Introduction
The early 1990s ushered in the era of endovascular aneurysm repair (EVAR). Diffusion of this technology, although widespread, has been met with enthusiasm by some and caution by others. Advocates of traditional open surgical techniques maintain that EVAR is costly and that long-term outcomes for patients are inferior. The objectives of the present review are to use a case scenario to highlight the clinical problem, examine current data on the use of EVAR, and to describe the principles behind the safe application of this therapy to patients.
Case Scenario
A 78-year–old white man involved in a motor vehicle crash is evaluated in the emergency department and is found to be without serious injury. Computerized tomography (CT) reveals the incidental finding of a 4.8-cm infrarenal abdominal aortic aneurysm. He has been unaware of the aneurysm but recalls an older brother who had undergone repair of an aneurysm previously. He smokes 2 packs of cigarettes daily, has moderate chronic obstructive pulmonary disease, and has hypertension that is well controlled on a β-Blocker. He is referred to a vascular surgeon, who recommends ultrasound surveillance with a 6-month follow-up visit. The subsequent visit reveals aneurysm growth to 5.6 cm, which is confirmed by repeat CT. The patient remains asymptomatic and endovascular aneurysm repair is offered.
Clinical Impact of Abdominal Aortic Aneurysm
Abdominal aortic aneurysm (AAA) is a significant health risk in older populations, representing the 14th-leading cause of death for the 60- to 85-year–old age group in the United States. Necropsy studies from Europe and the United States suggest an overall prevalence of the condition of 2% to 4% for men and 1% to 2% for women. The prevalence is greatly affected by case definition, however, with less stringent definitions of AAA in population-based screening studies demonstrating a prevalence of nearly 9% in men and 2% in women. Universally noted are the graded increase in prevalence with advancing age and the increased prevalence with male gender.
Noninvasive screening programs and a dramatic rise in the elderly population have led to an overall increased incidence of asymptomatic AAA. Despite an aggressive surgical posture toward elective repair before rupture, the incidence of ruptured AAA has also continued to increase.Annually 35 000 to 40 000 aneurysms are repaired surgically in the United States.A steady upward trend in stent graft use reflected in administrative databases implies that EVAR now represents the majority of cases.1 With Markov modeling of hospital costs ranging from $16 016 to 18 484 for open repair and $20 083 to 20 716 for EVAR, AAA represents a considerable economic burden.
Pathophysiology and Current Therapies
AAAs are conventionally defined as a 50% increase in aortic diameter compared with the normal proximal aorta. The inciting events leading to AAA formation are not well understood. Some common themes of maturing aneurysms include
- proteolytic degradation of aortic wall
- connective tissue
- transmural inflammation immune responses
- increased biomechanical wall stresses
Once AAA formation is initiated, a slow steady growth of the aneurysm until rupture is typical, although on an individual basis this growth rate may be quite variable. Larger-diameter aneurysms in particular, along with female gender, advanced age, smoking, and hypertension have been associated with rapid growth. Rapid growth has consequently been associated with increased risk of rupture, although initial aneurysm size at diagnosis has the strongest association with rupture. Open aneurysm repair, initially using homografts, has successfully been employed to prevent rupture since the 1950s. The traditional open surgical approach in the modern era is performed either via a retroperitoneal or transperitoneal exposure to obtain proximal and distal aortic control. The aneurysm is then opened, back-bleeding branch arteries are ligated, and a prosthetic graft is sutured from the normal proximal aorta to the normal distal aorta or iliac segments. Flow is then restored to the lower extremities and the aneurysm sac is closed over the newly placed synthetic graft. Although effective and durable in treating aneurysms and preventing rupture, this operation has been associated with national mean mortality rates> 4% since the 1980s.
The excessive mortality associated with open aneurysm repair and a strong trend in surgery toward minimally invasive techniques led to the concept that a covered stent graft might be delivered endoluminally, effectively sealing off the aneurysm wall from systemic pressures, preventing aneurysm rupture, and decreasing associated mortality. In 1991, the first published report of stent graft implantation for AAA in humans suggested that this approach was feasible. Subsequent years have seen a tremendous surge in both the number of endovascular aneurysm repairs performed and technological improvements in stent graft design. Four Food and Drug Administration (FDA)–approved devices, each with a slightly different design, are currently being marketed . Rather than relying on sutures to provide fixation, as in open repair, endovascular stent grafts rely on radial forces of self-expanding stents for fixation or self expand in concert with active fixation using hooks or barbs at the proximal aorta fixation site. One FDA-approved device uses bare-wire suprarenal support, as well. With appropriate positioning and adequate fixation, each of these devices redirects the transmission of aortic pulsatile flow and shear forces from the wall of the aneurysm sac to the graft itself. The responses of aneurysm sacs to these changes are variable. The great majority will cease growth or shrink over time. With longer follow-up now being achieved after EVAR, >97% 5-year and >94% 9-year rupture-free survival has been observed.
