Steve Henao, MD, FACS, FACC New Mexico Heart Institute Albuquerque, New Mexico
Approximately 20 to 30 percent of patients with abdominal aortic aneurysm are unsuitable anatomic candidates for standard EVAR. Within this subgroup, 50 to 60 percent of patients are ineligible for EVAR because of proximal aortic neck anatomy limitations. Over the last decade, vascular specialists have incorporated “off the shelf” techniques for this juxtarenal and pararenal abdominal aortic aneurysm pathology. Greenberg is credited for first describing this technique, known also as chimney, snorkel or periscope; This strategy allows for preservation of the potentially compromised visceral vessel by preemptive stent grafting of the target followed by the subsequent proximal, parallel deployment of the main body of the endograft.
The primary advantages of the chimney strategy is the immediate availability of the device components and lower cost. Patients that are unsuitable for open repair that may be symptomatic and are unable to wait for the manufacturing of a conventional fenestrated or branched device may be exceptionally suited for this therapy.
The primary weakness of the chimney technique is the incidence of type I endoleak, secondary to so called “gutters” formed by the pairing of endograft and chimney grafts. Incomplete apposition between the main body and the chimney graft may produce continued flow between the devices and lead to subsequent pressurization of the sac, compromising the successful exclusion of the aneurysm.
The vast majority of experience with these techniques involves traditional EVAR devices that depend of self-expanding stent technology. It is the very nature of these wire and fabric endografts to require long overlap between the aortic neck and the main body to achieve proximal seal. Similarly, these same principles come into play when attempting to seal at the graft-graft interface of the chimney combination; Unfortunately, longer chimneys used to achieve improved seal may inadvertently lead to higher rates of thrombosis, compromising patency and long term outcome.
A new generation of EVAR devices is becoming more widely accepted world wide that achieve seal through the use of polymer as a principal step in deployment that molds to the aortic anatomy in a more customized and predictable fashion. The Endologix Trivascular Ovation Abdominal Stent Graft Platform and Nellix Endovascular Aneurysm Sealing System are two novel devices that appear to be ideal devices for parallel endografting applications based on their ability to conform to the both the aortic anatomy and the chimney device, thus excluding the troublesome “gutters” that plague current self expanding EVAR technology in these applications (FIGURE 8).
The Ovation Abdominal Stent Graft Platform is an ultra low profile trimodular implant comprised of low permeability polytetrafluoroethylene (PTFE) and a suprarenal nitinol stent for active fixation. Aortic seal is created via a novel O-ring design filled with low viscosity radiopaque polymer. The customized seal created by the O-ring molds and conforms to virtually any irregular luminal surface, such as mural thrombus, calcium and other endovascular devices, such as stents (FIGURE I).
In some cases, a chimney case may not necessarily require disturbing the O-ring on the Ovation platform, but only deviate the sealing collar. This PTFE material is located just above the sealing ring, and is easily deflected distally towards the ring by a wire or stent, effectively elevating the sealing ring to the juxtarenal aorta (FIGURE 2). This technique is known as “venting” and is very effective in maximizing the sealing capabilities of the device (FIGURE 3). When a stent crosses the O-ring, this is termed a true “chimney” as it is known the conventional sense.
The technical details of chimney EVAR utilizing the Ovation platform are similar to those previously described elsewhere. At our institution, ultrasound guided brachia! artery access or surgical
cut down are established as the initial step. As an alternative, a left axillary artery cut-down and placement of a single large sheath through which the visceral stents can all be delivered may be a consideration. One or both renal arteries are then selectively catheterized and Rosen wires are delivered to secure positioning of 7 French long (90 cm) sheaths (FIGURE 4). Covered balloon expandable stents are then delivered so as to provide adequate placement within the visceral vessel while also providing enough length to avoid coverage by the O-ring or sealing collar (FIGURE 5). Once these are positioned, femoral access is obtained. We prefer bilateral percutaneous access with Proglide Perclose systems, but femoral cutdown may be appropriate in select cases, such as severe calcification or femoral occlusive disease. The Ovation device is then delivered over a stiff wire so that the device markers indicating the PTFE fabric are placed at the level of the visceral stents. An oblique projection is than obtained to delineate the position of the superior mesenteric artery to avoid overlap of the ostia. At this point, the ballon expandable covered stents are deployed and the balloon(s) inflated to nominal. The mid-crown and full-crown deployments are then performed, again, in lateral/oblique projection to ensure SMA patency, and the polymer is subsequently infused. The curing time is 14 or 20 minutes, depending on the polymer used for the Ovation device (FIGURE 6). In our institution, the visceral balloons are brought down
at the 3 to 5 minute mark. Of course, the balloons are left in place in the event that a rare stent deformity should occur during polymer curing. The contralateral limb is then selectively catheterized and a limb positioned and deployed, depending on the length determined with a retrograde marking pigtail iliac angiogram. This is then followed by proximal placement of a pigtail catheter to confirm successful proximal seal (FIGURE 7). Following the appropriate time for polymer curing, the main body delivery system is disengaged and removed. A retrograde iliac angiogram is then performed to determine the appropriate length of the ipsilateral limb, which is then subsequently deployed. Angioplasty of the iliac limbs is typically performed in a “kissing” manner using 12 mm diameter devices. A final completion angiogram is performed to confirm patency of the visceral stent(s) and the trimodular device, as well as absence of a type I endoleak. In the event that a type I endoleak is encountered, an aortic balloon is used to further mold the O-rings to the aortic wall. If this is still unsuccessful, a Palmaz stent is mounted on an aortic balloon and a long 16 French sheath delivered transfemorally to above the proximal seal ring. The Palmaz is then situated so that it will sit on the marks of the device, indicating the location of the sealing collar, which will serve as a primary mechanism of seal in this situation.
The Nellix Endovascular Aneurysm Sealing System is a 17 French device made up of 10 mm balloon expandable stents and integrated ‘endobags’ that receive polymer that serve to exclude the entire aneurysm. The amount of polymer is carefully determined with a ‘pre-fill’ step that involves administration of saline into the endobags, which serves to slowly unroll and conform them to the aortic anatomy and to also predict the amount of polymer that will optimally seal the aneurysm. This process is largely determined by the pressure of saline and, eventually, polymer infused into the endobags; a pressure that should ideally be at 180 mmHg. The polymer typically cures at about 7 to 8 minutes.
The technical details of chimney EVAR utilizing the Nellix system are also very similar. Brachia! or axillary access are obtained in an identical manner as described above. Visceral artery access is then performed and long sheaths situated. Balloon expandable stent grafts also seem to be preferred in this setting. Once the visceral stents are positioned, femoral access is obtained, either by ultrasound guided percutaneous pre-close technique or femoral cutdown, depending on anatomic factors. The Nellix stent system is then delivered so that bare metal, uncovered segment of the stents is above the chimney devices. As previous, an oblique/lateral projection is obtained to ensure preservation of the SMA, and the pre-fill and polymer steps are performed to ensure an optimized sealing of the aneurysm while preserving the integrity of the visceral stent grafts. To ensure precise positioning throughout the entirety of the procedure, it is essential that one operator maintain position of the Nellix systems manually while a second operator perform the pressure-sensitive steps of pre-fill and polymer infusion (FIGURE 9). In the event of an endoleak, a secondary fill option is performed to deliver additional polymer for further expansion and filling of crevices within the endobag system.
Parallel endografting utilizing next generation polymer based devices such as the Ovation and Nellix systems appear to offer a new option for juxtarenal and pararenal abdominal aortic aneurysm therapy. The nature of these devices offers customized conformability to the chimney devices and appears to offer a more reliable means to eliminate “gutter” leaks seen with traditional wire and fabric devices traditionally used in this setting. More experience and long term data are needed to better understand the role these technologies will play in patients with complex aortic anatomy.
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