What is an abdominal aortic aneurysm (AAA)?
An abdominal aortic aneurysm, also called AAA or triple A, is a bulging, weakened area in the wall of the aorta resulting in an abnormal widening or ballooning greater than 50 percent of the vessel’s normal diameter (width). The aorta extends upward from the top of the left ventricle of the heart in the chest area (ascending thoracic aorta), then curves like a candy cane (aortic arch) downward through the chest area (descending thoracic aorta) into the abdomen (abdominal aorta). The aorta delivers oxygenated blood pumped from the heart to the rest of the body. The most common location of arterial aneurysm formation is the abdominal aorta, specifically, the segment of the abdominal aorta below the kidneys, or an infrarenal aneurysm. An aneurysm can be characterized by its location, shape, and cause.
The shape of an aneurysm is described as being fusiform or saccular, which helps to identify a true aneurysm. The more common fusiform-shaped aneurysm bulges or balloons out on all sides of the aorta. A saccular-shaped aneurysm bulges or balloons out only on one side.
The aorta is under constant pressure as blood is ejected from the heart. With each heart beat, the walls of the aorta distend (expand) and then recoil (spring back), exerting continual pressure or stress on the already weakened aneurysm wall. Therefore, there is a potential for rupture (bursting) or dissection (separation of the layers of the aortic wall) of the aorta, which may cause life-threatening hemorrhage (uncontrolled bleeding) and, potentially, death. The larger the aneurysm becomes, the greater the risk of rupture. Because an aneurysm may continue to increase in size, along with progressive weakening of the artery wall, surgical or endovascular intervention may be needed. Preventing rupture of an aneurysm is one of the goals of therapy.
It is estimated 1 to 4 % of persons over age 50 years are affected. Rupture of an AAA is the presenting feature of this disease in roughly 2/3rds of patients, and it remains the 13th most common cause of death in the United States. The best treatment for AAA is elective repair of pre-symptomatic abdominal aortic aneurysms. Such a therapeutic strategy depends on effective identification of patients with AAA, and the subgroup of patients in whom there is a real risk of aneurysm rupture.
Abdominal aortic aneurysm (AAA) risk factors include:
As the vast majority of patients with AAAs are asymptomatic, timely identification of AAA may be achieved through targeted screening of the at risk populations, typically with outpatient abdominal ultrasound. Over the last two decades longitudinal studies of patients with smaller AAAs have provided insights into the ideal timing of AAA repair and the need for and frequency of ultrasound surveillance if an expectant management strategy is followed.
Abdominal aortic aneurysm is usually the result of degeneration in the wall of the vessel, resulting in a slow and continuous dilatation of the vessel lumen (where the blood flows). In fewer than 5 % of cases, AAA is caused by other mechanisms like infection. Abdominal aortic aneurysms are usually not symptomatic until they expand or rupture.
Presence of a pulsatile abdominal mass is virtually diagnostic, but is found in less than 50 % of cases. Rupture is uncommon if aneurysms are less than 5 cm in diameter, but ruptures are dramatically more common for aneurysms greater than 6 cm in diameter. Without prompt intervention, ruptured aneurysms are often fatal. Thus, elective repair is usually recommended for all aneurysms greater than 6 cm unless repair is contraindicated. In patients who are good procedural risks, elective repair is generally recommended for aneurysms between 5 and 6 cm (mortality, about 2 to 5 %).
For an AAA, the standard open approach to surgical repair involves a long midline abdominal incision, and placement of a graft in the aneurismal sac. It is now possible to secure a bifurcated graft within an aneurysm at the latter site using a femoral approach from within the vessel. The use of an endovascular graft may be considered as indicated by any of the following criteria: (i) diameter of aneurysm is greater than 5 cm; or (ii) diameter of aneurysm is 4 to 5 cm and has increased in size by 0.5 cm in the past 6 months; or (iii) diameter of aneurysm is twice the diameter of the normal infrarenal aorta.
Targeted screening for AAA
In the past 40 years with the advent and generalized use of abdominal ultrasonography there has been an accurate, cheap and non invasive tool for the diagnosis of abdominal aortic aneurysms. Abdominal ultrasonography has been found to be an accurate and reproducible modality in measuring the dimensions of AAA. This has led to the concept of
its use for screening of at risk populations. In the last 20 years there have been four population based randomized controlled trials which have assessed the value of targeted screening in reducing mortality from abdominal aortic aneurysms in the unselected elderly male population.
What is an abdominal aortic aneurysm (AAA) repair?
Repair of an abdominal aortic aneurysm may be performed surgically through an open incision or in a minimally-invasive procedure called endovascular aneurysm repair (EVAR). No aspect of vascular disease management has changed as much in the past decade as the management of abdominal aortic aneurysm (AAA). One might also argue that the advent of endovascular aneurysm repair changed the entire field of vascular surgery by introducing vascular surgeons to a host of endovascular techniques, already used by interventional cardiologists, which have applications throughout the vascular tree in the management of both occlusive and aneurysmal disease. This new superspeciality of endovascular therapy draws on a broad heritage derived from the pioneering work of many physicians Dotter (arterial dilatation – cardiologist), Fogarty (arterial balloons – surgeon), Gruentzig (balloon dilation angioplasty – cardiologist), Palmaz (arterial stents – cardiologist), and Parodi (arterial stent-grafts – vascular surgeon).
The role of endovascular stent-graft implantation is still evolving. The endovascular approach is associated with less physiological derangement, lower morbidity and mortality, and more rapid recovery than open surgical repair. In assigning a role for endovascular technique in the management of AAA, these short-term advantages must be weighed against uncertain long-term performance. Early stent-grafts were unstable, but more recent devices appear to be durable and effective.
While the surgical and endovascular techniques both aim to bridge the aneurysm with a fabric conduit, or graft, there are important differences in the means of graft insertion and attachment. The endovascular approach employs a transfemoral route (from arteries in the groin retrograde to the aorta) and stent-mediated attachment, whereas the surgical approach employs a trans-abdominal route (open surgical exposure) and suture-mediated attachment. But surgery and stent-graft implantation are not just different ways to achieve the same effect. One cannot operate on an open aneurysm without eliminating all sources of flow into the treated segment. In endovascular stent-graft implantation, on the other hand, immediate aneurysm exclusion is both unnecessary (while the aneurysm remains intact) and unachievable (while the lumbar and inferior mesenteric arteries remain patent). The efficacy gap between the two approaches has narrowed over the years with the development of durable stent-grafts and secure, hemostatic methods of stent-graft attachment.
The scope of endovascular aneurysm repair has also been limited by the anatomical requirements for successful insertion and attachment. The open surgical approach allows extensive intra-operative variation in technique. In contrast, endovascular technique restricts the choice of access route, implantation site, stent-graft dimensions, and adjunctive maneuvers. Consequently, many early systems of endovascular repair proved to be incapable of dealing with various common anatomical distortions, such as tortuosity of the iliac arteries or aorta and dilatation of the aorta near the renal arteries. This is another area in which the gap between the two approaches is narrowing. Better guidewires, more trackable delivery systems, and bifurcated stent-grafts have greatly expanded the applicability of the endovascular technique. Other innovations, such as complex modules, fenestrations, and multibranched stent-grafts, promise to increase the scope still further.
Over the past decade, endovascular methods of AAA repair have benefited from clinical experience with a wide range of early stent-grafts. Observed changes in stent-graft position, structure, and function have been the primary impetus behind the steady improvement in stent-graft design. The goal was (and still is) to achieve surgical results using endovascular techniques.
Types of abdominal aortic aneurysm repair
There are two approaches to abdominal aortic aneurysm repair. The standard surgical procedure for AAA repair is called the open surgical repair. A newer procedure is the endovascular aneurysm repair (EVAR).
- AAA open surgical repair
Open surgical repair of an AAA involves an incision of the abdomen to directly visualize the aortic aneurysm. The procedure is performed in an operating room under general anesthesia. The surgeon will make an incision in the abdomen either lengthwise from below the breastbone to just below the navel or across the abdomen and down the center. Once the abdomen is opened, the aneurysm will be repaired by the use of a long cylinder-like tube called a graft. Grafts are made of various materials, such as Dacron (textile polyester synthetic graft) or polytetrafluoroethylene (PTFE, a non textile synthetic graft). The graft is sutured to the aorta connecting one end of the aorta at the site of the aneurysm to the other end of the aorta. Open repair remains the standard procedure for an abdominal aortic aneurysm repair.
- Endovascular aneurysm repair (EVAR)
EVAR is a minimally-invasive (without a large abdominal incision) procedure performed to repair an abdominal aortic aneurysm. EVAR may be performed in an operating room, radiology department, or a catheterization laboratory. The doctor may use general anesthesia or regional anesthesia (epidural or spinal anesthesia). The doctor will access the femoral arteries in each groin and with the use of special endovascular instruments, along with X-ray images for guidance, a stent-graft will be inserted through the femoral artery and advanced up into the aorta to the site of the aneurysm. A stent-graft is a long cylinder-like tube made of a thin metal framework (stent), while the graft portion is made of various materials such as Dacron or polytetrafluoroethylene (PTFE) and may cover the stent. The stent helps to hold the graft in place. The stent-graft is inserted into the aorta in a collapsed position and placed at the aneurysm site. Once in place, the stent-graft will be expanded (in a spring-like fashion), attaching to the wall of the aorta to support the wall of the aorta. The aneurysm will eventually shrink down onto the stent-graft.
The doctor will determine which surgical intervention is most appropriate, either open surgical repair or EVAR.
Reasons for the procedure
Reasons an abdominal aortic aneurysm repair may be performed include, but are not limited to, the following:
- To prevent the risk of rupture
- To relieve symptoms
- To restore a good blood flow
- Size of aneurysm greater than 5 centimeters in diameter (about two inches)
- Growth rate of aneurysm of more than 0.5 centimeter (about 0.2 inch) over one year
- When risk of rupture outweighs the risk of surgery
- Emergency life-threatening hemorrhage (uncontrolled bleeding)
Risks of the procedure
As with any surgical procedure, complications can occur. Some possible complications may include, but are not limited to, the following:
- Open repair
- Myocardial infraction (heart attack)
- Irregular heart rhythms (arrhythmias)
- Bleeding during or after surgery
- Injury to the bowel (intestines)
- Limb ischemia (loss of blood flow to legs/ feet)
- Embolus (clot) to other parts of the body
- Infection of the graft
- Lung problems
- Kidney damage
- Spinal cord injury
- Damage to surrounding blood vessels, organs, or other structures by instruments
- Kidney damage
- Limb ischemia (loss of blood flow to leg/feet) from clots
- Groin wound infection
- Groin hematoma (large blood-filled bruise)
- Endoleak (continual leaking of blood out of the graft and into the aneurysm sac with potential rupture)
- Spinal cord injury
EVAR Brief Early History
The development of stent-grafts started with a period of rapid innovation, followed by a longer period of refinement and convergent evolution. Although modern systems of endovascular aneurysm repair differ in important ways, they share features, such as the combination of a metallic stent and a fabric graft, remote insertion, fluoroscopic guidance, over-the-wire delivery, and secure attachment mechanisms, all of which were already in clinical use by 1994.
Successful stent-graft implantation depends on atraumatic transfemoral insertion, accurate orientation and deployment, and secure, hemostatic attachment. All are affected by common distortions of aortoiliac anatomy. In this context, the best stent-graft is the most versatile, the one that can overcome anatomical obstacles and achieve success in the largest number of cases.
The ease of delivery system insertion depends on size, profile, flexibility, and, most of all, track ability. Most modern systems are either extremely flexible or have a long, tapered tip with a smooth gradient of stiffness. Delivery systems with these features seldom fail to traverse tortuous iliac arteries, except when tortuosity is combined with rigidity (calcification), stenosis (atherosclerosis), or both. The majority of current systems have external diameters of 20 to 22 French.
Stent Graft Implantation
Refinements in operative technique are largely the consequence of improvements in stent-graft technology, which have made transfemoral insertion far easier, even in cases of iliac tortuosity, calcification, and narrowing.
Modern tapered, low-profile delivery systems permit over-the-wire insertion into the artery, thereby minimizing the risk of dissection, reducing blood loss, and shortening the period of femoral occlusion. Percutaneous arterial closure devices have even made it possible to dispense with surgical exposure of all but the smallest, most calcified femoral arteries. In the “pre-close” technique, sutures are placed within the arterial wall (and/or femoral sheath) before up-sizing the puncture site during delivery system insertion. The theoretical advantages include less pain and a shorter recovery time. Most surgeons still prefer surgically exposed femoral arteries, although few still use a traditional longitudinal incision.
A variety of adjunctive maneuvers were developed to help a large, blunt-ended delivery system traverse tortuous iliac arteries. Most have been made obsolete (for AAA repair) by the advent of better delivery systems and stiffer guidewires.
Implantation Site Compromise
Secure, hemostatic implantation remains an absolute requirement for successful endovascular aneurysm repair, and the lack of a suitable proximal implantation site below the renal arteries has become the commonest exclusion criterion. Improvements in the accuracy, flexibility, attachment, and diameter range of the available manufactured stent-grafts have all helped to expand the number of candidates, yet juxtarenal, pararenal, thoracoabdominal, and bilateral CIA aneurysms require specific provision for branch artery perfusion.
Like an untreated aneurysm, an inadequately treated aneurysm may produce no symptoms until disaster strikes. The goal of routine image-based follow-up is to identify silent problems, such as migration, endoleak, or kinking, while they are still amenable to endovascular correction. Contrast-enhanced CT has been the mainstay of most follow-up programs because it is capable of showing whether the aneurysm has been excluded from the circulation (no perigraft flow) and whether the natural history of the aneurysm has been altered, as evidenced by shrinkage. The main threat to device performance, and therefore the main risk factor for aneurysm rupture, is not endoleak (leak into the original AAA from branch vessels or through the graft) or aneurysm dilatation, but migration. Of course, this is a generalization; failure modes vary from device to device. The ideal imaging study for any particular stent-graft is the one that best detects the serious problems known to occur most often with that device, while incurring the least pain, expense, and renal damage. Serial ultrasound and plain abdominal radiography appear to satisfy these criteria and will probably replace CT as the routine study of choice for long-term follow-upii. It is also conceivable, given the apparent stability of some devices, that routine imaging will be performed on a far less rigorous schedule or even abandoned altogether when the initial findings are promising.
The past decade has seen a series of improvements in identifying patients at risk for AAA, the approaches for surgery and endovascular AAA repair, and a steady expansion of EVARs role in disease management. Current stent graft designs appear to be durable enough for use in healthy young patients, and new models are versatile enough for use in the presence of compromised implantation sites. However, there is still room for improvement.
It is hoped that during the next decade advances will come from a growing understanding of the endovascular environment, leading to more predictive preclinical testing and lower failure rates, even when new approaches call for a departure from existing technology.
At First Coast Heart and Vascular Center, we are ready to evaluate AAA and the need for interventions. Our vascular laboratory and staff are accredited and registered respectively to perform ultrasound testing for AAA. Dr. Van Crisco works side by side with other surgeons to repair these aneurysms with minimal invasive approach. For evaluation and or treatment, please call our office at (904) 423-0010.