AAA
Abdominal Aortic Aneurysm minimum views and accredidation standards.
Overview
Ultrasound to detect abdominal aortic aneurysm has long been part of the armamentarium of the emergency physician.
Emergency ultrasound improves the time to diagnosis and definitive treatment of AAA¹. ED Physician performed US has been shown in several studies to have a high sensitivity and specificity for diagnosis². Pooled data from these studies shows a +LR of >10.8 and –LR of <0.025.
It is important to visualise the entire aorta in both the transverse and longitudinal planes.
Areas of the scan
- proximal aorta — at level of porto-splenic confluence
- mid aorta — at level between proximal and distal aorta
- distal aorta — just proximal to the bifurcation
- bifurcation of common iliac
Transverse plane
Probe position
- Probe marker to patient's right
- Subxyphoid to periumbilical
- In the epigastrium, tilk the probe up and under the xyphoid to get a good view of the roximal aorta. Subsequent images are with the probe held vertically on the patient
Begin in the midline just below the xyphisternum. Have the depth as high as possible eg 20cm. Visualise the convex dome of the vertebral body with acoustic shadowing posteriorly.
Once the vertebral body is identified, decrease the depth to have the aorta just above the focus.
The aorta is immediately superior to the vertebral body and to the right of screen. The IVC will be to the left of the aorta on the screen.
Visualise the entire aorta from here to the umbilicus (L4) which is usually the site of bifurcation. Visualise the iliac arteries.
Take still images of proximal, mid and distal aorta and iliac vessels.
Then turn the probe 90 degrees so that the probe marker is placed cranially. Visualise the aorta longitudinally and save a still image.
Measure an aneurysm at its maximal diameter in the transverse view. Measurement should be from outer edge to outer edge in the vertical plane.
Important measurements
Aneurysmal aorta: >3cm
Aneurysmal iliac arteries: > 1.5cm
Note: a ruptured aneurysm may be difficult to visualise. The haemorrhage is often retroperitoneal and not visualised by EFAST.
Distinguishing Aorta from IVC
| Aorta | IVC |
|---|---|
| Thick walled | Thin walled |
| Not collapsible with probe pressure | Collapsible |
| To the left of patient (right screen) | To the right of patient (left screen) |
1 Plummer D et al. Emergency department ultrasound improves time to diagnosis and survival in ruptured abdominal aortic aneurysm. Acad Emerge Med [Abstract] (1998) 5: 417
2 Rubano E et al. Systematic review: emergency department bedside ultrasonography for diagnosing suspected abdominal aortic aneurysm. Acad Emerg Med. (2013) Feb;20(2):128-38.