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IS082

Endovascular Stent Grafts for Descending Thoracic Aortic Pathology
Marc R. Moon, M.D.
Department of Cardiothoracic Surgery
Washington University
St. Louis, MO, USA
 
  • Laboratory experience
  • Clinical experience
  • Closing

  • The initial laboratory experience that created interest in endovascular treatment of thoracic aortic disease was reviewed in this presentation, as well as the initial clinical experience.

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    Laboratory experience







    Figure 1. Intravascular ultrasound image demonstrating a dissection with the true lumen (small) and false lumen (large). (Moon 2000)

     

    Stenting alone without a graft comprised the initial experience with endovascular treatment of thoracic aortic disease.1 An experimental acute type B dissection of the descending thoracic aorta was created in 12 dogs through a left lateral thoracotomy, manually creating a dissection plane in the aorta. To create the dissection, a spatula was used to create a tear in the aorta, and once flow was reconstituted distally, the dissection propagated into the abdominal aorta in most cases. A complete radiologic evaluation was done followed by intravascular stent placement to determine the effects on the true lumen and false lumen over time. Figure 1 is an intravascular ultrasound (IVUS) image demonstrating a dissection with the true lumen (small) and false lumen (large).

    Fluoroscopy and IVUS was initially used to guide the placement of the balloon-expanding stents used to reconstitute flow. In some animals it was not possible to reconstitute complete true lumen flow, thus a residual false lumen was present despite reconstitution of a normal true lumen. In some animals stents were placed all along the aorta to eliminate the false lumen. In other animals, stents were placed only proximally to determine whether the false lumen would remain patent.


    Figure 2. Intravascular stent that is well incorporated at 6 weeks. (Moon 2000)

     

    All intercostal and visceral arteries were patent at the 6-week evaluation, which included angiographic, IVUS, and histologic studies. No stent migration occurred despite the high pressure of normal, distal aortic flow. The stented true lumens were all patent without thrombus formation on the stent, despite the fact that no anticoagulation was used. The aortic wall had healed in most instances, and neointimal formation was occurring over the stents themselves. Figure 2 demonstrates an intravascular stent well incorporated at 6 weeks.

    Fate of the distal false lumen

    In the four animals with complete proximal to distal obliteration of the dissection, no false lumen was present and the aortic wall had healed. In the three animals with complete obliteration of only the proximal portion of the dissection, the false lumen was patent and beginning to develop pseudo-endothelialization. In the two animals with proximal partial compression, one false lumen was patent and one was thrombosed. This is similar to what is seen in clinical dissections with some false lumens becoming thrombosed without treatment. Two of the three control animals (no stents placed) had a patent false lumen and one was thrombosed.

    This experimental study showed that intravascular stents: 1) restored distal flow; 2) obliterated the false lumen if positioned proximally to distally throughout the aorta; and 3) promoted aortic wall healing without thrombus formation; however, 4) stenting limited to only the proximal dissection did not prevent the development of a chronic patent distal false lumen.

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    Clinical experience


     

    Initial human studies

    The first patient in whom a descending aortic stent-graft was placed was elderly with aneurysms of the ascending aorta and the proximal descending aorta, as well as chronic obstructive pulmonary disease (COPD). Initially the patient underwent surgical intervention with replacement of the ascending aorta and had a relatively long post-operative course because of his COPD. Because of hesitancy to perform a thoracotomy in this patient, a self-expanding stent was placed in a delivery sheath and positioned under fluoroscopic guidance into the proximal descending thoracic aorta. Dacron had been sutured over four stents (each 2.5cm in length) to create the stent-graft. The post-operative angiogram demonstrated complete occlusion of the aneurysm with reconstitution of distal flow.

    "First-generation" stent grafts

    At Stanford University, thoracic aortic stent grafting was completed in 103 patients who were often high-risk for conventional surgery.2 Notably, 60% of these patients were deemed unsuitable for a standard thoracic aortic replacement by a surgeon either in their group or outside their institution. The average diameter of the aneurysm was 6 cm, ranging from 4 to 11 cm. The average length was about 9 cm, ranging from 1 cm in a post-traumatic aneurysm to 22 cm extending nearly the entire length of the descending thoracic aorta. Most (n=64) were atherosclerotic in origin, with some traumatic, anastomotic pseudo-aneurysms, ulcers, and dissections.

    One technical issue is whether or not there is a sufficient landing zone to position the graft due to the aneurysm being very close to the subclavian artery. In 8 of the 103 patients a pre-operative subclavian-to-carotid bypass or transposition was performed to allow the orifice of the subclavian artery to be covered with the stent-graft without obstructing flow to the arm. The femoral artery was used for access in most cases, although in some patients it was too small, tortuous, or diseased. Access to the abdominal aorta was gained through a retroperitoneal approach in these patients. Intercostal arteries presumably were covered from T9 to T12 in about 18% of patients.

    The mortality rate was 9%. This high-risk group had a 3% incidence of paraplegia. However, in that 3%, the intercostal arteries were not considered to be covered. Most of the patients that experienced paraplegia postoperatively had simultaneous super renal abdominal aortic replacement, and the paraplegia was presumed to be due to the combined intervention.


    Figure 3. Computed tomographic scan following stent-graft placement demonstrating complete exclusion of the thoracic aorta. (Moon 2000)

     

    Complete thrombosis occurred in 84% of the patients. Figure 3 is a computed tomographic scan of one patient following stent-graft placement demonstrating complete exclusion of the thoracic disease. Note the presence of contrast within the stent-graft in the descending thoracic aorta and the absence of contrast within the aneurysm sac. A phenomenon called "endoleak" developed in which flow was still present around the stent-graft itself following placement. This occurred in 24% of patients. Endoleak was successfully treated in 11 of the 25 patients with complete exclusion, generally with a combination of either further stent-graft placement or coiling of a portion of the aneurysm. Short-term survival was acceptable, 81% at 1 year and 73% at 2 years. Late rupture occurred in 2% of patients at 22-month average follow-up.

    Acute aortic dissections

    In human studies, covered stent-grafts have been used to treat acute descending thoracic aortic dissections,3 in contrast to the non-covered stents that were used in the laboratory study described above.1 The covered stent-grafts positioned only at the proximal portion of the dissection. In the initial human experience, also from Stanford University, the primary tear was covered in the proximal descending aorta in 19 patients.3 The mortality rate was not insignificant at 16%. These patients, however, all had significant preoperative complications that prompted stent-graft placement. Symptomatic branch vessel involvement (renal, visceral, femoral, iliac) was very common at 74%, and was corrected in 75%. Complete thrombosis of the false lumen occurred in 80%. No late deaths occurred after the initial period of treatment, and no late aortic rupture or aneurysmal development occurred at the 13-month follow-up. After placing the stent-graft in the aorta, the true lumen increased substantially after stent grafting at all levels despite treatment only at the proximal level. The proximal aortic diameter (true and false lumen combined) also shrank over time and did not become an aneurysm, as often occurs with medical treatment.

    Simultaneous abdominal aortic aneurysms and thoracic stent-grafting

    Five percent of patients with abdominal aortic aneurysms have a descending thoracic aneurysm, and 13-29% of patients with descending thoracic aortic aneurysms also have an abdominal aortic aneurysm. Conventional surgical options include simultaneous open repair or staged repair. However, 30% of post-operative deaths in these patients occur secondary to rupture of the second aneurysm while awaiting staged repair.4

    A novel treatment approach for multiple aneurysms was investigated in 18 patients.5 A standard abdominal aortic repair through a retroperitoneal approach was performed. Then a 10mm side graft was sewn onto the abdominal aortic graft. A delivery sheath was then placed through the side graft and a stent-graft positioned into the descending thoracic aorta under fluoroscopic guidance.

    One death due to multiple organ failure occurred at 31 days in a redo suprarenal aneurysm. One patient developed paraplegia of unknown etiology; it is thought to have been due to simultaneous loss of critical lumbar and thoracic intercostals. At 14-month follow-up, 17 of the 18 patients were alive and well. In later studies, another patient with simultaneous suprarenal aneurysm developed paraplegia. It may be that these patients are at increased risk for paraplegia.

    The development of endoluminal stents to treat aneurysms has changed practice patterns at Washington University. For abdominal aneurysms, in the second quarter of 1999, 16 open procedures and 5 closed procedures were performed. Now that endoluminal stents are commercially available for infrarenal abdominal aortic replacement, the number of endoluminal stent-graft placed far outweighs that of open infrarenal aortic replacement. 100 endoluminal abdominal procedures were performed from September 1999 to April 2000, compared to about 15 open procedures. Commercially-produced stent-grafts are now being tested at Washington University and other centers, and may potentially be available in 18-24 months.

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    Closing


    Stent-graft placement is feasible and can be used to exclude aortic pathology. However, the first-generation studies demonstrated that it was associated with morbidity and mortality that is not insignificant. The new commercially available devices will hopefully reduce morbidity and mortality due to less intra-aortic trauma. The short-term results are acceptable, although certainly there is room for improvement. Future goals are to develop more "user friendly" device systems that minimize trauma and simplify the technique, increase the precision of stent graft deployment, and decrease the risk of cerebral vascular incidents. Long-term results must be evaluated to determine whether this approach will be applicable to younger patients and provide the same long-term results as with standard surgical replacement.

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    References
    1. Moon MR, et al. Intravascular stenting of acute experimental type B dissections. J Surg Res 1993;54:381-388.
    2. Dake MD, et al. The "first generation" of endovascular stent-grafts for patients with aneurysms of the descending thoracic aorta. J Thorac Cardiovasc Surg 1998;116:689-704.
    3. Dake MD, et al. Endovascular stent-graft placement for the treatment of acute aortic dissection. N Engl J Med 1999;340:1546- 1552.
    4. Crawford ES, et al. Graft replacement of aneurysm in the descending thoracic aorta. Results without bypass or shunting. Surgery 1981;89:73-85.
    5. Moon MR, et al. Simultaneous abdominal aortic replacement and thoracic stent-graft placement for multilevel aortic disease. J Vasc Surg 1997;25:332-340.


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