Abdominal Aortic Aneurysm Repair

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CHAPTER 25 Abdominal Aortic Aneurysm Repair

Benjamin M. Jackson, Jeffrey P. Carpenter

Case Study

A 70-year-old female with a medical history of coronary artery disease, hypertension, and tobacco use presents to the emergency department complaining of 10 hours of severe abdominal and back pain. Physical examination shows a pulsatile, tender epigastric mass. A computed tomography (CT) scan is performed without contrast and shows a 7-cm abdominal aortic aneurysm (AAA) (Fig. 25-1).

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Figure 25-1 Noncontrast computed tomography scan of a symptomatic 7-cm abdominal aortic aneurysm.

BACKGROUND

Abdominal aortic aneurysms, defined as focal dilations of the abdominal aorta to a 50% or greater increased diameter, are the most common true aneurysms. Mortality from AAAs results from aneurysm rupture; half of all patients with ruptured AAAs die before arrival at a hospital and those patients who do reach a hospital alive have a 50% in-hospital mortality rate. In the United States, ruptured AAAs are the 15th leading cause of death, accounting for 15,000 fatalities per year. In contrast, elective open (as opposed to endovascular) AAA repair has a surgical mortality rate of less than 5%. Most deaths from AAAs are, therefore, preventable with prophylactic surgery.

True aneurysms involve all layers of the arterial wall, whereas pseudoaneurysms do not.

The majority of AAAs are infrarenal (i.e., they involve the aorta below the renal arteries). Approximately 5% of AAAs also involve the suprarenal aorta, and are termed suprarenal AAAs. A higher percentage of aneurysms are juxtarenal. Approximately 25% of AAAs also involve one or both iliac arteries (aortoiliac aneurysms). Aneurysm morphology may be described as fusiform (i.e., relatively symmetric and spindle-shaped) or saccular (i.e., focal asymmetric outpouching). Atherosclerotic ulcers or “blebs” sometimes involve the AAA, and this may increase the risk of rupture.

Most AAAs are degenerative (sometimes described as atherosclerotic). Other, much less common, etiologies are infection (mycotic aneurysms), arteritis, cystic medial necrosis (as in Marfan’s disease), trauma, and inherited connective tissue disorders. Risk factors for the development of degenerative AAAs include smoking history, male sex, age greater than 50 years, and family history of aneurysm disease.

Aneurysm size is the most significant determinant of the risk of aneurysm rupture. Aneurysms that are between 5 and 6 cm in diameter have a yearly rupture risk of approximately 10%. In addition, chronic obstructive pulmonary disease (COPD) and hypertension confer an increased rupture risk. Female sex is sometimes considered a risk factor for rupture as well, most likely because aneurysms of the same diameter are larger in comparison to the normal diameter of the abdominal aorta in women than in men.

Abdominal aortic aneurysms may be associated with popliteal aneurysms, femoral artery aneurysms, and thoracic aortic aneurysms.

INDICATIONS FOR ELECTIVE SURGERY

Indications for elective AAA repair include:

I. Asymptomatic AAAs: Asymptomatic aneurysms 5 to 5.5 cm or greater in diameter and those that are expanding by 1 cm or more in diameter per year are repaired because of the relatively high associated risk of rupture.
II. Symptomatic AAAs: Symptoms of abdominal or back pain without other evident etiology in the presence of an AAA suggest imminent rupture and are an indication for expeditious aneurysm repair.
III. Symptomatic embolism of an aneurysm thrombus.

PREOPERATIVE EVALUATION

Most AAAs are asymptomatic and are diagnosed on the basis of physical examination findings or, more often, are incidental findings on abdominal imaging performed for another reason. Patients with symptomatic but nonruptured aneurysms most often present with pain, suggesting expansion of the aneurysm. Patients with ruptured aneurysms present with some combination of abdominal or back pain, hypotension, syncope, or a pulsatile abdominal mass. Because morbidity and mortality rates after aneurysm surgery are significantly increased in patients with coronary artery disease and COPD, evaluation of patients before elective AAA repair commonly includes stress testing, echocardiography and, if pulmonary disease is suspected, pulmonary function testing. In addition, patients frequently undergo carotid ultrasound to exclude carotid stenoses, which would predispose to stroke in the perioperative period.

I. Ultrasound is an excellent imaging modality for the diagnosis and surveillance of aortoiliac aneurysms; however, it does not provide sufficient objective anatomic information to enable operative planning and cannot reliably identify aneurysm rupture. Moreover, the presence of bowel gas and an obese body habitus may decrease the sensitivity of a sonographic evaluation.
II. Magnetic resonance imaging (MRI) and magnetic resonance arteriography (MRA) have been used frequently as diagnostic modalities and in operative planning, especially in the evaluation of patients with chronic renal insufficiency. The incidence of nephrogenic systemic fibrosis in patients with impaired renal function after gadolinium exposure and the frequency of other contraindications to MRI (e.g., pacemakers and defibrillators), however, limit the utility of this modality.
III. Arteriography is not sensitive in detecting AAAs or in measuring aneurysm extent or diameter because of the frequent presence of thrombus in the aneurysm sac, which narrows the lumen through which blood can flow. Angiography, however, is useful for delineating the anatomy and patency of aortic branches, including the celiac, superior and inferior mesenteric, renal, and hypogastric arteries.
IV. Computed tomography and, in particular, computed tomography arteriography (CTA) are extremely useful for both diagnosis and operative planning. They also are highly sensitive for the detection of rupture. Most surgeons use CT or CTA as their imaging modality of choice.
V. Three-dimensional modeling of aortic aneurysms after cross-sectional imaging (MRI or CT scan) has become more popular with the increased application of stent grafting for the treatment of AAAs. Such modeling allows for the detailed evaluation and measurement of segments of the aorta and aortic branches. In addition, center-line lengths along the course of the aorta and iliac arteries can be measured, obviating the geometric corrections necessary when grafts are sized or designed with axial imaging. Some software even allows insertion of a virtual stent graft and visualization of the projected completed repair.

COMPONENTS OF THE PROCEDURE AND APPLIED ANATOMY

The standard treatment for patients with AAA is open endoaneurysmorrhaphy. This has traditionally been undertaken either through a transperitoneal (anterior) approach or a retroperitoneal approach. Since its introduction in 1991, endovascular AAA repair (EVAR) has largely supplanted open aneurysm repair for the treatment of infrarenal AAAs. These operative approaches, along with their attendant advantages, are addressed in the next section.

Preoperative Considerations

I. Open AAA Repair

A. Patients undergo open AAA repair under general endotracheal anesthesia, almost exclusively. Frequently, this is supplemented with epidural anesthesia to ensure intraoperative and postoperative analgesia. Some studies suggest a decrease in cardiac complication rates with the use of epidural anesthesia.
B. Open aneurysmorrhaphy requires aortic cross-clamping. The associated hemodynamic variability necessitates continuous arterial line blood pressure monitoring with a radial artery catheter. Furthermore, obligate blood loss (from lumbar arteries arising from the aortic sac) calls for large-bore intravenous access for resuscitation.
C. Any repair that will require a suprarenal (or more proximal) aortic clamp entails a period of renal, and possibly visceral, ischemia, frequently resulting in oliguria at some stage of a patient’s intraoperative or early postoperative course. A pulmonary artery catheter may facilitate monitoring of intravascular volume and resuscitation.
D. Urinary catheter placement allows continuous monitoring of urine output, and placement of a nasogastric tube allows for gastric decompression during, and after, aneurysm repair.

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