The Abdominal Aorta Bradley W. Frazee Abdominal aortic aneurysm (AAA) is a potentially life-threatening process that is present in up to 5% of elderly patients seen in an ambulatory clinic or emergency department. Because physical exam is unreliable for the detection of AAA, an objective test is required for diagnosis. Among the diagnostic tests available, ultrasound (US) is unique in that it is highly accurate yet simple to perform at the bedside. In one study of emergency department ultrasound, evaluation of possible AAA was the third most commonly performed study and was found to have 100% concordance with a formal exam by a radiologist.l In the emergency management of critically ill patients with abdominal pain, immediate evaluation for possible AAA has been shown to provide highly useful information.2 Whenever an abdominal ultrasound exam is performed on an elderly patient in the ambulatory setting, thorough inspection of the aorta should be routinely included. This chapter reviews the anatomy, pathology, and technique of ultrasound scanning of the abdominal aorta and discusses the role of the limited ultrasound exam to diagnose AAA in the ambulatory and emergency department setting. CASE HISTORY A 70-year-old man comes to the emergency department complaining of left flankpain. There is a history of myocardial infarction. Vital signs are blood pressure of 160/95, pulse of 110, respirations of 20, and temperature of 99.4 F. On physical exam the patient is obese, diaphoretic, and in mild distress. The cardiopulmonary exam is normal, except for diminished pedal pulses. The abdomen is protuberant without tenderness or palpable mass, and stool is heme negative. The ECG shows sinus tachycardia and evidence of an old inferior MI. Urinalysis reveals hematuria. The diagnosis of a kidney stone is strongly entertained. An intravenous pyelogram is obtained and interpreted as normal. Repeat blood pressure is 100/60. An ultrasound exam of the abdomen is performed and reveals a 7-cm AAA. Based on new abdominal symptoms and relative hypoten sion in the setting of sonographically documented AAA, the patient is taken immediately to surgery. A leaking AAA is found and successfully repaired. NORMAL ANATOMY The descending aorta enters the abdominal cavity through the aortic hiatus of the diaphragm at the level of the T12 vertebral body and the tip of the xiphoid process (Figure 8-1). The abdominal aorta is a retroperitoneal structure and lies immediately anterior to the lumbar spine. It courses inferiorly to the level of the L4 vertebral body, the superior iliac crests, and 1 to 2 cm below the umbilicus, where it bifurcates into the left and right common iliac arteries. The aorta tapers slightly throughout its course from the aortic root to the bifurcation; in the abdomen, a diameter of 3 cm is considered the upper limit of normal. Immediately to the right of the abdominal aorta lies the inferior vena cava (IVC). The aorta gives off five main branches between the diaphragm and the bifurcation. The first, or most cephalad, is the celiac trunk, which arises anteriorly from the aorta 1 to 2 cm below the diaphragm. It trifurcates close to its origin to form the splenic, hepatic, and left gastric arteries. The next aortic branch, approximately 2 cm below the celiac trunk, is the superior mesenteric artery (SMA). The SMA arises anteriorly, then courses inferiorly, parallel to the aorta. Inferior to the SMA, and arising laterally, are the right and left renal arteries. The small inferior mesenteric artery arises anteriorly near the aortic bifurcation. The right and left common iliac arteries divide to form the internal iliac arteries, which supply the pelvic viscera, and the external iliac arteries, which course beneath the inguinal ligament to become the right and left femoral arteries. FIGURE 8-1. Normal abdominal vascular anatomy. FIGURE 8-2. Longitudinal view of upper abdominal aorta and normal vascular anatomy. The IVC is a retroperitoneal structure, which courses parallel and to the right of the aorta and vertebral column. In the pelvis, the iliac veins converge to form the IVC deep to the iliac at approximately the level of the T8 vertebral body. The largest tributaries of the IVC in the abdomen are the hepatic veins, which enter the IVC at the level of the diaphragm. The renal veins connect to the IVC in the midabdomen; the left renal vein courses between the aorta and the SMA as it crosses the midline. The splenic vein courses anteriorly to the SMA at approximately the same level as the left renal vein. These relationships are clearly seen on the ultrasound image in Figure 8-2. SCANNING APPROACHES The abdominal aorta is best examined with a 2.5- or 3.5-MHz transducer, with the patient in a supine position. Images should be obtained in the longitudinal and transverse plains along its entire course from the diaphragm to the bifurcation. Ordinarily, the aorta is easily visualized because of the marked difference in acoustic impedance between the vessel wall and the blood-filled lumen. The major obstacle to adequate imaging is the interposition of fat and bowel gas between the transducer and the vessel. In this setting, several maneuvers may be employed to gain a better image: applying gentle pressure to the transducer, modifying the angle of the transducer, and pushing panniculus to the side. The liver may be used to gain an acoustic window to the aorta by placing the patient in left lateral decubitus position and imaging during sustained deep inspiration when the liver projects farther into the abdominal cavity. It is important to differentiate the aorta from the vena cava. Both vessels are pulsatile. On transverse images, the aorta is round and has a thick, echogenic wall, whereas the vena cava is frequently. almond shaped in cross-section and thin-walled and varies in diameter with the respiratory cycle and valsava maneuver. Typically, the aorta lies directly anterior and adjacent to the spinal column, while the vena cava lies to the right of the spinal column and courses posteriorly to anteriorly through the liver. The aortic diameter is measured from outside wall to outside wall. Diameter must be measured in two planes at a given level, particularly in the case of a tortuous aorta, because inadvertently measuring an oblique image will result in overestimation of diameter. When an AAA is found, an attempt should be made to define its proximal relationship to the renal arteries and to identify any involvement of the iliac arteries. Intraluminal thrombus, if present, may be seen as an echogenic substance that casts a shadow, usually located on the anterior and lateral walls. If AAA rupture has occurred, a hypoechoic retroperitoneal hematoma may rarely be appreciated. The branches of the aorta may also be visualized, although this is not a requirement of the limited exam. The celiac trunk is best seen on longitudinal scans but may be difficult to appreciate because it is short and courses anteriorly. The SMA is readily identified on longitudinal and transverse scans, because it parallels the aorta and is characteristically surrounded by echogenic retroperitoneal fascia. The renal arteries are difficult to visualize but are best seen on transverse scans. The inferior mesenteric artery (IMA) is small and difficult to visualize and the iliac arteries are frequently obscured by bowel gas (Box 8-1). Tributaries of the IVC that are frequently visualized at the level of the SMA are the splenic vein and left renal vein. Box 8-1. ABDOMINAL AORTA SCANNING TIPS Problem-bowel gas inferior to the edge of the liver Patience with steady gentle pressure may eventually displace bowel gas and allow visualization. With the patient in the left lateral decubitus position, try scanning through the liver during sustained deep inspiration. Scanning coronally from the left side may be helpful. Figures 8-2 and 8-3 are ultrasound images of normal abdominal vascular anatomy. Longitudinal and transverse scans were obtained at the level of the left lobe of the liver, 1 cm superior to the umbilicus. Note the location of the cursors for measuring aortic diameter. The distal aorta and vena cava should likewise be imaged in the transverse and longitudinal planes (Figures 8-4 and 8-5). PATHOLOGY An aneurysm is defined as a localized abnormal dilation of a vessel in which all three layers of the wall are involved. In the abdominal aorta, this is most often due to atherosclerosis; rare causes include syphilis, cystic medial necrosis, Marfan's syndrome, and trauma. In one screening study of patients over the age of 65, the prevalence of AAA was 4%.6 With clinically evident vascular disease elsewhere, the prevalence rises to approximately 10%.7,g An aneurysm of the abdominal aorta is defined by a diameter > 3 cm or >1.5 times the diameter of the proximal uninvolved segment.9>10 The majority of AAAs are fusiform, or spindle shaped, with the accentric portion usually projecting anteriorly and to the left (Figures 8-6 to 8-8). The less common saccular aneurysm is spherical and attached to the main vessel lumen by a mouth. Thrombus is often found along with anterior wall of fusiform aneurysms and tends to partially or completely fill saccular aneurysms.5 AAAs usually begin below the level of the renal arteries (>90%) and commonly extend beyond the bifurcation to involve the iliac arteries (40%).11 The natural history of the untreated AAA is variable. When aneurysm diameter is less than 5 cm, expansion proceeds at a rate of 0.2 to 0.4 cm per year and gradually accelerates. 12,13 While an AAA of any size may rupture, risk of rupture is directly related to diameter. Rupture usually results in a rapidly expanding retroperitoneal hematoma, which causes hypovolemic shock and death if untreated (Figure 8-9). Unusual manifestations of AAA include chronic contained rupture, inflammatory aneurysms, infected AAA, complete aortic thrombosis, lower extretnity atheroembolism, and aortoenteric fistula. FIGURE 8-3. Transverse view of upper abdominal aorta and normal vascular anatomy. FIGURE 8-4. Transverse view of lower abdominal aorta. FIGURE 8-5. Longitudinal view of lower abdominal aorta. FIGURE 8-6. Illustration of two types of abdominal aortic aneurysms. FIGURE 8-7. Fusiform abdominal aortic aneurysm. A, Transverse view. B, longitudinal view. Note echogenic thrombus. C, CT scan. Note circumferential thrombus. FIGURE 8-8. Saccular abdominal aortic aneurysm. A, Transverse view. B, Longitudinal view. FIGURE 8-9. Ruptured AAA. A, Transverse ultrasound view demonstrates AAA with no signs of rupture. B, CT scan demonstrates AAA and left periaortic hematoma. (H, hematoma.) CLINICAL PERSPECTIVE Ultrasound is the imaging modality of choice for detection and measurement of AAA, because it is sensitive and risk free and accurately estimates diameter. 9,15,16 Sensitivity is 97% to 100%,2~gn7 with the recognized limitation that in a small percentage of patients (4 out of 71 in one studyl7), aortic imaging will be impossible due to an abundance of bowel gas. Ultrasound is as accurate as CT and superior to angiography in estimating aneurysm diameter.l~1g Measurements correlate within 3 mm of surgical specimens. It has the obvious advantage that it can be performed at the bedside in an emergency department and on an unstable patient. The disadvantages of ultrasound are poor ability to detect extraluminal blood, which is indicative of rupture (Figure 8-9), and to delineate the proximal and distal extent of the aneurysm.2ng In one study, bowel gas prevented ultrasound visualization of the iliac arteries in 19 of 25 cases. How does ultrasound compare to other modalities for detection and evaluation of AAA? Screening for AAA by physical examination alone has an estimated sensitivity of less than 50% and is dramatically affected by patient habitus.19 On plain abdominal radiographs, calcified media forms an outline of the AAA in 50% to 85% of cases, but this finding may be obscured by overlying stool or bone shadows. CT scan is a highly accurate test for detecting and measuring AAA, with a sensitivity approaching 100%. However, ultrasound is preferred to CT as a screening test for AAA and for serial measurements, because of the cost of CT and the risk associated with exposure to radiation and contrast agents.9,14,15 A CT scan is usually obtained before AAA repair, because the CT can delineate the proximal and distal extent of an aneurysm and assess periaortic inflammation of associated retroperitoneal pathology. 15 In the rare stable patient with suspected AAA rupture, CT is the test of choice because it reliably identifies retroperitoneal hematoma. Angiography is superior to CT and ultrasound in defining the relationship of an aneurysm to the renal and iliac arteries and, for this reason, is used routinely by some surgeons for preoperative planning.11 However, angiography is invasive, does not detect AAA rupture, and underestimates aneurysm diameter if mural thrombus is present. Therefore it is not used for screening or serial mea- surements of diameter. Digital subtraction angiography reduces dye load and eliminates the need for arterial catheterization. MRI, while expensive and not widely available, is sensitive and accurate at detecting and measuring AAAs. MRI demonstrates aneurysm rupture and retroperitoneal pathology and is comparable to angiography in defining renal and iliac involvement. 20 Most AAAs discovered by ultrasound in a nonreferral setting will be asymptomatic. Management, which must take into account surgical risk and comorbid conditions as well as characteristics of the AAA, will be dictated by the consulting surgeon. The operative mortality after AAA rupture is nearly 50%, while that of elective repair is 3.5%.12 The risk of rupture is directly proportional to diameter and rate of expansion. However, the best time for elective repair remains controversial. The following simple rules of thumb are generally accepted.6,10,12,13 An AAA is considered significant, meaning it warrants close surveillance and surgical consultation, when the diameter is 4.0 cm or twice that of the proximal uninvolved segment. Surveillance of a small asymptomatic AAA usually consists of serial ultrasound exams every 6 months. At :!~;5 cm, the growth rate is <.5 cm/yr, with a 1-year risk of rupture of < 5%. At > 5 cm the risk of rupture at 1 year probably exceeds the operative mortality of elective repair, and surgical consultation should be considered urgent. The presence of new or changing abdominal symptoms in the setting of AAA demonstrated by ultrasound should always raise the suspicion of rupture and should be evaluated further by CT scan or by laparotomy if the patient is unstable. In the symptomatic patient who has not yet been diagnosed with AAA, it is a formidable clinical challenge to diagnose an expanding or leaking AAA as the cause of symptoms. Symptomatic or ruptured AAAs are notorious for masquerading as renal colic and. may be mistaken for diverticullitis, a gastrointestinal hemorrhage, MI, and musculoskeletal back pain.21,22 Suspicion of occult AAA should be elevated in the elderly and obese patient and in the presence of macrovascular disease elsewhere.1g If the diagnosis of AAA is entertained, physical exam and plain radiography are not sufficiently sensitive to exclude the diagnosis, and limited ultrasound should be performed. The utility of ultrasound in the setting of possible acute ruptured AAA was studied by Shuman and others.2 Sixty patients with the constellation of abdominal or back pain, pulsatile abdominal mass, and hypotension or tachycardia underwent rapid (< 1 min) ultrasound exam of the aorta by a sonographer during their emergency department evaluation. Of 32 patients with AAA confirmed surgically or angiographically, 31 were correctly identified by limited ultrasound (aortic diameter of >-4 cm); one patient had an indeterminate exam due to obesity and abundance of bowel gas. Twenty-eight patients without AAA were correctly identified, and unnecessary emergency surgery was prevented. In 24 patients with extraluminal blood at surgery, ultrasound findings suggesting rupture were present in only one case. In summary, there are three clinical settings in which limited ultrasound of the abdominal aorta should be performed.1 In the patient with abdominal pain and hypotension, a limited ultrasound exam should be performed as early as possible. Demonstration of a normal aorta reliably excludes the diagnosis of ruptured AAA and should prompt a search for other diagnoses, whereas presence of AAA, regardless of sonographic signs of rupture, strongly suggests the need for emergency surgery.2 In the stable elderly patient with unexplained abdominal or back pain, symptomatic AAA is one of the few life-threatening possible diagnoses and should be ruled out immediately with a limited ultrasound exam of the aorta.3 Every elderly patient, especially those who are obese or with vascular disease elsewhere, represents an opportunity to screen for AAA; a limited exam of the aorta should be considered a routine part of the abdominal ultrasound exam in this patient population. REFERENCES 1. Schlager D, Lazzareschi G, Whitten D, Sanders AB: A prospective study of ultrasonography in the ED by emergency physicians, Am J Emerg Med 12(2):185-189, 1994. 2. Shuman WP, Hastrup W, Kohler TR, et al: Suspected leaking abdominal aortic aneurysm: use of sonography in the emergency room, Radiology 168(1):117-119, 1998. 3. Hagen-Ansert SL: Textbook of diagnostic ultrasonography, ed 4, St Louis, 1995, Mosby. 4. Rumack CM, Wilson SR, Charboneau JW: Diagnostic ultrasound, St Louis, 1991, Mosby. 5. Reynolds T, Santos T, Weidemann J, et al: The evaluation of the abdominal aorta: a "how-to" for cardiac sonographers, JAm Soc Ecbo 3(4):336-346, 1990. 6. Scott RAP, Wilson NM, Ashton HA, Kay DN: Is surgery necessary for abdominal aortic aneurysm less than 6 cm in diameter? Lancet 342:1395-1396, 1993. 7. Cabellon S, Moncrief CL, Pierre DR, Cavanaugh DG: Incidence of abdominal aortic aneurysms in patients with atheromatous arterial disease, Am J Surg 146:575-576, 1983. 8. Graham M, Chan A: Ultrasound screening for clinically occult abdominal aortic aneurysm, Can Med Assoc J 138:627-629, 1988. 9. Laroy LL, Cormier PJ, Matalon TAS, et al: Imaging of abdominal aortic aneurysms, AJR 152:785-792, 1989. 10. Crawford ES, Hess KR: Abdominal aortic aneurysm, New EnglJMed 321(15):1040-1043, 1989. 11. Vowden P, Wilkinson D, Ausobsky JR, Kester RC: A comparison of three imaging techniques in the assessment of an abdominal aortic aneurysm, j Cardiovasc Surg 30:891-896, 1989. 12. Hollier LH, Taylor LM, Oshsner J: Recommended indications for treatment of abdominal aortic aneurysms, J Vasc Surg 15(6):1046-1056, 1992. 13. Nevitt MP, Ballard DJ, Hallett JW: Prognosis of abdominal aortic aneurysms, New EnglJMed 321(15):1009-1014, 1989. 14. Bower TC, Cherry KJ, Pairolero PC: Unusual manifestations of abdominal aortic aneurysms, Surg Clin Nortb Am 69:745-754, 1989. 15. Ernst CB: Abdominal aortic aneurysm, New EnglJMed 328(16):1167-1172, 1993. 16. Siegel CL, Cohan RH: CT of abdominal aortic aneurysms, AJR 163:17-29, 1994.