Endovascular Repair of Abdominal Aortic Aneurysms

Frank William Sanchez, M.D., Kurt Wick Mori, M.D., Drs. Mori, Bean & Brooks, P.A.
 
Abdominal Aortic Aneurysms (AAA) are a common clinical problem (Figure 1). They are the tenth leading cause of death in men and are responsible for 8,500 deaths per year in the USA. AAA primarily affect the elderly and their incidence increases with age. While the exact etiology of AAA is unknown, clinical features imply a systemic connective tissue disorder with familial tendencies. Patients with AAA frequently have other aneurysms and ectasias.1 The caliber of the normal aorta is 1.2 to 2 cm. Aneurysms are defined as a dilation of an arterial segment, where the diameter is 1.5 times the diameter of the normal vessel. AAA are generally fusiform and infrarenal, but may be saccular juxtarenal or suprarenal. The lower extent of the aneurysm is usually into the aorto-iliac bifurcation and iliac arteries.4 95% of all AAA are found below the level of the renal arteries and associated with aortic atherosclerotic disease.1-4

Asymptomatic AAA are frequently found on physical exam or are an incidental finding on x-ray, CT or MRI studies. Symptomatic aneurysms present with a diversity of signs and symptoms from vague abdominal pain to acute rupture, shock and death. AAA are very frequently accompanied by other significant comorbid medical conditions which makes their management complex. These commonly include: coronary artery disease (45-55%), chronic obstructive pulmonary disease (25-50%), and chronic renal insufficiency (4-17%).2,4

The primary objective in the treatment of AAA is to prevent death from rupture. Once an asymptomatic AAA is discovered, the number one question becomes the probability of rupture. The probability of rupture correlates well with the size of the aneurysm at the initial diagnostic exam (usually ultrasound) and is directly related to the diameter of the aneurysm. Rupture rates are 25-40% at 5 years for aneurysms greater than 5 cm in diameter, 5-7% for aneurysms 3.5-5.0 cm in diameter, and approach 0% for those aneurysms less than 3.5 cm.2,4

Proven standard surgical therapy for infrarenal AAA has been open repair of the aneurysm with placement of an interposition graft from the infrarenal aorta to the iliac or femoral arteries.5 40,000 Abdominal Aortic Aneurysms are repaired surgically every year. Standard surgical repair is frequently complicated by the high likelihood of associated comorbidities and has an average mortality of approximately 2% increasing to 6% in higher risk patients.3 As the population ages so does the number of high risk candidates. Average blood loss is 2-3u in 95% of patients with an average hospitalization of 6 days.3,4

Endovascular repair of AAA is an emerging technology. This technology has evolved in an effort to avoid major intraabdominal (or retroperitoneal) surgery and the related morbidity and mortality associated with standard surgical repair. As with conventional surgical therapy of AAA, the goal of endovascular treatment of AAA is to provide a durable repair maintaining prograde flow in the graft while excluding flow within the aneurysm. Endovascular treatment offers a less invasive alternative to standard surgical repair with the potential to reduce hospitalization, morbidity and mortality.6
 
Endovascular grafts for repair of AAA fall into three broad categories: bifurcated or tubular unibody grafts; modular multicomponent grafts; and aorto unilateral grafts with a contralateral iliac occluder which are followed by surgical femoro-femoral bypass.6 The suitability (eligibility) of a patient with an infrarenal AAA for endovascular therapy is determined by the morphology of the aneurysm (Figure 2). Pre-procedure evaluation is performed by a combination of contrast enhanced spiral CT and abdominal aortography with special marker catheters. A few of the critical anatomic determinations which must be made in planning graft implantation are: diameter and length of the infrarenal aorta and neck of the aorta; diameter and length of the iliac attachments (landing zones); tortuosity and size of the access vessels and critical accessory vessel anatomy. These anatomic factors are used to address three essential questions: Are there suitable upper and lower attachment sites? Can the aneurysm be excluded completely? Can the graft be implanted safely?


Figure 2. Morphology of Abdominal Aortic Aneurysms.

The proper assessment of these complex anatomic variables differ by graft type and manufacturer. With these restrictions in mind, it is not surprising that only 20-50% of all patients are suitable candidates for endovascular repair of their infrarenal AAA. The thorough assessment of aneurysm morphology, native vessel anatomy and device selection will assure atraumatic endograft insertion and secure hemostatic implantation.6

Two devices have now been FDA approved for the endovascular treatment of AAA and many others are under investigation.4 The two FDA approved devices are the Ancure (Guidant/EVT) and the AneuRx (Medtronic, AVE).6

The Ancure device is available in a tubular or bifurcated (most commonly used) configuration. The device is a non-supported polyester graft with proximal and distal hook fixators. The main delivery catheter size is 27.5F.7 The AneuRx device is a modular bifurcated or aorto unilateral graft made of woven dacron with an external supported skeleton of a self-expanding thermal memory alloy (Nitinol). The main delivery catheter is placed through a 21F sheath.8 Because of the large size of the main delivery catheters, both the Ancure and AneuRx devices are placed by surgical cutdown and exposure of the common femoral artery. In the case of the AneuRx, this is the aorto unilateral limb module. In the case of the Ancure, the entire bifurcated graft is housed in the main delivery catheter. The contralateral femoral artery is then accessed by either direct puncture or cutdown for placement of the contralateral sheath. The contralateral sheath in the AneuRx is 16F and is used to deliver the modular contralateral limb, which is then attached to the main aorto-unilateral module within the patient.8 The Ancure contralateral sheath is 12F and used to retract and deploy the contralateral limb from the main graft. Once implantation is completed, follow-up intra-procedural angiography is performed to evaluate graft placement and assess for technical problems. Various ancillary procedures may be required at the time of implantation including angioplasty and endovascular stent placement in the iliac arteries to assure a good hemostatic seal and patent graft limbs. If satisfactory, the femoral arteriotomies and punctures are then closed surgically. Failed procedures may lead to open surgical conversion and standard aneurysm repair.

Aortic endograft placement begins a process of post-procedure follow-up. The primary modality for follow-up is contrast enhanced spiral CT which is performed in the immediate post-procedure period (0-2 weeks) and then at 3 months, 6 months and then yearly.7,8

To accurately characterize the outcomes of endovascular therapy for Abdominal Aortic Aneurysms and determine the place for this technology in AAA, a careful review of the available data must be undertaken. Successful implantation can be achieved in approximately 98% of cases with an open surgical conversion rate of 12%.7-9 Minor technical deployment problems are encountered in 17% of cases and the overall in-hospital mortality is approximately 2.5% and associated with multiple medical risk factors. This compares favorably to surgical series with mortalities in the 2-6% range depending on the associated comorbidities. Estimated blood loss during endovascular repair averages 300 ml compared to 800 ml for surgery and hospitalizations average 3 days compared to 6 days for surgery. However, the rapid recovery and decreased length of stay benefits are offset by the relatively high cost of the devices—approximately $10,000/device.6-9

A problem unique to endovascular AAA repair are endoleaks (Figure 3). Endoleaks are defined as the presence of flow outside the lumen of the endograft but within the aneurysm sack. Presumably, this would continue to put the patient at risk for rupture by maintaining arterial pressure within the aneurysm sack. The rate of endoleak following endovascular repair of AAA is approximately 14%.10 There are four types of endoleaks: Type I _ anastomotic at the attachment sites; Type II _ collateral back bleeding into the aneurysm sac; Type III _ modular disassociation; Type IV _ graft material porosity. All endoleaks may not be of equal significance. The most dangerous is the Type I with failure to achieve a proximal seal. Such leaks require treatment at the time of procedure or at follow-up and may require elective open surgical conversion.

Since Parodi undertook the first endograft repair of an AAA in 1990,11 there has been great excitement. The new treatment and supporting technologies generated much optimism which must now be tempered by clinical experience and practical considerations. This new procedure is currently constrained by both technical limitations and patient selection criteria and, as such, is not applicable to the majority of patients with AAA. The endovascular repair of AAA requires significant commitments from the participating physicians and the facilities in both time and logistic support. The promise of decreasing mortalities and shortened hospital stays are countered by extremely high device costs and the application of aneurysm repair to a higher risk patient population. Large delivery catheters exclude many women whose small iliac arteries will not accommodate these devices.

Endovascular repair of AAA is an exciting but demanding technology which holds great promise. The technology and the demands of proper implantation has spurred the formation of interdisciplinary endovascular teams working together for a common goal. The hope for the future is less costly, smaller profile devices with wider applicability, improved stability and fixation, and the elimination of endoleaks providing a durable repair comparable to a surgical graft. Continued research and development will hopefully someday move endovascular repair of AAA to a totally percutaneous procedure and attain these goals, thus improving patient care.

References

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  8. Schwarten DE. Treatment of abdominal aortic aneurysms with a Medtronic AneuRx endovascular grafting system: Techniques and problem solving. Tech in Vasc Int. Radiol. 1999; 2:124-126.
  9. Harris PL, Buth J, Miahle C, et al. The need for clinical trials of endovascular abdominal aortic stent-graft repair: The Eurostar project. J Endovasc Surg. 1997; 4:491.
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December, 2000/ Jacksonville Medicine
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