mean (MV) and end diastolic (EDV) velocities, pulsatility index (PI) and volume flow in the graft.

Results

All IPC modes significantly enhanced MV, PSV, EDV and volume flow in both graft types; IPCfoot+calf was the most effective. IPCfoot+calf enhanced median volume flow, MV and PSV in femoropopliteal grafts by 182, 236 and 49 per cent, respectively, and attenuated PI by 61 per cent. Enhancement in femorodistal grafts was 273, 179 and 53 per cent respectively, and PI attenuation was 63 per cent.

Conclusion

IPC was effective in improving infrainguinal graft flow velocity, probably by reducing peripheral resistance. IPC has the potential to reduce the risk of bypass graft thrombosis.

Does an Adjuvant AV-fistula Improve the Patency of a Femorocrural PTFE bypass with Distal Vein Cuff in Critical Leg Ischaemia?—A Prospective Randomised Multicentre Trial

INTRODUCTION

A lack of suitable veins can cause serious problems when attempting to revascularise critically ischaemic legs. Prosthetic grafts have much worse patency in the femocrural position, despite the use of distal anastomotic cuffs. The use of adjuvant AV-fistula at the distal anastomosis should increase the graft flow above the thrombotic threshold velocity and thus increase prosthetic graft patency.

AIM

The aim of the study was to evaluate the benefit of an adjuvant AV-fistula on the patency of a femorocrural PTFE bypass with a distal vein cuff.

MATERIALS AND METHODS

This prospective randomised multicentre trial was conducted in four centres. A total of 59 patients with critical leg ischaemia and no suitable veins for grafting were randomised to receive a femocrural PTFE bypass and distal vein cuff, with or without an adjuvant AV-fistula. Thirty-one patients were randomised to the AV-fistula group (AVFG) and 28 to the control group (CG). Six patients were lost to follow-up during the 2-year study time.

RESULTS

There were six immediate occlusions in each treatment group, but half of these were saved by re-operation. The mean postoperative ankle-brachial index (ABI) was 0.85 in the AVFG and 0.94 in the CG. The primary and secondary patency rate at 2 years was 29 and 40% for the AVFG and 36 and 40% for the CG (NS). Leg salvage at 2 years was 65 and 68%, respectively (NS).

CONCLUSION

Adjuvant AV-fistula does not improve the patency of a femorocrural PTFE bypass with a distal vein cuff.

A Comparison of Distal End-to-side and End-to-end Anastomoses in Femoropopliteal Bypasses

OBJECTIVES

to compare end-to-side (ETS) and end-to-end (ETE) distal anastomoses. Design retrospective cohort study.

METHODS

retrospective cohort study. Between 1988 and 1992, 204 femoropopliteal bypasses (188 patients) were performed for claudication (55%), rest pain (22%) and tissue loss (23%). One hundred and eighteen ETS were compared with 86 ETE in terms of patency or a mean (range) follow-up of 68 (0.5-120) months.

RESULTS

overall patency was 86%, 66% and 57% at 1, 5 and 8 years, respectively. Multivariate analysis showed ETE anastomoses (p =0.04), and also knee bypass ( p =0.05) and venous conduit ( p =0.004) to be significantly associated with impaired patency.

CONCLUSIONS

ETE may improve femoropopliteal bypass patency.

Local haemodynamics of arterial bypass graft anastomoses

One of the main causes of failure of expanded polytetrafluoroethylene (PTFE) bypass grafts used in the lower limbs is the development of myointimal hyperplasia (MIH). Clinical studies show that higher patency rates can be obtained with the use of an autologous vein cuff (the Miller cuff) interposed between the graft and artery. The reasons for the improved performance are still unclear, but preliminary studies suggest that the change in local haemodynamics due to the cuff geometry may be the significant factor rather than the presence of autologous material. If this is the case, then PTFE grafts can be produced with an integral cuff, i.e. a precuffed graft, with similar haemodynamic patterns to that of the Miller cuff. In this paper, two different types of precuffed graft are presented and their flow patterns are compared with those recorded in the Miller cuff and the conventional end-to-side anastomosis. The haemodynamic studies were carried out using optically clear silicone rubber models under simulated in vivo pulsatile flow conditions. Flow structures similar to those observed in the Miller cuff were seen in the precuffed grafts.

Prospective randomised trial of distal arteriovenous fistula as an adjunct to femoro-infrapopliteal PTFE bypass

OBJECTIVES

To compare graft patency and limb salvage rate following femoro-infrapopliteal bypass using ePTFE grafts with and without the addition of adjuvant arterio-venous fistula.

DESIGN

A prospectively randomised controlled trial.

MATERIALS

Patients referred to two teaching hospital vascular surgery units in the U.K. for the treatment of critical limb ischaemia.

METHODS

Eighty-seven patients (M:F; 2.3:1) undergoing 89 femoro-intrapopliteal bypass operations with ePTFE grafts for critical limb ischaemia were randomly allocated to have AVF included in the operative procedure (n = 48) or to a control group without AVF (n = 41). An interposition vein-cuff was incorporated at the distal anastomosis in all patients.

RESULTS

The cumulative rates of primary patency and limb salvage at 1-year after operation for patients with AVF were 55.2% and 54.1% compared to 53.4% and 43.2%, respectively, for the control group. The differences between the AVF and control groups did not reach statistical significance, in terms of either graft patency or limb salvage, at any stage after operation (Log-Rank test).

CONCLUSIONS

AVF confers no additional significant clinical advantage over interposition vein cuff in patients having femoro-infrapopliteal bypass with ePTFE grants for critical limb ischaemia.

Polytetrafluoroethylene (PTFE) femorodistal grafts with a distal vein cuff for critical ischaemia

OBJECTIVE

To analyse the results of PTFE femorodistal bypass grafts using a distal anastomotic vein cuff in the treatment of critical ischaemia.

DESIGN

Prospective study of consecutive patients under the care of a single Vascular Surgeon.

SETTING

A dedicated Vascular Surgical Unit in a University teaching hospital.

SUBJECTS

Fifty-one consecutive femorodistal PTFE grafts were performed on 50 patients (median 74 years, range 58-94 years, 25 men; median ankle:brachial index 0.4). In 28 (55%) a common ostium arteriovenous fistula (AVF) was fashioned at the distal anastomosis.

RESULTS

The 30-day postoperative mortality was 8%, with 10 (21%) major complications. The 1 and 2 year primary patency was 64% and 51%, respectively, with corresponding limb salvage rates of 85% and 80%. The presence of an AVF did not significantly affect graft patency or patient survival, but appeared to hinder limb salvage. The latter effect may be due to bias in patient selection.

CONCLUSION

PTFE femorodistal grafts with a distal vein cuff are a suitable alternative to autogenous saphenous vein in distal reconstructions for critical ischaemia. No conclusions can be made about the efficacy of an AVF in this context.

Role of medial smooth muscle cell function in antithrombogenicity of vein grafts

OBJECTIVES

To determine the significance of prostaglandin I2 (PGI2) production by medial smooth muscle cells (SMCs), we assessed PGI2 production from denuded vein grafts and their antithrombogenicity.

MATERIALS AND METHODS

A total of 30 dogs were divided into two groups: in group I, 14 dogs (27 veins, 188 segments) were used to assay PGI2 production, which was measured as 6-keto-PGF1 alpha. In group II, 16 dogs were used for an implantation study and the thrombus-free surface (TFS) score was calculated 24 h after implantation. Both groups contained the following subgroups: subgroup A, freshly harvested veins; subgroup B, endothelial denuded veins; subgroup C, veins frozen and cryopreserved; subgroup D, veins treated with protease.

RESULTS

Values of 6-keto-PGF1 alpha (pg/mg/min) for basal and stimulated states in subgroup I-A through I-D were 58.0 +/- 8.9 and 530.6 +/- 74.7, 26.3 +/- 4.7 and 82.3 +/- 11.4, 17.9 +/- 1.3 and 39.4 +/- 3.3, and 13.3 +/- 1.8 and 32.2 +/- 6.2, respectively. The PGI2 production in subgroup I-A were significantly higher than those in the other three subgroups (p < 0.01 for basal and stimulated), while the production in subgroup I-B was also significantly higher than those in the remaining two (p < 0.05 for basal and stimulated). The TFS scores in subgroup II-A through II-D were 98 +/- 2%, 90 +/- 2%, 38 +/- 5%, and 15 +/- 7%, respectively, showing significantly superior antithrombogenicity in subgroup II-B, as well as in II-A, when compared with the remaining two (p = 0.014).

CONCLUSION

The amount of PGI2 generated by the medial SMCs may be sufficient for maintaining the antithrombogenicity of the endothelial denuded AVGs.

Effects of albumin coating of knitted Dacron grafts on transinterstitial blood loss and tissue ingrowth and incorporation

Transinterstitial blood loss at implantation and the degree of graft incorporation and inner capsule thickening was compared in serial explants of albumin-coated Dacron versus blood preclotted Dacron grafts in the canine thoracoabdominal aortic position (8 mm internal diameter x 30 cm length). The coated grafts were Bard DeBakey Vasculour II knitted Dacron prostheses impregnated with carbodiimide-cross-linked human albumin. Control grafts were otherwise identical and preclotted with the recipients' whole blood before heparinization during surgery. Transinterstitial blood loss after establishing flow was measured by weighing sponges wrapped around the grafts. Albumin pretreatment resulted in significantly less median blood loss (5.1 g versus 11 g, P=0.04; Mann-Whitney rank sum test). Grafts were explanted at 1 week, 4 weeks, 10 weeks, and 20 weeks. Patency was 100% in both groups. Graft incorporation at explantation was graded by the surgeon as: 1 = none, 2 = minimal, 3 = moderate, or 4 = extensive. No significant differences were noted at any time period. Inner capsule thickness measurements were made every 2.5 mm along the length of all explants. Grafts explanted at 1 week displayed no inner capsules. By 20 weeks, median inner capsule thickness was significantly less in albumin-coated grafts (190 microm versus 235 microm; P<0.0001). These inner capsules in both groups formed as islands, containing abundant myofibroblasts and collagen, covered by endothelial cells and surrounded by residual fibrin coagula. In conclusion, albumin-coated knitted Dacron grafts displayed less transinterstitial blood loss at implantation, and qualitatively similar incorporation, but significantly thinner inner capsules at 20 weeks.

Pantaloon vein graft technique in tibial revascularization with arteriovenous fistula for limb salvage

Graft patency and limb salvage in 91 successive patients operated upon between April 1989 and April 1994 with a human umbilical vein graft combined with an adjunctive arteriovenous fistula at the distal anastomosis of the prosthetic graft in a single limb were evaluated. All the patients operated on were in the 'limb salvage' category. On discharge from hospital 91% of the revascularized limbs were salvaged. Secondary cumulative patency at 57 months was 57.3% with a 61.5% limb salvage rate. This technique, which has anatomical and haemodynamic advantages, results in improved graft patency and foot salvage rates in patients with critical leg ischaemia and poor distal run-off when autologous saphenous vein is absent or cannot be used.

Compliance and diameter mismatch affect the wall shear rate distribution near an end-to-end anastomosis

The development of intimal hyperplasia near the anastomosis of a vascular graft to an artery may be related to changes in the wall shear rate distribution. Mismatches in compliance and diameter at the end-to-end anastomosis of a compliant artery and a rigid graft cause shear rate disturbances that may induce intimal hyperplasia and ultimately graft failure. The goal of this study is to determine how compliance mismatch, diameter mismatch, and impedance phase angle affect the wall shear rate distribution in end-to-end anastomosis models under sinusoidal flow conditions. Wall shear rates are obtained through flow visualization using a photochromic dye. In a model with a well-matched graft diameter (6% undersized), the compliance mismatch causes low mean wall shear rates near the distal anastomosis. Considering diameter mismatch, the wall shear rate distributions in 6% undersized, 16% undersized, and 13% oversized graft models are markedly different at similar phase angles. In the two undersized graft models, the minimum mean shear rate occurs near the distal anastomosis, and this minimum is lower in the model with greater diameter mismatch. The oversized graft model has a minimum mean shear rate near the proximal anastomosis. Thus in all three models, the minimum mean wall shear rate is observed at the site of the divergent geometry. The impedance phase angle, which can be altered by disease states and vasoactive drugs, has a minor effect on the wall shear rate amplitude far from the anastomosis but a more pronounced effect closer to the anastomosis. Mean wall shear rates under sinusoidal flow conditions are significantly lower than under steady flow conditions at the same mean flow rate, but they are fairly insensitive to phase angle changes. In order to avoid the divergent geometry that may cause lower wall shear rates, we recommend that compliance mismatch be minimized whenever possible and that graft diameter be chosen to match the arterial diameter at the relevant physiologic pressure, not at the reduced pressure present when the graft is implanted.

Effect of differential shear stress on platelet aggregation, surface thrombosis, and endothelialization of bilateral carotid-femoral grafts in the dog

PURPOSE

The purpose of this study was to determine the effects of increased shear stress on the aggregability of platelets as they traverse a long, small-caliber (6 mm) Dacron graft in the dog and on the surface thrombosis and endothelialization of such a graft.

METHODS

Each of nine dogs received bilateral carotid-femoral artery grafts, approximately 75 cm long, for 3 months; one graft of each pair had a distal femoral arteriovenous fistula to produce a higher shear rate than the contralateral graft. Platelet aggregation scores were determined on blood withdrawn from the external jugular vein and from the proximal and distal ends of the grafts in each animal. Graft flow rates, which were used in the computation of shear stress, and luminal pressure gradients through grafts were measured during surgery and specimen retrieval. Specimens were studied with light microscopy after hematoxylin and eosin and immunocytochemical staining and by scanning electron and transmission electron microscopy to evaluate the nature, composition, and thickness of the flow surface lining, as well as the transmural healing.

RESULTS

Two high-shear stress and two low-shear stress grafts occluded unilaterally; five dogs had bilaterally patent grafts, allowing comparative analyses. All subjects had low platelet aggregability with aspirin. Platelet aggregation scores taken from proximal and distal ends of the grafts were not significantly different. The high-shear stress grafts had significantly more endothelial-like cell coverage (p < 0.0371) than the low-shear stress grafts, less flow-surface thrombus (p < 0.0056), and a thinner surface lining (p < 0.0029), on both the neointima and pseudointima.

CONCLUSIONS

In subjects with low platelet aggregation scores, long Dacron grafts do not elevate platelet aggregability of blood flowing through them. High-shear stress grafts have less flow surface thrombus, more endothelialization, and a thinner surface lining than do low-shear stress grafts.

Differential effect of the retropleural and retroperitoneal environments on healing of the inner wall of porous fabric prostheses in the thoracic and abdominal aorta of the same dog

Healing of the inner wall of the same preclotted knitted Dacron arterial prostheses was compared in the descending thoracic aorta (DTA) and the abdominal aorta (AA) of the same dog. Each of 16 dogs received this dual implantation with study periods of 4 weeks for five dogs, 8 weeks for five dogs, and 16 weeks for six dogs. Healing was studied with light microscopy, scanning electron microscopy, transmission electron microscopy, and immunocytochemistry identification. The outer capsule was firmly adherent to the Dacron framework of all grafts implanted in the DTA; advanced healing of the inner wall of all thoracic grafts was present by 4 weeks, nearly complete healing by 8 weeks, and complete healing by 16 weeks. In contrast, the outer capsule was either not attached or only loosely adherent to the Dacron framework in eight AA grafts (50%), and in these implants no healing of the inner wall occurred beyond the limited perianastomotic pannus zone. In the other eight implants in which the outer capsule was firmly adherent to the Dacron framework, healing was roughly comparable to that in the grafts implanted in the DTA. This study demonstrated that (1) DTA grafts heal faster and more completely than AA grafts, (2) healing and endothelialization are related to the tightness of the outer capsule, (3) there is a high incidence of loose tissue attachment in the AA, and (4) healing of aortic grafts is site specific.

Hemodynamics of stenotic infrainguinal vein grafts: theoretic considerations

We developed a theoretic model of arterial stenosis to study the relationship between perfusion pressure and regional hemodynamics in stenotic infrainguinal vein grafts in an attempt to identify grafts at high risk for failure. Our model was based on the concept of energy and mass conservation of the flowing blood. We used the modified Bernoulli equation (delta P = 4 delta V2) to calculate the maximum possible intrastenotic peak systolic velocity (PSV) from the systolic blood pressure. PSV was measured by means of duplex ultrasonography in infrainguinal bypasses up to the time of revision (nine grafts) or spontaneous thrombosis (two grafts). We related arm systolic blood pressure, intrastenotic PSV, and prestenotic PSV obtained from duplex examinations conducted prior to graft thrombosis or revision and applied our model to these stenotic vein grafts. Intrastenotic PSV was consistently lower than maximum PSV predicted from the Bernoulli equation. The highest measured intrastenotic PSV of 600 cm/sec would require a minimum perfusion pressure of 144 mm Hg. The lowest measured PSV (20 cm/sec) was considered the minimum "thrombotic threshold velocity." This model predicts that for parabolic profile flow in an 80% diameter-reducing axisymmetric stenosis (96% cross-sectional area reduction), a prestenotic PSV of 20 cm/sec would produce an intrastenotic PSV of 500 cm/sec requiring the equivalent potential energy of 100 mm Hg systolic blood pressure. Our theory implies that in patients with nocturnal hypotension thrombosis of stenotic vein grafts may occur.

Effect of geometric taper on thrombosis in vascular prostheses: an ex vivo study

A common method of assessing the thrombogenicity of prosthetic arterial grafts is to determine the amount of platelets and fibrin that accumulates on their surfaces. This study was designed to compare thrombus formation in cylindrical and tapered vascular grafts (angle of taper 0.5 deg) made from the same biomaterial. Turbulence was generated upstream of the grafts by means of a 75% axi-symmetric stenosis. In general, turbulence is dissipated by the effect of blood viscosity, whereas in a tapered graft, there is an additional reduction in the intensity of turbulence owing to the effect of converging flow through the taper. The aim of this study was to assess the effect of turbulence on thrombus formation in cylindrical and tapered grafts. After blood had circulated for 2 and 4 h through the cylindrical and tapered grafts, which formed part of two parallel ex vivo circuits, platelet deposition and fibrinogen uptake were measured by means of a radioisotope technique. Although the flow disturbances were less in the tapered grafts, no significant difference in platelet and fibrin deposition in the two types of graft was observed at 2 h. For the longer perfusion time, however, increased platelet and fibrin activity was recorded in the annular vortex region in the tapered grafts. In general, neither platelets nor fibrin accumulated in areas of high turbulence intensity.

Vascular prostheses for aortocoronary bypass grafting: a review

Alternative conduits must be chosen when autologous grafts are not available for coronary artery bypass grafting (CABG). Viable grafts do not always have perfect characteristics for CABG, and homologous venous conduits have been used with unsatisfactory results. Many small caliber vascular grafts used for coronary bypass conduits have been developed in the past, but most of them have failed except in rare instances. In this paper the current problems in available conduits, new technologies for improvement, animal models, and possibilities for the future for CABG conduits are discussed.

Proof of fallout endothelialization of impervious Dacron grafts in the aorta and inferior vena cava of the dog

PURPOSE

The purpose of this study was to prove directly whether cells from the blood stream contribute to endothelialization of isolated, impervious Dacron vascular grafts in the dog.

METHODS

We designed an 18 cm, three-component graft with two parallel central Dacron limbs; one was made impervious with silicone rubber, and the other was preclotted. This model was implanted in the canine descending thoracic aorta with 30 microns polytetrafluoroethylene grafts anastomosed at each end. An 8 cm, three-component graft completely coated with silicone rubber was implanted in the canine abdominal aorta and inferior vena cava. Implant periods ranged from 4 to 12 weeks. Flow surfaces were studied by use of stereomicroscopy after being stained with silver nitrate, and by use of scanning and transmission electron microscopy, the inner wall and flow surface were studied by light microscopy after hematoxylin-eosin and immunocytochemical staining (the latter for endothelial and smooth muscle cells), and the full wall was studied by light microscopy after hematoxylin-eosin staining.

RESULTS

Effective prevention of pannus and transmural ingrowth into the impervious central test grafts was achieved, and scattered islands of endothelial cells were conclusively demonstrated on flow surfaces in each of the three implant sites 4 weeks after implantation. In the descending thoracic aorta, where these grafts were also implanted for 8 and 12 weeks, alpha-actin-positive cells and microvessels were found beneath some of the endothelial islands.

CONCLUSION

Fallout endothelialization of Dacron vascular grafts occurs in both the arterial and venous systems of the dog.

Femoropopliteal bypass with externally supported knitted Dacron grafts: a follow-up of 200 grafts for one to twelve years

PURPOSE

This article reports our experience with externally supported, preclotted knitted Dacron grafts in femoropopliteal bypass.

METHODS

This is a retrospective analysis of a consecutive series of 154 patients who received 200 grafts (175 above knee and 25 below knee). Follow-up extended to 12 years (mean 59 1/2 months).

RESULTS

Primary patency rates for the entire series were 75%, 70%, and 47% at 3, 5, and 10 years, respectively. Above-knee grafts had 76%, 71%, and 50% rates and 3, 5, and 10 years, respectively. Below-knee grafts had 65% and 57% at 3 and 5 years, respectively. Limb-salvage rates were 87%, 79%, and 73% at 3, 5, and 10 years, respectively, for the 57 limbs operated on because of critical ischemia. The most significant predictor of graft failure was poor runoff as determined by preoperative arteriography. The effect of poor runoff was most pronounced in the first 3 months.

CONCLUSION

Externally supported, preclotted knitted Dacron grafts provide encouraging primary patency rates for above-knee femoropopliteal bypass. Poor leg vessel runoff is a major determinant of early graft failure.

The uptake of platelets, fibrinogen and leucocytes in ePTFE vascular grafts in relation to blood flow--an experimental study in sheep

The uptake of platelets, fibrinogen and leucocytes in relation to blood flow was studied in ePTFE arterial grafts inserted in both common carotid arteries of 10 adult sheep. Autologous platelets labelled with In-111, autologous leucocytes labelled with Tc-99m and homologous fibrinogen labelled with I-125 were used. The leucocyte uptake was studied in five of the animals. The uptake was measured with a gamma scintillation technique. The blood flow in the graft on one side was restricted to 25 ml/mm, and in the other graft the blood flow was unrestricted (median 170 ml/min). One out of 10 grafts in the reduced flow group occluded but none of the grafts in the unrestricted flow group. The mean thrombus weight was significantly higher in the reduced unrestricted flow (0.28 vs. 0.19 g). At the proximal anastomosis the uptake of platelets, fibrinogen and leucocytes was higher in the reduced than unrestricted flow. The differences were significantly higher for fibrinogen for the whole duration of the experiments and for platelets and leucocytes during the last 48 min. Distally the platelet and fibrinogen uptake were higher in the reduced than in unrestricted flow. The leucocyte uptake did not differ distally. The differences were significant for fibrinogen for the whole duration of the experiments and for platelets during the last 120 min. A comparison between the proximal and distal anastomosis showed a higher uptake distally.

ePTFE grafts for femoro-crural bypass--improved results with combined adjuvant venous cuff and arteriovenous fistula?

Patency rates for long prosthetic bypass grafts with standard anastomoses to single tibial or peroneal arteries are very poor. Adjuvant techniques employed with the aim of improving patency rates include arteriovenous fistula (AVF) at the distal anastomosis to accelerate blood flow above thrombotic threshold velocity (TTV) and a venous cuff (VC) or patch which may reduce or modify anastomotic myointimal hyperplasia within the recipient artery. In a consecutive series of 43 femoro-crural bypasses with ePTFE grafts, adjuvant AVF and VC procedures have been applied in combination. The results are compared with those of an antecedent series of 76 similar grafts with AVF alone and a contemporaneous series of 179 autologous vein grafts. All operations were undertaken for critical limb ischaemia with anastomosis to a single calf or pedal artery. The three groups were well matched for age, sex, diabetes, smoking history, previous surgery and the proportion with rest pain and tissue necrosis. The cumulative patency rate at 2 years for ePTFE grafts with combined AVF and VC was 62% compared to 28% for those with AVF alone and 68% for autologous vein grafts. The patency rate for prosthetic grafts with AVF and VC was significantly higher than AVF alone (p < 0.01) and did not differ significantly from vein grafts. Cumulative limb salvage rates for ePTFE grafts with AVF and VC were 68% at 1 year and 55% at 2 years compared to 38 and 35% for AVF alone and 78 and 69% for vein grafts.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative measurement of wall shear rate by pulsed Doppler ultrasound

A 20 MHz pulsed Doppler ultrasound system was used to obtain steady flow velocity profiles in cylindrical tubes. A deconvolution technique was employed to improve the spatial resolution and the corrected results were compared with Poiseuille's theory. Frequency domain windowing was used to suppress noise in the deconvolution procedure, using a method which reduced distortions at the window boundaries. Wall shear rate was calculated from the measured velocity profiles both before and after deconvolution, and compared with theory to quantify the accuracy of the technique. The results illustrate the significant improvement provided by deconvolution, with the average error in measured wall shear rate reducing from 46 to 12% after applying the technique. Some of the discrepancy is in part attributed to the method used to determine wall shear rate.

Contralateral external carotid-to-middle cerebral artery graft using the saphenous vein. Case report

A variation of the extracranial-intracranial arterial bypass, using a long saphenous vein graft, is presented. The saphenous vein graft was inserted from the contralateral external carotid artery to the distal middle cerebral artery to replace the common and internal carotid arteries in a patient with a large neck tumor that invaded the common and internal carotid arteries, the esophagus, and the trachea. The patient had a positive balloon Matas' test. The saphenous vein was covered with an artificial vascular graft so that turning of the head or movement of the mandible did not displace or compress the graft. A large volume of flow began immediately after anastomosis. A description of the case and the operative technique is presented herein.

Engineering design of vascular prostheses

The replacement and bypass of arteries of diameter greater than 6 mm with textile vascular prostheses has proved very successful since they were first introduced forty years ago. Although manufacturers continue to improve their products and make them of consistent quality for increased safety and performance and to facilitate their use by surgeons, most of the research work in this area is concerned with the development of small-diameter prostheses. Current expanded PTFE and textile prostheses do not perform satisfactorily when their diameters are reduced to less than 6 mm. For the small-diameter prostheses it will be necessary to develop less thrombogenic materials and to design the structure of the prostheses more closely to match the mechanical properties of the natural arteries. The purpose of this paper is to discuss the design requirements and to review the development of large- and small-diameter vascular prostheses.

The effect of a combined thromboxane receptor- and synthesis-antagonist on platelet, fibrinogen and leucocyte uptake in ePTFE grafts: an experimental study in sheep

Arterial expanded polytetrafluoroethylene (ePTFE) grafts were interpositioned in the common carotid arteries of 20 adult sheep. This model was used to study the effect of a combined thromboxane receptor- and synthesis-antagonist, Ridogrel (R 68070), on acute graft patency and platelet, fibrinogen and leucocyte uptake. The animals were randomised to treatment (n = 10) or control groups (n = 10). Treatment was given as an intravenous injection with Ridogrel of 8 mg kg body weight-1. The flow in one of the two inserted grafts was restricted to 25 ml min-1. Autologous 111In-labelled platelets, 125I-labelled fibrinogen and 99m-Tc-labelled leucocytes were injected intravenously and the radioactivity over the vessels measured before and after graft insertion using the gamma scintillation technique. After graft insertion the measurements continued for 4 h at two separate points over the proximal and distal anastomosis areas. In the treatment group six out of 10 grafts with restricted flow remained patent compared with nine out of 10 grafts with flow reduction in the control group (N.S.). The median thrombus weights did not differ significantly. There was no difference in the platelet and fibrinogen activities at the proximal anastomosis but distally the animals receiving treatment had a significant reduction during the first 2 h of the experiment. The leucocyte activity in the treatment group compared to the control group did not differ proximally but distally the activity was significantly higher during the last 3 h of the experiment. In the group with unrestricted flow all grafts in both groups remained patent. The thrombus weight was significantly lower in the treatment group compared with the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Intensive blood flow monitoring following femoro-distal bypass: can early outcome be reliably predicted?

Forty-two femoro-distal vein grafts were studied in the early post-operative period for up to 1 week using a newly developed Doppler monitoring system designed to make frequent, automatic assessments which were stored on tape. Thirty-three grafts remained patent at 1 month with clinical improvement but in nine cases, all performed for critical ischaemia, reconstruction was unsuccessful with six grafts thrombosing and three remaining patent without clinical improvement (haemodynamic failure). The system was shown to provide good quality data, subsequent analysis of which with respect to the time-averaged mean velocity (TAMV) and pulsatility index (PI) revealed good separation between successful and failed grafts. In 30/33 cases a good result was predicted by the early establishment of hyperaemic flow (TAMV greater than 10 cm s-1, PI less than 2), whereas in three a "low flow" state was observed (TAMV less than 10 cm s-1, PI 2-3) which also resulted in a successful outcome. Graft thrombosis was heralded by the rapid development of high pulsatility (PI greater than 6) or a progressive departure from initial hyperaemia towards more pulsatile flow within a few days. Haemodynamic failure was associated with poor flow (TAMV less than 10 cm s-1) accompanied by a PI greater than 3.5. Intensive post-operative monitoring with this new system offers a means whereby early graft outcome can be reliably predicted and also has the potential to study the effects of any intervention used to salvage high risk grafts.

Acute thrombogenicity of collagen coating of dacron grafts: an experimental study in sheep

The early stages of platelet accumulation in two types of sealed interposition Dacron grafts implanted in the carotid arteries of sheep have been studied. One type was externally coated with collagen (Haemaguard) while the other experimental conduit had an additional inner lining of the same substance. 32P-labelled platelets were used to assess platelet accumulation and corrections for wall absorption were calculated. The activities in both types of grafts were highest proximally and decreased towards the distal anastomosis. The increases in the doubly-sealed grafts were larger than in those that had been single sealed, presumably reflecting activation of platelets in contact with collagen at the graft-wall and bloodstream interface. In addition, a significantly larger amount of thrombus was formed in the doubly-sealed grafts 4 h after reperfusion. It is reasonable to assume that increased acute thrombogenicity due to direct collagen-blood contact on graft surfaces is unfavourable to long term patency.

Assessment of pressure drop flowrate relationship of axillobifemoral bypass grafts

An in vitro study was conducted to measure the pressure distributions in knitted polyester axillobifemoral bypass grafts under steady and pulsatile flow of a Newtonian fluid. The relationship between pressure drop and flowrate was determined across the outflow branches of two types of axillobifemoral grafts. Pressure losses were greater across the 90 degrees side branch of the T-type graft than in the Y-type graft at all Reynolds numbers studied. Mean pulsatile pressure drops in the T graft were greater than those measured in steady flow at the same Reynolds number. Estimates of the likely in vivo pressure drops are made under resting and exercise flow conditions. It is suggested that the Y graft is less likely to have a significant effect on blood supply to the lower limbs, even at high blood flowrates.

Angiographic assessment of arterial outflow: predictive value of a new classification system

A prospective study with 4 years of follow-up involving 127 consecutive symptomatic patients (60.6% with claudication, 39.4% with critical ischemia) who underwent aortobifemoral bypass surgery is described. A new grading system for the classification of arterial outflow was applied to determine its usefulness in predicting the outcome of surgery. Preoperative angiograms were numerically scored according to the arterial outflow status at the level of main segmental involvement. Higher scores corresponded to worse outflows. Outflow scores ranged between 1 and 10 with a mean of 3.6 +/- 0.24. The main comparison was between patients with scores of less than 5 (group A, n = 80) and patients with scores of 5 or more (group B, n = 47). Better outflow was associated with higher postoperative mean increases in the ankle-brachial index (ABI) (group A, 0.35 +/- 0.03; group B, 0.17 +/- 0.04; P less than .001) and transcutaneous oximetry (PtcO2) (group A, 15.4 mm Hg +/- 1.8; group B, 8.4 mm Hg +/- 3.0; P = .01). At 4-year follow-up, group A had higher cumulative rates of patency (98.3% vs 78.0%, P less than .001), symptomatic relief (84.0% vs 23.3%, P less than .001), and palliation (67.0% vs 19.9%, P less than .001). In conclusion, angiographic outflow, as evaluated with the system described, successfully helped predict postoperative increases in ABI and PtcO2 and the cumulative rates of graft patency, symptomatic relief, and palliation.

The effect of a thromboxane receptor antagonist on acute ePTFE arterial graft thrombogenicity--an experimental study in sheep

ePTFE interposition grafts in the common carotid arteries of 16 adult sheep were used to study the effect of a thromboxane receptor antagonist. Vapiprost (GR 32191, Glaxo research group, London, UK). After insertion of the grafts the sheep were randomised to treatment (n = 8) or control (n = 8). Treatment was given as an intravenous injection with GR 32191 of 1 mg/kg body weight. The flow in one of the two inserted grafts was restricted from normal (190 ml/min) to 25 ml/min. Autologous 111-In labelled platelets and human 125-I labelled fibrinogen were injected intravenously. The radioactivity over each graft was measured for 4 h at two separate points with a gamma scintillation technique. Due to technical complications immediately before the start of the measurements one sheep in each group was excluded from the study. In the treated group three out of seven grafts with restricted flow occluded compared to all seven grafts with a flow reduction in the control group (p less than 0.05). The grafts with unrestricted flow occluded in two of seven in both groups. In the open grafts with unrestricted flow the fibrinogen activity was significantly reduced in the treated group compared to the control group. The platelet activity was not significantly reduced. It is concluded that GR 32191 significantly reduced the fibrinogen uptake as well as graft occlusions of ePTFE grafts in a low flow situation.

The use of an adjuvant arterio-venous shunt in prosthetic femoro-crural bypass

In limb threatening ischaemia the use of a long prosthetic graft to the distal calf vessels may be the only alternative to a primary amputation. Patency rates in these situations are relatively poor and the use of an adjuvant arteriovenous fistula (AVF) at the distal anastomosis to increase the velocity of flow above the thrombotic threshold seems logical. We present follow-up of up to 8 years in a series of 80 consecutive reconstructions to the distal crural vessels using human umbilical vein with an adjuvant AVF. The mean age of the patients was 68.3 years and 12 (15%) were diabetic. The indication for surgery was limb threatening ischaemia in all cases; 41 (51.2%) for established necrosis and the remaining 39 (48.8%) for severe ischaemic rest pain. The mean ankle:brachial index was 0.23. The common ostium (CO) configuration of AVF was used in 50 cases (62.5%) and pre-anastomotic in 30 cases (37.5%). The volume blood flow in 49 cases, measured peroperatively using an electromagnetic flowmeter, was 116 ml/min (+/- 62.5) with the shunt closed and 283 ml/min (+/- 132.2) with the shunt open (p less than 0.01, Student's t-test). Cumulative patencies of all grafts were 39% at 2 years and 29% at 4 years. The age of the patient and the type of AVF, common ostium or pre-anastomotic, had no significant effect on patency rates, but grafts under 70 cm in length had a cumulative patency at 2 years of 42.5% compared to 13.5% for those over 70 cm in length (p less than 0.01). Cumulative patencies were derived by life table analysis and compared using the log rank test. The size of the recipient vessels at the distal anastomosis is thought to be crucial to the outcome of these operations. An increase in velocity of flow produced by an AVF, increases the wall shear stresses at this site and may lead to an increased tendency for intimal hyperplasia. It may be possible to mitigate these effects by the use of additional surgical or pharmacological techniques and further studies are required.