Abdominal Aortic Ultrasound 3 min read Peter Luckow, MD

Abdominal Aortic Ultrasound

Background

  • Abdominal aortic aneurysms are typically asymptomatic and will progressively expand until they rupture. Ruptured aortic aneurysms have a mortality rate of 85-90% (Kent 2014).
  • Prevalence rates are estimated to be 1.3-8.9% in men and 1.0-2.2% in women with main  risk factors of smoking, age, family history, and hypertension (Sakalihasan 2005).

Indications

  • In the primary care setting, the US Preventive Services Task Force recommends screening for abdominal aortic aneurysms (AAA) in men 65-75 years old who have ever smoked (grade B). 
  • In the ED, AAA should be on the differential in the setting of pain, hypotension and a pulsatile mass. AAA should also be considered in at-risk patients who present with syncope or signs of retroperitoneal hemorrhage. 
  • Emergency ultrasound of the abdominal aorta by emergency medicine physicians has been shown to be sensitive and specific. One prospective study in 2008 found bedside ultrasound to be 100% sensitive and 98% specific in detecting a AAA (Tayal 2008).

Technique

  • The low-frequency, 3.5 MHz curvilinear probe is typically used for imaging of the aorta (Dean 2008).
  • Beginning in the epigastric area, with the probe in the transverse plane, identify the vertebral body. The aorta should be visualized above the vertebral body on patient’s left.
  • Short axis images should be obtained of the patient’s proximal abdominal aorta as well as the distal abdominal aorta, just above the site of the bifurcation into the iliac arteries. 
  • A long axis view should be obtained with the probe in the sagittal plane. 
  • A short axis view of the iliacs should be obtained distal to the bifurcation.
  • Use the measurement calipers to measure the anterior-to-posterior diameter of the abdominal aorta using the outer walls.

Short axis. “Ultrasound Assessment of Abdominal Aortic Aneurysms.” Medscape, reference.medscape.com/features/slideshow/ultrasound.

Long axis.  “POCUS Exams – Aorta.” Ultrasound Idiots, www.ultrasoundidiots.com/aorta.

Evaluation

  • An abdominal aortic aneurysm is diagnosed when the diameter is greater than 3.0 cm.
  • The aorta should taper as the vessel is traced distally. An aorta that dilates should prompt further evaluation, even if the measurements remain within normal limits.
  • Complete evaluation should trace past the bifurcation and include measurements of the common iliac vessels. These should measure <1.5 cm in women and <1.8 cm in adult men.

8x8cm AAA including thrombus.  Bookatz, Allen, et al. “Aortic Ultrasound.” WikEM, 30 Dec. 2017, wikem.org/wiki/Aortic_ultrasound.

Measure from outer edge to outer edge.  “Ultrasound Assessment of Abdominal Aortic Aneurysms.” Medscape, reference.medscape.com/features/slideshow/ultrasound.

Common Pitfalls

  • Bowel gas will occasionally obscure the image. If this occurs, place additional pressure on the probe and gently fan up and down in an attempt to displace the bowel gas.
  • Accurate measurements require measurement from the outer wall to the outer wall. Take care not to confuse the inner edge of a thrombus with the outer wall of the abdominal aorta.
  • Be sure to not confuse the IVC with the abdominal aorta. The IVC is on the patient’s right and is typically non-pulsatile and compressible. Additionally, doppler mode can help to identify pulsatile waveforms consistent with aortic blood flow.

Take Home Points

  • Consider AAA in setting of pain, hypotension, pulsatile mass, or syncope, especially in older men with a smoking history
  • AAA is diagnosed when the outer wall to outer wall measurement exceeds 3.0 cm. Be careful not to mistake the inner rim of a thrombus for the outer wall of the abdominal aorta. 
  • Take the time necessary to differentiate the abdominal aorta from the IVC. If bowel gas is obscuring the image, apply increased pressure and slowly fan up and down.

References


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Source: coreem.net

Abdominal Aortic Ultrasound