Thrombogenicity of metallic vascular stents in arteries and veins--an experimental study in pigs

Treatment of venous stenosis in oncologic patients

Symptomatic obstruction related to malignant involvement of large veins may occur in central veins both in the thoracic and pelvic regions, and in the abdominal region of the body. Both cases represent a therapeutic challenge, and endovascular revascularization, consisting of angioplasty and stent placement, is safe and effective. Superior vena cava stenting has become widespread in the management of occlusive venous disease. The percutaneous placement of large expandable metal stents allows rapid restoration of normal blood flow in the majority of patients, thus improving symptoms. Published data on the diagnosis and treatment of symptomatic cancer-related iliocaval obstructions are limited and mainly consist of case reports and small case series. The present review reports the current state of endovascular treatment for both superior vena cava occlusion and iliac compression syndrome in cancer patients.

Platelet Function and Pharmacologic Inhibition

Platelets have traditionally been understood within the context of hemostasis and hemorrhagic disorders. However, with increasing procedures being performed in smaller vessels and with an increasing incidence of atherosclerosis, the often critical role platelets play is more evident. Platelets are no longer viewed as “scaffolding” for the events of the coagulation cascade but rather as important catalysts in hemostasis, thrombosis, and fibrinolysis. Improved understanding of platelet physiology has led to developments of pharmacologic adjuncts resulting in improved patency rates and improved patient outcomes. This review addresses the physiology of platelet function and the impact of new pharmacologic agents in percutaneous intervention.

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.

Animal models for the study of lower extremity chronic venous disease: lessons learned and future needs

The purpose of this review is to define animal models of chronic venous disease and to demonstrate how animal studies can impact our understanding and treatment of this disorder. To this end an extensive literature search was conducted highlighting potential animal models of chronic lower extremity venous disease. Scientific investigations using animals to study particular aspects of this disease are also reviewed. This review was conducted by members of the Committee on Research of the American Venous Forum to help provide direction for future venous research endeavors. Useful models of chronic venous occlusive disease involve controlled ligation of a major lower limb vein and multiple tributaries. Such a model can provide sustained venous hypertension and studies using this model have confirmed that an isodiametric graft can provide early hemodynamic relief. Models of primary, postphlebitic, and isolated chronic deep venous insufficiency are available for study. Valve repair or transplantation can positively impact the insufficiency observed in these models. Investigations into valve substitutes have generally been disappointing or are undergoing early evaluation. In conclusion, animal models for the study of some aspects of chronic venous disease do exist and have already affected our clinical approach to patients. The scientific study of basic pathophysiology, diagnostics, end-organ response, and long-term surgical treatments of this disorder in well-controlled animal experiments have not been conducted.

The effect of endovascular aortic stents placed across the renal arteries

OBJECTIVES

To investigate renal artery patency and renal function after deployment of aortic stents covering the orifices of renal arteries.

DESIGN

Prospective open animal study.

SETTING

Department of Experimental Surgery at a university hospital.

MATERIALS

Twenty-three pigs were used.

METHODS

Ten pigs were observed for 1 h after graft-anchoring aortic stents, Gianturco (5) and Palmaz (5), were placed so that the stents covered the renal arterial orifices. In 13 pigs, Gianturco (6) and Palmaz (7) stents without grafts were placed over the renal arteries and left in situ for 7 days. Renal function and blood flow were measured by renograms, iohexol clearance and ultrasonic blood flow meter and patency was verified by angiograms. The kidneys were microscopically examined for signs of ischaemia and microemboli.

RESULTS

One renal artery covered by a graft-anchoring Gianturco stent occluded. The remaining renal arteries remained patent without any significant decrease in renal blood flow after stent deployment. Normal renal function and histology was maintained.

CONCLUSIONS

Aortic stents placed at the level of the renal arteries do not affect renal blood flow within 1 week in this experimental model. This may prove valuable in endovascular treatment of aortic aneurysms and in other procedures involving stents.

Venous recanalization by metallic stents after failure of balloon angioplasty or surgery: four-year experience

PURPOSE

This retrospective study describes our updated experience in treating venous stenoses and occlusions with metallic endovascular stents.

METHODS

Gianturco, Palmaz, and Wallstent stents were placed in 55 patients over a 4-year period. Stent sites included the subclavian veins (9), innominate veins (3), superior vena cava (4), inferior vena cava (3), iliac veins (29), femoral veins (5), and portal veins (6). The most common indications for stent placement were malignant stenoses and chronic pelvic venous occlusions. Venoplasty and/or urokinase were used as ancillary therapy. Patients were anticoagulated for 3-6 months. Follow-up included clinical assessment and duplex ultrasound.

RESULTS

Lifetable analysis shows 59%, 63%, and 72% primary, primary assisted, and secondary 1-year patency rates, respectively. The 4-year primary patency rates were the same. Duration of patency depended on the venous site. Death was a complication of stent placement in 2 patients and 12 patients died within 6 months after stent placement from primary disease progression. Although early failures were more common in stents placed across occlusions than stenoses, 1-year secondary patency rates were comparable. Primary patency rates were only slightly lower in patients with malignant obstruction than in patients with benign disease.

CONCLUSION

Endovascular stent placement provides a nonsurgical alternative for reestablishment of venous flow and symptomatic relief in patients with benign as well as malignant venous obstruction.

Hydrophilic surface modification of metallic endoluminal stents

PURPOSE

Stainless steel endovascular stents are inherently thrombogenic so that thrombus accumulates on these devices, leading to acute vessel occlusion. A potential solution to this problem is stent surface modification with hydrophilic polymers, which might limit platelet adhesion and reactivity.

METHODS

N-vinylpyrrolidone (NVP) and potassium sulfopropyl acrylate (KSPA) hydrophilic monomers were gamma graft polymerized onto 1 cm2 stainless steel slabs and 4 mm Palmaz stainless steel stents. Surface characteristics of modified and plain stainless steel stents were then investigated with contact angle and x-ray photoelectron spectroscopy measurements, and in vitro and in vivo platelet reactivity was assessed as 111Indium platelet accumulation expressed as counts/min/cm2.

RESULTS

Surface modification of stainless steel slabs and stents with both NVP and KSPA hydrophilic polymers significantly reduced in vitro platelet adhesion (plain = 2249 +/- 723 counts/min/cm2, NVP = 428 +/- 156 counts/min/cm2, KSPA = 958 +/- 223 counts/min/cm2) and in vivo platelet accumulation after 1 hour of blood flow exposure (plain = 1407 +/- 796 counts/min/cm2, NVP = 426 +/- 175 counts/min/cm2, KSPA = 399 +/- 124 counts/min/cm2. In addition, platelet accumulation on modified stents indexed to plain stents was lowest in KSPA-modified stents (NVP = 79.3% +/- 31.7% of plain, KSPA = 51.2% +/- 36.2% of plain). Surface analysis confirmed surface grafting with both monomers, and SEM documented smoothing of the irregular surfaces of the stainless steel stents after grafting.

CONCLUSION

Hydrophilic polymer surface modification of stainless steel stents decreases initial stent surface platelet accumulation, which may decrease the risk of vessel thrombosis associated with the use of these devices.

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.

Porcine platelets in vitro and in vivo studies: relevance to human thrombosis research

This review summarizes present knowledge on porcine platelets in vitro and recent studies on in vivo activation of platelets in the pig. There are certain differences compared to human platelets: Platelet aggregation and secretion cannot be achieved by epinephrine, and the arachidonate pathway seems poorly developed in porcine platelets. Genetic models for von Willebrand disease (vWD) and storage pool deficiency (SPD) have been developed in the pig. Several models for the study of in vivo platelet deposition and early thrombus formation have been developed. Platelet radio-labeling techniques (with 111In) have been used extensively. We conclude that the pig seems to be a good choice for the investigation of in vivo platelet activation and deposition based on present knowledge of porcine platelets and on already established animal models.