Ultrasound Assessment of the Abdominal Aorta

Introduction

The abdominal aorta is the continuation of the thoracic aorta and the major conduit artery distributing blood to the abdominal organs and then to the lower extremities. Pathologic processes that affect the abdominal aorta, in order of decreasing incidence, are: atherosclerosis (mostly nonhemodynamic significant plaques), abdominal aortic aneurysm formation, various forms of vasculitis, genetically based degenerative disease of the aortic wall, and the extension of proximal thoracic aortic dissections.

Anatomy

The abdominal aorta extends from the level of the diaphragm to the common iliac artery bifurcation (i.e., roughly at the level of the umbilicus). It is an elastic artery and elastin fibers give it mechanical strength. Oxygen is supplied to the arterial wall from the aortic lumen and by the small arterioles that comprise the vasa vasorum of the adventitia. Because of this dual supply and the limits of oxygen diffusion, a region of relatively poor perfusion lies approximately at the middle third of the media. This portion of the aortic wall is prone to the development of cystic medial degeneration (necrosis). The abdominal aortic wall is less prone to cystic medial degeneration than the thoracic aortic wall. Genetic defects in the FBN1 gene also make the media susceptible to dissections as seen in Marfan or Ehlers-Danlos syndromes. The presence of cystic medial degeneration increases the risk of thoracic aortic wall dissection and of intraluminal hematoma formation.

The major branches of the abdominal aorta include the celiac axis, the superior mesenteric artery, the renal arteries, the inferior mesenteric artery, the gonadal arteries, and the lumbar arteries. The celiac axis has an anterior origin, most often near the midline at approximately the level of the 12th thoracic vertebral body and the first lumbar artery. The superior mesenteric artery originates at approximately the level of the first and second lumbar vertebral arteries and is slightly to the left. The renal arteries have their origins just inferior to the superior mesenteric artery and are located to the side of the aorta. The inferior mesenteric artery has its origin just above the level of the aortic bifurcation and oriented slightly to the left of the midline. Both gonadal arteries arise from the aorta between the superior and inferior mesenteric arteries. The paired lumbar arteries are located on both sides of the posterior aspect of the aorta. There are normally four pairs of lumbar branches, the lower one possibly arising from iliac artery branches.

Normal Sizes

The abdominal aorta gradually tapers as it courses through the abdomen. The average diameter of the aorta is smaller in women than in men. Normal values vary with age, progressively increasing from childhood into adulthood. These size ranges are shown in Table 24.1 . By adulthood, the average diameter of the abdominal aorta is approximately 27 mm at the diaphragm, tapering to approximately 21 mm at the iliac bifurcation. For women, diameters are smaller by approximately 3 to 5 mm.

Normal Doppler Velocity Profiles

Peak systolic velocities in the abdominal aorta average 110 cm/s in a population with an average age of 12 years. With aging, the average velocity decreases, ranging from 70 to 100 cm/s. The aortic blood flow patterns change with distance from the diaphragm. Near the diaphragm, the waveforms exhibit a low-resistance blood flow pattern during diastole ( Fig. 24.1A ). This diastolic component is a reflection of the low-resistance vascular beds located slightly lower in the abdominal aorta: the liver, spleen (from the celiac axis), and the kidneys. These organs have low-resistance blood flow patterns, in essence monophasic (see Chapter 23 ). Below the renal arteries, the aortic Doppler tracings will show a typical peripheral waveform appearance (see Fig. 24.1B ). This triphasic appearance with early diastolic reversal is caused by the high impedance of the lower extremity arteries. The low-resistance component of blood flow to the kidneys, spleen, and liver no longer contribute to decreasing the high outflow resistance/impedance seen in the distal abdominal aorta. The inferior mesenteric artery has a minor contribution to the waveform appearance, if any, given that it is small and has a high resistance pattern in the fasting state.

Blood flow patterns in the proximal abdominal aorta will therefore be affected, to a certain extent, by the state of the renal arteries and the celiac axis. There have not been any systematic studies of the variations of adult abdominal aorta Doppler tracings linked to pathologic changes in these organs. For example, no specific studies have looked at the Doppler aortic waveforms in the presence of chronic renal failure.

Pathologic States

Atherosclerosis and occlusive arterial disease

The atherosclerotic process affects the aortic wall relatively early in life and is prevalent by the late 50s in men ( Fig. 24.2 ) with a lag of about 10 years in women. Most autopsy studies of the distribution of arterial disease show that the abdominal aorta is one of the first affected arteries along with the thoracic aorta. Involvement of the abdominal aorta alone is uncommon in the spectrum of atherosclerotic disease. Historically, isolated symptomatic aortic lesions were described in relatively young patients (mean age of 55 years) and more often in women, with smoking being a significant risk factor. Recent studies suggest that this gender bias might not be the case. Clinically, high-grade lesions develop slowly, are located below the renal arteries, and are associated with the concurrent development of collateral channels. However, acute aortic occlusions can occur rapidly and acutely compromise lower extremity blood flow. Progressive aortic occlusive narrowing tends to be accompanied by slowly increasing lower extremity symptoms. Symptoms can be very restrictive, and the patients adapt by decreasing their walking distance. Ultimately, symptoms progress to pain at rest and may result in impotence in men. This condition is referred to as Leriche syndrome.

Total occlusion of the abdominal aorta is rare and tends to occur in the lower portion of the abdominal aorta with thrombus spreading upward (cephalad). Although the occlusive process and the associated thrombus formation may extend to the level of the renal arteries, they generally stay patent and rarely occlude. The superior mesenteric artery is rarely affected and serves as a source of collateral blood flow to the lower extremities via collaterals to internal iliac artery branches. The right and left inferior epigastric arteries are also major collateral pathways.

Doppler imaging at the level of the common femoral arteries can suggest the diagnosis of either occlusion or severe stenosis of the abdominal aorta ( Fig. 24.3 ). This indirect approach will not fully document the type and extent of aortic disease or isolated involvement of both common iliac arteries. Doppler tracings will typically show a monophasic (tardus-parvus) pattern common to any severe proximal arterial occlusion with peak velocities of 45 cm/s or less. With acute occlusions, diastolic velocities may also be depressed prior to the development of collaterals. The diastolic component of the waveform will become more prominent as collateral pathways develop and the relative magnitude of diastolic blood flow is a reflection of the ability of the peripheral circulation to recruit collaterals.

Doppler ultrasound can confirm the presence of an aortic occlusion by the absence of blood flow in the involved aortic segment. No large series has evaluated the accuracy of this sonographic evaluation. Grading of stenosis severity in the aorta by Doppler criteria has also not been extensively studied because of the rarity of isolated abdominal aortic stenosis. The diagnostic criteria currently used to grade aortic stenosis severity were adapted from the peak systolic velocity ratio criteria used for the determination of lower extremity arterial stenosis ( Fig. 24.4A and B ). As discussed in the chapter dealing with the lower extremity arteries (see Chapter 15 ), the Doppler peak systolic velocity ratio is determined by performing a Doppler velocity measurement at the site of maximal blood flow velocity and then dividing this peak systolic velocity by a normal aortic peak systolic velocity. This normal velocity is typically measured in a contiguous segment of the aorta located above the stenosis. In cases of diffuse disease or in the absence of a good acoustic window, distal peak systolic velocities can be used to determine the peak systolic velocity ratio. This can be used only in the absence of turbulent or disturbed flow related to a distal stenosis. As in the lower extremity, a velocity ratio of 2 or greater is used to indicate the presence of at least 50% diameter stenosis.

Abdominal Aortic Aneurysm