Points of Attention in Branched EVAR
Branched endovascular aortic aneurysm repair (B-EVAR) was developed for the treatment of thoracoabdominal aortic aneurysms (TAAA). With increasing physician experience and device evolution we can at the moment treat almost all types of aneurysms extending from the aortic arch to the iliac bifurcation. In connective tissue disease, open reconstruction remains the first choice of treatment. There is a multitude of different configurations for branched devices, including downward- or upward-oriented branches, internal or external configurations, and a combination of branches and fenestrations. A typical branched-only device has four downward-facing branches intended for the celiac artery (CA), the superior mesenteric artery (SMA), and two renal arteries (RA). This common design has led to the development of the single currently available off-the-shelf branched device (T-branch, Cook Medical, Bloomington, IN, USA). Other companies (WL Gore & Associates, Newark, DE, USA; Jotec GmbH, Hechingen, Germany) are developing comparable systems that will enable physicians to treat patients even in an acute setting. Despite increasing standardization, applicability of off-the-shelf endografts is not universal, and a significant proportion of patients with TAAA will still require treatment with custom-made devices (CMD) to address specific anatomical characteristics. This underlines the need for meticulous planning of this complex endovascular procedure, which is the first step to avoid intraoperative complications. Nevertheless, a number of technical challenges can arise during B-EVAR, requiring experience with specific endovascular techniques and bail-out measures to achieve technical success.
Preoperative planning of B-EVAR requires good-quality imaging with thin cut (≤1 mm) spiral computerized tomography angiography (CTA) from the chest to the groin with the option of axial, coronal, and three-dimensional reconstructions. To this purpose, physicians have an increasing software arsenal for multiplanar reconstructions and center-line measurements enabling precise placement and orientation of side branches as well as selection of proximal and distal components in composite systems. If a tube graft is required proximally in the descending thoracic aorta, it should be designed to land 3 cm above the orifice of the CA. This facilitates catheterization of the target arteries, while ensuring sufficient overlap between components. In compromised proximal neck, additional fixation with endoanchors can be considered. Typical branch orientation includes clock positions of 01:00 for the CA, 12:00 for SMA, 09:00–11:00 for the right RA and 01:00–03:00 for the left RA. Usually, the endograft main body consists of 10–12 bare metal stents covered with graft material. Branches originate at the midportion of the endograft, and each single stent can accommodate up to two branches; this means that in cases of three or more vessels originating at the same level, one branch has to be positioned higher and the target vessel secured with a longer bridging stent. In situations when the aorta has an hourglass shape, with narrowing of the renomesenteric segment or narrow true lumen in postdissection TAAA, fenestrations or branches can be an option. Specific points of attention are supraaortic artery anatomy in association with proximal landing and intraoperative stroke risk, shaggy aorta, which is the main contraindication for B-EVAR, and access vessels that may restrict endovascular solutions or mandate use of low-profile devices.
Landing zone: Maximal diameter is 40 mm. In hostile anatomy, adequate sealing length is mandatory with possible additional fixation with endoanchors.
Access-vessels tortuosity, calcification, or stenosis: surgical exposure for conduit.
Kinked anatomy (aorta, target vessels): Plan longer overlap and consider ways for accurate device deployment.
Focally narrow aortic segment: Consider a combination of branches and fenestrations to accommodate individual anatomy.
Shaggy aorta: Main contraindication for B-EVAR.
In order to exclude the aneurysm completely with endovascular methods, physicians need to extend the level of repair well above and below the aneurysm to a parallel segment of the aorta to achieve a safe and durable seal. A problem arising particularly in the case of B-EVAR is the risk of spinal cord ischemia (SCI) with increasing length of covered aorta. Preservation of the left subclavian artery and pelvic circulation should always be targeted. Proximally, this can be achieved with the use of carotid–subclavian or carotid–carotid–subclavian bypass and debranching or arch stent grafts with branch fenestrations and, in some cases, chimneys for supraaortic vessels. In cases of common iliac artery involvement, the distal sealing zone can be extended below the iliac bifurcation with iliac branched devices that preserve flow to the external and internal iliac arteries. Despite these measures, published studies have demonstrated that extensive TAAA repair is associated with considerable SCI rates. It is therefore advisable to try to reduce the length of covered descending aorta to the minimum required for an adequate seal. In the case of descending aortic diameters of 32–38 mm, where the branched component usually has to be extended proximally with deployment of a tapered tube-graft and additional aortic coverage of approximately 10 cm, we now have the option to use a double-barrel “2-in-1” CMD device (Cook Inc.). This consists of two coaxial stent grafts with a maximal outer diameter of 44 mm, allowing for adequate seal at the level of the branched component, without extension of the proximal landing zone. Aside from stent graft planning, prevention of SCI requires care with operative timing, pre-, intra-, and postoperative management.
An additional consideration during TAAA repair is the risk of bowel and liver ischemia, caused by embolic complications or malperfusion of main tributaries and important collaterals. Studies have reported an estimated risk of 2%–3% for bowel ischemia in aortic endovascular repair, with a respective mortality of 50%. Specific planning to avoid this in extensive TAAA repair can include additional branches or fenestrations in case of separate orifices of intestinal vessels, an additional branch to prominent (≥4 mm) inferior mesenteric arteries (IMA) when mesenteric circulation is compromised, and branches for the preservation of the internal iliac arteries.
SCI Prevention: Preserve flow to LSA and IIA, do not unnecessarily extend aortic coverage.
Bowel ischemia: Preserve all important tributaries.
Although B-EVAR is considered a less invasive procedure than open TAAA surgery, it still constitutes a major and extensive procedure and therefore requires thorough preoperative medical assessment and evaluation of overall patient status and life expectancy, e.g., in our experience, significantly comorbid patients (ASA IV) have a higher rate of midterm all-cause mortality.
Preoperative patient management should maintain preexisting medication with the exception of aggressive antihypertensive treatment to avoid perioperative hypotension that could increase the risk of SCI. Preoperative antiplatelet therapy with aspirin is standard.
Recent studies have demonstrated that patients with extensive thoracoabdominal disease benefit from staged exclusion of the TAAA. There are several staging strategies that can be applied in atherosclerotic and postdissection TAAA. One consists of primary TEVAR and deployment of the branched device at a second stage. The advantage of this method in patients treated with CMD is that a standard off-the-shelf thoracic device can be implanted immediately while the branched component is in production. An additional benefit in postdissection TAAA with narrow true lumen is to expand the thoracic aorta and create more space prior to implantation of the branched device. A possible disadvantage of this method in atherosclerotic aneurysms is the free-floating distal end of the stent graft, which may interfere with the aortic thrombus and cause embolization. Alternatively, staging can be carried out by means of temporary aneurysm sac perfusion (TASP) either through an open branch for the celiac artery or by iliac extension, which is secured in a second stage following the initial procedure. The PAPA-ARTIS study which is currently being carried out, with preemptive coiling of spinal arteries prior to TAAA repair, might provide further insights to possible SCI prevention protocols. Despite the obvious advantage of staging strategies in relation to SCI, it has to be noted that any continuing perfusion of the aneurysm sac may be associated with an unclear rupture risk in the time interval to completion.
In order to reduce time to complete aneurysm exclusion, intraoperative monitoring of motor evoked potentials (MEPs) may play an important role. Our strategy is to monitor MEPs intraoperatively during the initial procedure, after deployment of the stent grafts. Following removal of sheaths and restoration of flow to the limbs, the remaining open branch and the respective target vessel are selectively occluded with a balloon from 30–40 min, while sack pressure is monitored. Uneventful MEPs in normotension and induced hypotension indicate that completion of the procedure in a single stage is feasible. If MEPs decline during induced hypotension and recover in normotension, the procedure is staged, 7–10 days after initial procedure, during the same hospital stay. On the contrary, a significant decrease in MEPs with recovery only after balloon deflation indicates that the completion interval should be extended. We have observed reduction of perfused aneurysm sac volume and visible intercostal arteries during the TASP interval, but the exact delay to maximize TASP protection while minimizing the risk of rupture is still not precisely defined.
Preoperative evaluation: General status and comorbidities should play an important role in patient selection.
Staging: Lowers paraplegia risk in extensive TAAA repair. MEPs monitoring may determine time of completion.
Operative Basics, Tips, and Tricks
As a result of the long procedural duration, B-EVAR is usually carried out under general anesthesia. Regional or local anesthesia can be an option in staged procedures, especially during the TEVAR stage or during TASP completion. If local anesthesia is applied for the branch/iliac completion step, monitoring of MEPs is not necessary. Cerebrospinal fluid (CSF) drainage is generally indicated for patients with extensive TAAA. Monitoring of spinal fluid pressure and drainage should be carried out with an automated system (e.g., LiquoGuard, Möller Medical GmbH, Fulda, Germany) and is usually continued for 72 hours postoperatively. Monitoring of intraoperative lower limb perfusion is advisable because of the use of occluding sheaths in the femoral arteries and can be carried out with somatic oximetry sensors (INVOS).
Ischemia Monitoring: Apply Invos and Meps
Standard access for B-EVAR includes one or both common femoral arteries and, in our center, the left axillary artery in order to avoid crossing of the aortic arch and apply shorter sheaths for branches and target vessels. In some centers the subclavian/brachial artery is used instead, alternatively left- or right-sided. Percutaneous access can be applied in patients without atherosclerotic lesions; alternatively, cut-down access is used.
If repair extends to the level of the left subclavian artery or further proximally, the first thoracic component is introduced over a super-stiff wire (e.g., Lunderquist, Cook Medical, Bloomington, IN, USA), placed in the ascending aorta. In all other cases, a through-and-through wire from the common femoral to the axillary artery is used. The patient is heparinized aiming for an activated clotting time equal to or in excess of 300 seconds. During introduction via the through-and-through wire, tension has to be applied and a sheath or catheter advanced through the axillary artery to prevent damage of the left subclavian artery and the proximal descending aorta from the wire.
In cases of angulation or kinking of the aorta or iliac vessels, with difficulty advancing, the wire can be fixated at the distal end of the deployment system and the device pulled via the transaxillary end of the wire. In case stent graft advancement through heavily calcified or stenosed iliac arteries is not possible, different methods to facilitate passage have been described. These include balloon dilatation of the iliac stenosis, the “paving and cracking” technique, or our method of preference, which consists of gradual dilatation of the access vessel using dilators of increasing sizes. Regardless of the method used, the risk of iliac artery rupture should not be ignored and appropriate covered stents should therefore be readily available. To minimize blood loss, these maneuvers should be carried out under temporary proximal balloon occlusion. In some patients a proximal surgical conduit may be necessary.
Access problems: Specific endovascular techniques (i.e., gradual sheath dilatation, PTA, “paving and cracking”) may be required to avoid open surgical exposure/conversion.
Correct alignment of the branches needs to be controlled under X-ray prior to introduction, and the stent graft needs to be advanced correspondingly. Further corrective maneuvers might need to be undertaken to achieve the same orientation during advancement, especially in kinked anatomies. Rotation of the delivery system over 90 degrees should be avoided, otherwise there is a risk of “candy-wrap” torsion of the endograft between two stents ( Fig. 15.1 ). In cases where extensive rotation of the device is required, the system should be retracted and reintroduced in adjusted orientation.