Bedside Ultrasonography Evaluation of Abdominal Aortic Aneurysm
- Author: Timothy Jang, MD; Chief Editor: Caroline R Taylor, MD more...
Ultrasonography for the detection of abdominal aortic aneurysm (AAA) has been shown to be sensitive, specific, and relatively simple to perform.[1, 2, 3, 4] A 1998 study by Plummer et al demonstrated its utility as a point-of-care test in the emergency department (ED) by showing faster time to diagnosis and higher rate of survival of ruptured AAA. Further validation studies have shown that emergency medicine (EM) residents and, by extension, appropriately trained EM physicians, are able to quickly and accurately identify AAA using ultrasound at the bedside.
It is important to remember, however, that while bedside ultrasonography can be especially useful in an unstable patient who cannot leave the ED or other acute care patient setting for computed tomography (CT) of the abdomen, it should never delay potentially lifesaving abdominal surgery when such surgery is immediately available and the diagnosis is strongly suspected.
A leaking or ruptured AAA classically presents in patients who are older than 60 years who have a history of hypertension and atherosclerosis and who experience severe abdominal, back, or flank pain after an episode of syncope. Although AAA is more common in men than in women, the incidence of rupture is greater in women because the growth rate of AAA is significantly greater in women than in men.
Unfortunately, less than 50% of patients with ruptured AAA present with the classic triad of syncope followed by back, flank, or abdominal pain and a pulsatile abdominal mass. Up to 30-60% of patients with AAA are initially misdiagnosed.[11, 12, 13, 14] Be careful not to confuse AAA with more benign diagnoses such as renal colic, mechanical back pain, or diverticulitis. Maintain a high index of suspicion in patients older than 60 years who present with pain in the abdomen, back, flank, or groin.
Additionally, in at-risk patients who may present with an unrelated chief complaint, emergency ultrasonography can be a fast and accurate method for identifying patients with AAA who may benefit from follow-up or intervention. The main disadvantages in bedside ultrasonography are that a leaking or ruptured aneurysm can be difficult to distinguish from an enlarged but nonruptured aneurysm. The aorta can also be difficult to visualize when the patient is obese or bowel gas is present.
The abdominal aorta is said to have an aneurysm when the distal aorta is dilated to a diameter larger than 3 cm. An AAA typically enlarges at a rate of 2-8 mm/y. Because enlargement results in an increasing incidence of rupture (eg, a 7-cm AAA has a 19-32% rate of rupture per year) and because the mortality rate is much lower with elective repair than with emergency repair (3-5% vs 50%), the general recommendation is that AAAs larger than 5.0-5.5 cm should be electively repaired.[17, 19]
Go to Abdominal Aortic Aneurysm and Emergent Management of Impending Rupture/Rupture of Abdominal Aortic Aneurysm for complete information on these topics.
Indications for bedside ultrasonography include the following:
Suspicion of AAA
As a diagnostic aid in the evaluation of patients older than 60 years who present with nonspecific pain in the back, flank, abdomen, or groin
For rapid evaluation of an unstable patient in the ED or other acute care patient setting while the surgical team is assembling and resuscitation is ongoing
The U.S. Preventive Services Task Force (USPSTF) recommends the following[20, 21] :
One-time screening for AAA with ultrasonography in men aged 65-75 years who have ever smoked.
Selective screening for AAA in men aged 65-75 years who have never smoked.
Current evidence is insufficient to assess the balance of benefits and harms of screening for AAA in women aged 65-75 years who have ever smoked.
Recommendation against routine screening for AAA in women who have never smoked.
Abdominal ultrasonography should not be performed if it would delay definitive care by means of abdominal surgery.
Anesthesia is generally not necessary for sonographic evaluation, because the procedure is not usually painful. The patient should be warned, however, that a considerable amount of pressure is sometimes necessary to obtain quality images. This pressure may be unanticipated and mildly uncomfortable.
Equipment includes the following:
Ultrasonograph with an abdominal transducer (the standard probe used for this examination is a 3.5- to 5-MHz transducer)
Acoustic gel for application on the patient’s upper abdomen, the transducer, or both
The procedure should be done with the patient in the supine recumbent position.
Although abdominal ultrasonography in the emergency department (ED) or other acute care patient setting is a good test for suspected abdominal aortic aneurysm (AAA), do not delay operative care to obtain images in an unstable patient.
Explain the procedure, benefits, risks, and complications to the patient and/or the patient’s representative. Ask if he or she would like others to be present for the procedure.
Disrobe the abdomen from xiphoid to pubis. Apply a generous amount of acoustic gel to the patient’s upper abdomen (from xiphoid to umbilicus), the transducer, or both.
Scan the upper abdomen in the transverse plane while holding the probe perpendicular to the patient’s long axis. Identify the aorta and trace it to its bifurcation with the iliac vessels (see the image and the videos below).
The abdominal aorta can be distinguished from the vena cava (see the image below) by its thicker wall, pulsations, noncompressible nature, and ability to detect pulsatile Doppler flow.
Acquire pictures in the transverse plane, and use the external diameter (outer wall to outer wall) to measure the diameter of the aorta at its widest point (see the image and the video below). An accurate measurement of the aorta’s size is important because the risk of rupture is related to the overall size of the aorta.
Scan the upper abdomen in the longitudinal plane, holding the probe parallel to the long axis of the patient’s body. Acquire pictures in the longitudinal plane, and measure the diameter at the widest point (see the images and the video below). Remember that a diameter larger than 3 cm is considered to represent abnormal dilatation.
Once images have been acquired in both transverse and longitudinal planes and the aorta’s diameter has been measured, emergency bedside ultrasonographic imaging for AAA is complete. Wipe the gel from the patient and transducer, cover the patient, and place him or her in a position of comfort.
Visualizing an enlarged (>3 cm) abdominal aorta is diagnostic of AAA, though rupture cannot be definitively diagnosed with ultrasonography alone. The first two videos below depict AAA, while the third depicts normal aortic anatomy.
In the appropriate clinical context, ultrasonography alone can rule in the diagnosis of rupture when rupture appears to be present. However, if adequate images cannot be obtained (eg, because of patient obesity or bowel gas), the diagnosis of aortic rupture cannot be excluded.
Common pitfalls in performing bedside ultrasonography to evaluate AAA include the following:
Failing to compress the overlying bowel adequately with probe pressure
Mistaking the vena cava for the aorta because of transmitted pulsations
Overestimating the aneurysmal width because of the lack of a true transverse measurement (cross-section)
Confusing an imaging artifact with thrombus
Failing to measure the external diameter (outer wall to outer wall)
Failing to move the transducer off the sagittal plane while following a tortuous aorta
Misinterpreting acoustic enhancement distal to the aorta as evidence of leakage. (Remember that ultrasonography is not good at detecting rupture or leakage. Contrast abdominal computed tomography [CT] is the criterion standard. However, contrast-enhanced ultrasonography may be a reasonable alternative to CT, if CT is unavailable.  )
Being reluctant to move the transducer far laterally in an attempt to visualize an aorta that is obscured by overlying bowel gas
Bedside ultrasonography for evaluation of abdominal aortic aneurysm (AAA) has no major complications, aside from the potential complications that could ensue if immediately available surgical repair is delayed for the purpose of obtaining imaging studies.
Although no data suggest that palpation of an AAA by physical examination or with the probe increases the rate of rupture or worsens an already rupturing AAA, repeated examinations for teaching purposes are not advised.
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