Steady, graded pressure can help to push bowel gas away. When truncal obesity is the impediment to visualization of the aorta, use the lowest-frequency probe and lowest probe settings available to increase the penetrance of the sound waves. Attempt any position that visualizes the aorta.

In the right midaxillary line, the liver can be used as an acoustic window with the patient in the left lateral decubitus position. In this image, the aorta will lie deep to the IVC.

The distal aorta and common iliacs may be better visualized with the probe placed in the left paraumbilical region, in the supra-umbilical position, and pointing caudad, or in the infra-umbilical position pointing cephalad.

Begin first with transverse visualization of the distal aorta and then rock the probe more inferiorly to visualize its bifurcation. Alternatively, begin infra-umbilically and rock the probe superiorly to visualize the iliac arteries.

Pitfalls

Ultrasound is operator dependent and subjective to interpretive error, particularly when measurement errors are made. Aortic dimensions should be taken perpendicular to the axis of the vessel; this may be more difficult in a tortuous aorta. Oblique imaging planes may exaggerate the true aortic diameter, while tangential planes may underestimate true diameter.

Measuring only the lumen of the vessel may be misleading if there is intra-luminal clot, making it appear smaller than its true caliber. The correct measurement should be taken from outer wall to outer wall.

Longitudinal scanning should include images of the distal aorta, as an AAA occurs most commonly below the renal arteries. In longitudinal imaging, the IVC may be mistaken for the aorta. Check for a thicker wall and pulsatile appearance for confirmation.

Scan through the aorta in its entirety, with a particular suspicion for a saccular aneurysm. The distal aorta should taper gradually. An aorta that increases in size, even within the normal measurement range, may still be aneurysmal.

Conclusion

Bedside emergency ultrasound of the abdomen to look for AAA can be accurately performed by emergency physicians. Bedside ultrasound should always be used in the context of the clinical scenario and should never replace the physical exam.

References

1. American College of Emergency Physicians Policy Statement on Emergency Ultrasound Guidelines. Approved October 2008.
2. American College of Emergency Physicians Policy Statement on Emergency Ultrasound Imaging Criteria Compendium. Approved April 2006.
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