Abdominal Aortic Aneurysm Repair
Two FDA-approved devices take AAA treatment beyond surgery.
Once a physician determines that a patient with an abdominal aortic aneurysm (AAA) requires intervention, the traditional approach has been an open repair utilizing a transabdominal or retroperitoneal surgical exposure. In addition, we currently have the ability to offer some patients a less invasive alternative approach known as endovascular or endoluminal stent graft repair.1-3 This article takes a detailed look at two FDA-approved devices that make endovascular AAA repair an option.
ENDOVASCULAR AAA REPAIR
The recent development of catheter-based grafts to repair AAAs represents an exciting advance in the history of vascular surgery. At the present time, there are two FDA-approved devices, Ancure (Guidant/EVT, Menlo Park, CA) and AneuRx (Medtronic, Santa Rosa, CA). Many other devices are awaiting FDA approval.
The Ancure Graft
The two approved devices display fundamental differences. The Ancure is a unibody bifurcated graft, meaning the entire design comes loaded on one delivery system that is inserted through bilateral femoral artery access. This polyester graft attaches to the proximal aorta and the common iliac arteries using sharp Elgiloy wire hooks (Elgiloy Specialty Metals, Elgin, IL) that penetrate the arterial wall. The physician uses a balloon to secure fixation at the hook attachment sites. It is considered an unreinforced graft design because the fabric is not structurally supported with stents. This design was developed to mimic the grafts used for conventional open repair.
For successful access, one external iliac artery must measure at least 7.9 mm in diameter. The graft can be deployed in a proximal aortic neck measuring up to 26 mm in diameter and at least 15 mm in length. The largest available iliac graft limb diameters are 13 mm, which means that larger diameter common iliac arteries cannot be used as the distal attachment site. The two anatomic issues that contraindicate device insertion are circumferential calcification in the proximal aortic neck, which makes hook penetration impossible and calcified tortuous stenotic external iliac arteries, which will prevent device access. A two-physician team enhances successful deployment.
The AneuRx Stent Graft
The AneuRx device is a modular system whereby a bifurcated graft is essentially assembled in vivo using multiple catheter exchanges. A reinforced stent graft design utilizes polyester graft material inside an exoskeleton consisting of diamond-shaped nitinol stent rings. The stent graft is self-expanding and is held in position by its own radial force. The system comes with proximal and distal covered extensions that add to its versatility, allowing for intraoperative design modifications. The main bifurcated section is mounted on a 21F catheter delivery system that requires one 7-mm external iliac artery for access. The AneuRx device can be deployed in a proximal aortic neck that measures up to 26 mm in diameter and ideally at least 10 mm in length. These devices are held in place by their own radial and columnar force, therefore, oversized graft designs, especially proximally, are essential to create a seal and prevent distal migration.
PROS AND CONS
Both the Ancure and AneuRx clinical trials had a technical success rate of over 90%. These positive findings stem from a number of factors. Patients do not require general anesthesia or postoperative ICU stay. The devices require only femoral artery exposure for insertion and eliminate the need for a laparotomy, reducing the length of hospital stay and operative blood loss compared with open surgical controls. The endovascular approach eliminates the major perioperative intravascular fluid shifts resulting in hemodynamic fluctuations that are associated with open repair. This technology can successfully treat elderly and frail patients with multiple comorbidities. Additionally, patients undergoing endovascular AAA repair recover quicker and with less postoperative pain when compared with those receiving open aneurysm repair.4
On the other hand, endovascular procedures are not shorter in duration than conventional open surgeries, and in some cases, may actually take longer. Successful device deployment requires state-of-the-art intraoperative fluoroscopic imaging, and X-ray exposure must be monitored regarding the patient as well as the operating room team. Contrast dye is required to place these grafts, and contrast dye-induced nephropathy is a possibility in patients with renal dysfunction.
CURRENTLY LIMITED APPLICATION
Despite its advantages, endovascular technology is not yet an option for all AAA patients. These devices are not customized and are only available in limited sizes. In addition, there are numerous potential anatomic exclusions including patients with juxtarenal aortic aneurysms, complicated proximal aortic necks (short, angulated, ulcerated, calcified, or dilated), small tortuous diseased external iliac arteries, calcified narrowed aortic bifurcations, and coexistent extensive iliac artery aneurysms in which endovascular repair would compromise pelvic perfusion. There are also gender-specific issues—women’s smaller iliac arteries can often preclude access. In our experience, however, women are also more likely to be excluded due to short, dilated, and angulated proximal aortic necks.
In addition to the Ancure and AneuRx grafts, there are many investigational graft designs. The design modifications include various forms of suprarenal fixation, endograft customization, lower-profile delivery systems, simplified deployment techniques, percutaneous access, and alternatives to the polyester fabric. Data from these ongoing clinical trials are being collected and analyzed, and several of these investigational endografts will be presented to the FDA for approval next year.
THE ENDOLEAK DILEMMA
One of the most striking differences between open conventional repair and the endovascular approach relates to postoperative follow-up. Open AAA repair is considered a durable surgery, which does not require intensive follow-up surveillance. Although there is the potential for progressive dilatation of the pararenal and suprarenal segments, as well as the development of anastomotic pseudoaneurysms, open aneurysm repair is generally successful in both the short- and long-term.
Endovascular AAA repair cannot yet be considered a procedure with known durability. Success of this procedure is measured by an intraoperative completion arteriogram showing exclusion of the aneurysm sac, in addition to a postoperative CT scan that similarly demonstrates absence of contrast within the aneurysm sac. Many patients undergoing endovascular AAA repair will, however, not be able to “pass” these tests.
Varieties of Trouble
It is estimated that 20% of endovascular AAA patients will have contrast enhancement of the aneurysm sac demonstrated on a 1-month postoperative CT scan. This phenomenon following an endovascular repair is termed an endoleak. Endoleaks are classified according to their site of origin.5-7 Type I endoleaks arise from an incomplete seal at the proximal or distal attachment sites and therefore represent device or design failure. The type I endoleak is repaired by placing additional proximal or distal covered extensions; otherwise, the endograft must be explanted and the aneurysm repaired conventionally. Type II endoleaks result from patent perfusing inferior mesenteric and/or lumbar arteries. These endoleaks are a consequence of the current technology and are not caused by device or design failure. Successful endograft exclusion depends on spontaneous thrombosis of patent inferior mesenteric and lumbar arteries. Of note, surgeons often find these arteries to be vigorously backbleeding during open repair, necessitating individual suture ligation. Type III endoleaks develop at graft junctions, and type IV endoleaks result from graft porosity.
The Problem With Endoleaks
Type I endoleaks are by definition problematic because the aneurysm sac remains systemically pressurized from an incomplete seal at the attachment site. Until recently, type II endoleaks were thought to be less of a concern unless aneurysm sacs were growing over time. We detected systemic arterial pressure in the aneurysmal sac in nearly all of our patients with type II endoleaks. This finding has persuaded us to take a more aggressive approach; if an endoleak is found, we now intervene and repair early, usually following the 30-day CT scan. We have observed a 30-day endoleak rate of 17%; these have been predominantly type II in origin, which we have repaired using elective transarterial and translumbar endovascular approaches.8
We advise patients of an approximately 17% to 20% risk of needing further endovascular interventions relating to type II endoleaks in the short-term. Of greater concern is the late development of an endoleak, defined as one that occurs 6 months or longer after implantation. These late endoleaks must be meticulously studied with arteriography to determine the site of origin.
The morphology of the aneurysm sac and proximal neck changes over time following endovascular repair, with or without an endoleak. Sac shrinkage and neck dilatation lead to changes in diameter, length, and overall volume. These alterations may result in dislodgement and migration of endografts that depend on radial force for attachment. Modular junctions may be particularly vulnerable to separating as the aneurysm sac changes over time.
FOLLOW-UP IS CRUCIAL
Persistent endoleaks, late endoleaks, and aneurysm sacs that are not shrinking in size are all cause for concern. Patients must be followed indefinitely after stent graft repair; they should have both an unenhanced and enhanced CT scan including delayed imaging at 30 days postoperatively, 6 months, and yearly thereafter. Patients who are unwilling to adhere to indefinite follow-up should not undergo endovascular AAA repair that uses an aortic endograft. Endovascular failure may not become evident within the first few years, and follow-up data beyond 5 years are not yet available.
MUCH MORE TO LEARN
At the University of Pennsylvania Health System, we have performed more than 500 aortic endograft procedures over the 4 years using the devices previously described as well as several others still in various phases of FDA-approved IDE clinical trials. We perform the procedures in the operating room and patients spend that night in the stepdown ICU. The following morning, patients are moved to a regular ward bed and are typically discharged home on the second postoperative day.
In our experience, there have been no late ruptures or endograft migrations. Follow-up remains relatively short, however, and we have observed delayed graft limb occlusions occurring predominantly in unreinforced designs (ie, the Ancure device), but also to a lesser extent with reinforced endografts (ie, the AneuRx device). These limb interventions have also been addressed using endovascular techniques, although femoral-femoral bypass has been used successfully by other centers.
At this time, we try to dissuade relatively healthy low-surgical-risk patients from undergoing endograft repair. It is not uncommon for patients to arrive at my office with information regarding endovascular options gleaned from the Internet. Clearly, widespread knowledge concerning the short-term benefits of aortic endograft repair is driving this technology, despite the lack of follow-up data beyond 4 years.
Endovascular technology is evolving rapidly, and devices capable of percutaneous access are anticipated in the near future.9 In the meantime, there is much to learn about the longer-term implications of performing AAA repair using current endovascular devices.
Ronald M. Fairman, MD, is Associate Professor of Surgery and Radiology and Chief of the Division of Vascular Surgery at the University of Pennsylvania in Philadelphia. Dr. Fairman may be reached at (215) 614-0308; email@example.com.
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