Abdominal Aorta and the Inferior Vena Cava

Amber Jones and Monica Smith Pearl

Arterial supply to the abdominal viscera provided by the abdominal aorta and venous return of blood to the right atrium from all structures below the diaphragm via the inferior vena cava (IVC) are the subjects of this chapter. Classic as well as variant anatomy and relevant clinical scenarios are presented because a number of pathologic processes involve these vessels. The abdominal aorta, including its branches, is discussed from the level of the diaphragm to its termination into the common iliac arteries. The IVC, along with its tributaries, is discussed from its origin at the confluence of the common iliac veins to its termination in the right atrium.

Abdominal Aorta

The major blood supply to the abdominal viscera is derived from the aorta, which is constant in location and presence despite extensive variability in the anatomy of its branch vessels. The abdominal aorta begins as a midline structure at the level of the diaphragmatic crura, anterior to the lower border of the 12th thoracic vertebra, and has an average diameter of 1.5 to 2 cm. As it descends through the abdomen, it tapers slightly, with an average diameter of 1.5 cm below the renal arteries. Clinically, the abdominal aorta is often divided into suprarenal and infrarenal arterial segments. The rationale for this division is the higher incidence of atherosclerotic and aneurysmal disease in the infrarenal abdominal aorta and the increased complexity of interventions involving the suprarenal portion. It terminates in a bifurcation into two common iliac arteries at the lower region of the fourth lumbar vertebra, just slightly to the left of midline. On the anterior abdominal wall, this bifurcation may be visualized as a point approximately 2.5 cm below the umbilicus or at the level of a line drawn between the highest points of the iliac crests.

Imaging of the abdominal aorta may be accomplished by ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI). Ultrasound is widely available, portable, and lacks ionizing radiation, but it is heavily operator dependent and thus its reproducibility is variable.¹ Other challenges arise when there is excess bowel gas and obesity. Multidetector CT is a noninvasive imaging technique that allows for rapid image acquisition and high spatial resolution, including nearly isotropic submillimeter resolution in the X, Y, and Z planes.² Other advantages of multidetector CT include widespread availability, detailed evaluation of the aortic wall and lining mural thrombus, detection of unsuspected extraluminal abnormalities, and multiplanar postprocessing for interventional planning.² MRI is another noninvasive imaging modality that has the potential to illustrate the arterial anatomy of the abdominal aorta and its branches. Time-of-flight (TOF) and phase-contrast (PC) magnetic resonance angiography (MRA) are two well-known techniques that rely on flowing blood to image the arterial system. These techniques, however, are prone to artifacts related to disruptions or variations in blood flow, as seen with pulsatile and turbulent flow.³ Gadolinium (Gd)-enhanced three-dimensional (3D) MRA, introduced in the early 1990s, relies on the T1 shortening effects of circulating Gd-chelate contrast media instead of on flowing blood. The technique is quick, easy to perform, and provides high-resolution 3D images with minimal artifacts related to spin saturation, slow flow, or turbulent flow associated with TOF.³

As the aorta descends through the abdominal cavity, it gives off anterior, lateral, and posterior branches. Anterior branches include the celiac trunk, superior mesenteric artery, and inferior mesenteric artery, which supply the gastrointestinal viscera, as well as the phrenic and gonadal arteries (Fig. 46-1).

FIGURE 46-1 Abdominal aorta. (Redrawn from Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier/Churchill Livingstone; 2005.)

The bilateral inferior phrenic arteries may arise together as a trunk or as independent vessels just above or at the origin of the celiac trunk. They pass upward from the abdominal aorta to the diaphragm and give off multiple branches to the adrenal glands. The inferior phrenic artery also supplies the Glisson capsule of the liver through anastomoses at the bare area of the liver within the triangular ligaments.

The celiac trunk is the first wide anterior branch of the abdominal aorta; it arises just below the diaphragmatic hiatus and anterior to the superior part of the first lumbar vertebra. It supplies the foregut, which includes the abdominal esophagus, stomach, duodenum superior to its major papilla, liver, pancreas, and gallbladder. Blood supply to the spleen also develops in relation to that of the foregut region (Fig. 46-2). (See Chapter 52 for further discussion of the celiac trunk and its branches.)

FIGURE 46-2 Celiac artery and its branches. Liver has been raised and lesser omentum and anterior layer of greater omentum removed.

The common hepatic artery, a medium-sized branch of the celiac trunk, is directed forward and to the right and divides into its two terminal branches, the hepatic artery proper and the gastroduodenal artery. The hepatic artery proper, named from the origin at the gastroduodenal artery to its bifurcation into right and left hepatic branches, ascends toward the liver in the free edge of the lesser omentum. It runs to the left of the bile duct and anterior to the portal vein to the porta hepatis, where it divides into the right and left hepatic arteries. As the right hepatic artery approaches the liver, it gives off the cystic artery, which passes downward and forward along the neck of the gallbladder. Alternatively, the cystic artery may arise from the common hepatic, left hepatic, or even the superior mesenteric artery. Additionally, there may be more than one cystic artery. The right gastric artery arises from any site along the hepatic artery, before or after the gastroduodenal artery. It descends to the pyloric end of the stomach, passes to the left along its lesser curvature, and anastomoses with the left gastric artery.

In the event of occlusion of the proper hepatic artery, collateral blood supply is provided by small unnamed collateral arteries in the porta hepatis and by reversal of flow in the right gastric artery. If the occlusion involves a proximal hepatic artery branch, intrahepatic collaterals provide additional blood flow, an important consideration during embolization of intrahepatic bleeding.

In general, 45% of patients have some variation in their hepatic arterial supply. Variations may involve a replaced artery that substitutes for the absent normal vessel, or an accessory artery that appears in addition to the one normally present.

Eight different types of variations have been described, the most common being replacement of either the right hepatic artery from the superior mesenteric artery (type 3) or the left hepatic artery from the left gastric artery (type 4). Other replaced variants include the common hepatic artery from the superior mesenteric artery (type 1). Accessory variants include an accessory right hepatic artery from the superior mesenteric artery (type 5) and an accessory left hepatic artery from either the left gastric artery (type 6) or the right hepatic artery (type 7). A type 2 variant involves early bifurcation of a short common hepatic artery or separate origins of the right and left hepatic arteries from the celiac trunk, with the gastroduodenal artery arising from the right hepatic artery. Additionally, variant anatomy may involve the pathway of the artery, with a type 8 variant involving the right hepatic artery passing anterior to the common hepatic duct instead of posteriorly. These variants can occur in isolation or together (Fig. 46-3).

FIGURE 46-3 Hepatic artery variations: types 1 through 8. Most frequent variation is replacement of hepatic artery. Variations occur in about 40% of the population. *CHA,* Common hepatic artery; *HA,* hepatic artery. (Redrawn from Uflacker R. Atlas of vascular anatomy: an angiographic approach. Baltimore: Lippincott Williams & Wilkins; 1997.)

The gastroduodenal artery is a short but large branch of the common hepatic artery that descends near the pylorus between the superior part of the duodenum and the pancreatic neck. (See Chapter 52 for further discussion of the gastroduodenal artery.)

The superior pancreaticoduodenal artery divides into anterior and posterior branches as it descends between the duodenum and pancreatic head, and supplies these structures. (See Chapter 52 for further discussion of the superior pancreaticoduodenal artery.) The pancreaticoduodenal arteries enlarge and provide important collateral pathways between the celiac trunk and superior mesenteric artery in the case of stenosis or occlusion of either aortic branch.

Classic celiac trunk anatomy is present in up to only 70% of individuals; a wide range of variants may exist. Awareness of these variations is essential for visceral vascular imaging. Couinaud described eight different types of variations of the celiac trunk (Fig. 46-4). Type 1 involves a classic configuration of the celiac trunk, with all three branches arising as a trunk from the abdominal aorta. The variation exists in the possible subtypes: a hepatosplenic trunk, with the left gastric artery arising from the trunk; a hepatogastrosplenic trunk, with the three arteries arising simultaneously in a trifurcation; or a gastrosplenic trunk in which the splenic artery is dominant and the hepatic artery arises from the splenic artery. Another variant described involves a common trunk for two arteries, the hepatic and splenic arteries for type 2 and the hepatic and left gastric arteries for type 3, whereas the third artery, the left gastric artery in type 2 and the splenic artery in type 3, arises directly from the aorta. Alternatively, the splenic artery in type 3 may arise from the superior mesenteric artery. Similar to a type 2 variant is type 4, in which the left gastric artery arises directly from the aorta; however, the common trunk comprises the hepatic, splenic, and superior mesenteric arteries. The most complex configuration is type 5, in which the left gastric and splenic arteries form a common trunk, and the common hepatic artery, when present, may arise from the aorta or superior mesenteric artery. When the hepatic artery does not exist, it is replaced by the right, left, or both hepatic arteries at the same time. Types 6 and 7 involve additional arteries arising from the celiac trunk, including the superior mesenteric artery in type 6 and the left colic or middle colic arteries in type 7. Additionally, the three main arteries may arise directly from the aorta as in type 8.