Bedsdie Ultrasound of abdominal aorta

 

Teaching Points – Bedside Ultrasound of the Abdominal Aorta

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I. Clinical Relevance of AAA

• AAA definition: Aortic diameter > 3 cm; iliac > 1.5 cm; 150% of normal size.

• 90% of AAAs are infrarenal (below the renal arteries).

• Surgical indications:

  • Diameter > 5.5 cm

  • Rapid expansion

  • Symptomatic AAA (e.g., pain, hypotension)

• AAA presentations:

  • Often asymptomatic until rupture

  • Symptoms may include vague abdominal, flank, or back pain, hypotension, syncope, weakness, or cardiac arrest.

• High mortality rate; survival decreases 1% per minute post-rupture.

• Rupture risk correlates with size: >7 cm = up to 50% rupture risk/year.

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II. Utility of Bedside Ultrasound

• First ED-diagnosed AAA via ultrasound: 1989.

• High sensitivity and specificity for detecting AAA.

• Cannot rule out rupture—CT or other imaging required.

• Essential for rapid evaluation in unstable patients.

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III. Aortic Anatomy for Ultrasound

• Retroperitoneal structure, enters at T12, bifurcates at L2.

• Aorta follows the spine’s curvature, becomes more superficial distally.

• Key aortic branches:

  • Celiac artery → “Seagull sign” (hepatic + splenic)

  • SMA (anterior branch)

  • Renal arteries (lateral, hard to see)

  • Bifurcation into iliac arteries at umbilicus level

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IV. Scanning Technique

• Probe: Low-frequency curvilinear preferred; phased array acceptable.

• Views: Both transverse and longitudinal.

• Scan from: Diaphragm to iliac bifurcation.

• Transverse scanning:

  • Start at epigastrium, marker to patient’s right.

  • Use liver edge as a window.

  • Identify vertebral body (echogenic “horseshoe sign”) to orient.

• Measurement:

  • Always outer wall to outer wall.

  • At least 3 levels: proximal, mid (below SMA), and distal (above bifurcation).

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V. Differentiating Aorta from IVC

Feature	Aorta	IVC
Wall	Thick, non-compressible	Thin, compressible
Pulsatility	Consistently pulsatile	Respiratory variation
Branching	Anterior branches (Celiac, SMA)	No anterior branches caudal to hepatic veins
Entry	Dives posteriorly behind heart	Drains into RA directly
Location	Left of midline	Right of midline

Pulsation is not reliable alone—look for branching and wall structure.

• Use probe rotation and vertebral body identification to verify vessel identity.

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VI. Pathologic Examples

• Large AAA >10 cm: Risk of underestimation if only intraluminal diameter is measured.

• Saccular aneurysms: Small, localized—require full scan to avoid missing.

• Distal aneurysms: Near iliac bifurcation; easily missed.

• Dissection: Flap may be seen, but ultrasound is limited—CT needed.

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VII. Imaging Challenges & Optimization

• Gas interference (transverse colon): Use graded compression and adjust probe position laterally.

• Obesity: Use firm pressure and left lateral decubitus positioning to displace pannus.

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VIII. Common Pitfalls

• Underestimating size: Always measure outer wall to outer wall.

• Mistaking IVC for aorta: Verify location, branching, compressibility.

• Missing aneurysms:

  • Failing to scan full aorta

  • Not scanning below SMA

  • Inadequate depth may lead to misidentifying SMA as aorta.

• FAST exam not sufficient to rule out rupture (retroperitoneal bleeds).

• When bedside ultrasound is inconclusive, CT abdomen is required.

What is the Seagull Sign?

The Seagull sign refers to the sonographic appearance of the celiac trunk branching off the abdominal aorta.

• The celiac trunk arises anteriorly from the aorta and divides into:

  • Common hepatic artery (to the right)

  • Splenic artery (to the left)

These two branches create a Y-shaped or seagull-wing appearance on transverse ultrasound, resembling a bird in flight.

Explanation:

✅ Most Common Type:

• Fusiform aneurysm: A uniform, circumferential dilation of the aorta

  • Most common type of abdominal aortic aneurysm (AAA)

✅ Most Common Location:

• Infrarenal: Occurs below the renal arteries

  • ~90% of AAAs are located in this region

Correct answer: False

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Explanation:

• The IVC lies to the patient’s right of the aorta, not the left.

• It can be differentiated from the aorta by several features:

  • Location: IVC = right of midline, Aorta = left of midline

  • Wall structure: IVC = thin-walled, Aorta = thick-walled

  • Compressibility: IVC = collapsible, Aorta = non-compressible

  • Branching:

    • Aorta gives off celiac trunk and SMA (both anterior branches)

    • The IVC receives hepatic veins; it does not branch like the aorta