Ultrasound in Abdominal Aortic Aneurysm

Aka: Ultrasound in Abdominal Aortic Aneurysm, Ultrasonography of Abdominal Aortic Aneurysm, Ultrasound of AAA, Abdominal Aorta Ultrasound, Ultrasound of Abdominal Aorta

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II. Indications

  1. Cases in which Ultrasound is AAA imaging modality of choice
    1. Screening for Abdominal Aortic Aneurysm
      1. See Abdominal Aortic Aneurysm for screening indications
    2. Monitoring Abdominal Aortic Aneurysm rate of change
  2. Other Indications
    1. Emergent Bedside Ultrasound in suspected AAA rupture

III. Efficacy

  1. Efficacy when performed by Radiology Ultrasonographer
    1. Test Sensitivity: 94%
    2. Test Specificity: 98%
    3. Owens (2019) JAMA 322(22):221-18 [PubMed]
  2. Highly accurate even when performed by Emergency Physicians
    1. Test Sensitivity: ~100% (in practice, likely approaches 94% as study above)
    2. Test Specificity: 98%
    3. Accurate to within 0.3 cm in sizing aneurysm (compared with CT)
    4. Tayal (2003) Acad Emerg Med 10(8): 867-71 [PubMed]
  3. Good efficacy has been found for medical students after only 3 hours of training
    1. Mai (2018) Ann Vasc Surg 52:15-21 [PubMed]
  4. Primary care physicians were trained 25 hours using hand-held Ultrasound, and could perform accurate exam in 4 minutes
    1. Siso-Almirall (2017) PLos One 12(4):e0176877 [PubMed]
  5. Contrast with with physical exam whose accuracy is notoriously poor until AAA reaches a size that is at high risk for rupture
    1. Lynch (2004) Accid Emerg Nurs 12:99-107 [PubMed]
  6. Limitations
    1. Does not define periaortic Vascular Anatomy
    2. Reduced image quality in some patients
      1. Obese patients
      2. Increased intestinal gas

IV. Precautions

  1. Apply firm pressure to push bowel gas out of the way (especially in obese patients)
  2. Err on the side of overestimation of AAA size due to difficulties in estimating size of clot
  3. Consider color doppler to assist in highlighting aorta
    1. But probe most be directed with the flow or against the flow (not perpendicular)

V. Imaging: Approach

  1. Transducer
    1. Curvilinear-Array Transducer (3.5 MHz)
    2. Indicator to 9:00 (patient's right, with probe in transverse or short axis)
    3. Set depth to include Vertebrae with posterior shadowing (aorta lies immediately anterior to Vertebrae)
  2. General landmarks
    1. Start in upper Abdomen at the epigastrium, and slide probe inferiorly
      1. Liver may be used as acoustic window for proximal abdominal aorta
    2. Vena cava will be to the patient's right side of the aorta (left side of screen)
      1. Has no major trunks in upper Abdomen below hepatic vein (contrast with Celiac and SMA of aorta)
      2. Portal Vein will be seen superficial to vena cava, but will not intersect
    3. Aorta sits immediately superficial to the Vertebrae
    4. Aorta bifurcation into iliac arteries occurs near Umbilicus
      1. Most critical site of Ultrasound is immediately superior to the Umbilicus (most common AAA site)
  3. Overview in long axis (other views are short axis)
    1. Consider starting with a long axis or longitudinal view (probe indicator to 12:00)
    2. Position probe in each of 2 locations
      1. Epigastrium
        1. Use liver as acoustic window and orient probe inferiorly
      2. Umbilicus
        1. View includes infrarenal aorta and bifurcation
    3. Differentiate aorta from the IVC at its right lateral side
      1. Aorta will have celiac and SMA trunks exiting from its superficial surface
        1. In long axis, SMA will course immediately superficial to the aorta for several centimeters
      2. Vertebrae will lie immediately deep to the aorta
        1. Vertebrae will appear with bright line and posterior shadowing
  4. Level of proximal aorta at Celiac Artery (short axis)
    1. Seagull appearance (immediately anterior to aorta)
      1. Celiac Artery forms the seagull's head
      2. Hepatic artery forms the wing on the patient's right
      3. Splenic artery forms the wing on the patient's left
  5. Level of proximal aorta at superior Mesenteric Artery (short axis)
    1. Left renal vein crosses anterior to the aorta and posterior to the superior Mesenteric Artery
    2. Eyebrow over eye appearance
      1. Splenic vein forms the eyebrow
      2. Superior Mesenteric Artery forms the eye
  6. Level of proximal aorta at renal arteries (short axis)
    1. Difficult to visualize renal arteries at the aortic origin
  7. Level of mid-aorta, infrarenal, above bifurcation (short axis)
    1. No unique landmarks
    2. Most common site of AAA
  8. Level of distal-aorta at Umbilicus level or L4 (short axis)
    1. Aorta bifurcates into Iliac arteries
  9. Images
    1. Abdominal aorta anatomy
      1. abd_aortaWithKidney@0,25x.jpg
    2. Abdominal Aorta Long Axis
      1. ultrasoundProbePositionAbdAortaLong.jpg
    3. Abdominal Aorta Short Axis - Proximal
      1. ultrasoundProbePositionAbdAortaShortProx.jpg
      2. ultrasoundBMP_abdAortaSAXatCeliac.jpg
      3. ultrasoundBMP_abdAortaSAXatSMA.jpg
    4. Abdominal Aorta Short Axis - Mid
      1. ultrasoundProbePositionAbdAortaShortMid.jpg
    5. Abdominal Aorta Short Axis - Distal
      1. ultrasoundProbePositionAbdAortaShortDistal.jpg

VI. Imaging: AAA Screening Protocol

  1. Obtain 3 short axis views (proximal, mid and distal aorta including iliac bifurcation)
  2. Obtain one longitudinal view (90 degrees from short axis view) at level to include distal aorta

VII. Technique: Aorta Meaurements

  1. Measure aorta in short axis in AP diameter from outer wall to outer wall
  2. Pitfall: Thrombus within aorta may be confused with aortic wall
    1. Thrombus typically forms anterior and lateral within the aorta
    2. Measure from anterior to posterior aorta walls (including the thrombus)
    3. Do not limit measurement to open lumen (underestimates aneurysm)
  3. Aorta diameter >3 cm is consistent with Abdominal Aortic Aneurysm
    1. Aorta diameter >5.5 cm meets criteria for elective repair
    2. Aorta diameter >7-8 cm is at high risk of rupture

VIII. Resources

  1. Abdominal Aortic Aneurysm Ultrasound Video (Dr. Perera, SonoSite)
    1. http://www.youtube.com/watch?v=WKnFD6KeO4c

IX. References

  1. Jorgensen (2012) Introduction and Advanced Emergency Medicine Ultrasound Conference, GulfCoast Ultrasound, St. Pete's Beach
  2. Noble (2011) Emergency and Critical CareUltrasound, Cambridge University Press, New York, p. 115-30
  3. Reardon (2016) Abdominal Aorta, Stabroom.com online video, accessed 4/1/2016
  4. Reardon (2011) Pocket Atlas Emergency Ultrasound, McGraw Hill, New York, p. 107-128
  5. Sparks (2002) Am Fam Physician 65(8):1565-70 [PubMed]

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