Patient Selection for Endovascular AAA Repair
Interventionalists should strive to balance physiologic and anatomic risk factors.
If given a choice, most patients diagnosed with abdominal aortic aneurysm (AAA) would opt to undergo the least invasive procedure possible. Physicians, however, must decide how to select those most suitable for endovascular aortic aneurysm repair (EVAR) considering the data available.
There is compelling evidence that using rupture and death as end points in properly selected patients, early results of EVAR are comparable to conventional repair.1-3 EUROSTAR data from 88 institutions deploying more than 3,000 endografts indicate that the 30-day mortality rate of EVAR is 2.1%. However, as patients were followed longer, various complications arose. For example, freedom from persistent endoleak at 18 months was 90%, the rate of rupture approached 1.5% per year, and the conversion rate to open repair was 3% per year.4
CHOOSING THE RIGHT OPTION
Proper patient selection requires an assessment of both physiologic and anatomic risk factors. The treating physician must take into account the patient’s life expectancy, the risk posed by conventional surgery, and the anatomic factors that make endovascular repair possible or impossible, safe or risky. Most surgeons suggest open repair for young, healthy patients and turn to EVAR for older, sicker patients if they are anatomically suitable.
The Size Threshold
The lower morbidity and mortality rates of EVAR compared with conventional surgical repair may tempt some interventionalists to lower the size threshold for such repair. We should resist the urge to perform procedures on patients with AAAs that would not be candidates for open repair based on size criteria.
The risk of dying from a ruptured aneurysm versus the risk of death from a non-AAA cause probably does not differ provided that the aneurysm has a diameter of <6 cm. The UK Small Aneurysm Trial and the VA’s Aneurysm Detection and Management Trial have recently demonstrated that there is no benefit from early surgery versus frequent surveillance for AAAs that are <5.5 cm. Patients whose aneurysms met the size criteria for surgery in the UK Small Aneurysm Trial, but were medically unfit, had a mortality rate of 22% at 10 months and 50% at 2 years.5
Using annual rupture rates for infrarenal aortic aneurysms measuring <4 cm (0%), 4.5 cm (1%), 5.5 cm (11%), and 6.5 cm (26%), mortality rates of 1% for EVAR and 3.5% for conventional repair (age 70 years), and an immediate conversion rate from EVAR to open repair of 5% and 1% per year, mathematical models show that the benefit of EVAR increased with advancing patient age. In addition, these models showed that the benefit of EVAR disappeared with procedural mortality rates exceeding 3.5% and long-term endovascular graft failure rates exceeding 6% per year. In these models,6 lowering the threshold for EVAR was only justified in patients over 80 years of age and in poor health, where the data supported reducing the diameter threshold from 8.1 cm to 5.7 cm.
Assessing Surgical Repair Risk
The patient’s age, cardiac, pulmonary, and renal function as well as the experience of the surgeon and the volume of the hospital all have an effect on the overall medical risk of elective surgical AAA repair. Single-center studies exceeding 100 patients report the most favorable results (mortality rates of 0% to 3.7%). Surgical mortality rates from multi-institutional series of more than 300 patients are higher, ranging from 3.6% to 4.9%. Population-based studies report still higher mortality rates ranging from 6% to 7.3%. Patients randomized to surgery in the UK Small Aneurysm Trial had a 30-day mortality rate of 5.8%, although a 2% mortality rate was predicted during study design. A multivariate analysis of data from Maryland showed that patient age, low hospital volume, and very low surgeon/procedural volume were independent variables of mortality. Specifically, patients over 80 years of age had a 7.3% mortality rate after aneurysm repair compared with a rate of 2.2% for patients younger than 65 years old. Furthermore, the mortality rates for hospitals with high volumes (>50 during the study period) and surgeons with very high volumes (>100 during the study period), were roughly half the rates of hospitals with low procudure volume and surgeons with little experience in surgical aneurysm repair.7-9
ANATOMIC CONSIDERATIONS FOR EVAR
The availability of devices that fit any given patient’s anatomy ultimately dictates the applicability of EVAR. Anatomic factors affecting patient selection include the length, shape, and angulation of the infrarenal neck; any aneurysmal or occlusive disease in the common iliac arteries; occlusive disease or marked tortuosity of the ilio-femoral access vessels; or intrinsically small iliac arteries. Figure 1 shows a suitable anatomic candidate for EVAR.
Stable proximal fixation is the key to long-term AAA repair durability. The downward forces on the proximal attachment site are related to the diameter and curvature of the aorta. Increasing angulation of the proximal neck is associated with significant type I endoleaks. As the degree of angulation increases, the proximal attachment area must lengthen. Patients should not be considered EVAR candidates if the proximal diameter of their aorta progressively increases (conical neck). The reconstructed CT scan in Figure 2 illustrates a proximal neck deemed unsuitable for EVAR.
In addition, the iliac arteries must be of sufficient diameter to allow access to the aneurysm, yet still provide suitable deployment sites within the ranges of the available devices. The distal attachment sites should not interfere with the existing internal iliac arteries. The angle between longitudinal axis of the aorta and common iliac arteries should be less than 45º for successful deployment of a bifurcated endograft, and there should be a landing zone of at least 2 cm.
EVAR is not recommended for heavily calcified iliac arteries. However, the presence of a single aneurysm in a common iliac artery is not a contraindication provided that one hypogastric artery can be preserved, and that the possible complications of intentional sacrifice of a hypogastric artery are acceptable to the patient (Figure 3).
Distal deployment sites up to 20 mm in diameter can usually be utilized provided that reverse tapering of the iliac limb is achieved (bell-bottom technique). Deploying the device into the external iliac artery and occluding the ipsilateral hypogastric artery can exclude iliac vessels >20 mm. However, this technique is not without risk: Buttock, thigh, and pelvic claudication or ischemia occurred in 30% of patients. In addition, the surgeon should avoid performing a bilateral hypogastric artery occlusion.
As the size of the aneurysm increases and the medical comorbidities worsen, the physician may be justified in offering endovascular repair to patients with anatomically less favorable situations, provided that an experienced team is available to handle any unforeseen complications.
Conventional surgical repair should be considered in patients with a perioperative mortality of <5% and a life expectancy exceeding 10 years. Endovascular repair should be considered when the surgical mortality exceeds 5%, when the patient has a shortened life expectancy, and when other factors (such as a hostile abdomen) are present. Once the patient is considered physiologically appropriate for endovascular repair, anatomic suitability becomes the determining issue. Patients who receive endografts must understand the palliative nature of this procedure and be willing to trade off the proven long-term security of an open repair against the convenience and reduced recovery time of the less-invasive procedure.
Richard M. Green, MD, is Professor and Chair in the Division of Vascular Surgery at the University of Rochester School of Medicine in Rochester, New York. Dr. Green may be reached at (585) 275-6772; Richard_Green@urmc.rochester.edu.
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9. Dardik A, Lin JW, Gordon TA, Williams M, et al. Results of elective abdominal aortic aneurysm repair in the 1990s: A population-based analysis of 2335 cases. J Vasc Surg. 1999;30:985-995.