Ultrasound Assessment Following Endovascular Aortic Aneurysm Repair

General principles

As discussed in Chapter 24 , intervention on the enlarging AAA is undertaken to reduce, if not eliminate, the risk of catastrophic aneurysm rupture.

Endografts are a combination of metallic stents and affixed prosthetic graft materials. The stent portion gives radial strength to the stent-graft, and the graft material covers it and makes it impermeable. In essence, a perfectly anchored stent-graft only permits blood to flow through its conduit. The stent portion also stabilizes the position of the stent-graft by anchoring it to healthy portions of the aorta and iliac arteries. Instead of using sutures to stabilize the position of the graft and its limbs, the net radial force exerted by the metallic components of the stent pushes outward against the aortic and arterial segments selected for placement. The stent can be made of nitinol, stainless steel, or a cobalt-chromium alloy. The fabrics typically used as the prosthetic graft material component of the endograft include Gore-Tex (PTFE) and Dacron.

Once the endograft is fixed into position above and below an aneurysm, blood should only flow through the graft. The endograft effectively excludes the aneurysm sac from the effects of blood pressure and blood flow ( Fig. 25.1 ). Endovascular grafts come in many types and configurations ( Fig. 25.2 ).

The endovascular placement of a stent-graft is currently favored over open surgical intervention because it reduces the relatively high morbidity and mortality associated with traditional open operative repair. The first series of endovascular placed stent-grafts for the repair of AAAs in high-risk patients was reported in 1991. Since then, significant advances in the design of endografts have facilitated their deployment for the treatment of aortic and aortoiliac aneurysms.

Although many devices have been granted US Food and Drug Administration (FDA) approval for the treatment of aortoiliac aneurysms and are available for widespread use in the USA, the list keeps growing with various fenestrated models now available for suprarenal implantation.

Because most aortic aneurysms are infrarenal, the typical stent graft is deployed in an infrarenal location. The proximal anchor point is in the infrarenal aorta immediately below the lowest renal artery and the inferior anchor points are in both common iliac arteries ( Fig. 25.3 ). It is not uncommon for endovascular aortic aneurysm repair to be supplemented by other ancillary procedures such as graft extension, femorofemoral artery bypass, intra-arterial coil vessel occlusion, or other vessel occlusion procedures ( Fig. 25.4 ). The device has an uncovered metal component when there is need for suprarenal fixation. The uncovered proximal metal component of the stent can cross the orifices of the renal arteries. This design is thought to provide better fixation of the graft to the surrounding aortic wall, thereby reducing the potential for graft migration and providing a better proximal seal. The uncovered component also allows perfusion of the kidneys. Newer endovascular grafts can treat a variety of complex arterial pathologies, and their surveillance becomes more complex. Examples include fenestrated endografts that allow coverage of the renal arteries as well as the celiac axis and superior mesenteric arteries if needed.

Although the endovascular repair of AAAs offers many benefits ( Table 25.1 ), there are several potential complications specific to this technique. The most significant of these complications are endoleaks and, in early series, graft migration. Although endoleaks are still common, endograft migration, deformation, or occlusion are uncommon because of new designs and broader experience with the technique.

Endograft placement is performed to reduce pressure in the aneurysm sac and therefore prevent aneurysm rupture. An endoleak is an undesired source of pressure within the aneurysm sac and is defined as blood flow outside the endovascular graft but within the aortic aneurysm sac. These leaks might increase intrasac pressures and make it more likely for the aneurysm sac to rupture because the pressure can cause aneurysm enlargement, and the presence of blood flow makes it likely to have active hemorrhage when the aneurysm sac ruptures. The presence of an endoleak therefore negates the primary goal of the endovascular procedure and indicates that the aneurysm remains inadequately treated. Considerable progress in patient selection and endovascular technique has reduced the overall rate of all types of complications seen after EVAR ( Table 25.2 ). The optimal method of follow-up following endograft placement is contrast-enhanced CT with delayed imaging. However, this method is relatively invasive because of the need of intravenous access, is expensive, involves radiation exposure to the patient, and has increased risk for possible contrast nephrotoxicity. Color Doppler ultrasound and contrast-enhanced ultrasound are increasingly and successfully being used to follow patients following endograft placement. Patients who may require further intervention can be readily identified.

It is important to try to determine the origin of any blood flow signals identified within the aneurysm sac ( Table 25.3 ) because the source of the blood flow signals (endoleak) and associated characteristics will help determine subsequent treatment options.

Cross-sectional diameter measurements are recorded at each visit to determine maximum aneurysm size. When an aneurysm sac is excluded from the circulation, it should remain stable or decrease in size over time. Any increase in size suggests that there is preserved blood flow into the aneurysm sac (endoleak) with associated increase in blood pressure, which is a continued risk for rupture. However, increases in size have been reported without CT, angiographic, or color Doppler evidence of endoleak (endotension). Endotension is defined as persistently elevated pressures within the aneurysm sac, without a known source, and can be documented by direct pressure measurements made within the sac.

It is also important to determine whether the distal arterial circulation has been preserved by ensuring that there are no kinks or obstructions within the body of the endovascular graft, the graft limb(s), and the inflow and outflow arteries. This can be done using a well-defined protocol. When abnormalities are detected by color Doppler imaging, contrast arteriography and CT may be used to characterize the abnormality further. The following section describes a protocol for assessing endovascular grafts performed for the repair of isolated aneurysms of the abdominal aorta as well as aortoiliac aneurysms.

Patient preparation

As with all abdominal scanning, the quality of the examination may be degraded in patients with a large body mass index or when abundant bowel gas is present. Patient preparation may be necessary. The patient should fast overnight or for at least 8 hours before the study. This will decrease the amount of intestinal gas and facilitate visualization of the graft and attachment sites. Usually, no other patient preparation is necessary.