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IS082
Endovascular Stent
Grafts for Descending Thoracic Aortic Pathology |
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Marc R. Moon, M.D.
Department of Cardiothoracic
Surgery
Washington University
St. Louis, MO, USA |
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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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Figure
1. Intravascular ultrasound image demonstrating
a dissection with the true lumen (small) and false
lumen (large). (Moon 2000) |
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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.
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Figure
2. Intravascular stent that is well incorporated
at 6 weeks. (Moon 2000) |
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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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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.
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Figure
3. Computed tomographic scan following stent-graft
placement demonstrating complete exclusion of
the thoracic aorta. (Moon 2000) |
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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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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
- Moon MR, et al. Intravascular stenting of
acute experimental type B dissections. J Surg Res 1993;54:381-388.
- 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.
- Dake MD, et al. Endovascular stent-graft placement
for the treatment of acute aortic dissection. N Engl J Med
1999;340:1546- 1552.
- Crawford ES, et al. Graft replacement of aneurysm
in the descending thoracic aorta. Results without bypass or
shunting. Surgery 1981;89:73-85.
- 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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