Abdominal Vasculature Overview

Abdominal Vasculature

• The abdominal vascular system is responsible for transporting gases, nutrients, and essential substances to tissues and removing waste products.
• Arteries are elastic tubes that carry blood away from the heart, often enclosed with a vein and nerve. Smaller arteries have less elastic tissue and more smooth muscle compared to larger ones.
• Arteries' elasticity is critical for maintaining steady blood flow. The pulsation of the abdominal aorta does not change with respiration.
• Veins are collapsible tubes with less tunica media. They appear collapsed and generally have a larger diameter than arteries. Veins have valves to prevent backflow and carry blood toward the heart. The IVC slightly expands with suspended respiration.
• Arteries branch into arterioles, then capillaries. Capillaries form a network where materials exchange between blood and tissue fluid.
• Venules collect blood from capillaries and form larger vessels that return blood to the heart.
• Arteries and veins consist of three layers: Tunica intima (inner), Tunica media (middle), and Tunica adventitia (outer). The vasa vasorum supplies the walls of blood vessels.
• The aorta is the largest artery in the body and divides into five sections: root, ascending aorta & arch, descending thoracic aorta, abdominal aorta, and bifurcation into iliac arteries.
• The abdominal aorta supplies blood to various organs via branches (e.g., celiac trunk, superior mesenteric artery, renal arteries, inferior mesenteric artery).

Sonography of the Aorta

• Clinically, sonography is used to evaluate conditions like pulsating abdominal masses, abdominal pain, abdominal bruits, and hemodynamic compromise in the lower legs.
• The arterial system can be affected by calcification, aneurysm, infection, rupture, or thrombosis.
• Normal aorta and iliac diameters vary based on sex (Men 2.0 ± 2.5 cm, Women 1.7 ± 1.5 cm for aorta, Men 1.3 ± 2.0 cm, Women 1.2 ± 1.3 cm for common iliac arteries)

Pathology of the Aorta: Abdominal Aortic Aneurysm (AAA)

• AAA involves dilation of the abdominal aorta exceeding 3 cm and affecting all three vessel layers (bulbous, saccular, fusiform).
• Aortic locations are categorized as suprarenal, at renal level, and infrarenal.
• 95% of AAAs are infrarenal.
• Risk factors include tobacco use, hypertension, vascular disease, COPD, family history of AAA, and genetic disorders (Marfan's, Ehlers-Danlos syndrome).
• Symptoms include palpable abdominal mass, back pain, and a drop in hematocrit (rupture).
• "Pseudo-pulsatile" abdominal masses include retroperitoneal tumors, large fibroid uteri, and paraaortic nodes (transmissions of pulsations from the aorta).
• Aortic dissection involves separation between the vessel's layers, often diagnosed with an ultrasound showing an "intimal flap".

Size and Treatment

• Treatment options depend on the aneurysm size. Aneurysms <4 cm are monitored every 6 months; symptomatic aneurysms are considered for intervention.
• 4-5 cm aneurysms usually undergo surgical intervention.
• Aneurysms > 6 cm are considered high risk for surgery.
• Endovascular stent grafts (less invasive) can be utilized for patients who are higher risk for surgery; endovascular grafts are often recommended with renal and iliac vessel involvement.

Pseudoaneurysm

• A pseudoaneurysm is a pulsatile hematoma resulting from blood leakage into soft tissue surrounding a ruptured artery, contained by deeper vessel layers.
• Doppler ultrasound is employed to visualize the neck and internal anatomy of the aneurysm, guiding potential thrombin injection treatments

Rupture of Aortic Aneurysm

• Aortic rupture has a 50% mortality rate. Rupture risk increases with aneurysm size (>5 cm has a 25% cumulative rupture chance over 8 years).
• Rupture symptoms include excruciating abdominal pain, shock, and expanding abdominal mass.

Aortic Dissection

• Aortic dissection is diagnosed by identifying vessel wall layer separation, particularly the visualization of "intimal flap" on ultrasound scans.

Abdominal Aortic Branches: Visceral

• Celiac axis/trunk, superior mesenteric, renal, and inferior mesenteric arteries comprise visceral branches, vital for visceral organ perfusion.

Abdominal Aortic Branches: Parietal

• Inferior phrenic arteries, lumbar arteries, median sacral artery, gastroduodenal artery, and splenic artery provide necessary parietal branches.

Abdominal Veins

• The major abdominal vein is the inferior vena cava (IVC), which receives blood from the lower body, carrying it to the heart.
• The IVC formation occurs at the union of common iliac veins at L5 vertebral level, then ascends through the retroperitoneum on the right, passing behind the liver, and entering the heart's right atrium at the inferior thoracic region.
• Tributaries of the IVC include three anterior hepatic veins, three lateral (suprarenal, renal, and testicular/ovarian), five lateral abdominal wall, and three veins of origin (common iliac veins, median sacral veins).

Portal Venous System

The portal venous system carries blood from the digestive tract to the liver. This system consists of various veins, including the portal vein, superior and inferior mesenteric veins, and splenic vein. Crucially, it should be noted that the portal venous system differs from the systemic veins in its flow towards the liver, unlike systemic blood flow towards the heart.

Renal Veins

The renal veins carry blood from the kidneys to the inferior vena cava. The right renal vein typically flows directly into the posterior aspect of the IVC without significant tributaries.

Doppler Flow Patterns

• Abnormal portal vein flow patterns can be identified using Doppler examinations. A chronic obstruction of the portal vein, as well as various other conditions, can manifest with hepatofugal flow (opposite direction from normal, towards the liver).
• Various conditions, including liver cirrhosis, hepatic vein obstruction, and IVC blockage or prolonged heart failure, can contribute negatively toward the flow characteristic.