Ultrasound - Abdominal aorta

Indications

To assess and exclude the presence of:

Abdominal aortic aneurysm

Consider in the following presentations (particularly in men over 50):

Abdominal pain, back or loin pain

Syncope

Hypotension

Pulsatile abdominal mass

Contraindications (absolute in bold)

Airway management or resuscitation required

Alternatives

CT angiogram of abdomen and pelvis (if haemodynamically stable)

Laparotomy without imaging (unstable with likely ruptured abdominal aortic aneurysm)

Informed consent

Medical emergency

Consent is not required if the patient lacks capacity or is unable to consent

Brief verbal discussion is recommended if the situation allows

or

Verbal consent

Less complex non-emergency procedure with low risk of complications

Potential complications

Failure to visualise aorta from diaphragm to bifurcation (excess tissue, bowel gas)

Failure of image interpretation (false negative or positive)

Procedural hygiene

Standard precautions

PPE: non-sterile gloves

Area

Any bed

Staff

Procedural clinician

Equipment

Towel

Ultrasound machine and gel

Curvilinear ultrasound probe (abdo)

Positioning

Supine with hips flexed to relax the abdominal muscles

Ultrasound depth set to 15cm (or abdominal pre-set)

Probe inferior to xiphoid directed towards patient’s back (at right angle to the skin)

Marker to patients right for transverse view, and head for longitudinal view

Sequence (scanning)

Place probe inferior to xiphoid directed towards patient’s back (marker to patients right)

Ask patient to take a deep breath (lowering the diaphragm and liver improving view)

Identify the hyperechoic midline vertebral body

Identify the circular hypoechoic aorta anterior to the left side of the vertebral body

Slide probe upwards to visualise the aorta as proximally as possible

Slide probe inferiorly viewing the whole of aorta to the point of bifurcation (at he umbilicus)

If the view is lost (due to bowel gas) maintain constant downwards pressure to dissipate gas

Return to mid-aortic view (just above umbilicus) and rotate 90 degrees into the longitudinal view

Rock or slide the probe to obtain a longitudinal view from as proximal as possible to bifurcation

Measure the aortic diameter from outside wall margins at its widest point in both views

If a abdominal aortic aneurysm is identified (diameter >3cm) perform a FAST scan to evaluate for free abdominal fluid

Sequence (improving view over a section of aorta obscured by bowel gas)

Maintain constant downward pressure with the probe over the area of poor view

Jiggle the probe side to side attempting to displace bowel loops

Fan probe in the windows above and below the sonographic obstacle to obtain a view

Try a right mid-axillary line intercostal view using the liver as an acoustic window (aorta deep to IVC)

Try a right mid-axillary line intercoastal view (with patient lying on left side to improve view)

Post-procedure care

Clean ultrasound gel from patient

Initiate further management if required

Document (indication, structures identified, interpretation)

Tips

Any measurement greater than 3 cm is abnormal

Do not mistake a lumen through thrombus as the wall diameter (adjust gain until lumen anechoic)

Take care to differentiate the Aorta from the IVC in the longitudinal view

Discussion

Ultrasound in the emergency department can accurately identify or exclude the presence of infrarenal abdominal aortic aneurysm. In some case it may also identify a suprarenal abdominal aortic aneurysm or distal aortic dissection (with floating intimal flap).

Most aneurysms are fusiform extending over a long section or the aorta, however saccular aneurysms also occur confined to a few centimetres. These can be easily missed. To be reliably excluded aneurysm the aorta must be visualised in both in transverse and longitudinal planes without any missing sections.

The inferior vena cava can be mistaken for the aorta, with both appearing pulsatile. You can differentiate the brighter, thicker, rounder and non-compressible aorta lying to the left of the inferior vena cava. The aorta also has anterior branches distal to the liver whereas the inferior vena cava does not.

Gas in the transverse colon commonly obscures the aorta in band inferior to the liver, which prevents an uninterrupted view of the aorta. Downward pressure, fanning techniques and use of different windows may be successful, but some studies will be inconclusive for this reason.

The absence of free intraperitoneal fluid does not rule out acute abdominal aortic aneurysm rupture as most acute patients presenting to the emergency department do not have free peritoneal fluid. Leakage is most commonly retroperitoneal and cannot be reliably identified on ultrasound.

Peer review

This guideline has been reviewed and approved by the following expert groups:

Emergency Care Institute

Please direct feedback for this procedure to ACI-ECIs@health.nsw.gov.au.

References

Roberts JR, Custalow CB, Thomsen TW. Roberts and Hedges’ clinical procedures in emergency medicine and acute care. 7th ed. Philadelphia, PA: Elsevier; 2019.

Dunn RJ, Borland M, O’Brien D (eds.). The emergency medicine manual. Online ed. Tennyson, SA: Venom Publishing; 2019.

Jim J, Thompson RW. Clinical features and diagnosis of abdominal aortic aneurysm. UpToDate, 2020 Jan 15. Available from: https://www.uptodate.com/contents/clinical-features-and-diagnosis-of-abdominal-aortic-aneurysm

Emergency ultrasound imaging criteria compendium. Ann Emerg Med. 2016;68(1):e11-e48. doi:10.1016/j.annemergmed.2016.04.028

Dawson M, Mallin M. Introduction to bedside ultrasound: volume 1. New York NY: Apple Books; 2013.

Dawson M, Mallin M. Introduction to bedside ultrasound: volume 2. New York NY: Apple Books; 2013.

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