Transverse View of Aortic Aneurysm

US is very helpful in identifying AAA because it is non-invasive, can be rapidly performed, and patient does not have to leave resuscitation area. A technically adequate US has virtually 100% sensitivity. Other imaging modalities include x-ray, CT, and MRI all of which have advantages and disadvantages.

 
Scanning Technique:

Equipment – curvilinear abdominal probe or phased array probe.  2-5 MHz multi-frequency is ideal.

General – scan from diaphragmatic hiatus to the bifurcation

Identification – this is done most easily and accurately in transverse plane.  First identify echogenic vertebral body and aorta will lay adjacent to left anterior surface of vertebral body.  May use doppler or M-mode to identify pulse if needed.

Real time scanning technique- Surface anatomy that corresponds with the diaphragmatic hiatus is the xiphoid process and the surface anatomy that corresponds with the bifurcation is the umbilicus.  Start at xiphoid with probe midline and at 90 degrees to skin and marker to patients right to get transverse view, then slid down to bifurcation.  Then rotate probe 90 degrees so marker points to approx. the patients head to obtain longitudinal views, probe may need to be rocked gently back and forth to get full view.

Additional windows- may need these in obese patients or those with increased bowel gas.  Can use right midaxillary line intercostals views using liver as window, may help if patient is in LLD position.  Another approach is to place the probe in the left paraumbilical region.

Measurement- measure from outside wall to outside wall in two planes (transverse and long) at its maximum diameter.  Best to measure each view in anterior to posterior and side-to-side.  Best to measure in two places: proximal (may use liver as window, transverse and long measurement) and distal (just above bifurcation, again transverse and long).  Upper limit of normal is 3cm, > 5cm = increased risk of rupture.  If AAA is seen a FAST scan should be preformed to look for free peritoneal fluid.  If high index of suspicion, but no AAA seen, may attempt to evaluate iliac vessels.

Limitations- does not identify all abnormalities or diseases of aorta, exam may be limited by technical difficulties including:

1. obesity

2. bowel gas – may need to apply gentle pressure to probe or rock probe back and forth to get better views.

3. abdominal pain

Pitfalls-

1. A small aneurysm does not mean that it cannot rupture, if patient is symptomatic and abdominal aorta greater then 3cm, then should be treated as AAA.

2. Absence of free intraperitoneal fluid does not preclude AAA diagnosis.

3. Retroperitoneal hemorrhage cannot be reliably identified on US.

4.Even if AAA is seen may not be cause of patients symptoms.

5.Saccular aneurysms may be easily overlooked since they may be confined to a small area.

6.Oblique or angled cuts may exaggerate true diameter, if aorta is tortuous or ectatic, be sure to measure at right angle to vessel.

7.Large para-aortic nodes may be confused with aorta or AAA, they are usually located anterior to aorta.  May be able to identify due to nodular shape or using color flow doppler.

Sources:

“ACEP Policy Statement on Emergency US imaging Criteria Compendium”.  April 2006.  Pgs 1-5.

South Carolina College of Emergency Physicians.  “Emergency Ultrasound Course Abdominal Vascular US”.  April 2005.  Pgs.1-16.

Tintinalli, Judith; Kelen, Gabor; Stapczynski, Stephan.  “Emergency Medicine – A Comprehensive Guide”.   6^th Edition.2004.  Pg.404-408.

Aorta at bifurcation into iliacs

For more information on repair of the AAA, link here

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