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Ann Thorac Surg 1995;59:914-915
© 1995 The Society of Thoracic Surgeons
Cardiovascular and Interventional Radiology Department of Radiology Stanford University Medical Center 300 Pasteur Dr, H-3647 Stanford, CA 94305-5105
Transluminally placed stent-grafts offer an alternative approach to standard surgical treatments for a variety of vascular pathologies. The clinical feasibility of transluminal endovascular grafting for the treatment of abdominal aortic aneurysm, thoracic aortic aneurysm, subclavian artery aneurysm, arteriovenous fistula, and femoral occlusive disease is well documented. These newer procedures are potentially less invasive and less expensive, with a lower risk than standard operative repair.
Now Kato and colleagues describe their experience with a new transcatheter-placed self-expandable stent-graft for treatment of an experimentally created type B dissection in a canine model. Their report supports the feasibility of this approach in an animal model and corroborates the previously published studies by Kato and associates [1] and Yoshida and colleagues [2] using similar endovascular devices. These authors and others previously demonstrated the limitations of bare metal stents for managing type B dissection and advocated the use of stent-grafts in this setting.
Beyond demonstrating the technical success of their procedure for obliterating an artificially created dissection, Kato and colleagues provide new information regarding the effectiveness and biostability of a microporous polyurethane graft material. This stent covering of thin open-cell structured polyurethane appears to support endothelialization from 1 to 8 months after implantation. Although their initial histopathologic examinations of this surface appear promising, they should be viewed with caution because the durability of this type of vascular implant has been questioned in the past. Clearly, only long-term studies will answer confidently concerns over possible erosion or degeneration of chronically implanted polyurethane surfaces.
In terms of the structural plumbing issues often complicating aortic dissection, it now appears the stage is almost set to address the most compelling question raised by this research-namely, can this experimental application of endovascular techniques successfully translate to the real-world clinical setting? It is anticipated that a number of speed bumps and eddy currents will need to be negotiated successfully before patients will realize any incremental benefits relative to traditional surgery.
I suspect the initial clinical application for this type of endovascular stent-graft technology will focus on treatment of acute type B dissections. It will be used not so much as a means of improving ischemic peripheral sequelae of complex dissections but more in a ``prophylactic'' manner to prevent subsequent progressive dilatation of the false lumen, which could lead to hemodynamically significant compromise of the true lumen or aneurysm formation, which may increase the difficulty of eventual surgical treatment. As noted by Kato and colleagues, in 20% to 50% of patients who survive the acute stage of a type B dissection with medical treatment ``enlarged aneurysms'' will develop within 1 to 5 years.
Why choose acute type B dissections as the initial proving ground for this type of therapy? The answer pertains to the relative anatomic complexity of various types of aortic dissection and the currently available stent-graft technology.
When considering the exact nature of an aortic dissection there are two aspects that may make it more or less amenable to stent-graft therapy. First is the age of the dissection. Is it acute or chronic? Second, what is the extent of the dissection? Is it a type A or type B dissection? A type A dissection extending through the transverse arch is potentially more complicated than a type B dissection owing to the possible involvement of the great vessels, as well as the coronary arteries and aortic valve. The anatomic orientation of the flap relative to these branches is clearly idiosyncratic and the hemodynamic implications are often serious. Kato and colleagues acknowledge the limitations of their self-expandable stent-graft to adapt to a curved segment of the aorta. This obviously restricts its use to the straight descending portion of the thoracic aorta. Likewise, reports by other investigators using relatively rigid devices concluded they are inappropriate for therapy of type A disease. Clearly, flexible stent-graft combinations as well as delivery systems are requisite to advance beyond the current device limitations, which prohibit stent-graft treatment of type A dissection.
What about acute versus chronic dissections? The treatment of the primary aortic pathology, including the primary tear, is more problematic in the chronic setting than with an acute dissection. It may be virtually impossible to obliterate the false lumen of a chronic aortic dissection using an endovascular stent-graft. This is because with time, the once relatively flaccid, torn intimal flap becomes fibrotic, thickened, and tough. It is resistant to reapproximation by a stent-graft, especially a self-expandable device with less intrinsic hoop strength than a relatively rigid balloon-expandable prosthesis. Furthermore, the task of reapproximating the contracted flap against the outer wall of a dilated false lumen may not be structurally possible due to a mismatch in anatomic dimensions. Without obliteration of the false channel, flow via multiple distal reentry tears may maintain patency in this lumen and progressive enlargement may ensue. Alternatively, thrombosis of a relatively large residual false lumen can perpetuate hemodynamically significantly compromise of the true lumen and distal organ ischemia.
The work of Kato and associates should be lauded as an important investigational step toward the eventual clinical application of endovascular stent-grafts for the treatment of aortic dissection. The ultimate suitability and effectiveness of stent-grafts for this application are unclear, but the challenge to elucidate their role is indeed an interesting one.
References
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