Re-expansion of balloon-expandable stents after growth.

OBJECTIVES: The purpose of this study was to evaluate the feasibility of re-expansion of balloon expandable intravascular stents and to examine the gross and histologic effects of re-expansion on vascular integrity. BACKGROUND: Intravascular stents have been used successfully as an adjunct to balloon dilation of congenital pulmonary artery branch stenosis and postoperative stenosis of the pulmonary arteries in children. However, use of rigid stents in children could result in development of relative stenosis at the site of stent implantation with subsequent growth of the child. METHODS: Stainless steel "iliac" stents were placed in the thoracic aorta of 10 normal juvenile swine by a transcatheter technique. Angiography and re-expansion were performed at a mean of 11 weeks (n = 9) and again at 18 weeks (n = 5). After euthanasia, the aortic specimens were removed for gross and histologic examination. RESULTS: Stents were successfully implanted in 10 swine. Re-expansion was successfully performed in each animal at 11 weeks and at 18 weeks. Aortic growth produced a relative constriction of the aorta of 20% +/- 10% (mean +/- SD) at the site of stent implantation at both 11 and 18 weeks. Re-expansion produced a significant increase in mean stent diameter from 10.1 +/- 1 mm to 12.3 +/- 1.2 mm at 11 weeks and from 11.2 +/- 0.7 to 13.5 +/- 1.1 mm at 18 weeks after implantation (p < 0.001). Balloon dilation produced a relative increase in stent diameter of 21% +/- 7% at 11 weeks and 18% +/- 4% at 18 weeks. Stent re-expansion was accompanied by plastic deformation of the neointima without neointimal dissection. Where neointima was thick, there was no evidence of neointimal abrasion, but where neointima was thin, areas of localized neointimal abrasion were observed with focal fibrin and platelet adherence to the stent struts. There was no evidence of medial or adventitial hemorrhage or dissection produced by re-expansion. CONCLUSIONS: Re-expansion of intravascular stents is feasible after growth in juvenile swine without significant injury to neointima, media or adventitia. The results of this study support careful and selective use of intravascular stents as an adjunct to balloon dilation of congenital stenoses in children.

Nonselective digital subtraction angiography. Compact contrast material bolus for improved image quality.

Digital subtraction angiography (DSA) examinations of major aortic branch vessels traditionally have been performed with aortic injections of dilute contrast material (70-150 mg I/mL) over approximately 2 seconds. This study examines a technique employing rapid boluses of undiluted contrast material (282-300 mg I/mL) in small volumes and compares the quality of the images to those obtained using conventional methods. Twenty intra-aortic DSA angiograms of the renal arteries were performed in 10 patients. In each patient, both compact bolus and conventional injections were performed. Injections of 12 or 15 mL of diatrizoate meglumine-60% at 30 mL/second (duration = 0.4 or 0.5 seconds) were compared with injections of 24 or 30 mL of diatrizoate meglumine-30% at 12 mL/second (duration = 2 seconds). Aside from injection technique and image projection, no other variables were altered, and the iodine loads were the same for the two injections in each patient. The images were paired and rated by four radiologists without knowledge of the technical parameters. The radiologists overwhelmingly preferred the images corresponding to the compact bolus technique (Chi-square analysis P less than .001). Visualization of the main renal arteries and the intrarenal branches was improved (P less than .001 for both). A reduction in misregistration artifact is suggested but not statistically significant. We conclude that image quality in nonselective, intra-arterial DSA can be improved without an increase in iodine dosage by the rapid administration of undiluted contrast material in small volumes.

The hemodynamic effects of the administration of ionic and nonionic contrast materials into the pulmonary arteries of a canine model of acute pulmonary hypertension.

Clinical studies have shown pulmonary and right ventricular hypertension to be important factors increasing the risk to patients during pulmonary angiography. This experiment was undertaken to define the hemodynamic changes induced by the administration of contrast material into the pulmonary arteries of dogs with embolic pulmonary hypertension, and to compare the effects of ionic and nonionic agents. Ten closed-chest dogs under light halothan anesthesia were subjected to pulmonary embolization with sephadex microspheres until severe pulmonary hypertension occurred and the cardiac output decreased to 50%-60% of the pre-embolization baseline. Intra-pulmonary injections of contrast material were performed in eight animals while hemodynamic indices were measured. Sodium methylglucamine diatrizoate induced severe, transient, hypotension associated with a large decrease in systemic vascular resistance and little change in the cardiac output. Hypotension is especially undesirable in the presence of pulmonary hypertension because it worsens the preexisting coronary ischemia and compromised right ventricular function. No elevation in mean pulmonary artery pressure was seen, and pulmonary vascular resistance decreased. Iohexol induced milder effects, perhaps because it exerts a less severe systemic vasodilatory effect and is not a negative inotrope. These findings suggest iohexol may be safer in the high risk patient, however, these data may not be directly applied to unanesthetized humans.

Male hypoplastic infrarenal aorta and premature atherosclerosis.

Hypoplasia of the abdominal aorta distal to the origin of the renal arteries may be a contributing factor in the development of early atherosclerosis. The abnormal aortic configuration and resultant increased stress forces may account for the localization of the disease to the aortoiliac segment. The increased frequency of single bifurcating lumbar arteries at the L4-L5 level, easily demonstrable on conventional arteriograms, suggests that the aortic hypoplasia may result from excessive fusion of the paired dorsal aortas during embryonic life. Hypoplasia of the infrarenal aorta is present in 9% of females with aortoiliac disease, and it is believed to be uncommon in males. A review of 408 peripheral arteriograms of males revealed 18 cases (4.4%) of hypoplasia of the abdominal aorta. Males with hypoplastic aortas develop symptoms 10 years earlier than those with normal-sized aortas and 10 years later than females with similar narrowing of the distal aorta.

Variceal hemorrhage associated with portal vein thrombosis: treatment with a unique portal venous stent.

A patient with an uncommon cause of portal venous hypertension, pancreatitis, is depicted. The patient had an equally uncommon pattern of symptoms and signs consisting of abdominal pain and lower gastrointestinal hemorrhage caused by colonic varices. A unique treatment, with angiographic placement of an expandable intraluminal stent within the portal vein, was employed to reopen the portal venous system and reduce portal pressure. Relief of bleeding was accomplished and sustained for more than 1 year.

Expandable intraluminal vascular graft: a feasibility study.

An expandable intraluminal graft mounted coaxially over an angioplasty balloon catheter was used in dog arteries. The graft, a wire mesh tube that has the ability to retain its expanded shape, opposes elastic recoil of the arterial wall after maximum balloon inflation. Eighteen grafts were placed in the abdominal aorta and iliac femoral, renal, superior mesenteric, and carotid arteries of eight dogs through femoral or carotid arteriotomies. Two grafts were placed in areas of artificially induced stenosis, completely restoring the lumen. Overall patency rate at 35 weeks was 77%. Histopathologic examination of patent grafts showed complete endothelialization at 3 weeks. The smaller caliber grafts and those that had outflow obstruction showed significant degrees of intimal hyperplasia.

Surgically induced carotid subclavian steal syndrome. Diagnosis by video dilution technique.

A rare occurrence of carotid subclavian steal syndrome following carotid subclavian bypass for arm ischemia was described. Blood flows in the carotid artery and carotid subclavian bypass, at rest and following arm exercise, were determined by video dilution technique during the angiographic procedure. There was no obstruction of the inflow or outflow of the proximal or distal anastomoses to account for the steal (55%). Rather, increased arterial flow to the subclavian artery due to the patient's status as a bilateral amputee was thought to be the cause. The diagnosis and subsequent correction by takedown of this bypass and conversion to an axillary-to-axillary bypass were performed. Video dilution technique offers a unique and accurate way to study steal phenomena in conjunction with routine angiography and does not add to the patient's cost or risk.

The etiology of symptoms in patients with recurrent carotid stenosis.

We performed 33 carotid endarterectomies in 29 patients for recurrent carotid stenosis. The interval between the initial and second operations ranged from six weeks to 11 years with a mean of 56 months. Three types of pathologic lesions were identified: (1) recurrent atherosclerosis (RA), (2) neointimal fibromuscular hyperplasia (NFH), and (3) lesions with elements of both RA and NFH (complex lesions). Histologic examination of early-recurring lesions (less than three years) revealed NFH in 17 patients and one complex lesion. Late-recurring lesions (three years or later) were due to atherosclerosis in eight vessels, NFH in four, and both RA and NFH in three. Focal neurologic symptoms occurred in 25 (76%) of 33 vessels, and an embolic source could be identified in 16 (64%) of 25 patients. Embolic events rather than reduced blood flow due to progressive stenosis are more frequent causes of symptoms in patients with recurrent carotid stenosis than was formerly believed.

[Balloon-expandable stents in arterial changes of the iliac arteries. Early and follow-up results of 65 interventions]. / Ballonexpandierbare Stents bei arteriellen Veränderungen der Beckenstrombahn. Früh- und Nachuntersuchungsergebnisse von 65 Interventionen.

69 balloon-expandable vascular endoprostheses (Palmaz stents) were placed in 65 iliac arteries of 57 patients. The indications for stent placement were total occlusions (n = 15), restenosis after PTA (n = 13), dissections (n = 4), post-operative strictures (n = 3) or others (n = 30). The peak systolic pressure gradient dropped from 43.9 to 2.9, the mean pressure gradient to 1.9 mmHg after stent placement. This was significantly lower than conventional PTA (residual peak systolic pressure gradient: 5.8). Complications occurred in 7.7% of all interventions. 78.4% of the patients showed no limiting claudication after stent placement. Angiographic controls after 47 weeks demonstrated an average intima proliferation of 0.4 mm, not leading to significant restenosis. Cumulative patency after 16 months was 96%. Iliac stents became an effective and in certain situations indispensable adjuvant to PTA.

[Animal experimental studies on the effect of flow restriction on the thrombogenicity of the Palmaz stent using 111Indium-marked thrombocytes]. / Tierexperimentelle Untersuchungen über den Einfluss der Flussrestriktion auf die Thrombogenität des Palmaz-Stentes mittels 111Indium-markierter Thrombozyten.

The purpose of this experimental study in dogs is to evaluate the influence of restricted flow on the thrombogenicity of balloon-expandable intravascular stents (Palmaz type). We implanted 24 Palmaz stents in the femoral artery of mongrel dogs. 12 dogs experienced a 75% flow restriction by means of an artificial stenosis distal to the start in the outflow tract. We used 111In labelled thrombocytes as a marker for thrombus detection within the stent. The results presented demonstrate that there is no considerable risk of thrombus formation if the stent is implanted into vessels with good outflow tracts and therefore high blood flow velocities within the stent. Under conditions of highly impaired flow, a 68% rate of stent occlusion occur if no anticoagulation therapy is performed. After i.v. administration of 100 IU/kg body weight of heparin the occlusion rate drops to zero. It can be concluded that even under low-flow conditions as a result of a restricted outflow tract situation the Palmaz stent can be implanted without any risk of stent occlusion as long as a sufficient anticoagulation protocol is maintained.

New advances in endovascular technology.

Future research on and development of intravascular stents, stent grafts, and other implantable devices will be aimed largely at improving their blood and tissue biocompatibility. Most of the modifications resulting from this research and development will alter the surface properties we currently understand: surface energy or "wetability," electrical surface charge, surface texture, and surface chemistry. However, as knowledge of the physical, chemical, and biological properties of bioprosthetic surface materials expands, new areas for exploration will develop, increasing the opportunities for improvement.

Reverse ventilation--perfusion mismatch.

Patients having lobar airway obstruction or consolidation usually have decreases of both ventilation and perfusion on lung scans. We report three patients in whom hypoxic vasoconstriction was apparently incomplete, resulting in a "reversed" ventilation-perfusion mismatch. Perfusion of the hypoxic lobe on the radionuclide scan was associated with metabolic alkalosis, pulmonary venous and pulmonary arterial hypertension in these patients.

Protein interactions with endovascular prosthetic surfaces.

Blood protein interaction with prosthetic surfaces seems to be the initial step in the chain of events leading to tissue incorporation of endovascular devices. This paper focuses on the relationship between surface free energy and protein adsorption on metals and polymers commonly used for fabricating vascular prosthetic devices. Our results support a relationship between surface energy and protein adsorption. Albumin was more easily eluted than fibrinogen and fibronectin from most metals and all polymeric surfaces considered. Following elution, metals retained a larger fraction of protein as compared to polymers.

Influence of topography on endothelialization of stents: clues for new designs.

To evaluate the influence of stent design on endothelialization of the stented surface, we placed trapezoidal objects of variable thickness on a confluent culture of endothelial cells and subjected the assembly to flow and shear conditions similar to those found in arteries. After 24 h, we measured and analyzed the area on top of the objects covered by cells and the maximum migration distance from the borders. In addition, we evaluated areas devoid of cells surrounding the objects, which developed after exposure of the assembly to flow. The cell-covered area and migration distance significantly decreased on objects 75 microns thick, and it was nonexistent on objects 250 microns thick. Areas devoid of cells or gaps were largest adjacent to the downflow side of the object, disposed transversely to flow. Cell gaps were smallest along the side aligned with flow. In conclusion, endothelial cell coverage may be impaired by stent wall thickness larger than 75 microns. It is likely that this impairment is related to flow disturbances impairing cell attachment.

Endothelial cell migration onto metal stent surfaces under static and flow conditions.

Restenosis associated with intimal hyperplasia and thrombosis at sites of balloon angioplasty or stent placement remains an important clinical problem. It is likely that loss or damage to the arterial endothelium associated with these interventional procedures as well as the rate of its restoration plays a critical role in the extent of restenosis. Migration of arterial endothelial cells from adjacent intact endothelium is the predominant source of cells involved in re-endothelialization of the injured site. In this paper, we review the influence of hemodynamics on endothelial cell migration, both in vivo and in vitro. In addition, we present recent in vitro studies demonstrating the importance of the nature of metal substrates in modulating endothelial cell migration rate. Finally, we review the cellular and molecular mechanisms likely involved in governing endothelial cell migration, and relate them to a possible scenario of endothelial response to injury at sites of arterial intervention. Understanding the important factors regulating endothelial migration may provide insights that will ultimately lead to methods to accelerate endothelial healing and reduce the occurrence of arterial restenosis.

Electrostatic forces on the surface of metals as measured by atomic force microscopy.

Electrostatic forces play an important role in modulating the interaction of plasma proteins and blood cellular components with the surface of the vascular endothelium. Based on the concept that electrostatic forces residing on the surface of metal intravascular prostheses, such as the stent, also are critical in influencing blood interactions with those surfaces and the vascular wall, these studies were designed to measure these forces on 4 metals using atomic force microscopy (AFM). AFM measurements performed in a low saline aqueous medium at physiological pH indicate a similar net electronegative surface charge level for gold and 316l stainless steel that is significantly higher than the level measured on an electropolished Nitinol surface. Heat oxidation of the Nitinol surface increased the overall electronegativity and created a more homogeneous surface charge distribution. This study demonstrates that AFM force measurements can be a valuable approach to understanding the electrostatic surface of metallic as well as other biomaterials that may be important in understanding how these surfaces influence vascular healing at intravascular interventional sites.

Blood flow and thrombus formation determine the development of stent neointima.

To establish a relationship between flow, acute thrombus formation, and late intimal formation in implanted arterial stents, canine femoral arteries with normal blood flow were compared to contralateral femoral arteries with restricted blood flow. Thrombocyte activity over the stent segment was evaluated for 3 h after stent placement with nuclear scanning, following administration of In-111 labeled platelets. To evaluate long-term stent patency in relationship to arterial flow, an additional group of dogs were subjected to long-term observation. Matched, symmetrically implanted femoral stents with normal and restricted flow were explanted at 1, 12, and 24 weeks for histological analysis and comparative measurement of neointimal thickness. Angiographic studies were performed before and after nuclear scan in the acute animals and before explant in the chronic animals. Acutely, heparinization prevented subocclusive or occlusive thrombus, regardless of flow. In the absence of heparinization, normal arterial blood flow did not prevent thrombus formation on stents. Chronically, stents with flow restriction had significantly greater neointimal formation in comparison with unrestricted stents. Histological studies suggested that the stent neointima resulted from progressive replacement of stent thrombus.

[Treatment of acute iliac artery occlusion by catheter lysis, catheter dilatation and implantation of a new kind of metallic vascular endoprosthesis]. / Die Behandlung eines akuten Beckenarterienverschlusses durch Katheterlyse, Katheterdilatation und Implantation einer neuartigen metallischen Gefässendoprothese.

Methods of interventional radiology have achieved a significant importance for the treatment of acute iliac artery occlusion if either local or general clinical controindications preexclude routine surgical means of vessel repair. The newly developed balloon-expandable metal mesh vascular endoprosthesis is shown to accomplish percutaneously a definitive reconstruction of heavily diseased iliac artery vessel segments which very well compares with the hemodynamic and morphologic aspect of surgical treatment such as thrombectomy combined with TEA or bypass surgery. Both, short and long-term results one year after the implantation of two balloon-expandable metallic vascular stents reflect a much higher efficacy in functional treatment of iliac artery related claudication than a routine percutaneous transluminal angioplasty alone. In addition, it is shown that local lysis based on a standard protocol of selective intrathrombus instillation of urokinase helps to clear totally clogged iliac arteries.

Intravascular stenting: from basic research to clinical application.

Although the use of intravascular stents is approaching widespread clinical use, several questions remain regarding their safety and efficacy. Several factors have direct bearing on the immediate and long-term results of intravascular stenting. The technical results of stenting are dependent on the mechanical characteristics of the device and long-term patency is closely related to thrombogenicity and endothelialization. Although stents are used most frequently for the treatment of atherosclerotic disease there is no clear understanding of how they influence the atheromatous lesion.