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Advanced Procedures in Interventional Radiology
Endovascular Aneurysm Repair (EVAR)
1
What is the basic composition of a stent graft?
A stent graft is composed of a metallic stent structure covered by a medical-grade fabric. The stents are typically made from stainless steel or nitinol (nickel titanium alloy), and the fabric is either Dacron (polyester) or expanded polytetrafluoroethylene (ePTFE). A stent graft comes compressed and loaded in a delivery catheter. Stent graft designs include straight tubes and bifurcated “pant leg” designs to accommodate the iliac branch. Bifurcated designs can either be unibody (one piece) or modular (multiple pieces).
2
Who is at risk for abdominal aortic aneurysm (AAA), and what are the major complications of AAA?
The incidence of AAA increases after age 50 and peaks in the eighth decade of life, when the frequency rate of AAA ranges from 0.5% to 3% in autopsy studies. The prevalence of AAA is higher in older men with risk factors (4.5%) and lower in older women with risk factors (1%). The main risk factors for AAA include male gender and age older than 65, smoking history, atherosclerosis, hypertension, and family history. Major complications of AAA include rupture, thromboembolic disease, and compression of adjacent organs when large in size. There are more than 15,000 deaths annually due to AAA rupture (the thirteenth leading cause of death in the U.S.).
3
What is the natural history of AAA, and when is intervention indicated?
Aneurysms grow over time. Increased expression and activity of matrix metalloproteinases (MMPs) and decreased activity of their inhibitors result in the loss of aortic wall structural integrity, which leads to AAA formation and expansion. The rate of growth is generally 1 to 4 mm per year for aneurysms less than 4 cm in diameter, 4 to 5 mm per year for aneurysms 4 to 6 cm, and 7 to 8 mm per year for aneurysms greater than 6 cm. Intervention is typically indicated when the aneurysm reaches 5.5 cm, although many clinicians will intervene at the 5-cm threshold. The risk of rupture increases as the aneurysm grows: diameter greater than 5 cm = 20% eventual, 4% annual; diameter greater than 6 cm = 40% eventual, 7% annual; diameter greater than 7 cm = 50% eventual, 20% annual. Given that aneurysms grow, studies are ongoing to determine the benefit of earlier intervention when aneurysm diameters are smaller and patients are relatively healthier.
4
How are most AAAs detected, and what is the role of screening?
Symptomatic aneurysms manifest with back, abdominal, buttock, groin, testicular, or leg pain. Most AAAs are asymptomatic, however. AAAs can be easily detected via ultrasonography (US), which has been shown to be cost-effective in targeted, high-risk patients. If AAA ruptures, the mortality rate is greater than 85%, and the morbidity rate and cost are significant for patients who survive. Elective repair of aneurysms is associated with low rates of mortality and morbidity, so aneurysmal disease is well suited to screening. Many hospitals and private, mobile screening companies offer aneurysm screening, often in conjunction with screening for other conditions such as peripheral and carotid arterial disease. The U.S. Congress passed the SAAAVE Act (Screen Abdominal Aortic Aneurysms Very Efficiently) in late 2005. Medicare-funded AAA screening is limited to male ever-smokers and to men and women with a positive family history of AAA.
5
In general, when is endovascular aneurysm repair (EVAR) favored over open surgical repair?
EVAR is preferred over operative repair when the surgical operative risk is higher because of comorbidities and older age. EVAR has been shown in studies to have lower short-term rates of death and complications, although the survival curves merge in the long term. Generally, open surgical repair is preferred for younger, healthier patients, in whom longer term durability is a primary concern. EVAR versus open repair is essentially a matter of proper patient selection based on physiologic and anatomic risk factors and is often a matter of patient preference for less invasive therapy.
6
Describe briefly the traditional open surgical repair of AAA.
With the patient under general anesthesia, a vascular surgeon makes an incision in the abdominal wall and exposes the aorta and the aneurysm. The incision is either down the center of the abdomen from immediately below the sternum to below the umbilicus or across the abdomen from underneath the left arm across to the center of the abdomen and down to below the umbilicus. Clamps are placed above and below the aneurysm to arrest blood flow; the surgeon then butterflies the aneurysm and removes blood clots and plaque. A surgical tube graft is sewn to the healthy sections of the aorta connecting both ends of the aorta together. The clamps are removed, the wall of the aneurysm is wrapped around the graft, and the incision is closed. Recovery from AAA surgery is typically 7 to 10 days.
7
Name some findings on preprocedure imaging of AAA that may preclude a patient from undergoing EVAR.
A major preclusion to EVAR is proximal neck complexity or lack of a suitable landing zone just below the renal arteries to ensure fixation and seal of the stent graft. A short neck is defined as one that is less than 10 cm from immediately below the renal arteries to the beginning of the aneurysmal zone. An angulated neck is defined as greater than 45 degrees from immediately below the renal arteries to the beginning of the aneurysmal zone. A wide neck is greater than 32 mm. A neck that flares immediately below the renal arteries or an aneurysm that extends into and above the renal arteries (juxtarenal) would also preclude safe landing of a stent graft. Another contraindication is complex iliac arteries. The arteries may be tortuous or calcified, precluding passage of the delivery catheter, or they may be aneurysmal in which case it may be difficult to achieve fixation and seal of the iliac “legs” of a bifurcated stent graft. Lower profile delivery systems, branched and fenestrated grafts, and more flexible and conformable designs are under development to attempt to solve some of these contraindications to EVAR.
8
What is an endoleak? What are the different types of endoleaks, and how are they treated?
An endoleak is persistent blood flow into the aneurysmal sac after placement of a stent graft. A type I endoleak results from poor attachment to the vessel wall. It can be caused by poor apposition of the stent graft to the aortic or iliac wall, often owing to tortuosity, angulation, or disease (e.g., thrombus or calcification). Stent graft undersizing and aortic neck dilation are also causes of type I leaks. Type I leaks are typically treated with balloon dilation or the placement of an additional stent graft or balloon-expandable stent. Type II endoleaks are not caused by the graft itself but rather by retrograde flow into the aneurysm sac from collateral arterial branches such as lumbar arteries or the inferior mesenteric artery. Type II endoleaks sometimes resolve spontaneously or are monitored to see if they contribute to sac expansion or pressurization. If intervention is indicated, they are treated using coils or embolic glues. Type III endoleaks are caused by modular disconnections of stent graft pieces or by graft or metal fatigue causing tears in the fabric. This type of endoleak was more common in the early days of EVAR but is now decreasing in incidence because of more durable designs and more overlap of stent graft pieces in the initial procedure. Type IV endoleaks are transgraft endoleaks caused by fabric porosity or microabrasion caused by graft or metal fatigue. Type III and type IV endoleaks are treated by placement of additional stent graft pieces or through open repair. Type V endoleaks (also called endotension) occur when there is continued aneurysm sac expansion without evidence of a leak site and are poorly understood. Treatment with additional endoluminal components or with open surgical repair may be performed as needed.
9
What follow-up imaging tests do patients treated with EVAR undergo?
Patients require lifelong imaging surveillance to monitor for endoleak, aneurysm expansion, and graft integrity. Imaging follow-up after EVAR is usually performed by periodic contrast-enhanced computed tomography (CT). The typical regimen is a baseline CT study at 1 month, with a follow-up study at 6 months and annually thereafter. To reduce the need for expensive, radiation-exposing serial CT scans, some experts advocate for follow-up via color flow duplex US scanning in the absence of endoleak.
10
What are some of the complications related to EVAR?
Complications of EVAR include endoleak, continued enlargement or pressurization of the aneurysm sac without endoleak, delayed aneurysm rupture, graft migration, graft limb occlusion, graft infection, stent-graft structural breakdown, and groin or access complications.
11
What are the major studies that have looked at the outcomes of EVAR?
The major randomized controlled studies include (1) the Dutch Randomized Endovascular Aneurysm Management (DREAM) trial, (2) the EVAR versus open repair in patients with abdominal aortic aneurysm randomized controlled trial (EVAR trial 1), and (3) the EVAR and outcome in patients unfit for open repair of abdominal aortic aneurysm randomized controlled trial (EVAR trial 2). Major registry data include the European EUROSTAR database, the U.S.-based Lifeline Registry of Endovascular Aneurysm Repair, and the Medicare matched cohort data published in the New England Journal of Medicine . Short-term and long-term data from Investigational Device Exemption (IDE) studies from all U.S. Food and Drug Administration (FDA)-approved stent grafts are available at the respective manufacturers' websites.
12
What are the results of the major EVAR studies?
The EVAR 1 trial showed a 3% lower initial mortality for EVAR, with a persistent reduction in aneurysm-related death at 4 years. Improvement in overall late survival was not shown. Similarly, the DREAM trial observed an initial mortality advantage for EVAR, but overall 1-year survival was equivalent in both groups. The EVAR 2 trial did not show a survival advantage of EVAR with respect to nonoperative management, although the design of this trial has been heavily debated. The Medicare matched cohort study concluded that, compared with open repair, endovascular repair of AAA is associated with lower short-term rates of death and complications. The survival advantage was more durable among older patients. In the Lifeline Registry, patients receiving endovascular grafts were older and had more cardiac comorbidities compared with surgical controls (e.g., open repair), but there was no difference in the primary end points of all-cause mortality, AAA death, or aneurysm rupture between the endovascular graft and surgical control groups up to 3 years.
13
What devices are currently commercially available in the United States for EVAR, and how do they differ?
Current FDA-approved devices include the Cook Zenith, the Endologix Powerlink, the Gore Excluder, the Medtronic AneuRx, and the Medtronic Talent. Except for the unibody Powerlink platform, all platforms are modular in design. Each manufacturer offers a slightly different range of widths and lengths, including tapered and flared iliac extensions. The Zenith and the Excluder are equipped with tiny hooks or barbs to aid in fixation of the graft to the aortic wall. Some stent grafts are suprarenal, meaning they have uncovered stent structures that extend across and above the renal arteries. The Zenith and the Talent are suprarenal stent grafts. The AneuRx and the Excluder are infrarenal stent grafts. The Powerlink offers infrarenal and suprarenal aortic cuffs and is the only graft that is designed to “sit” on the iliac bifurcation. All stent grafts are self-expanding, but each has a different delivery catheter and actuator mechanism.
14
What advantages will future generations of stent grafts have over ones that are currently available?
Currently approved stent grafts have catheter delivery profiles ranging from 20 to 24 Fr. New designs currently in clinical trials have profiles ranging from 14 to 18 Fr, which would decrease access complications and significantly improve the ability to treat patients (especially women) with complex and narrow iliac arteries, for which EVAR is currently contraindicated. Patients with complex proximal aortic necks and juxtarenal aneurysms would benefit from branched and fenestrated grafts that permit endovascular repair, while preserving renal patency. More durable fixation methods (e.g., endostapling) and more flexible stent graft designs would expand the applicability of EVAR further.
15
What are advantages of EVAR over traditional open surgical repair?
The advantages of EVAR are that it is less invasive than open surgery, has a lower surgical morbidity and mortality rate, and reduces the length of postoperative hospital stays. The disadvantages of EVAR are the initial cost of the devices, the need for lifelong follow-up imaging, and the question of long-term durability.
Transarterial Chemoembolization (TACE)
16
What is meant by transarterial chemoembolization (TACE)?
TACE is a procedure that involves blocking (embolizing) the arterial blood supply to a tumor and injecting chemotherapeutic drugs directly into the artery that feeds the tumor. A catheter is placed in the femoral artery and used to select the hepatic artery. The catheter in the hepatic artery is used for delivery of the embolic agent and drugs. The catheter is removed immediately after treatment.
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