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A thorough understanding of the anatomic considerations is essential
for a successful outcome. presented
By Luis A. Sanchez, MD
Certain basic anatomic considerations must be understood before choosing the device or procedure to use in the endovascular treatment of patients with AAAs. There are four AAA endografts on the market at this time, and they each fit different anatomical features and accommodate patients uniquely. A thorough understanding of these devices is necessary before a good clinical decision can be made about which device will fit a particular patient. Poor device adaptation and patient selection lead to poor results and significant complications, something we have unfortunately seen over and over again as we have tried to accommodate some of these devices in patients who may not be suitable anatomic candidates for EVAR.
When assessing the anatomy of an EVAR candidate, several factors need to be considered. These include the quality of iliofemoral access, the proximal attachment site (infrarenal neck), the anatomy of the aneurysm itself, the anatomy of the distal aorta, and the distal attachment site, which is most commonly in the iliac arteries, but can be more distal, depending on the patient’s pelvic anatomy.
ILIOFEMORAL ACCESS
When assessing iliofemoral access, we need to take into account the tortuosity of these vessels, their calcification, degree of stenotic disease, and their size. Because each of the stent grafts has a different introducer system, you may need to choose the size most appropriate for both the main trunk of the device and its contralateral portion. It is also important to know which guidewire will best get you from your femoral or other access to the site of the stent graft’s implantation because the delivery systems are relatively bulky devices (Figure 1).
Issues involving iliofemoral access can usually be resolved in the majority of patients being considered for EVAR. Currently, tortuosity is not a major problem because there are a variety of stiff guidewires available that can navigate tortuous anatomy. Focal stenoses can usually be overcome, but when you encounter the combination of some of these problems (calcified vessels, relatively small vessels, and tortuous vessels), which are much more common in elderly women, it should raise a warning flag that there may be problems associated with access during the endovascular intervention in this patient.
If the anatomy of the pelvis is difficult and you still think the patient can be best treated in an endovascular fashion, retroperitoneal access, even with a conduit and a reconstruction, is sometimes a better option (Figure 2). Rupture of the iliac vessel can be an unpleasant and life-threatening complication and the last thing you want is to turn an elective procedure into an emergent one.
PROXIMAL ATTACHMENT SITE (INFRARENAL NECK)
If you have chosen the wrong patient for EVAR, the infrarenal neck is where failure is most likely to occur. Improper device placement or poor device selection will not easily accommodate suboptimal infrarenal neck anatomy. Know the diameter and length of the infrarenal aortic neck, and oversize the device by approximately 10% to 15%. Necks that are 15 mm in length in a portion that is reasonably healthy are preferred. If the neck is shorter, you run a greater risk of reconstruction failure and the need for further endovascular or open interventions.
Patients with long infrarenal necks appear to be perfect candidates for these devices, but it must be kept in mind how the devices are constructed. Two of the four stent graft devices currently available have relatively short bodies from which two large limbs extend. In a relatively long and narrowed neck, those two limbs may not fit inside that narrow distal neck and may lead to limb failure. It is better to choose a device with a longer body to create a longer seal zone and move the bifurcation of the device lower and closer to the native aortic bifurcation.
It is also important to know whether there is a significant amount of thrombus, plaque, or calcification in the infrarenal neck. None of these is an absolute contraindication for endovascular repair, but they will make a difference when deciding how to place the device, where to expect a good seal, and what adjunctive measures you may need to take to ensure the best possible result and avoid a type 1 endoleak or migration of the device.
Renal pathology is also a consideration, especially now that renal artery interventions have become commonplace. If your patient has significant renal artery stenosis, when working very close to the renal arteries or even placing a transrenal device, you run the risk of partially or totally covering a renal artery. You should consider renal artery angioplasty and stenting before placing your AAA endograft, but you need to be very accurate in your stent placement to allow accurate and safe placement of the endovascular graft.
Too much angulation of the infrarenal neck can be problematic and is a contraindication for EVAR. By its nature, the larger the AAA is, the more anterior angulation the proximal neck usually has. It is easy to underestimate the length of the infrarenal neck when evaluated purely by cross-sections, which is why three-dimensional reconstructions are needed. If you evaluate your patients with angiography, try to obtain a lateral view to get a better sense of anterior angulation. Patients ideally should have angulations of less than 45º, with 60º being the cut-off at which most investigators believe that the risk of endovascular repair failure is high.
The device must be accurately placed just below the renal artery. To do this, you need to change the angle of your image intensifier to see the renal arteries and the length of the infrarenal aortic neck. To ensure a good result, you want to be close to the renal arteries. If you do not adjust your image intensifier appropriately, you may lose the ability to see a portion of the neck, and when you think you’re deploying the device very close to the renal arteries, in fact, you may be a centimeter or more farther away from the renal arteries, setting up the patient for poor long-term graft durability, an increased risk of migration, poor apposition of the device, type 1 endoleak, and the need for further intervention. In general, craniocaudal angulation of 15º is reasonable as a starting point, but many patients will require further angulation to be perfectly perpendicular to the aortic neck. Devices that allow slow, controlled deployment permit readjustment of the device and the image intensifier during deployment depending on the specific location of the renal arteries.
Many potential EVAR patients have one or two suboptimal factors related to their infrarenal neck, but a patient with multiple issues, especially if the neck is somewhat short and angled, diseased and somewhat irregular, is not a good candidate for EVAR. In the future, as newer devices such as grafts with transrenal configurations, fenestrated devices, and branched devices become available, we will then be able to treat patients with more complex proximal neck anatomy.
AAA ANATOMY
If there are large patent branches coming out of the aneurysm (eg, a large accessory renal artery or inferior mesenteric artery), which the graft itself is not going to seal, consider embolization to try to prevent some of the type 2 leaks or persistent outflow from even a type 1 endoleak that may occur in that situation.
Aneurysm length might also be an issue. The bodies of most of the modular devices currently available are approximately 7 cm to the bifurcated portion of the device. If the aneurysmal segment is shorter than 7 cm, it will not accommodate a bifurcated device. An aorto-uni-iliac graft reconstruction or a shorter bifurcated graft would be needed for an endovascular repair.
Aortic stenosis in the middle of the aneurysm, or what we refer to as a bilobed aneurysm, can be challenging, and you will need to set yourself up so that the gate or the contralateral limb of the device opens in the widest portion so cannulation is easier. Otherwise, you might be forced to convert to an aorto-uni-iliac reconstruction or fail to complete your procedure because you cannot enter the contralateral gate. A longer device, such as the Zenith, may work in some of these situations, or your best choice may be to defer EVAR and opt for surgical repair.
DISTAL AORTA
If the aortic bifurcation is challenging (Figure 3A,B), narrowed (Figure 3C), or calcified, a scenario more often encountered in female patients, EVAR may not be your best option. I look for a minimum diameter of approximately 18 mm to be able to accommodate two limbs on those patients. There are a few techniques to consider when trying to accommodate a bifurcated device: (1) maintain access to the aneurysm sac at all times with a sheath, and (2) do not jail yourself out of the aneurysm from the contralateral side. The majority of patients can accommodate a bifurcated device, but if you do not think the anatomy is suitable, aorto-uni-iliac reconstructions are a good option.
If there are already stents in the common iliacs or in the bifurcation of the aorta, these are much more difficult to traverse with the large sheaths required for an endograft device. If the patient has a stenosis distal to the AAA, avoid treating it until after you treat the aneurysm. Besides the obvious issue of metal against metal, it is technically much more difficult to place and seal your stent graft appropriately if the iliac arteries have been stented in the past.
DISTAL ATTACHMENT SITE (ILIAC ARTERIES)
Ideally, there should be a landing zone of at least 2.5 cm for the distal attachment, whether in the common or external iliac arteries. The devices available can accommodate iliac artery diameters up to 21 mm. Most people will consider diameters up to 25 mm ectatic, not truly aneurysmal, but in the event that your common iliac artery is aneurysmal or greater than 21 mm in diameter, it is reasonable to consider occlusion of one of the hypogastric arteries in order to seal the stent graft in one of the external iliac arteries. Try to avoid occluding both hypogastric arteries, which incurs a risk of buttock claudication and other potential pelvic ischemic complications.
If you encounter bilateral common iliac artery aneurysms and you want to maintain patency of at least one hypogastric artery, there are a variety of options. One is a small retroperitoneal access with a short bypass to salvage one of the internal iliac arteries. An endovascular external iliac artery–internal iliac artery bypass is another possibility, and it is probably the most commonly employed one in combination with an aorto-uni-iliac endovascular reconstruction. If one system is completely occluded on one side, aorto-uni-iliac devices are readily available. People have even recanalized short common iliac artery occlusions and have been able to accommodate a bifurcated device. In the near future, branched components will allow reconstructions that maintain hypogastric artery patency.
PATIENT SELECTION GUIDELINES
At least initially, patient selection guidelines are similar for all the available prostheses. Because there is considerable variation among devices in terms of size, trackability, and flexibility of the introducer systems, it is important to be very familiar with the devices and their differences. Table 1 shows a basic comparison of the four available devices. As far as aortic neck diameter is concerned, the Zenith device (Cook Incorporated, Bloomington, IN) goes larger than any of the others, up to 28 cm. For neck angulation, the Zenith is the only endograft with a suprarenal component. And, if you look at the introducer systems, the main component of each system is fairly similar, from the 18-F Excluder sheath (W. L. Gore & Associates, Flagstaff, AZ) (inside diameter) to the 21-F AneuRx (Medtronic, Inc., Santa Rosa, CA) (outside diameter). As far as the contralateral limb is concerned, the Excluder has the lowest profile, which is its biggest advantage.
CONCLUSION
EVAR is a great procedure when performed appropriately and with good preplanning. You need to be very familiar with appropriate patient selection because your outcomes will depend heavily on it. Do not underestimate the value of a thorough understanding of the parameters of the different endograft devices and the different technique options for accurate deployment and to navigate through tough situations. If you place the device from the renal arteries to the hypogastrics, have good seal zones, and if you have chosen the patient well, your likelihood of achieving good short- and long-term results is excellent.
Luis A. Sanchez, MD, is from the Washington University School of Medicine, St. Louis, Missouri. He has disclosed that he is a paid consultant to Medtronic AVE.
1. Greenhalgh RM, Brown LC, Kwong GP, et al. Comparison of endovascular aneurysm repair with open repair in patients with abdominal aortic aneurysm (EVAR trial 1), 30-day operative mortality results: randomised controlled trial. Lancet. 2004;364:843-848.