Endovascular Therapy for Abdominal Aortic Aneurysm

Imaging

Successful endograft placement is completely dependent on adequate and accurate preoperative planning. One of the major elements distinguishing preoperative planning in open aneurysm repair from endovascular repair is the latter’s increased dependency on imaging to provide information necessary for clinical decisions. Preprocedural imaging allows the surgeon to determine whether a patient is an acceptable candidate for endovascular aortic grafting and which device is best suited for a particular patient; this ultimately allows for determining the proper size of the endograft.

Historically, contrast aortography was used as a routine adjunct to axial imaging, as it was thought to allow for a more accurate determination of vessel length and angulation before CT reformatting was widely available. Preoperative angiography is now rarely employed and reserved for cases where an adjunctive therapeutic intervention (i.e., coil embolization) is necessary. Recently, the advent of flat panel technology has allowed for the development of rotational angiography techniques that facilitate the construction of three-dimensional images found to be comparable to standard multidetector CTA. This technique has been termed C-arm cone-beam CT or FluoroCT. FluoroCT utilizes a modified C-arm with specialized software. This technique uses the spin rate of the C-arm around the gantry to acquire images. Most machines take between 6 and 8 seconds. To get good contrast imaging, it may require a large contrast bolus. This technology appears valuable in complicated aortic cases, such as fenestrated branched endovascular aneurysm repair, due to its ability to image graft to graft and graft to aorta apposition without the use of contrast. However, concerns have developed regarding overall radiation doses delivered to patients and staff, due to pre-, intra-, and postoperative imaging and therefore FluoroCT is not routinely used.

Spiral CT of the abdomen and pelvis is the mainstay of aortoiliac imaging. The imaging protocol is different from the standard protocol for most abdominal CT scans. Acquisition should use a 1:1.5 helical pitch and 3 to 5 mm collimation. Slices of 1.5 mm are ideal for providing adequate information for stent graft planning. The two-dimensional (2-D) images, however, can often be misinterpreted. The axial images may “cut” vessels at an angle, particularly iliac arteries that have some degree of tortuosity—thus creating an ellipse as opposed to visualization of the true lumen diameter. Due to this problem, some physicians recommend three-dimensional (3-D) image processing as a better method to evaluate aortoiliac anatomy for endograft therapy. Although it was common initially to perform an angiogram in addition to CT scan, recent evaluations suggest that high-quality 3-D CT scans alone may provide sufficient data for endovascular graft planning. Currently, proprietary products for CT postprocessing provide the ability to evaluate the 3-D reconstruction of the aortoiliac system rapidly, rotate the images on the screen to obtain better vessel diameter measurements, and provide “virtual endograft” simulation.

Magnetic resonance angiography (MRA) can provide information similar to that of a CT scan. It, too, can provide thin-slice reconstructions and 3-D postprocessing. Its usefulness, however, is often limited by availability and physician expertise. MRA may provide a useful modality in patients in order to avoid the use of iodinated contrast agents. However, use of gadolinium-based contrast agents in MR is not without risk and may result in nephrogenic systemic fibrosis, and gadolinium deposition in bone and parts of the brain.

Intravascular ultrasound (IVUS) is not routinely used in the preoperative evaluation of an endograft candidate. It is an invasive procedure, often performed at the time of angiography. Images produced by IVUS have a similar problem as viewing axial images on a 2-D CT scan. Unless the catheter remains centerline within the aorta, the images produced will be elliptical, which may also provide shorter than required length measurements. Its primary use is at the time of stent graft placement to assess graft position relative to the renal artery ostia; this can help to diminish the amount of contrast agent required.

Aortic Neck

The aortic neck is defined as the area of the aorta cephalad to the aneurysm in which the aortic endograft is placed ( Fig. 38.1 ). This zone of the aorta is important for two reasons during aortic endografting. First, it is the site of proximal fixation that will prevent the device from migrating distally. Second, a circumferential seal must be obtained between the graft and the aorta in this area in order to prevent leakage of blood into the aneurysm sac. The exact length of aortic neck required is somewhat device dependent, but most commercially available devices require a 10- to 15-mm length of aortic neck below the level of the most caudal renal artery. Some investigational devices may allow for shorter necks. One such device is the Medtronic Heli-FX EndoAnchor System, which is indicated for patients with aortic neck lengths < 10 mm and > 4 mm and < 60-degree infrarenal angle. Prospective analysis of 1 year outcomes from 19 US and 3 EU sites demonstrated a rate of type Ia endoleak of 1.9%; however, these results come from a small-sample–size trial ( n = 70). The ANCHOR study demonstrated an overall rate of type Ia endoleak 9.9% in 202 patients who underwent EVAR for various indications. Overall, EndoAnchors are useful in EVAR to prevent proximal site complications, and therefore has been given intent for use by the US Food and Drug Administration.

Additionally, several devices employ the use of a suprarenal, uncovered (or bare) stent to provide additional protection against graft migration. Suprarenal stent fixation may be useful, particularly in patients who have a shorter aortic neck, as it transfers protection against migration to a more normal segment of aorta. The suprarenal stent, however, does not provide any function with regard to creating a circumferential seal.

In addition to the length of the neck, other anatomic characteristics are important when determining whether patients are suitable candidates for endovascular aneurysm repair. These include aortic neck angulation, the shape of the neck, and the quality of the neck. Neck angulation refers to an alteration in the direction the aorta takes with regard to the centerline pathway. Acute angulation of the aortic neck can greatly affect the endograft’s ability to obtain a proximal seal. Aortic neck angulation of greater than 60 degrees compared with the centerline is often considered prohibitive for endovascular aneurysm repair. There are devices undergoing preliminary trials that would allow for greater neck angulation, up to 90 degrees. The shape of the aortic neck also affects the ability of the graft to obtain a seal as well as fixation. A conical-shaped neck ( Fig. 38.2 ) is generally thought to be unstable and predisposes to distal migration. An increase in diameter from the top of the neck to the bottom of greater than 10% is often believed to be a contraindication to routine aortic endografting. The presence of circumferential thrombus or aortic calcification can also negatively affect an endograft’s ability to obtain a proximal seal.

Iliac Arteries

The iliofemoral arterial system is important in endograft placement for two reasons. First, most endografts are placed through the common femoral artery and must traverse the iliofemoral system to reach the aorta. Iliac artery diameter and tortuosity can adversely affect the ease with which the endograft traverses this course. This topic is covered in more detail below. Certainly, the presence of significant atherosclerotic disease can cause arterial narrowing that inhibits the placement of the device. In addition, tortuosity of the iliac arteries can hinder placement of the grafts ( Fig. 38.3 ). Second, the iliofemoral system is important because it is the site of the distal seal between the endograft and the iliac artery, preventing retrograde flow of blood into the aneurysm sac. Many of the features necessary for an adequate aortic neck are also necessary for the distal landing zone. The presence of thrombus, calcification, and tortuosity can significantly hinder the iliac limb seal. Ectatic or aneurysmal iliac arteries obviously affect the ability of the graft to seal against the iliac limb. Most available endograft systems require a femoral artery of 8 mm and common iliac artery diameter of 8 to 25 mm. Some endograft systems, such as Medtronic Endurant, require at least a 15-mm segment of iliac artery to be of adequate caliber and free of significant disease in order to obtain a distal seal, whereas others, such as Zenith from Cook Medical and Excluder from Gore, only require 10-mm distal landing zone. If this is not present, adjunct interventions can be performed to assist in placing the device (i.e., iliac artery conduit placement or coil embolization of the internal iliac artery). Management of these complicated situations is discussed in more detail later.

Delivery System

Standard endograft insertion involves placement of the device through an arteriotomy in the common femoral artery, from where the graft traverses the external iliac and common iliac arteries. The ability to deliver the endograft safely and effectively in this fashion is a prerequisite for effective repair. Three factors are important determinants of device delivery.