In vitro model to test the thrombogenicity of coronary stents

A macrofluidic model to investigate the intrinsic thrombogenicity of clinically used stents and develop less thrombogenic stents

Microfluidic blood flow models have been instrumental to study the functions of blood platelets in hemostasis and arterial thrombosis. However, they are not suited to investigate the interactions of platelets with the foreign surfaces of medical devices such as stents, mainly because of the dimensions and geometry of the microfluidic channels. Indeed, the channels of microfluidic chips are usually rectangular and rarely exceed 50 to 100 μm in height, impairing the insertion of clinically used stents. To fill this gap, we have developed an original macrofluidic flow system, which precisely reproduces the size and geometry of human vessels and therefore represents a biomimetic perfectly suited to insert a clinical stent and study its interplay with blood cells. The system is a circular closed loop incorporating a macrofluidic flow chamber made of silicone elastomer, which can mimic the exact dimensions of any human vessel, including the coronary, carotid or femoral artery. These flow chambers allow the perfect insertion of stents as they are implanted in patients. Perfusion of whole blood anticoagulated with hirudin through the device at relevant flow rates allows one to observe the specific accumulation of fluorescently labeled platelets on the stent surface using video-microscopy. Scanning electron microscopy revealed the formation of very large thrombi composed of tightly packed activated platelets on the stents.

The Ex-Vivo-Shunt-Model: Novel Approach for Assessing the Thrombogenicity of Vascular Implants

OBJECTIVE

Disadvantages associated with commercially available vascular implants necessitate alternative strategies to develop new vascular prostheses. Although many tissue characterizing strategies have been defined, no valid test for thrombogenicity exists. Here we introduce a novel concept for thrombogenicity testing of vascular implants

METHODS

Silastic tubes were implanted into the carotid arteries of 12 sheep. After placing these shunts, tc99m-labeled platelets were administered and test-vessels were put in between the shunts. Native autologous (n=6), as well as native/acellularized allogeneic (n=6/n=6), and xenogeneic (n=6/n=6) carotid arteries and allogeneic (n=6/n=6) and xenogeneic (n=6/n=6) carotid arteries reseeded with allogeneic endothelial-cells, fibroblasts and myocytes were evaluated. Number and time course of intra-operatively deposited platelets were evaluated with a Geiger-counter; certain areas of platelet deposition located, envisioned and characterized by a gamma-camera and scanning electron-microscopy afterwards.

RESULTS

Counter results revealed no significant different platelet depositions when comparing silastic tubes with either autologous or allogeneic native carotid arteries. However, starting 5 minutes after placement, acellularized/reseeded allogeneic (p=0.001/p=0.00004), and xenogeneic (p=0.0001/p=0.01) carotid arteries showed significantly more platelet depositions than native autologous carotides. Moreover, it was possible to show that almost no platelets adhere to native vessels or silastic tubes, thus proving the test method itself.

CONCLUSION

The Ex-Vivo-Shunt-Model is a valid method to measure and envision the intrinsic thrombogenicity of vascular implants.

Comparison of modified chandler, roller pump, and ball valve circulation models for in vitro testing in high blood flow conditions: application in thrombogenicity testing of different materials for vascular applications

Three different models, a modified Chandler loop, roller pump, and a new ball valve model (Hemobile), were compared with regard to intrinsic damage of blood components and activation of platelets. The Hemobile was used for testing of polymer tubes. High flow was not possible with the Chandler loop. The roller pump and the Hemobile could be adjusted to high flow, but he pump induced hemolysis. Platelet numbers were reduced in the roller pump and Chandler loop (P < 0.05), but remained high in the Hemobile. Platelet aggregation was reduced in all models. The Hemobile was applied for testing vascular graft materials, and allowed different circuits circulated simultaneously at 37°C. ePTFE, Dyneema Purity UHMWPE fiber and PET fiber based tubes, all showed hemolysis below 0.2% and reduced platelet count and function. Binding of fibrin and platelets was higer on PET, inflammatory markers were lowest on Dyneema Purity UHMWPE. We concluded that the Hemobile minimally affects blood and could be adjusted to high blood flows, simulating arterial shear stress. The Hemobile was used to measure hemocompatibility of graft material and showed Dyneema Purity UHMWPE fiber in many ways more hemocompatible than ePTFE and PET.

Heparanase regulates thrombosis in vascular injury and stent-induced flow disturbance

OBJECTIVES

The purpose of this study was to examine the role of heparanase in controlling thrombosis following vascular injury or endovascular stenting.

BACKGROUND

The use of endovascular stents are a common clinical intervention for the treatment of arteries occluded due to vascular disease. Both heparin and heparan sulfate are known to be potent inhibitors of thrombosis. Heparanase is the major enzyme that degrades heparan sulfate in mammalian cells. This study examined the role of heparanase in controlling thrombosis following vascular injury and stent-induced flow disturbance.

METHODS

This study used mice overexpressing human heparanase and examined the time to thrombosis using a laser-induced arterial thrombosis model in combination with vascular injury. An ex vivo system was used to examine the formation of thrombus to stent-induced flow disturbance.

RESULTS

In the absence of vascular injury, wild type and heparanase overexpressing (HPA Tg) mice had similar times to thrombosis in a laser-induced arterial thrombosis model. However, in the presence of vascular injury, the time to thrombosis was dramatically reduced in HPA Tg mice. An ex vivo system was used to flow blood from wild type and HPA Tg mice over stents and stented arterial segments from both animal types. These studies demonstrate markedly increased thromboses on stents with blood isolated from HPA Tg mice in comparison to blood from wild type animals. We found that blood from HPA Tg animals had markedly increased thrombosis when applied to stented arterial segments from either wild type or HPA Tg mice.

CONCLUSIONS

Taken together, this study's results indicate that heparanase is a powerful mediator of thrombosis in the context of vascular injury and stent-induced flow disturbance.

A novel in vitro model for preclinical testing of the hemocompatibility of intravascular stents according to ISO 10993-4

Subacute stent thrombosis, caused by undesired interactions between blood and the stent surface, is a major concern in the first few weeks following coronary artery stent implantation. The aim of this study was to establish a novel in vitro model for hemocompatibility testing of coronary artery stents according to ISO 10993-4. The model consists of a modified Chandler-Loop design with closed heparin-coated PVC Loops and a thermostated water bath. The tests were performed with anticoagulated human whole blood. After incubation in the loop, blood was analyzed for coagulation and inflammatory activation markers (TAT, β-TG, sP-selectin, SC5b-9 and PMN-elastase). Three different stent types with varying thrombogenicity were tested; statistically significant differences were found between the three stent types in measures of coagulation and platelet activation. The new Chandler-Loop model can be used as an alternative to animal and current in vitro models, especially for the determination of early events after stent implantation.

Frequency and consequences of early in-stent lesions after carotid artery stent placement

PURPOSE

To examine the prevalence of in-stent lesions 1 month after carotid artery stent placement with multidetector computed tomography (CT) angiography and to evaluate their possible causes and their consequences during 1-year follow-up.

MATERIALS AND METHODS

Sixty-nine patients with symptomatic carotid artery stenosis underwent multidetector CT angiography of the carotid arteries 1 month after carotid artery stent placement. Patients were followed-up until 1 year after stent placement, when duplex ultrasonography (US) was performed. In-stent lesions were defined as hypo- or hyperattenuating lesions at the stent wall found with multidetector CT. Significant restenosis (70%) at 1 year was defined as a peak systolic velocity of more than 300 cm/sec at duplex US. The Fisher exact test was used to assess the relationship between early in-stent lesions and ischemic events and restenosis.

RESULTS

At 1 month, 14 of the 69 patients (20%) were found to have in-stent lesions. In one patient, the stent was occluded. The other 13 in-stent lesions did not result in significant lumen reduction. In the year following stent placement, no difference in ischemic events was found between patients with (14%) and those without (13%) early in-stent lesions (P = .99). There was no difference in the occurrence of restenosis at 1 year (7% vs 4%, P = .59).

CONCLUSIONS

At 1 month after carotid artery stent placement, in-stent lesions are found in about one-fifth of patients. These lesions do not appear to be related to recurrent ischemic events or to restenosis at 1 year.

Sulfatides activate platelets through P-selectin and enhance platelet and platelet-leukocyte aggregation

OBJECTIVE

Sulfatides are sulfated glycosphingolipids present on the surface of a variety of cells; however, their exact physiological function is not known. Recently, we have shown that the inhibition of sulfatide-P-selectin interactions leads to disaggregation of platelet aggregates.

METHODS AND RESULTS

In this study, we show that sulfatides activated platelets as they increased activation of GPIIb/IIIa (PAC-1 epitope) and expression of P-selectin on the platelet surface. Furthermore, sulfatides aggregated washed platelets in a dose-dependent manner and enhanced platelet aggregation in platelet-rich plasma. Previous activation of platelets was necessary for this effect. Monoclonal anti-P-selectin antibodies inhibited not only sulfatide-induced PAC-1 binding to platelets but also sulfatide-induced platelet aggregation, suggesting that sulfatides activate platelet GPIIb/IIIa via signaling through P-selectin. The proaggegatory effect of sulfatides was also observed in an ex vivo thrombosis model using whole blood and pulsatile flow at 37 degrees C. In this model, sulfatides significantly enhanced platelet aggregation and the formation of platelet-leukocyte aggregates.

CONCLUSIONS

We show that sulfatide-P-selectin interactions lead to subsequent platelet activation and P-selectin expression, forming a positive feedback loop that can potentiate formation of stable platelet aggregates. In addition, sulfatides enhance the aggregation of platelet-leukocyte aggregates. These mechanisms may play a significant role in hemostasis and thrombosis.

Radiopacity of Current Endovascular Stents: Evaluation in a Multiple Reader Phantom Study

PURPOSE

To evaluate the radiopacity of endovascular stents based on the fluoroscopy mode in a phantom of the human pelvis.

MATERIALS AND METHODS

The following stents were included in this study: Medtronic AVE Bridge, Medtronic AVE Bridge X, Cordis Covered Nitinol (Covent), Guidant Dynalink, Luminexx, Guidant Megalink, Memotherm Flexx, Palmaz Medium, Palmaz-Schatz Long-Medium, Palmaz Corinthian PQ394Q and PQ294Q, SelfX, SMART without markers, SMART with radiopaque markers, Easy Wallstent. To evaluate radiopacity, images of the stents placed in four different positions (lumbosacral junction left and right, iliosacral joint left and right) of a pelvic phantom were taken at the following modes: spotfilm, continuous fluoroscopy, 15 pulses per second, 7.5 pulses per second, and 3 pulses per second. Images were presented at random to four independent readers and radiopacity scores were assessed: 0 = not visible, 1 = poor visibility, 2 = average visibility, 3 = good visibility, and 4 = very good visibility.

RESULTS

The Covent stent had the highest overall radiopacity score (3.25), followed by the Luminexx (3.04) and the Medtronic AVE Bridge X (2.74) stents. At the spotfilm mode, the best visible stents were the Medtronic AVE Bridge X, the Covent and the Easy Wallstent stents and at the continuous fluoroscopy mode, the Covent, the Luminexx, and the Medtronic AVE Bridge X stents. Decreasing the fluoroscopy mode went hand in hand with a reduction of the radiopacity scores of all stents. At the standard fluoroscopy mode of 7.5 pulses per second, the Covent stent was seen well or very well in 96.9%, followed by the Luminexx (76.9%), and the Medtronic AVE Bridge X (41.25%) stents.

CONCLUSIONS

Stent radiopacity directly depends on the fluoroscopy mode; if the pulse frequency decreased, detecting the stents became more difficult. Stent mass correlates with stent radiopacity (e.g., Cordis Covered Nitinol, Bridge X). Radiopaque markers may improve stent radiopacity dramatically (e.g., Luminexx vs Memotherm Flexx).

[Comparative analysis of in vitro test procedures for evaluating hemocompatibility of cardiovascular stents]

Within the scope of this study existing in vitro techniques for testing the hemocompatibility of coronary stents were analysed and optimised. Static and quasi-stationary systems were compared to a pulsed flow model with respect to platelet activity. The streamlines were visualized by dye injection. Blood flow was measured by ultrasonic Doppler velocity meter and electromagnetic flow meter. Uncoated stainless steel (316 L) stents were tested. Surrogate parameters of the hemocompatibility were the change in surface morphology after blood contact and the rise of biomechanical activation markers as C3a and beta-thromboglobulin. The results were correlated to the stent design and to the flow characteristics of the test systems.

Low background, pulsatile, in vitro flow circuit for modeling coronary implant thrombosis

We have developed an in vitro method for creating pulsatile flows to mimic coronary type flow patterns on a beat-to-beat basis. The flow is created by accelerating fluid loops about an axis, inducing relative wall motion. Using this technique, a variety of oscillating flow patterns can be generated and modulated. Such flow generation offers the potential to monitor sensitive, flow-dependent, biological parameters like thrombosis while minimizing background disturbances from pump action and circuit effects. We examined this potential by measuring the loop occlusion time for loops stented with stainless steel 7-9 NIR stents and stentless control loops.

Thrombogenicity of various endovascular stent types: an in vitro evaluation

PURPOSE

The aim of this study was to evaluate the thrombogenicity of different peripheral stent types in a standardized in vitro model with fresh human whole blood.

MATERIALS AND METHODS

Different stents (N = 77; n = 7 of each of 11 types) were implanted in polyvinyl chloride tubing loops and filled with donor blood samples. After 120 minutes of blood circulation, the thrombin-antithrombin III complex (TAT) levels, beta-thromboglobulin (beta-TG) levels, and platelet counts were assessed.

RESULTS

After 2 hours, significant differences were seen. TAT values (+/- SD) with the investigated stents were 31 micro g/mL +/- 20 (control, no stent), 328 micro g/mL +/- 206 (Saxx stent, peripheral medium CrNi31 L), 651 micro g/mL +/- 760 (Palmaz Corinthian Stent, 316 L stainless steel, electropolished), 1,609 micro g/mL +/- 1,264 (Palmaz Corinthian Stent, 316 L stainless steel, not electropolished), 810 micro g/mL +/- 578 (Palmaz Schatz long medium stent), 569 micro g/mL +/- 347 (Smart Nitinol stent), 1,037 micro g/mL +/- 577 (Megalink peripheral stent), 543 micro g/mL +/- 487 (peripheral stent, electropolished), 1,674 micro g/mL +/- 2,057 (peripheral stent, not electropolished), 3,128 micro g/mL +/- 1,812 (SelfX Nitinol stent, polished), 5,897 micro g/mL +/- 2,380 (SelfX Nitinol stent, unpolished), and 1,458 micro g/mL +/- 887 (bridge stent). The platelet count (x1,000/ micro L +/- SD) was 218 +/- 35 (control, no stent), 188 +/- 22 (Saxx stent), 187 +/- 20 (Palmaz Corinthian stent, electropolished), 135 +/- 37 (Palmaz Corinthian stent, not electropolished), 170 +/- 24 (Palmaz Schatz stent), 180 +/- 36 (Smart Nitinol stent), 159 +/- 26 (Megalink peripheral stent), 173 +/- 17 (peripheral stent, electropolished), 133 +/- 51 (peripheral stent, not electropolished), 123 +/- 37 (SelfX Nitinol stent, polished), 52 +/- 27 (SelfX Nitinol stent, unpolished), and 130 +/- 31 (bridge stent).

CONCLUSION

This standardized study showed a wide range of platelet activation after stent implantation. Electropolishing clearly reduced the thrombogenicity of the stents.

Stent struts and articulations: their impact on balloon-expandable stents' hoop strength, pushability, and radiopacity in an experimental setting

OBJECTIVES AND RATIONALE

To evaluate the hoop strength, the pushability, and the radiopacity of endovascular stents depending on their design.

MATERIAL AND METHODS

The Palmaz Medium (rectangular stent struts, no articulation = RE-NA), the Palmaz-Schatz Long-Medium (rectangular stent struts, central articulations = RE-CA), and the 39 mm and 29 mm version of the Palmaz Corinthian Stent (arch-like stent struts, omega hinges = AR-OH 1 and AR-OH 2) were included in the study. Hoop strength, pushability, and radiopacity were assessed in-vitro. A radiopacity score (RS)-ranging from 0 = invisible, 1 = poor, 2 = average, 3 = good, and 4 = very good-was used.

RESULTS

Mass adjusted hoop strength was nearly the same for the AR-OH 1 and the AR-OH 2 (AR-OH 1: 159.2 N/g*cm; AR-OH 2: 156.5 N/g*cm). Their hoop strengths per unit mass significantly exceeded that of a RE-NA (125 N/g*cm) and that of a RE-CA (75.3 N/g*cm). The most pushable stent was the AR-OH 2 (0.54 1/N) followed by the AR-OH 1 (0.47 1/N) and the RE-CA (0.169 1/N). Because of multiple stent dislocations, the RE-NA was considered nonflexible. At spotfilm mode the RE-NA was the most visible stent (RS 3.5) followed by the AR-OH 1 (RS 3.25), the RE-CA (RS 3.12), and the AR-OH 2 (RS 1.9). At continuous fluoroscopy the AR-OH 1 (RS 1.8) was better than the RE-CA (RS 1.125), the RE-NA (RS 1.0), and the AR-OH 2 (RS 0.6). At 7.5 pulses per second the RE-NA (RS 1.3) performed slightly better than the AR-OH 1 (RS 1.25), and clearly better than the RE-CA (RS 0.8), and the AR-OH 2 (RS 0.3).

CONCLUSION

Overall, the ideal stent-having high hoop strength, a low profile, a good pushability, and a good radiopacity-still does not exist. However, by changing strut design (from rectangular to arch-like struts) and by inserting articulations, hoop strength and pushability can be improved without reducing radiopacity.

In vitro analyses of diamond-like carbon coated stents. Reduction of metal ion release, platelet activation, and thrombogenicity

Coronary artery stents can induce platelet activation by shear forces, contact to the biomaterial, and release of metal ions. This activation is one important trigger for thrombosis. Coating of stents is a possible approach to prevent this side effect. The purpose of this study was to evaluate in vitro the biocompatibility of stents coated with diamond-like carbon (DLC). Semiquantitative energy-dispersive X-ray microanalyses showed a complete coverage of the DLC stents. Flow cytometric analyses revealed a significantly higher increase of mean channel fluorescence intensity for the activation-dependent antigens CD62p and CD63 in non-coated compared to DLC-coated stents (p<0.05). Atomic adsorption spectrophotometry analyses revealed a significant release of nickel and chromium metal ions by non-coated stents over a storage period of 96 hours in human plasma (p<0.05). In contrast, only minimal concentrations of released ions could be detected in the case of DLC-coated stents. Similar observations were made with inductively coupled plasma mass spectrometry analyses. Here, high concentrations of molybdenum and manganese ions were released from non-coated stents (p<0.05), while release of these ions from DLC-coated stents was virtually undetectable (p=0.1 for molybdenum and p=0.4 for manganese). Coating of intracoronary stents with diamond-like carbon significantly improves biocompatibility. This biocompatible coating may therefore contribute to a reduction in thrombogenicity.

Thromboembolization associated with sudden increases in flow in a coronary stent ex vivo shunt model

To observe the dynamics of thromboembolism (TE) in an animal model, a carotid-carotid arterial ex vivo shunt was developed. A coronary stent deployed in a 3.5 mm polyvinylchloride (PVC) tubing segment was used as a model device in the shunt. A light-scattering microemboli detector monitored the embolic content of the blood flowing through the shunt at 50-150 ml/min as determined by a clamp-on ultrasound flow probe. The stent was found to actively develop thrombi and release emboli for 1-3 hours when the activated clotting time (ACT) was maintained between 125 and 150 seconds. The shunt flow rate fluctuated considerably (from 50 to 150 ml/min) depending on the animal's activity. When the time profile of this fluctuating flow rate was super imposed on the time profile of embolization, it was noted that sudden increases in flow rate were associated with incidents of embolization. Statistically, sudden flow rate increases of 100% or more were accompanied by embolic events 95% of the time (p < 0.01). Based on the results of this study, it was postulated that the increased embolization may be due to the fluid forces associated with accelerating flow. To explore this postulate, in vitro studies were conducted to compare the effects of pulsatile flow with steady flow on stent induced TE. Results of this study suggested a significant increase (100%) in both stent thrombosis and embolism during pulsatile flow compared with steady flow.

Physical properties of endovascular stents: an experimental comparison

PURPOSE

Different endovascular stent types (AVE Bridge, AVE Bridge X, Memotherm, Palmaz Large, Palmaz Medium, Palmaz-Schatz Long-Medium, Perflex, S.MA.R.T., Symphony, and Wall-stent) of 4 cm length and 8 mm diameter were subjected to standardized physical tests.

MATERIALS AND METHODS

The metal mass of each stent was assessed by weighing. The balloon-expandable stents were pneumatically tested for hoop strength. In self-expanding stents, radial resistive force and chronic outward force were determined with use of a loop test. Stent delivery system pushability was assessed in a crossover model. Stent radiopacity was analyzed quantitatively.

RESULTS

The hoop strength of the balloon-expandable stents ranged from 15.8 N/cm (Perflex) to 28.9 N/cm (AVE Bridge X). The stent weight increased with greater hoop strength (Perflex, 0.046 g/cm vs. AVE Bridge X, 0.061 g(cm). The self-expanding stents had a radial resistive force between 0.39 N/cm (Wallstent) and 1.7 N/cm (Smart). The flexible balloon-expandable stents showed pushability values between 0.13/N (AVE Bridge) and 0.20/N (Perflex). The self-expanding stents had flexibilities between 0.13/N (Memotherm) and 0.24/N (Symphony). Radiopacity assessed with use of a phantom simulating the iliac region ranged from 92 (Palmaz Large) to 115 (AVE Bridge) on a 256-point gray scale (0 = black, 256 = white).

CONCLUSIONS

There is no stent with ideal physical properties. However, depending on the characteristics of the arterial lesion to be treated, the most appropriate stent can be chosen.

Influence of stent length and heparin coating on platelet activation: a flow cytometric analysis in a pulsed floating model

Platelets are involved in acute and subacute thrombotic occlusions of coronary stents and also may play a role in the pathophysiology of in-stent restenosis. This study sought to investigate the expression of activation dependent glycoproteins on platelets by flow cytometry and time until stent thrombosis in an in vitro model of stent thrombosis. Coronary stents were placed in parallel silicon tubings with circulating citrated platelet rich plasma to measure 1) influence of stent length on platelet antigens; 2) influence of heparin coating on platelet antigens; and 3) time until stent thrombosis. After recalcification aliquots of platelet-rich plasma were taken over 10 minutes in 2-minute intervals and immediately fixed and stabilized. For flow cytometric analysis monoclonal antibodies to CD41a (glycoprotein IIb/ IIIa), CD42b (glycoprotein Ib-V-IX), CD62p (P-selectin), and CD63 (glycoprotein 53) were used. Within 2 minutes after start of circulation, the expression of CD62p and CD63 increased. Longer stents resulted in more platelet activation than shorter stents (25 mm vs. 15 mm; p<0.001. Time until stent thrombosis was reduced (25 mm vs. 15 mm; p<0.05). Heparin coating did not significantly influence flow cytometry detectable platelet activation but prolonged time until stent thrombosis (coated vs. uncoated; p<0.005). In control tubing systems without stents platelet activation was less pronounced (p<0.0001). Antibodies to CD41a and CD42b did not show significant changes. In this model platelet activation detected by flow cytometry and time until stent thrombosis were dependent on stent length and coating. In vitro testing could be useful to optimize stent design and material.

Usefulness of abciximab for treatment of early coronary artery stent thrombosis

The clinical and angiographic outcomes of 10 patients who received abciximab as part of their therapy for early stent thrombosis was compared with 25 patients (using historical controls) who received conventional therapy. Although the angiographic outcome and the incidence of myocardial infarction in both groups was similar, there were no deaths or referral for emergency coronary bypass surgery in the abciximab-treated group versus 3 deaths and 10 referrals for emergency bypass surgery in the conventionally treated group.

Flow cytometric analysis of coronary stent-induced alterations of platelet antigens in an in vitro model

One of the limitations of coronary stenting is the subacute thrombotic occlusion. In an in vitro model, we examined the effects of tantalum wire stents (n = 12) on platelet antigens. Platelet-rich plasma (PRP) was circulated in PVC tubing systems. At fixed intervals over a 10-min time course, aliquots of PRP were drawn, stained with monoclonal antibodies (CD41a, CD42b, CD62p, and CD63), and analyzed by flow cytometry. Within 2 minutes of the onset of circulation, expression of the activation-dependent antigens CD62p and CD63 increased in all tubing systems with stents. This early increase was followed by a progressive rise in fluorescence intensity of these neoantigens over the course of 10 minutes (p < 0.05 vs.. control system without stent). Antigens CD41a and CD42b did not show significant changes in either system. The artificial surfaces and shear forces of stent meshes induce alterations in platelet antigens. Flow cytometry provides a sensitive technique for in vitro testing of the thrombogenicity of coronary stents, and may be useful in further improving stent biocompatibility.

Effects of GP IIb/IIIa receptor monoclonal antibody (7E3), heparin, and aspirin in an ex vivo canine arteriovenous shunt model of stent thrombosis

BACKGROUND

Thrombosis is an important limitation of metallic coronary stents, especially in smaller vessels in which shear rates are high. Monoclonal antibody to platelet glycoprotein IIb/IIIa receptor (7E3) has been shown to inhibit shear-induced platelet aggregation. In this study, we compared the effects of 7E3, heparin, and aspirin on stent thrombosis in an ex vivo arteriovenous shunt model of high-shear blood flow.

METHODS AND RESULTS

An ex vivo arteriovenous shunt was created in 10 anesthetized dogs. Control rough-surface slotted-tube nitinol stents (n = 72) expanded to 2 mm in diameter in a tubular perfusion chamber were interposed in the shunt and exposed to flowing arterial blood at a shear rate of 2100s-1 for 20 minutes. The animals were treated with intravenous murine 7E3 (Fab')2 (0.2, 0.4, and 0.8 mg/kg), heparin (100 U/kg), or aspirin (10 mg/kg). Effects of the test agents on thrombus weight, platelet aggregation, platelet P-selectin expression, bleeding time, and activated clotting time (ACT) were quantified. 7E3 reduced stent thrombosis by 95% (20 +/- 1 to 1 +/- 1 mg, P < .001) and platelet aggregation by 94% (14 +/- 2 to 1 +/- 1 omega, P < .001) at the highest dose (0.8 mg/kg). 7E3 significantly prolonged bleeding time but had no effect on ACT and platelet P-selectin expression. Heparin prolonged ACT but had no significant effect on stent thrombosis or platelet aggregation. Aspirin, although it inhibited platelet aggregation by 65%, had no effect on stent thrombosis (19 +/- 2 versus 20 +/- 1 mg in controls).

CONCLUSIONS

7E3 produced a dose-dependent inhibition of acute stent thrombosis under high-shear flow conditions. Stent thrombosis was resistant to heparin and aspirin. Thus, 7E3 may be an effective agent for preventing stent thrombosis.

Subacute thrombosis and vascular injury resulting from slotted-tube nitinol and stainless steel stents in a rabbit carotid artery model

BACKGROUND

Our objectives were to quantify the thrombogenicity and extent of vascular injury created by slotted-tube geometry stainless steel and nitinol coronary stents in a rabbit carotid artery model.

METHODS AND RESULTS

Stents were implanted in rabbit right carotid arteries without antiplatelet therapy. Stainless steel stents were implanted for 4 days while nitinol stents were placed for 4 and 14 days (n = 8, 8, and 6, respectively). Stent thrombosis was assessed by thrombus weight, grading thrombus encroachment of the lumen, and by blood flow in the stented and contralateral arteries. Stainless steel stents at 4 days contained more thrombus than 4- and 14-day nitinol stents (20.0 +/- 5.9 versus 2.5 +/- 0.6 and 2.7 +/- 0.3 mg, respectively; P < .000001). Stainless steel stents were more often occluded by thrombus (6 of 8) or contained more subocclusive thrombus (2 of 8) than nitinol stents (0 of 14, P < .002). Resting blood flow was reduced in arteries with stainless steel stents compared with 4- and 14-day nitinol stents (1.5 +/- 2.8 versus 24.0 +/- 2.0 and 25.5 +/- 1.9 mL/min, respectively, P < .000001). Stainless steel stents were less uniformly expanded, had deeper strut penetration into the vascular wall, and were associated with more extensive medial smooth muscle cell necrosis. There were strong correlations (r = .77 to .95) between variables of thrombosis extent (thrombus weight and grade) and histologically determined vascular injury (strut penetration and medial necrosis).

CONCLUSIONS

Slotted-tube stainless steel stents were more thrombogenic and created more extensive vascular injury than nitinol stents in a rabbit carotid artery model. The mechanisms underlying these differences probably are related to metallurgic and design geometry properties of the two stent types.

Multicenter evaluation of the Strecker tantalum stent for acute coronary occlusion after angioplasty

The Strecker stent is a balloon-expandable, flexible endoprosthesis constructed of knitted tantalum wire and has been implanted successfully in peripheral arteries. This study presents the first multicenter experience with implantation of this radiopaque device in the coronary arteries in 64 patients of 6591 consecutive percutaneous transluminal coronary balloon angioplasty (PTCA) procedures complicated by abrupt closure. In all except 1 patient the stents (n = 72) were correctly placed, and flow could be reestablished immediately. During hospitalization 12 (19%) patients had stent closures; 5 (8%) patients had Q-wave myocardial infarctions; and 13 (20%) patients underwent bypass surgery (4 on an emergency basis). The in-hospital mortality was 9%: 2 patients died after thrombotic stent occlusions; 2 patients had fatal bleeding complications; and 2 patients died after bypass surgery. Major bleeding complications at the puncture site were observed in 8 (12.5%) patients. Angiograms (n = 45) after 17 +/- 5 weeks revealed a stent patency rate of 89%. Thus the Strecker coronary stent proved to be helpful in the management of acute vessel closure during PTCA. However, in this first series a high incidence of early thrombotic occlusions and bleeding complications warrants close anticoagulation monitoring and limits broader indications.