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Ann Thorac Surg 2007;83:1640
© 2007 The Society of Thoracic Surgeons
Department of Cardiothoracic Surgery, Stanford University Medical Center, Falk CVRC, 300 Pasteur Dr, Stanford, CA 94305
(Email: rsmitch{at}stanford.edu).
As stated by the authors [1], the role of stent graft technology in the treatment of chronic type B aortic dissections is unclear. In the past decade, thoracic aortic stent grafts have made a major impact on the management of degenerative aneurysms of the descending thoracic aorta. Similarly, stent grafts have greatly facilitated our management of complicated acute type B aortic dissections, especially in regard to malperfusion, in which mortality for central aortic reconstructions approached 60%. Currently under investigation in the INSTEAD trial, the benefit of stent grafts for uncomplicated type B aortic dissections remains unproven.
For chronic type B aortic dissections, the usefulness of thoracic stent grafts remains similarly unclear. Perhaps thoracic stent grafts may be an effective strategy for focal aneurysmal change in a chronic dissection, but significant hazards and limitations abound. Manipulation of a large bore stiff introducer in the aortic arch is not without risk, including creating a new type A dissection (especially with uncovered proximal extensions) and stroke. Because the majority of primary intimal tears occur near the origin of the left subclavian artery orifice, it is usually necessary to cover that orifice, and frequently also to occlude it, to prevent a retrograde type II endoleak. Although arm complications can usually be managed expectantly, our understanding of the risk for posterior circulation stroke by covering the left subclavian artery with a dominant left vertebral artery is incomplete. An optimal landing zone greater than 2 cm in length, distal to the left carotid artery, is infrequently encountered, especially when measured along the lesser curvature of the arch.
The distal landing zone is similarly problematic. Unlike atherosclerotic aneurysms, distal intercostal artery pairs usually remain patent in chronic dissections, and stent graft coverage of the T-8 to T-12 regions has acutely been associated with an increased risk of paraplegia. Deployment in the true lumen is also absolutely essential, with sizing of the graft hugely problematic with a very compressed true lumen. It is also unclear how much remodeling of the true lumen can be expected with this thick fibrous dissection septum or how much radial force a stent graft should exert to promote that remodeling.
In the acute setting, our experience indicates that the true lumen can be acutely expanded for the length of the stent graft with false lumen thrombosis for a similar extent. Distally however, multiple fenestrations, especially at the level of the diaphragm and the abdominal vessels, promote continued patency of the false lumen. Our very limited experience with chronic dissections confirms these findings. Although a proximal aneurysmal segment may decrease in size after stent graft coverage of the proximal tear, more distally, both true and false lumens in the lower chest and upper abdomen tend to remain patent, and in fact, increase in size. Of these three chronic stent graft patients, 2 have required open repair for proximal endoleaks. Of approximately 30 patients who received a stent graft to treat a complication of an acute type B aortic dissection, we have serial follow-up aortic measurements in 16 patients at 1 year or greater. For the proximal aortic segment, 5 patients demonstrated a decrease in total aortic diameter, with the aorta unchanged or larger in 11 patients. Similarly, in the midaortic segment, the aorta was unchanged or larger in 13 patients, and for the distal segment, the aorta was unchanged or larger in 15 patients.
How then should we interpret this report from an active aortic surgery center in Vienna? First, as the authors acknowledge, "the patient number is terribly small." It would be interesting to know what percentage these cases comprised of chronic type B dissections followed at this center. As stated by the authors, this technology could be expected to "perform best in an isolated distal arch aneurysm originating from a chronic type B dissection with a short membrane and a regular diameter in the mid third of the descending aorta." This would seem to be a small percentage of all chronic type B dissections. Second, the mean follow-up of only 16 months is short, and further aortic dilation may mitigate any early decrease in size. Although the concept of coverage of the primary intimal tear with redirection of flow into the true lumen is appealing, its efficacy in the long term remains unknown. It is likely that stent grafts may evolve to play a major role in the management of these difficult patients, but specific devices designed for dissections with differing radial strengths, tapered graft bodies, separate branch grafts, and uncovered stents may be necessary for optimal usefulness. As our understanding progresses, and until we have data demonstrating significant benefit to stent grafting in the chronic situation, we must proceed with caution.
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