Graft material, length, and diameter determine in patency of small arterial prostheses in dogs

Induction of Neointima Formation by Platelet Derived Angiogenesis Fraction in a Small Diameter, Wide Pore, PTFE Graft

Enhancement of endothelialization and patency of a small diameter (2 mm), wide pore, PTFE graft was attempted by coating the luminal surface with a platelet derived angiogenesis fraction (PDAF) and implanting it in a rat model. PDAF was delivered to the grafts by combining it with a carrier polymer. PDAF-treated grafts were initially implanted in the retroperitoneum for 21 days followed by removal of one for histology and in situ end to side bypass to the infrarenal aorta for the other. Vascularized grafts were examined at 14 days for patency and 100 days for patency and histology. Significant differences were noted in transmural ingrowth of capillaries and tissue at 21 days post implantation in PDAF-treated verses untreated grafts. Similarly, near significance was noted in capillary ingrowth and significance was noted in tissue ingrowth at 100 days in PDAF-treated grafts. Despite favorable trends particularly early in the time course, no significant differences in graft patency, endothelialization, or hydroxyproline content was demonstrated between PDAF-treated and untreated grafts. Results of this preliminary study are encouraging for further study of PDAF-treated PTFE grafts and the potential that rapid vascularized neointima formation results improved in graft patency rates.

Venographic Changes Associated with Seeded and Nonseeded Vena Cava Grafting

Thirty-four mongrel dogs had their inferior vena cava replaced by a 10 cm porcine endothelial cell seeded or nonseeded polytetrafluroethylene graft. Graft patency at four days (documented by cavograms) was then correlated with that noted at the time of sacrifice (day 32). The overall day 32 patency rate was 32 %. Grafts seeded and treated with aspirin had the highest patency rate. Collaterals were noted on all cavograms showing clotted grafts and on none of the patent grafts. Large collaterals were seen in 25% of the subjects, and moderate and minimal collateralization occurred in 75% of the cases. No correlation was noted between graft occlusion and symptomatology.

Postoperative duplex scan surveillance of axillofemoral bypass grafts

OBJECTIVE

Duplex scan surveillance (DS) for axillofemoral bypass grafts (AxFBGs) has not been extensively studied. The intent of this study was twofold: 1, to characterize the flow velocities within AxFBGs; and 2, to determine whether postoperative DS is useful in assessment of future patency of AxFBGs.

METHODS

We identified all patients who underwent AxFBG procedures between January 1996 and January 2001 at our combined university and Veterans Affairs hospital vascular surgical service. All grafts were performed with ringed 8-mm polytetrafluoroethylene with the distal limb of the axillofemoral component anastomosed to the hood of the femoral-femoral graft. DS was every 3 months for 1 year and every 6 months thereafter. Duplex scan results were compared in primarily patent grafts with grafts that thrombosed. Graft failures from infection were excluded. Influences of ankle-brachial index, blood pressure, outflow patency, operative indication, and comorbidities on graft patency were analyzed.

RESULTS

One hundred twenty patients underwent AxFBG procedures. Twenty-eight were excluded because of infection or death before surveillance examination. Fourteen were lost to follow-up, 23 had failed grafts from occlusion, and 55 had grafts that remained patent. In the 78 patients evaluated during long-term follow-up period, the mean peak systolic velocities (PSVs) at the proximal (axillary) anastomosis during the first postoperative year ranged from 153 to 194 cm/s. Mean PSVs at the mid portion of the axillofemoral graft during the first postoperative year ranged from 100 to 125 cm/s, whereas those for the distal axillofemoral anastomosis ranged from 93 to 129 cm/s. Mean midgraft and distal anastomotic velocities obtained before thrombosis were significantly lower in the thrombosed grafts compared with the last recorded velocities at the same sites in the patent grafts (mean PSV, 84 versus 112 cm/s; P =.015; mean PSV, 89 versus 127 cm/s; P =.024, respectively). Forty-eight percent of occluded grafts had a mean midgraft PSV at last observation of less than 80 cm/s. Blood pressure correlated with midgraft velocity (r = 0.415; P <.05). With multivariate logistic regression analysis, a mean midgraft velocity less than 80 cm/s was the sole independent factor associated with graft failure (P <.01). No patients with midgraft velocities greater than 155 cm/s had occlusion.

CONCLUSION

Flow velocity varies widely within and among AxFBGs. Patency of AxFBGs is associated with higher midgraft PSV, and thrombosis with midgraft velocities less than 80 cm/s.

The role of vascular smooth muscle cell integrins in the compaction and mechanical strengthening of a tissue-engineered blood vessel

Vascular smooth muscle cells (VSMC) influence vessel structure and function during normal development, and in disease states. VSMC interactions with extracellular matrix, via cell surface integrins, play an important role in these processes. A greater understanding of the molecular basis of these interactions is also critical to advances in the field of cardiovascular tissue engineering. This study examined the role of VSMC integrins in the spontaneous compaction and eventual strengthening of a rudimentary tissue-engineered blood vessel (TEBV) consisting of a fibrillar type I collagen network populated by human aortic smooth muscle cells. Using integrin subunit-specific antibodies, we demonstrated that anti-beta1 (Mab13 and P4C10) and anti-alpha2 (P1E6) antibodies that inhibit aortic smooth muscle cell (AoSMC) adhesion to collagen, also significantly inhibit TEBV compaction during the 24-hour period following TEBV construction. However, no difference in the tensile stress of antibody-treated and control TEBVs was observed at this time point. In contrast, 72 hours after construction, the inhibitory effect of anti-integrin antibodies on compaction had been overcome but tensile stress was decreased in TEBVs treated with anti-alpha2/anti-beta1 antibodies when compared to controls. These data provide evidence linking VSMC integrins, specifically the alpha2beta1 integrin, with the initial compaction, as well as, the postcompaction strengthening of the TEBV.

Endothelial cell seeding: a review

The concept of endothelial cell seeding, designed to provide vascular grafts with a nonthrombogenic lining, has progressed from crude animal experiments during the past two decades to detailed in vitro functional studies using human cells. Although favorable results have been obtained in animal studies this has yet to be translated to humans, where current application of these techniques has been limited to a very few clinical trials. The history, current status and future directions are reviewed herein.

Radiofrequency plasma deposition of oxygen-containing films on polystyrene and poly(ethylene terephthalate) substrates improves endothelial cell growth

Polystyrene and poly(ethylene terephthalate) substrates were modified by radiofrequency plasma deposition with organic vapors comprised of carbon, oxygen, and hydrogen (acetone, methanol, glutaraldehyde, formic acid, allyl alcohol, and ethylene oxide). The treatments resulted in the deposition of a film at least 100 A thick containing up to 26% atomic oxygen at the surface. A high oxygen incorporation was observed for vapors with a large oxygen-to-carbon ratio. Bovine aortic endothelial cell growth measured on acetone, methanol, and glutaraldehyde films was linearly correlated with the oxygen content of the treated surfaces. Nitrogen was incorporated in the surface by blending nitrogen gas into the organic vapor used for plasma deposition. The resulting nitrogen-containing substrates exhibited a high affinity for serum fibronectin but a moderate cell growth.

An experimental collagen-impregnated Dacron graft: potential for endothelial seeding

This study evaluates the potential for endothelial seeding of a collagen-impregnated Dacron graft with or without surface modifiers (fibronectin, heparin) to attach and retain these cells during flow. Human umbilical endothelial cells were harvested, cultured, labeled with Indium111-oxine and seeded onto 30 mm X 4 mm diameter grafts. Six graft surfaces were studied: 1) a collagen-impregnated Dacron graft, HemashieldR (C); 2) C + fibronectin (C + F); 3) C + heparin (C + H); 4) C + F + H; 5) HytrelR + F (Hyt + F); and 6) Hyt + F + H. Radioactive loss determined the percentage attachment and then percentage retention of labeled inoculum after a one-hour in vitro perfusion. Scanning electron and light microscopy demonstrated the endothelium on the graft surface following perfusion. Fibronectin-coated grafts had a significantly higher percentage attachment than those without fibronectin (ANOVA, P less than 0.05). However, the percentage retention following perfusion was similar for all Dacron grafts and statistically inferior to the HytrelR grafts studied (ANOVA, P less than 0.05). SEM evaluation of the C + F + H graft surface was qualitatively the most impressive Dacron surface for seeding, yet was inferior to the HytrelR graft. We conclude that fibronectin benefits the initial attachment of endothelium to collagen-coated Dacron rivaling the HytrelR surface. Fibronectin does not improve percentage retention of the HemashieldR surface during perfusion, therefore, some of its initial benefit is lost.

The pony as an animal model for vascular implants

This study evaluated the pony as a potentially suitable model for vascular implant research. Healthy, conditioned ponies were randomly assigned to one of three groups: group I, carotid artery autografts (n = 6); group II, e-PTFE carotid interpositional grafts (n = 5); and group III, e-PTFE carotid interpositional grafts plus aspirin (10 mg/kg) and dipyridamole (3.5 mg/kg) drug administration. It was found that autografts remained patent longest (mean = 396.2 days; grafts were still patent at time of writing) followed by group III grafts (157.5 days), with group II grafts remaining patent for the shortest duration (61.1 days), (p less than 0.01). Patency was determined using two-dimensional real-time ultrasonography with Doppler velocimetry and/or arteriography. It was demonstrated that the pony's response to antithrombotic drugs was consistent and comparable to that in other animal models, both with respect to platelet function and affect on patency rate. The combination of the ease of surgical manipulation, drug administration, and platelet function testing, the comparable size of the pony and its heart and blood vessels to that of an adult human, the long life span of ponies, and the patency results of this study have demonstrated that the pony is a valuable animal model for vascular research.

The influence of endothelial seeding and platelet inhibition on the patency of ePTFE grafts used to replace small arteries--an experimental study

Previous studies on the influence of endothelial seeding on graft patency have shown that significant improvement has only been achieved with Dacron and an experimental, porous PTFE graft. Methods of assessing patency or showing statistical significance could be questioned in some of these studies. To determine if the combination of endothelial cell seeding and antiplatelet agents would improve patency in small-diameter, commercially available expanded polytetrafluorethylene (ePTFE) grafts, we placed ePTFE grafts into the left carotid position in two groups of mongrel dogs. All grafts were 4 mm internal diameter and 60 mm long, and were interposed in an end-to-end fashion. Both groups received aspirin (80 mg daily) and dipyridamole (25 mg daily) for 14 days, beginning immediately prior to surgery. In Group I (n = 12), the grafts were seeded with enzymatically harvested autogenous endothelium just prior to implantation; in Group II (n = 10) the grafts were not seeded. All grafts were removed at 30 days. Seven of 12 (58%) seeded grafts, but only one control graft (10%) remained patent (P = 0.03). Six of the seven seeded grafts exhibited surface endothelium, but the single patent control graft did not. The inner capsule of the seeded grafts consisted of a monolayer of endothelium and a thin acellular subendothelial matrix with an average thickness of 8 mu. We conclude that a 14-day course of anti-platelet agents combined with endothelial seeding of ePTFE resulted in significantly improved patency compared to controls, with most patent, seeded grafts developing an endothelial lining in 30 days.

Short-term in vivo stability of endothelial-lined polyester elastomer and polytetrafluoroethylene grafts

A fibronectin substrate will significantly enhance the strength of endothelial cell attachment on grafts constructed of polyester elastomer (PE) and polytetrafluoroethylene (e-PTFE). This experiment was undertaken to determine the short-term in vivo stability of endothelium on these fibronectin coated surfaces. Eight mongrel dogs underwent bilateral carotid artery replacement with both graft materials. All grafts were inoculated with 2,000 cells/mm2 using cultured autogenous venous endothelium labelled with Indium-111-oxine. The Indium-111 label in the grafts was measured immediately prior to implantation, after 1 hour of in vivo perfusion, and at explantation after 24 hours. The percentage of inoculated cells attached to the grafts before perfusion was similar for both materials, 93.3 +/- 3.0% versus 92.2 +/- 7.2%, for PE and e-PTFE respectively. All grafts were patent at one hour after implantation. PE grafts were found to have 93.8 +/- 3.9% of the attached cells present at one hour while e-PTFE grafts had only 54.5 +/- 10.8% remaining, p less than .001. After 24 hours, 5/8 (62.5%) e-PTFE grafts and 2/8 (25.0%) PE grafts remained patent, p = .13. Of the patent grafts however, endothelial cell retention was still superior on the PE grafts with 78.0 +/- 0.6% of the attached cells remaining compared to only 24.5 +/- 6.1% on e-PTFE, p less than .001. Occluded PE grafts had fewer cells remaining at 24 hours than patent ones, 78.0 +/- 0.6% versus 31.1 +/- 32.8%, respectively, p = .13. Histologically, patent PE grafts demonstrated nearly confluent endothelial monolayers while e-PTFE had patches of endothelial cells surrounded by a platelet-fibrin carpet.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro C5a generation by synthetic vascular prostheses: implications for graft incorporation in vivo

C5a, released during activation of the complement cascade, was measured by radioimmunoassay in human plasma incubated with Dacron, preclotted Dacron, glutaraldehyde treated human umbilical vein (HUV), polytetrafluoroethylene (PTFE), and Dacron-collagen composite vascular grafts. Expressed as percent of C5a in control plasma incubated without graft, C5a generation by preclotted Dacron, and by the HUV was similar to that by Dacron (941 +/- 206% S.E.M.), while that due to PTFE was markedly less (p = 0.005). The Dacron-collagen composite vascular graft also generated significantly less C5a than Dacron and was similar to PTFE in this respect. These results expand on previous work suggesting that lower C5a generation by PTFE explained the negligible polymorphonuclear infiltrate seen on its surface in vivo, allowing it to endothelialise as rapidly as Dacron despite poorer attachment of seeded endothelial cells. The role of complement as a factor limiting endothelialisation of synthetic vascular prostheses needs further investigation.