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Ann Thorac Surg 1995;59:908-914
© 1995 The Society of Thoracic Surgeons
Division of Cardiovascular Surgery, Osaka Prefectural Hospital, and Department of Bioengineering, National Cardiovascular Center Research Institute, Osaka, Japan
Accepted for publication November 7, 1994.
| Abstract |
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| Introduction |
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Acute aortic dissection is a life-threatening condition [1, 2]. Surgical intervention for type B dissection has not provided satisfactory results [37] because of the advanced age of patients at onset [8, 9] and the high incidence of postoperative pulmonary complications accompanying lateral thoracotomy [3, 5, 6, 8, 9]. It has been argued that medical treatment is superior to surgical treatment in acute type B dissection [24, 7, 10, 11]. However, in 20% to 50% of patients who have survived the acute stage with medical treatment, enlarged aneurysms develop within 1 to 5 years of onset [2, 3, 11]. In addition, in chronic type B dissection, distinct enlargement of the false lumen and narrowing of the true lumen occur in most cases, making surgical treatment at a later stage more difficult [12].
Thus, it is necessary to develop a less invasive therapeutic procedure to treat the type B dissection at an acute or subacute stage. Recently, intraluminal devices have been developed that may conceivably cure aortic dissection [13, 14]. These devices are designed to pressure the inner dissected wall from the aortic true lumen, and subsequently to cure the diseased vessel. These are metallic intravascular stents that are expanded by balloon or self-expand by their own elasticity.
In this article, we report on a transcatheter-placed, self-expandable intraaortic (IA) graft that we developed to accomplish less invasive obliteration of aortic dissection. Our IA graft consists of a tandem-type Gianturco stent with an outer jacket of thin, microporous polyurethane. The effectiveness and safety of this new graft and the vascular tissue regeneration process associated with the implanted graft were examined in acute and chronic phases.
| Material and Methods |
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To meet the first requirement, we used a self-expandable stent as the structural support of the graft. To meet the second and third requirements, we selected segmented polyurethane, the most durable and elastic material among the existing synthetic elastomers, as a material for a thin-wall graft which outer-jackets the stent. To promote tissue regeneration and ensure a nonthrombogenic potential similar to the native artery in the chronic phase, microporous structure was impregnated into a graft.
Preparation of IA Graft
A self-expandable stent (a Gianturco-type stent known as the Z-stent; William Cook Europe A/S, Bjaeverskov, Denmark) and a porous polyurethane jacket were prepared separately and these were then assembled into a one-piece graft.
POROUS POLYURETHANE JACKET.
A segmented polyurethane, Cardiomat 610 (Kontron Inc, Everett, MA), was used for fabrication of the graft. Crystals of sodium chloride (mean granular size, 0.1 mm) were mixed with 5% tetrahydrofuran solution of polyurethane. After the crystal-impregnated solution was cast in a cylindrical vessel by a rotation method at room temperature, impregnated crystals were eluted by washing with warm water to form an open-cell-structured, microporous polyurethane jacket (wall thickness, 0.2 mm; average pore size, 0.1 mm). Jackets with diameters of 12, 15, and 19 mm were prepared and their lengths were adjusted to the stent lengths. The luminal and external surfaces and the cross-section of porous polyurethane jacket were examined by a scanning electron microscope (Hitachi S-400; Hitachi Co Ltd, Tokyo, Japan).
STENT.
The device consisted of two or three self-expandable Gianturco stents connected in tandem with two stainless steel struts. Each stent was constructed of 0.35 mm stainless steel wire (Nippon Seisen Co Ltd, Osaka, Japan) bent into a nine-tip zigzag configuration that was 2 cm long and had an internal diameter of 15 or 20 mm (full length, 40 mm or 60 mm).
FABRICATION OF ONE-PIECE GRAFT.
The tetrahydrofuran solution of polyurethane was used as a glue to attach the stent to the porous polyurethane jacket as follows. The expanded form of the stent was inserted into the porous polyurethane jacket, and drops of solution were applied to the respective bending points of the stent and then air-dried. A structural support with two stents was covered with polyurethane jackets in full length (40 mm), and a support with three stents was covered only in proximal two stents (covered length, 40 mm; uncovered length, 20 mm). A 15-mm--diameter stent was used for a 12-mm--diameter graft, and a 20-mm--diameter stent was used for a 15-mm or 19-mm--diameter graft. The fabricated grafts were compressed by hand and inserted into a 12F catheter sheath.
In Vivo Performance
Ten adult mongrel dogs (body weight, 14 to 18 kg) were anesthetized and maintained under anesthesia with Nembutal (Abbott, Abbott Park, IL; pentobarbital sodium, 4 mg/kg). Mechanical ventilation was performed for dogs subjected to the experimental type B dissection. All animals were handled in compliance with the Principles of Laboratory Animal Care formulated by the National Society for Medical Research (USA) and the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health (NIH publication 85-23, revised 1985).
PREPARATION OF EXPERIMENTAL DISSECTION.
After systemic heparinization the descending aorta exposed by left lateral thoracotomy was simply clamped and a transverse incision was made in the adventitia. The aorta then was dissected semicircularly between the intima and adventitia and the resultant flap was separated toward the distal side using a special spatula. A reentry site was made at about 10 to 15 cm distal to the incision (the intima was punctured by a probe). Next, an entry site was made at the intima of the incision site using a 6-mm punch. After the dissected adventitia was sutured, the aorta was unclamped to complete the experimental dissection.
IA GRAFT IMPLANTATION.
After the entry site and false lumen were confirmed by aortography, the position of the entry site was marked on the body surface. A 12F catheter sheath (William Cook Europe A/S, Bjaeverskov, Denmark) inserted through the femoral artery was positioned at the entry site of the true lumen. After an IA graft was delivered up to the site of the entry through the catheter sheath using a pushing rod (William Cook Europe A/S), the graft was expanded from the sheath into the aorta by pulling the sheath without moving the push rod. On the basis of the preliminary aortogram study, the size of IA graft was selected to be 20% to 30% larger than the aortic diameter at the proximal side of the dissection. For acute experiment, 4 animals were sacrificed 2 hours after implantation. For chronic experiment, implantation was performed on 5 adult mongrel dogs that had normal descending aortas and 1 dog that had been subjected to experimental type B dissection prepared 3 weeks before implantation.
HISTOLOGIC EXAMINATION.
At the projected sacrifice periods, the entire graft complex and adjacent segments of aorta were excised. Each specimen was divided into three parts: proximal junction, middle portion, and distal junction. Each of the three parts was further divided into two parts, one for light microscopic examination, the other for electron microscopic examination. Specimens for light microscopic examination were immersed in 10% neutral buffered Formalin, and stained by hematoxylin and eosin, azan Mallory, elastica van Gieson, and peroxidase-antiperoxidase methods. The specimens for electron microscopic examination were fixed in a 3% solution of glutaraldehyde (0.1 mol/L cacodylic acid buffer solution, pH 7.2) for 2 hours at room temperature, after which the specimens were dehydrated using 1% tannic acid and an alcohol series method. After critical-point drying, the specimens were subjected to vapor deposition of platinum palladium (Polaron, scanning electron microscopy coating system) and then examined with scanning electron microscopy.
| Results |
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Aortography clearly revealed that entry sites and false lumens were created in the descending aorta of all the experimentally dissected animals (Fig 4A
). Grafting at entry sites was attained in all cases (Figs 4B, 4C![]()
); neither jumping at the time of grafting nor migration after grafting was observed. Aortograms at 30 minutes after the implantation revealed that the entry site was fully closed in all cases, whereas the false lumen was partly visualized retrogradely from the reentry (see Fig 4C
). Cineangiography displayed elastic motion of the graft in synchrony with aortic pulsation.
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Aortography showed that no migration took place in any of the animals in chronic phase. In animals with experimentally created dissection, the false lumen was completely obliterated 8 months after implantation.
Tissue Regeneration
Six IA grafts, five of which were implanted in normal descending aortas and the remaining one that was in an experimentally dissected aorta, were histologically examined as to how tissue regeneration proceeded after graft implantation. The vessel segments were removed at 1, 4, 6, and 8 months. Little complication (such as necrosis) was observed in any of explanted samples.
On gross examination at 1 month after implantation, minimal gross thrombus formation was observed on the luminal surface, most of which was covered with a thin, yellowish-white membrane. A small number of tiny thrombi were present on the distal portion of the graft. At 6 months (Fig 5A
), there was no thrombus on the IA graft, which was totally covered with a yellowish-white uniform tissue that resembled vascular endothelium. In the luminal surface of the dissected aorta removed at 8 months, the entry site was closed and the false lumen was completely filled with clot (Fig 5B
). A thrombotic layer was observed between the intima and the adventitia of experimentally created vessel wall.
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| Comment |
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Percutaneous transluminal angioplasty for atherosclerotic vascular disease has led to percutaneously implantable intraluminal devices which are either self-expandable or balloon-expandable [1619]. Catheter-aided methods based on these devices have been studied in recent years to develop new, minimally invasive techniques for treatment of type B dissection [13, 14, 20]. In 1986, Akaba and associates [20] devised a cylinder-type balloon catheter that was used for intraluminal compression of a dissection that had been induced by an inflated balloon. A metallic stent was later developed and introduced to prevent re-narrowing of the peripheral artery and the biliary tract after procedures were performed to dilate their lumens [1619]. These metallic stents were used not only for preventing stenosis of peripheral blood vessels and coronary arteries, but also for maintaining the true lumen and the closure of the false lumen in aortic dissection [13, 14]. Recently developed balloon-expandable stents for curing aortic dissection include the Trent stent [13], Palmaz stent [14], and Fontaine stent [14]. These intraluminal devices successfully induce clot formation as a result of compression at the false lumen of experimentally created aortic dissection. Due to the very nature of their design, however, rapid obliteration at entry site would not be expected and furthermore incomplete obliteration may occur.
We developed a new graft especially for treatment of acute type B dissection by covering a Gianturco stent, a typical self-expandable stent, with an elastomeric polyurethane jacket. We expect that its elasticity should make it function well not only at the time of insertion into the sheath and expansion in the lumen, but also during the chronic phase, so that it provides a constant pressure against the false lumen. We fabricated three different sizes of IA grafts with microporous segmented polyurethane. This material, extensively used for blood pumps such as intraaortic balloon pumps, assist devices, and artificial hearts, is elastic and durable. This microporous film is much more flexible than a nonporous film; it is so compressible that an IA graft can be inserted easily into a catheter sheath. In addition, the open-cell structure was designed to enhance tissue regeneration on and through the porous jacket.
We consider the diameter of the IA graft relative to that of the dissected aorta to be very critical. An oversized graft may result in pressure necrosis of the aortic wall, whereas an undersized graft may lead to graft migration. In this study, the diameter of the implanted IA graft was set at 120% to 130% of the diameter of the normal segment of the aorta proximal to the dissection. In all animal subjects, the IA graft was placed successfully in the predesignated part of the aorta, and closed the entry site of the dissection. Autopsy at 2 hours after implantation showed that the IA graft stretched tightly against the intima of the aortic wall and that blood was already coagulated in the false lumen, which was under stent pressure. Observations for up to 8 months revealed no thinning or pressure necrosis of the aortic wall, and no migration of the graft. Our selected proportions, therefore, seem to be appropriate for application in clinical cases. Although our IA grafts did not have any hooks to prevent migration, structural modification of stents to include hooks could greatly guarantee the stabilization of implanted grafts.
The tissue regeneration process on the IA graft, assessed by light and electron microscopic evaluation, was extremely encouraging. Endothelialization, probably initiated by ingrowth through micropores, began within several weeks. At 4 months, the entire surface of the IA graft was endothelialized, and micropores were filled with regenerated collagen fibers. In addition, the elastic fiber was regenerated. Fewer foreign-body giant cells were observed 6 months after implantation, indicating that tissue regeneration was completed within this period. Neovascularization also occurred beneath the endothelial layer through transinterstitial micropores. Thus, the microporous structure of segmented polyurethane film seems to enhance the tissue regeneration potential with minimal foreign-body reaction.
The essential requirement of this IA graft therapy is to close the entry point, which requires precise diagnosis of the entry site. In addition to aortograms used in this study, intravascular echography, magnetic resonance imaging spectroscopy, and other diagnostic techniques may help to precisely position IA grafts. Our current device uses a self-expandable Z-stent, which cannot adapt itself to a curved part of the aorta. This hampers application of the IA graft to the segment of the aorta immediately distal to the left subclavian artery, which is the most common site of the entry of type B dissection. Extended application to these dissections will require further modification of the IA graft. And as for the application of our IA graft to aneurysmal diseases, we think it is necessary to modify the covering material because our extremely elastic covering jacket may be the cause of graft aneurysm formation after implantation.
In summary, we developed a novel intraaortic prosthesis composed of a Gianturco stent and a microporous segmented polyurethane jacket, and successfully closed off the entry site of experimentally prepared type B dissection using the transcatheter technique. These results in dogs, together with satisfactory histologic biocompatibility in chronic experiments, suggest that our IA graft may be effective in treating the acute stage of type B dissection with minimal invasion.
| Acknowledgments |
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| Footnotes |
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| References |
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