AAA US Summary

Indications

  • Unexplained back/abdominal pain in older patients
  • Renal colic in older patients
  • Syncope in older patients
  • Risk factors: IHD, PVD, age

AAA Definition

  • Transverse aortic diameter >3cm
  • 90-95% infrarenal
  • Measure outer wall to outer wall
  • Ignore clot/false lumen in measurement

Aorta vs IVC

  • Aorta: Left, thick-walled, non-compressible, pulsatile, round
  • IVC: Right, thin-walled, compressible, transmitted pulse, almond-shaped

Scanning Protocol

  • Start at epigastrium (transverse view)
  • Identify coeliac axis, SMA, renal vessels
  • Longitudinal views of entire aorta
  • View IVC in longitudinal section

Pitfalls

  • "Salami slice" view overestimates diameter
  • Tortuous aorta affects transverse measurements
  • Clot may lead to underestimation
  • Must visualize entire aorta + iliac arteries

Emergency Action

  • AAA on US + unstable/symptomatic patient
  • Requires emergency vascular surgery consult
  • US cannot accurately detect leak
  • CT is gold standard but may delay treatment

Abdominal Aorta Ultrasound

Emergency assessment for abdominal aortic aneurysm

Indications for Abdominal Aorta Ultrasound

Features of a suspected abdominal aortic aneurysm (AAA) include:

Symptoms

  • Unexplained back or abdominal pain in an older patient
  • Renal colic in an older patient
  • Syncopal episode in an older patient

Risk Factors

  • Ischaemic heart disease
  • Peripheral vascular disease
  • Advanced age
  • Clinical examination findings

Emergency Protocol

Computed tomography (CT) is the gold standard investigation for suspected AAA, but can lead to delays. An early ultrasound is the primary investigation of choice. The combination of an aneurysm on ultrasound, together with an unstable or symptomatic patient, is enough to warrant an emergency vascular surgery opinion.

Focused Examination

Emergency Medicine AAA assessment is a focused examination to answer a single clinical question: Is an abdominal aortic aneurysm (with a diameter of greater than 3cm) present?

Clinical Anatomy

Approximately 90-95% of abdominal aortic aneurysms (AAA) are confined to the infrarenal aorta. In the acute setting, an AAA is defined as a transverse aortic diameter greater than 3 cm.

Key Anatomical Landmarks

Measurement Protocol

Measure from outer wall to outer wall of the vessel. Any clot or false lumen should be ignored when measuring.

Types of Aneurysms

Fusiform Aneurysms

  • Most common type in abdominal aorta
  • Most are true aneurysms
  • Weakness along extended section of aorta
  • Involves entire circumference
  • Appears as symmetrical bulge

Saccular Aneurysms

  • Appear like small blister on side of aorta
  • Asymmetrical
  • May be pseudoaneurysms from trauma
  • Can be caused by penetrating aortic ulcer

Dissection Aneurysms

  • Tear begins within wall of aorta
  • Wall layers separate
  • Dissections can cause aneurysms
  • Existing aneurysms can also dissect
  • More common in thoracic aorta

Understanding the Ultrasound

The inferior vena cava (IVC) and aorta are both seen in most cases. It is possible for a novice to confuse the two.

Feature Aorta Inferior Vena Cava (IVC)
Position Left of midline Right of midline
Wall Thickness Thick-walled Thin-walled
Compressibility Will not compress Compressible
Pulsation Pulsatile Transmitted pulse ('double bounce')
Shape Round (constant) Almond-shaped (varies)
Other Features Superior mesenteric artery demonstrated Size indicates right atrial pressure

Anatomical Relationships

The aorta is situated anterior to the vertebral bodies and left of the midline, whereas the IVC lies to the right of the midline. The aorta tapers and tends to be tortuous and move to the left. It can be calcified anteriorly which can make the ultrasound view more difficult.

Performing the Ultrasound

Patient Position and Preparation

Scanning Protocol

1

Transverse Section

  • Image the coeliac axis (resembles "seagull wings")
  • Identify the superior mesenteric artery
  • Locate renal vessels lower down
  • Find bifurcation and iliac arteries around umbilicus
  • Identify vertebral body (pre-vertebral stripe and acoustic shadow)
  • Aorta lies anterior to echo-bright pre-vertebral stripe
2

Longitudinal Images

  • Obtain longitudinal views of entire abdominal aorta
  • Aorta moves from deep at diaphragm to shallow at umbilicus (due to lumbar lordosis)
3

IVC Assessment

  • Move probe slightly right to view IVC in longitudinal section
  • IVC diameter at diaphragm indicates right heart load
  • In unventilated patients:
    • >25mm with minimal collapse = increased right atrial pressure
    • <15mm with complete collapse = hypovolemic shock
  • Less reliable correlation in ventilated patients
4

Measurement

  • Measure anteroposterior diameter in transverse view
  • From outer wall to outer wall
  • Ignore any clot or false lumen

Pitfalls and Challenges

Technical Errors

  • Non-perpendicular view creating "salami slice" overestimation
  • Probe may need to point caudally to compensate for lumbar lordosis
  • Failing to appreciate tortuous aorta affecting measurements

Interpretation Errors

  • Not recognizing clot leading to diameter underestimation
  • Excluding AAA without visualizing entire aorta and iliac arteries
  • Confusing IVC with aorta

Limitations

  • Ultrasound cannot accurately determine aneurysm leak
  • Difficult to define limits/relations of aneurysm in emergency setting
  • Bowel gas may obscure views

Critical Consideration

Never exclude an AAA without visualizing the entire abdominal aorta and proximal iliac arteries. A negative ultrasound does not completely rule out pathology if not all segments are adequately visualized.

Clinical Decision Making

Emergency Referral Criteria

Refer immediately for vascular surgery consultation if:

  • Aortic diameter >3cm on ultrasound
  • AND patient is symptomatic or unstable
  • Do not wait for CT confirmation in unstable patients

Documentation

Follow-up