Keywords
frozen elephant trunk - dSINEs - aortic dissection - stent-graft - aortic clinic
Introduction
Total arch replacement via the frozen elephant trunk (FET) technique has become a
well-established treatment option for patients with acute or chronic aortic dissections
(Type A, Type non-A and non-B, or Type B), those with aortic arch aneurysms, and even
for patients with penetrating aortic ulcers in whom thoracic endovascular repair is
unfeasible or unsafe.[1]
[2]
[3]
[4]
[5]
[6] There has also been a trend favoring the FET over isolated total arch replacement
or conventional elephant trunk in recent years. Factors include the excellent proximal
landing zone provided by the FET's stent-graft component, an anticipatory strategy
regarding any progression of the underlying aortic pathology, FET's potential to reexpand
the true lumen, and the ability to close proximal entries within the descending aorta.
Hence, there is now solid European consensus, and many aortic centers now favor the
FET for total arch replacement (“The elephant trunk is freezing”).[5]
[7]
[8]
Aortic Remodeling and Reinterventions
Aortic Remodeling and Reinterventions
In patients with acute or chronic aortic dissection who underwent FET implantation,
several groups demonstrated positive aortic remodeling of the descending and abdominal
aorta. Such studies were recently published by Berger et al, Shrestha et al, and Dohle
et al.[9]
[10]
[11]
Despite their promising results, there is evidence of a significant risk of aortic
reinterventions after FET implantation.[12] In competing risk regression analyses (competing risk: death), this risk rose to
as high as 64% after 3 years. Note that one needs to differentiate between intended
or expected secondary aortic reinterventions and those that are unexpected or unintended.[12]
Young patients, especially, suffering from any type of chronic or chronic residual
aortic dissection carry an inherent risk for aortic (re)interventions because of chronic
diameter progression.[13]
[14] This risk still remains after the FET procedure. Nevertheless, the FET procedure
is able to treat in one step the complete proximal aorta including the ascending aorta,
aortic arch, and proximal descending aorta. It also provides an excellent platform
for the dichotomous treatment of downstream aortic segments with comparably low risk.[6]
[15]
[16]
Hence, in a relatively simple step, distal stent-graft elongation is possible up to
the level of the celiac trunk following FET implantation.[16] In case this two-staged-approach fails to stabilize the remaining aorta, open thoracoabdominal
aortic replacement is simplified because the anastomosis site has been moved distally.
This procedure becomes comparable to a Crawford's type-4 scenario and is referred
to as the three-step approach.[15] Without distal stent-graft extension as the second step, surgical treatment would
amount to a more invasive Crawford's type-2 scenario. Lung compression can be significantly
minimized in this third step when the anastomosis site is moved distally through an
endovascular second step.[15]
For such conceptual, planned, or expected secondary aortic reinterventions following
the FET, the “risk” designation seems inappropriate. Yet, there is also a substantial
risk associated with unintended and unexpected aortic reinterventions following FET
treatment.[12] Distal stent-graft-induced new entries (dSINEs) have been identified as one significant
reason for such unintended secondary aortic reinterventions ([Fig. 1]).[17]
Fig. 1 Representative computed tomographic scan of a distal stent-graft induced new entry
(dSINE) that developed 1 year after frozen elephant trunk implantation.
Distal Stent-Graft-Induced New Entries following the Frozen Elephant Trunk Procedure
Distal Stent-Graft-Induced New Entries following the Frozen Elephant Trunk Procedure
Dong et al defined a stent-graft-induced new entry as “a new tear” caused by a stent-graft
regardless of natural disease progression or iatrogenic injury.[18] Because dSINE leads to false lumen perfusion, there is a significant risk for negative
aortic remodeling being exacerbated to trigger acute aortic rupture. Hence, a dSINE
occurrence should be considered as treatment failure. Because mortality rates for
untreated dSINE are known to be as high as 25%, diagnosis of a dSINE should also represent
a treatment indication.[19] Fortunately, dSINE therapy is quite simple via endovascular techniques.[16]
[17]
[19]
The risk for developing a dSINE after the FET procedure was recently reported by our
group to be up to 25% after 3 years.[17] The incidence of dSINE developing after the FET procedure is, thus, higher than
the 6% incidence of dSINE development following conventional thoracic endovascular
aortic repair.[20] Of note, dSINE may develop at any time following the FET procedure—even years after
hybrid-graft implantation.[16]
[17]
The interaction between a comparably stiff stent-graft and a flexible, thin dissection
membrane seems to be responsible for dSINE development.[17]
[21] In chronic dissections, the dissection membrane loses elasticity and may become
more likely to develop a dSINE.[21]
[22]
[23] Yet, the acuity of the dissection was not identified as a risk factor for dSINE
formation in our recent investigation.[17] To prevent dSINE, the stent-graft component should not be oversized. While oversizing
is necessary in conventional endovascular stent-graft treatment to anchor the grafts,
it is unnecessary in case of a FET implantation because the device itself functions
as an immobile stent-graft anchor. Hence, the stent-graft simply needs to re-expand
the true lumen via its radial force and sufficiently seal off the true lumen to prevent
a type IB endoleak.
To select the correct stent-graft size in acute scenarios, we apply a method first
described by Rylski et al[24]: we subtract 3 mm from the aortic diameter measured between the left carotid and
left subclavian artery, we add 8 mm to the true lumen diameter in the dissected aorta's
first quartile or we measure the maximum diameter of the dissected true lumen diameter
at the level of the anticipated FET stent-graft landing. In chronic scenarios, we
recommend measuring the true lumen's circumference at the level of the anticipated
FET stent-graft landing, always employing the smallest stent-graft sizes available.[12] In any case, one can always measure the true lumen's diameter by carefully inserting
intraoperatively a dilator into the true lumen.
Potential risk factors for developing a dSINE following stent-graft placement are
summarized in [Table 1]. Note that no clinical risk factors have yet been identified for dSINE development
in patients receiving a FET prosthesis.[17] Theoretically, a sharp angle between the stent-graft and native true lumen, particularly
after implanting the FET device in zone 2 and/or using shorter stent-grafts, may increase
the risk for dSINE formation (even though single-center studies have not proven this
to be a risk factor[17]
[22]
[23]
[25]). It seems plausible that placing the stent-graft within the native aortic arch
or in the very proximal descending aorta causes a sharp angle between the stent-graft
and the downstream descending aorta. In addition, when forcing a short stent-graft
downward into the straight descending aorta, the stent-graft may potentially return
to a straight alignment with the conventional arch component of the FET device causing
large dSINE as recently demonstrated by our group.[26] The same mechanisms may potentially be responsible for the higher dSINE incidence
following the FET procedure in comparison to conventional Thoracic Endovascular Aortic
Repair (TEVAR). Nevertheless, in our opinion, the advantages of a zone 2 implantation—namely
easier and faster surgical implantation and a lower risk of spinal cord ischemia—still
outweigh the theoretical disadvantage of dSINE formation.[17]
[27]
[28] After all, as long as patients are being routinely and closely followed, dSINE formation
is an easy-to-detect complication. Hence, in our center, patients are routinely followed-up
after 6 months, 12 months, and yearly thereafter. We perform computed tomography angiography
scans before FET implantation, before discharge, during every follow-up visit, and
whenever clinically warranted.
Table 1
Summary of potential risk factors for developing distal stent-graft induced new entries
|
Shown in meta-analysis:
|
|
Chronic aortic dissection
|
|
Oversizing/taper ratio
|
|
Single center studies/own clinical experience:
|
|
Short stent grafts
|
|
Sharp angle between the stent-graft and true lumen
|
|
Connective tissue disease
|
Radial Force of the Frozen Elephant Trunk Stent-Grafts
Radial Force of the Frozen Elephant Trunk Stent-Grafts
Since no clinical risk factors for dSINE development following the FET procedure have
been identified, and despite the fact that they occur in the absence of oversizing,
the radial force of the FET stent-graft's distal end should be as low as possible
to prevent injuries to the flexible dissection membrane.[17] Two FET prostheses are now on the European market: the Thoraflex (Ltd., Inchinnan,
United Kingdom) and the E-Vita Open (Cryolife Jotec Inc., Hechingen, Germany). We
demonstrated that the Thoraflex graft's distal stent-graft end is stiffer than the
E-Vita Open graft's in ex vivo mechanical tests.[17] Moreover, when the grafts were confined, the Thoraflex graft became even stiffer.
The relatively stiff, closed distal ring at the Thoraflex prosthesis stent-graft's
distal end is probably responsible for this increased stiffness compared with the
E-Vita graft's more flexible z-design.[17] However, we wish to emphasize that a large multicentric study relying on data from
three large European aortic centers failed to reveal any statistically significant
difference in the dSINE occurrence when comparing these two grafts. In fact, the numerical
incidence was actually lower in patients treated with the Thoraflex graft (Thoraflex
graft: 15% vs. E-Vita graft: 18%, p = 0.19).[29]
Distal Stent-Graft-Induced New Entries Treatment
Distal Stent-Graft-Induced New Entries Treatment
Endovascular stent-graft extension to cover the newly formed entry is usually possible,
and postoperative complications are rare.[16]
[17] In our center, we normally access the femoral arteries percutaneously using preclosure
techniques, and we usually extend TEVAR down to the level of the thoracoabdominal
transition in close proximity to the celiac trunk offspring. The FET stent-graft is
our proximal landing zone, and we normally oversize proximally by 2 mm (most proximal
stent-graft diameter to the stent-graft diameter of the FET). Distally, we avoid any
oversizing and use the above-mentioned methods to calculate the diameter of the true
lumen.[16]
[24] Because of the ideal artificial proximal landing zone of the FET graft, we follow
this standardized management also in patients with connective tissue disease when
the diameter threshold for the treatment of the descending aorta is met. Because these
patients have an inherent high risk for negative distal aortic remodeling, they may
potentially benefit most from the three-step-approach.[15]
Note that recurring dSINE may form after distal stent-graft extension at the distal
end of the newly implanted stent-graft.[20] Furthermore, to maximize spinal cord protection, we only carry out distal stent-graft
extension after implanting a cerebrospinal fluid drainage the day before surgery.[5]
[8] For the same reason, we generally avoid concomitant distal stent-graft extension
during the FET implantation. In case of planned intervention, we usually perform downstream
TEVAR 6 months following FET implantation. When clinically necessary (e.g., in case
of dSINE formation), we perform TEVAR as soon as possible.
Conclusion
Although the FET procedure is an excellent treatment option for patients with aortic
pathologies involving the aortic arch, surgeons need to be aware of the substantial
rate of planned, anticipated, but also unplanned or unexpected aortic reinterventions.
dSINEs are known to be one potential cause for aortic reinterventions, and they can
develop at any time during follow-up after FET implantation. Thus, it is mandatory
that all patients who have undergone the FET procedure be routinely followed-up, ideally
in a highly specialized aortic clinic.
