The effect of low molecular weight dextran on platelet deposition onto prosthetic materials

Efficacy of Dextran Solutions in Vascular Surgery

The purpose of this paper is to discuss the role and efficacy of dextran in vascular procedures using evidence-based data from the review of surgical literature. A MEDLINE search using “dextran,” “vascular surgery,” and “antiplatelet therapy” as keywords was performed for English-language articles. Further references were obtained through cross-referencing the bibliography cited in each work. Dextran is commonly used in carotid endarterectomy (CEA) patients where the embolic rate is reduced by 46%, resulting in fewer procedure-related strokes. As a prophylactic agent against thrombosis, multiple randomized studies have reported its benefit over other antithrombotic medications. Dextran is also particularly useful in “difficult” infragenicular lower extremity bypasses where artificial grafts (such as polytetrafluoroethylene [PTFE] or umbilical vein) are used in the setting of poor outflow vessels, or those with composite grafts and small-caliber venous conduits. Distal bypasses with adjunctive procedures (eg, arteriovenous fistula or anastomotic cuffs) also have a better outcome with the addition of dextran. Dextran has numerous important implications in vascular surgery, in particular with CEA patients or “difficult” infragenicular bypasses. Its effectiveness with endovascular stents remains unknown.

In vitro and in vivo evaluation of blood coagulation activation of polyvinyl alcohol hydrogel plus dextran-based vascular grafts

Polyvinyl alcohol hydrogel (PVA) is a water-soluble synthetic polymer that is commonly used in biomedical applications including vascular grafting. It was argued that the copolymerization of PVA with dextran (Dx) can result in improvement of blood-biomaterial interactions. The focus of this experimental study was to assess that interaction through an in vivo and in vitro evaluation of the coagulation system activation. The thrombogenicity of the copolymer was determined by quantification of platelet adhesion through the lactate dehydrogenase assay, determination of whole blood clotting time, and by quantification of platelet activation by flow cytometry. The thrombin-antithrombin complex blood levels were also determined. The obtained results for the in vitro assays suggested a non-thrombogenic profile for PVA/Dx. Additionally in vivo coagulation and hematological parameters were determined in an animal model after PVA/Dx vascular graft implantation. For coagulation homeostasis assessment, the intrinsic and extrinsic pathway's activation was determined by measuring prothrombin time (PT) and activated partial thromboplastin time (aPTT). Other markers of coagulation and inflammation activation including d-dimers, interleukin-6, and C-reactive protein were also assessed. The PVA/Dx copolymer tended to inhibit platelet adhesion/activation process and the contact activation process for coagulation. These results were also confirmed with the in vivo experiments where the measurements for APTT, interleukin-6, and C-reactive protein parameters were normal considering the species normal range of values. The response to those events is an indicator of the in vitro and in vivo hemocompatibility of PVA/Dx and it allows us to select this biomaterial for further preclinical trials in vascular reconstruction.

Intraoperative use of dextran is associated with cardiac complications after carotid endarterectomy

OBJECTIVE

Although dextran has been theorized to diminish the risk of stroke associated with carotid endarterectomy (CEA), variation exists in its use. We evaluated outcomes of dextran use in patients undergoing CEA to clarify its utility.

METHODS

We studied all primary CEAs performed by 89 surgeons within the Vascular Study Group of New England database (2003-2010). Patients were stratified by intraoperative dextran use. Outcomes included perioperative death, stroke, myocardial infarction (MI), and congestive heart failure (CHF). Group and propensity score matching was performed for risk-adjusted comparisons, and multivariable logistic and gamma regressions were used to examine associations between dextran use and outcomes.

RESULTS

There were 6641 CEAs performed, with dextran used in 334 procedures (5%). Dextran-treated and untreated patients were similar in age (70 years) and symptomatic status (25%). Clinical differences between the cohorts were eliminated by statistical adjustment. In crude, group-matched, and propensity-matched analyses, the stroke/death rate was similar for the two cohorts (1.2%). Dextran-treated patients were more likely to suffer postoperative MI (crude: 2.4% vs 1.0%; P = .03; group-matched: 2.4% vs 0.6%; P = .01; propensity-matched: 2.4% vs 0.5%; P = .003) and CHF (2.1% vs 0.6%; P = .01; 2.1% vs 0.5%; P = .01; 2.1% vs 0.2%; P < .001). In multivariable analysis of the crude sample, dextran was associated with a higher risk of postoperative MI (odds ratio, 3.52; 95% confidence interval, 1.62-7.64) and CHF (odds ratio, 5.71; 95% confidence interval, 2.35-13.89).

CONCLUSIONS

Dextran use was not associated with lower perioperative stroke but was associated with higher rates of MI and CHF. Taken together, our findings suggest limited clinical utility for routine use of intraoperative dextran during CEA.

In vitro biological performances of phosphorylcholine-grafted ePTFE prostheses through RFGD plasma techniques

Arterial prostheses made of microporous Teflon (ePTFE) are currently used in vascular surgery as bypasses for small and medium vessels. However, several clinical complications, such as thrombosis, frequently occur in these prostheses when implanted in humans. In this work, an original strategy was developed to improve the hemocompatibility of ePTFE prostheses, based on glow-discharge surface modification followed by chemical grafting of phosphorylcholine, known for its hemocompatible properties. This procedure leads to a covalent attachment of the molecules, therefore preventing their removal by shear stress induced by blood flow at the implant wall. The improvement of the blood compatibility properties of the modified ePTFE arterial prostheses have been investigated by in vitro tests such as thromboelastography, neutrophil adsorption, platelet aggregation, and cell cultures. These in vitro tests put in evidence that thrombogenicity index, platelet aggregation, and neutrophil adhesion were decreased by the molecule grafted on the prostheses. Moreover, the cell growth on the surface of the PRC-grafted prostheses was greatly enhanced in comparison to the virgin prosthesis. Based on these results, it could be concluded that PRC grafting on ePTFE prostheses permit to improve in vitro hemocompatibility and biocompatibility in comparison with their virgin counterpart.

Dextran 40 reduces in vitro platelet aggregation in peripheral arterial disease

Dextran 40 has been used to prevent post-operative thrombosis. However, its antithrombotic and antiplatelet effects in peripheral arterial disease (PAD) are poorly understood. We studied the in vitro effects of Dextran 40 on platelet function in control subjects and PAD patients using whole blood methods. Platelet function was assessed in 20 control subjects and 20 PAD patients. Spontaneous platelet aggregation (SPA) and agonist-induced platelet aggregation in response to increasing concentrations of Dextran 40 in vitro were measured by whole blood aggregometry. Flow cytometric measurements of platelet P-selectin, GpIIb/IIIa, GpIb and PAC-1 binding were also performed. There was no difference in SPA or ADP-induced aggregation in control patients with Dextran 40 in vitro. However, Dextran 40 inhibited collagen-induced aggregation in control patients (P < 0.05, Friedman test). In PAD patients, SPA and ADP (1 microM)-induced aggregation were significantly reduced by Dextran 40 in vitro (P < 0.001, Friedman test). In PAD patients, collagen-induced platelet aggregation (1 and 5 microg/ml) was significantly reduced by Dextran 40 in vitro (P < 0.01, Friedman test). GpIIb/IIIa, PAC-1 and P-selectin expression were significantly reduced in whole blood samples from PAD patients following incubation with Dextran 40 (P < 0.05, Wilcoxon rank test) but not in samples from control patients. Dextran 40 reduces spontaneous and agonist-induced platelet aggregation as well as the surface expression of markers of platelet activation in PAD patients. This antiplatelet effect may be of benefit to patients undergoing vascular surgical procedures where thrombosis is a significant risk.

Platelet activation in a circulating flow loop: combined effects of shear stress and exposure time

Measurement of small changes in platelet activation state (PAS) in circulating stenotic systems in vitro has been problematic because of a paucity of real-time assay methods and circulation systems of low platelet-activating potential. PAS was measured by a modified prothrombinase assay in which activated platelets provide the essential cofactors in the activation of prothrombin by factor Xa. Chemical modification of the prothrombin ensures that the thrombin produced, while assayable, does not activate platelets. Human platelets were circulated in loops in which exposure to shear stress was adjusted by independently varying flow rate, viscosity, and the time of exposure to shear. Although with some differences in platelet response to different conditions of stress, the PAS directly increased with time of circulation, shear stress, and time of exposure to shear. The results show that low-level platelet activation caused by shear stress in a circulation loop can be quantitatively assessed in near-real time in a system of tube geometry. They confirm previous results obtained in non-circulating systems that exposure of platelets to shear conditions on the same order as found in the vasculature causes significant platelet activation, and that this activation is dependent on both shear stress and time of exposure.

The role of platelets in peripheral vascular disease

Platelets play a major role in acute ischaemic syndromes and peripheral vascular disease. They are involved in the development and progression of atherosclerosis, native vessel and graft thrombosis. They have a central role in the development of restenosis and reocclusion after peripheral percutaneous transluminal angioplasty. Antiplatelet therapy has been shown to be beneficial in patients undergoing peripheral vascular surgery or radiological intervention. Yet current routine therapy, namely aspirin and dipyridamole are limited in their mode of action and efficacy. Recent developments in the understanding of platelet function has led to the development of new more potent drugs such as clopidogrel. Combination of drugs and more specific investigation of individual platelet function may well result in improved bypass and angioplasty patency rates. The results of proposed large randomised controlled trials on the role and safety of aspirin and clopidogrel are awaited with interest. Given the importance of platelets in peripheral vascular disease highlighted in this review, achieving an optimal safe anti-platelet effect for each patient with peripheral vascular disease should be the target of future research.

In vitro and in vivo studies of PEO-grafted blood-contacting cardiovascular prostheses

The initial step of thrombus formation on blood-contacting biomaterials is known to be adsorption of blood proteins followed by platelet adhesion. Poly(ethylene oxide) (PEO) has been frequently used to modify biomaterial surfaces to minimize or prevent protein adsorption and cell adhesion. PEO was grafted onto a number of biomaterials in our laboratory. Nitinol stents and glass tubes were grafted with PEO by priming the metal surface with trichlorovinylsilane (TCVS) followed by adsorption of Pluronic and y-irradiation. Nitinol stents were also coated with Carbothane for PEO grafting. Chemically inert polymeric biomaterials, such as Carbothane, polyethylene, silicone rubber, and expanded polytetrafluoroethylene (e-PTFE), were first adsorbed with PEO-polybutadiene-PEO (PEO-PB-PEO) triblock copolymers and then exposed to gamma-irradiation for covalent grafting. For PEO grafting to Dacron (polyethylene terephthalate), the surface was sequentially treated with PEO-PB-PEO and Pluronics followed by gamma-irradiation. In vitro studies showed substantial reduction in fibrinogen adsorption and platelet adhesion to the PEO-grafted surfaces compared with control surfaces. Fibrinogen adsorption was reduced by 70-95% by PEO grafting on all surfaces, except for e-PTFE. The platelet adhesion corresponded to the fibrinogen adsorption. When the PEO-grafted surfaces were tested ex vivo/in vivo, however, the expected beneficial effect of PEO grafting was inconsistent. The beneficial effect of the PEO grafting was most pronounced on the PEO-grafted nitinol stents. Thrombus formation was reduced by more than 85% by PEO grafting on metallic stents. Only moderate improvement (i.e. 35% decrease in platelet deposition) was observed with PEO-grafted tubes of polyethylene, silicone rubber, and glass. For PEO-grafted heart valves made of Dacron, however, no effect of PEO grafting was observed at all. It appears that the extent of thrombus formation on PEO-grafted biomaterials was directly related to the extent of tissue damage during implantation surgery. Platelets can be activated and form aggregates in the bulk blood, and the formed platelet aggregates may be able to deposit on the PEO monolayer overcoming its repulsive property. Our studies indicate that the testing of in vitro platelet adhesion should include adhesion of large platelet aggregates, in addition to adhesion of individual platelets. Furthermore, the surface modification methods should be improved over the current monolayer grafting concept so that the repulsive force by the grafted PEO layers is large enough to prevent adhesion of platelet aggregates formed in the bulk blood before arriving at the biomaterial surface.

Effects of heparin, desmopressin, and isovolemic hemodilution with dextran on thrombus formation in synthetic vessel grafts inserted into the vena cava of the rabbit

The objective of this study was to investigate the effects of isovolemic hemodilution with dextran-70 on thrombus formation and blood flow in synthetic venous vessel grafts. Polytetrafluoroethylene grafts (length, 11 mm; inner diameter, 3 mm) were inserted into the vena cava of rabbits. Six groups were studied: (1) the control group; (2) animals that underwent isovolemic hemodilution with dextran-70 to a hematocrit of about 30%; (3) animals that underwent isovolemic dextran hemodilution combined with a bolus injection of heparin; (4) animals that underwent heparin treatment only; (5) animals that underwent isovolemic dextran hemodilution combined with infusion of desmopressin; and (6) animals that underwent an identical treatment to group 3 but with a 2-week, instead of a 2-day, follow-up. Vena cava blood flow was measured before and after hemodilution and graft insertion and at the termination of the experiments at 2 days (groups 1 to 5) and 2 weeks (group 6) after surgery. Graft patency and thrombus mass weight were evaluated. In the control group, most of the vessels occluded within 2 days. Hemodilution with dextran improved blood flow and reduced thrombus mass weight significantly. Desmopressin, which increases factor VIII, did not influence the effects of hemodilution with dextran, which suggests that the effects of dextran are not mediated by a reduction in the level of this coagulation factor. A single bolus dose of heparin did not reduce thrombus formation in the grafts but did potentiate the effects of isovolemic hemodilution on thrombus mass and graft blood flow. We conclude that isovolemic dextran hemodilution combined with a single bolus of heparin had beneficial long-lasting effects. The grafts in groups 3 and 6 were all patent.

Pharmacologic intervention to prevent graft failure

Lower extremity vascular grafts, either vein or synthetic, fail for diverse reasons. Technical defects or poor surgical judgment doom a graft beyond any benefit pharmacotherapy can offer. Graft failure due to spontaneous thrombosis particularly affects prosthetic conduits, and use of antiplatelet agents (dextran, ASA) or anticoagulants (heparin, warfarin) is probably useful in this setting. An effective way to inhibit vein graft or anastomotic intimal hyperplasia remains elusive. Perhaps the most permanent and longstanding influence on lower extremity graft survival can be made through risk factor intervention aimed at arresting the progression of atherosclerosis. Aggressive treatment of hyperlipidemia, hypertension, smoking, and other known risk factors should be routinely and aggressively pursued in patients with lower extremity grafts, either autogenous or prosthetic. Lower extremity graft patency is optimally ensured by technically adept insertion of a proper autologous conduit in a well-selected patient. Pharmacotherapy may have a significant adjunctive role in the maintenance of graft patency, especially in high-risk settings such as limb salvage with associated poor outflow, a marginal vein graft, or the obligatory use of prosthetic material.

Deposition of platelets and neutrophils in porcine iliac arteries after angioplasty and Wallstent placement compared with angioplasty alone

PURPOSE

This study was designed to compare deposition of 111In-labeled platelets and neutrophils after balloon angioplasty (PTA) alone and PTA plus Wallstents.

METHODS

Histological investigation was performed with scanning electron microscopy (SEM). Fifty percent stenoses of both iliac arteries was created by resorbable ligature in 13 pigs. After 30 days, PTA was performed bilaterally with an additional stenting procedure done on one side. Autologous platelets were labeled and reinfused before the interventional procedure in six pigs, and labeled neutrophils were used in seven pigs. The deposition of the labeled cells was recorded in vivo over 270 min using a scintillation camera. The results were correlated with in vitro measurements.

RESULTS

Scanning revealed significant increase in platelet and neutrophil deposition at the site of the stent compared with the site where PTA alone was undertaken. In vitro measurements confirmed these differences. SEM demonstrated a fibrin lining on the stent surface and numerous adherent platelets. The adjacent arterial lumen was almost completely covered by fibrinous material. The PTA-alone site demonstrated denudation of endothelial cells and less fibrinous material, as well as platelets and leukocytes.

CONCLUSION

The complex interaction in the response of the vessel wall and flowing blood involves both platelet and neutrophil adhesion. The self-expandable vascular endoprosthesis contributes to increased deposition of platelets and neutrophils as seen in this experimental model of nonatheromatous stenosis.

Direct platelet effect of low molecular weight dextran in small calibre PTFE grafts

The thrombogenicity of synthetic vascular grafts is a major factor in occlusion of grafts when they are used to bypass small calibre arteries. In this paper, the effect of low molecular weight dextran (LMWD, Dextran-40) on graft surface-platelet interaction was studied using Indium-III-oxine labelled platelets. It was found that LMWD significantly reduced platelet deposition onto graft surfaces (P less than 0.001). Dextran had a direct antiplatelet effect independent of plasma volume expansion as dextran-soaked grafts significantly reduced platelet deposition when compared to systemic dextran administration (P less than 0.001). We therefore conclude that LMWD has a direct antiplatelet effect which is beneficial in reducing platelet deposition on synthetic PTFE grafts which may improve the early patency of such grafts.

Does dextran 40 reduce early graft thrombogenicity? An experimental investigation on patency and platelet deposition on prosthetic graft materials in sheep

The mechanism by which dextran 40 reduces early graft thrombogenicity has not been fully elucidated. Dextran improves haemaodynamics, reduces platelet aggregation, alters fibrin formation and enhances thrombus lysis. In this experimental investigation on sheep using a low flow model, the thrombogenicity of various grafts was studied when either a dextran or saline infusion was given. Bilateral carotid interposition grafts with expanded polytetrafluorethylene (ePTFE) on one side and dacron on the other (random allocation) were inserted in 12 sheep. The sheep were randomly divided into two groups, one given a dextran infusion and the other saline. A tendency for improved graft patency was seen in the dextran 40 treated animals (P less than 0.05 at 3 h). However, platelet accumulation did not differ markedly between the dextran 40 and saline treated groups. On the other hand there was a clear reduction of platelet accumulation on ePTFE grafts compared to dacron grafts (P less than 0.01). A large part of the radioactivity measured from the dacron graft was located within the graft wall. Further studies to clarify the mechanism of action of dextran are needed.