71 ABDOMINAL AORTIC ANEURYSM
Mark Nehler M.D., William C. Krupski M.D.

1. What is an abdominal aortic aneurysm (AAA)?

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A ≥ 50% increase in normal aortic diameter. Normal infrarenal aortic diameter is 2.0 cm for men. A definition of AAA as an aorta ≥ 3.0 cm in diameter is appropriate.

2. What is the incidence of AAA?

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* 3% in unselected adult patients screened with ultrasound
* 5% in patients with known coronary artery disease
* 10% in patients with known peripheral vascular disease

3. What is the etiology of AAA?

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Elastin is the primary load-bearing element of the aorta. In the normal human aorta, there is a gradual reduction in the amount of elastin present in the distal compared with the proximal aorta. Elastin fragmentation and degeneration are observed histologically in AAA walls. These observations help explain the predilection of AAAs in the infrarenal aorta. Absence of vasa vasorum in the infrarenal aorta has led to the suggestion of a nutritive deficiency. The degradation of aortic media in aneurysmal disease implies a disrupted balance between proteolytic enzymes and their inhibitors.

4. Do AAAs have a genetic component?

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Multiple reports describe a familial subgroup of AAAs. Therefore, screening of AAA patients’ first-degree relatives who are 50 years old and older makes sense. Two prospective studies demonstrated that approximately 30% of these relatives also harbor an AAA. The proposed genetic defect has been linked to abnormal type III collagen.

5. Are patients with AAA prone to aneurysms in other vascular beds?

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Yes. Forty percent of patients with a popliteal artery aneurysm harbor an AAA. Seventy-five percent of patients with a femoral artery aneurysm also have an AAA. Patients with thoracic aneurysms have a 20% chance of having a simultaneous AAA. Five percent of patients develop aortic aneurysms proximal to their graft at ≥ 5 years after infrarenal AAA repair.

6. Can AAAs reliably be detected on physical examination?

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No. The aortic bifurcation is at the level of the umbilicus. Therefore, the pulsatile mass of an AAA is located in the epigastrium. Thus, only relatively large AAAs can be detected in thin patients.

7. Can AAAs be detected by radiography?

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Plain abdominal or lumbar spine radiographs can detect occult AAA in about 20% of cases. A thin rim of calcification identifies the aneurysmal aortic wall. The majority of AAAs contain insufficient calcium to be visualized by radiography.

8. Which imaging method is the best for screening patients for AAA?

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Abdominal ultrasound (US) permits measurement accuracy within 0.3 cm and data in both cross-sectional and longitudinal dimensions.

9. What is the best single imaging modality to plan AAA repair?

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The contrast-enhanced computed tomography (CT) scan is the best one. Diameter measurements are accurate within 0.2 cm. Venous anomalies (i.e., retroaortic or circumaortic left renal vein, inferior vena cava duplication, and left-sided inferior vena cava) that dramatically alter the operative approach are well visualized on CT. Although CT is excellent at detecting aneurysmal rupture or leak (92% accuracy and 100% specificity), it is less useful for predicting suprarenal aneurysm involvement (sensitivity, 83%; specificity, 90%; positive predictive value, 48%).

10. What is the manifestation of a symptomatic AAA?

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Acute low back pain is the most common presenting symptom (82%), but only one third of AAAs are diagnosed before rupture. A hypotensive elderly man with acute onset of low back pain has a leaking AAA until proven otherwise.

11. What is the appropriate management of a patient suspected of a ruptured AAA?

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Just before emergent surgical exploration, patients who are hemodynamically unstable with a pulsatile abdominal mass should have an electrocardiogram to rule out myocardial infarction.

12. Should all patients presenting with AAA rupture undergo repair?

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Patients in profound shock or cardiac arrest at the time of presentation have little chance of survival. Extreme age, dementia, metastatic cancer, and other severe end-stage medical problems should force you to reassess this allocation of medical resources.

13. Do all patients with ruptured AAAs make it to surgery?

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Approximately half of patients with a ruptured AAA die before reaching the hospital. One fourth of those who make it to the hospital die before they can be brought to the operating room. Therefore, only 25% of patients make it to surgery.

14. How is a ruptured AAA treated operatively?

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The patient should not be anesthetized until completely prepped and draped and ready for immediate incision because the blood pressure may decrease dramatically upon induction of anesthesia. Rapid proximal aortic control is the key to successful outcome of operations for ruptured AAA. This can be at the diaphragm (in an unstable patient, with free intraperitoneal bleeding or a retroperitoneal hematoma that extends proximal to the left renal vein) or at the infrarenal aortic segment (in a stable patient with a lower retroperitoneal hematoma). Intraluminal balloon occlusion of the aorta is an option with free intraperitoneal rupture. As soon as control is obtained, the patient is resuscitated and clamps are moved to the more standard infrarenal location. Distal control can also be obtained with balloons or packs to prevent iliac venous injury.
KEY POINTS: ABDOMINAL AORTIC ANEURYSM

1. An AAA is defined as a ≥ 50% increase in normal aortic diameter.
2. Forty percent of patients with a popliteal artery aneurysm harbor an AAA.
3. CT is the single best imaging modality to plan an AAA repair.
4. AAA should be repaired electively when the size reaches 5.5 cm in diameter.

15. How should patients with symptomatic nonruptured AAAs be managed?

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Symptomatic AAAs are rapidly expanding and at high risk for rupture. Therefore, most vascular surgeons agree that symptomatic but intact AAAs should be repaired expeditiously (as early as is conveniently possible).
16. Are there any alternatives to open surgical repair for ruptured AAA? Show answer
Endovascular prosthetic grafts have been successfully placed in high-risk patients with symptomatic AAAs or contained ruptures both in the aortic and aortoiliac position.

17. What are the rupture rates of AAAs?

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A 5-cm diameter AAA has an annual rupture risk of < 1%. The risk of AAA rupture increases with size. Annual rupture risk is 10% for a 6-cm AAA and 30% for AAAs > 7 cm.

18. How fast do AAAs enlarge?

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The average expansion rate of all AAAs is 0.4 cm/year. However, 20% of all AAAs demonstrate no change in size over time. Conversely, 20% expand at a rate > 0.5 cm/year. Rapid expansion (0.5 cm/6 months) is considered to be predictive of rupture and an indication for repair.

19. When are angiograms helpful in the diagnostic workup for AAA?

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Traditionally, angiography has been indicated in patients when there is concern regarding the extent of the proximal neck, concomitant visceral occlusive disease, renal artery anomalies, a prior colectomy with need to visualize the visceral circulation, or lower extremity occlusive or aneurysmal disease.

20. What is the difference between extraperitoneal and transabdominal approach?

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Elective aortic graft placement can be carried out equally well via a transperitoneal or extraperitoneal approach. The former provides better pelvic exposure. The extraperitoneal approach provides superior exposure of the suprarenal aorta and facilitates postoperative pulmonary management.

21. What are endografts? Are they durable?

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Endovascular grafts are graft-covered stents that are placed via the femoral artery by interventional (i.e., radiographic) methods to exclude the aneurysm without the need for an abdominal incision or cross clamping the aorta. Multiple different series of successful endovascular AAA repair have been reported. Successful endograft placement has been reported in a wide variety of high-risk operative candidates. Many vascular surgeons and interventionalists are making aortic endograft placement their preferred treatment for patients with AAAs. The major drawbacks are late leaks or rupture from the graft, the cost of the procedure, and the need for long-term patient follow-up.

22. At what size should asymptomatic AAAs be repaired electively?

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They should be repaired electively when the AAA reaches 5.5 cm in diameter. The only benefit for repair of an asymptomatic AAA is to prevent subsequent rupture and death. Therefore, all candidates for elective repair must expect to live at least 5 years.

23. What are the technical aspects of AAA surgery?

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The two important decisions are the location of arterial clamps and the type of graft to place. The majority of cases can be managed by placing the arterial clamp below the renal arteries. This avoids prolonged ischemia to the kidneys. The aneurysm is opened after clamping proximally and distally. Lumbar artery orifices are oversewn to prevent bleeding from collateral arteries. The inferior mesenteric artery is often occluded, but when it is patent and not vigorously backbleeding, it may require reimplantation.

24. What are the major noncardiac complications of AAA repair?

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Renal failure (elevation in creatinine) and intestinal ischemia (bloody diarrhea).

References
WEB SITE
http://www.acssurgery.com
BIBLIOGRAPHY
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4. Holzenbein TJ, Kretschmer G, Dorffner R, et al: Endovascular management of “endoleaks” after transluminal infrarenal abdominal aneurysm repair. Eur J Vasc Endovasc Surg 16:208-217, 1998. Medline Similar articles
5. Killen DA, Reed WA, Gorton ME, et al: 25-year trends in resection of abdominal aortic aneurysms. Ann Vasc Surg 12:436-444, 1998. Medline Similar articles Full article
6. Lawrence PF, Wallis C, Dobrin PB, et al: Peripheral aneurysms and arteriomegaly: Is there a familial pattern? J Vasc Surg 28:599-605, 1998. Full article
7. Lederle FA, Johnson GR, Wilson SE, et al: Rupture rate of large abdominal aortic aneurysms in patients refusing or unfit for elective repair. JAMA 287:2968-2972, 2002. Medline Similar articles Full article
8. Lederle FA, Wilson SE, Johnson GR, et al: Immediate repair compared with surveillance of small abdominal aortic aneurysms. N Engl J Med 346:1437-1444, 2002.