Abdominal, back, and flank pain are some of the more common presenting complaints of patients for whom an emergency physician cares. In such cases, the physician must consider abdominal aortic aneurysm (AAA) in the differential diagnosis.

Competence in evaluating for an AAA is a core emergency ultrasound application as outlined by the 2008 American College of Emergency Physicians Policy Statement on Emergency Ultrasound. Studies demonstrate that emergency physicians are able to perform bedside ultrasound of the abdominal aorta with a high sensitivity and specificity for an abdominal aortic aneurysm.

Clinical Indications for Performing An Aortic Ultrasound Exam

The main indication for bedside aortic ultrasound examination is the rapid identification an AAA.

Patients older than 50 years with the classic presentation of abdominal, back, or flank pain, a pulsatile abdominal mass, and hypotension should have a bedside aortic ultrasound examination.

Pain can be referred to the scrotum, buttocks, thighs, shoulders, or chest, and many patients are misdiagnosed with renal colic, diverticulitis, or musculoskeletal pain. Any patient with unexplained hypotension, dizziness, or syncope should have a bedside aortic ultrasound.

Also consider that patients presenting in cardiac arrest may have a ruptured AAA. Patients with pulseless electrical activity may be in a state of severe hypotension that could be reversed if the cause is rapidly identified and aggressively treated.

Performing an Abdominal Aorta Ultrasound

Patient positioning: Place the patient in the supine position. When bowel gas or adipose tissue prevents adequate visualization, the patient can be placed in the lateral decubitus position. Consider bending the patient’s knees to decrease tension on the rectus muscles.

Anatomic landmarks: The abdominal aorta is a retroperitoneal structure beginning at the aortic hiatus of the diaphragm and then coursing anterior to the vertebral spine before dividing into the iliac arteries. The inferior vena cava (IVC) runs along the right side of the aorta, and sonographically the two structures must be distinguished from each other.

The IVC is thin walled, varies in size with respiration, and may be flattened with minimal pressure by an ultrasound probe. The aorta does not change in size with respiration and is non-compressible, thick-walled, and pulsatile.

The left lobe of the liver is located anterior to the proximal abdominal aorta and can be used to help delineate this portion of the aorta.

The first large branch of the abdominal aorta is the celiac trunk, which divides into the common hepatic artery, left gastric artery, and splenic artery. The superior mesenteric artery is the next branch, followed by the left and right renal arteries, which branch off laterally.

The inferior mesenteric artery is the final branch of the abdominal aorta, a few centimeters proximal to the iliac bifurcation.

Scanning the aorta. A low frequency probe (e.g., 3.5–5 MHz) should be used to scan the aorta. Either a curvilinear or small phased-array probe is appropriate to use.

With the patient supine and the knees flexed, the scan should begin in the epigastric region and end near the umbilicus.

The aorta should be scanned in its entirety in both short and long axis for evaluation of an AAA.

When the aorta is scanned in the long axis or longitudinal plane, the probe marker should point toward the patient’s head.

When the aorta is scanned in the short axis or transverse plane, the marker should point toward the patient’s right side.

A five-point measurement of the aorta should be performed:

1. The proximal aorta (see image 1);
2. The mid-aorta (see image 2);
3. The distal aorta proximal to the aortic bifurcation (see image 3); and
4. The common iliac arteries (see image 3).

Measure the lumen of the aorta in the anterior to posterior dimension, from the outer wall to the outer wall. Measurement of the lumen (inner wall to inner wall) can lead to a false negative result because of an intraluminal clot (see image 5 and image 6).

A fifth view of the aorta in a longitudinal plane should be demonstrated including the distal aorta (see image 4), because the majority of abdominal aortic aneurysms are infra-renal in location (see image 7).

The 2008 ACEP Policy Statement on Emergency Ultrasound Guidelines recommends measuring the maximal aortic diameter in both longitudinal and transverse planes when measuring the aorta and iliacs.

When scanning in the longitudinal plane, avoid inadvertently sweeping the beam into a right parasagittal plane, which may result in visualization of the IVC. Inaccurate measurements can occur when the longitudinal beam is directed at a tangent, resulting in a smaller AP diameter.

To avoid this operator error, measure the aorta in both longitudinal and transverse planes; verifying measurements in two planes ensures dimensions are consistent with the true size of the aorta.

Identifying Abdominal Aorta Pathology

An aorta measuring between 3.0 cm and 4.0 cm is suspicious for an abdominal aortic aneurysm.

In addition, the aorta tapers and becomes more superficial as it moves distally. An aorta that increases in size as it courses through the body, even if within the normal measurement range, may still be aneurysmal.

A common iliac artery measuring greater than 1.5 cm is concerning for an iliac aneurysm.

If an aneurysm is identified, evaluate the peritoneal cavity for free fluid using views similar to the FAST (Focused Assessment by Sonography in Trauma) exam.

Signs of rupture include peritoneal free fluid, retroperitoneal hematoma, and/or lateral displacement of the kidney on the side where the aorta is ruptured.

Most aneurysms that do rupture will leak into the retroperitoneum, which may contain the leak by tamponade and local clotting.

A saccular aneurysm can arise from an aorta with a normal sized lumen and may be missed if the aorta is visualized only in the median plane longitudinally or in intermittent areas transversely.

The major complication of AAA is rupture. Rupture leads to rapid hemodynamic deterioration and death from hemorrhagic shock.

The risk of rupture increases with female sex and increased AAA diameter. Other risk factors include tobacco smoking and hypertension.

The Difficult Aorta: Limitations to Visualization

The most common impediments to visualization of the aorta are bowel gas, obese habitus, and an uncooperative patient in pain. Remember that a focused exam is a rapid evaluation, and minimizing time spent scanning will help to limit the patient’s pain.

In the subxiphoid area, the liver can be used as an acoustic window to view the proximal aorta. In addition, using respiratory variation by asking the patient to take a deep inhalation will lower the diaphragm and liver margin, allowing better visualization of structures beneath.

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.
3. Bernstein EF, Chan EL. Abdominal Aortic Aneurysm in High-Risk Patients. Outcome of Selective Management Based on Size and Expansion Rate. Ann. Surg. 1984;200:255-63.
4. Blaivas M, Theodoro D. Frequency of Incomplete Abdominal Aorta Visualization by Emergency Department Bedside Ultrasound. Acad. Emerg. Med. 2004;11:103-5.
5. Brown LC, Powell JT. Risk Factors for Aneurysm Rupture in Patients Kept Under Ultrasound Surveillance. Ann. Surg. 1999;230:289-96.
6. Dent B, Kendall RJ, Boyle AA, Atkinson PR. Emergency Ultra-sound of the Abdominal Aorta by UK Emergency Physicians: A Prospective Cohort Study. Emerg. Med. J. 2007;24:547-9.
7. Kuhn M, Bonnin RLL, Davey MJ, et al. Emergency Department Ultrasound Scanning for Abdominal Aortic Aneurysm: Accessible, Accurate and Advantageous. Ann. Emerg. Med. 2000;36:219-23.
8. Moore CL, Holliday RS, Hwang JQ, Osborne MR. Screening for Abdominal Aortic Aneurysm in Asymptomatic At-risk Patients Using Emergency Ultrasound. Am. J. Emerg. Med. 2008;26:883-7.
9. Rodin MB, Daviglus ML, Wong GC, et al. Middle Age Cardiovascular Risk Factors and Abdominal Aortic Aneurysm in Older Age. Hypertension 2003;42:61-8.
10. Rowland JL, Kuhn M, Bonnin RL, et al. Accuracy of Emergency Department Bedside Ultrasonography. Emerg. Med. (Fremantle) 2001;13:305-13.
11. Tayal VS, Graf CD, Gibbs MA. Prospective Study of Accuracy and Outcome of Emergency Ultrasound for Abdominal Aortic Aneurysm Over Two Years. Acad. Emerg. Med. 2003;10:867-71.