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Abdominal Aortic Aneurysms (AAAs) are a relatively common pathology with a prevalence of 1.3% in patients over 50 years and an incidence in elderly men over 12%. Ruptured aneurysms have an exceptionally high mortality rate ranging 50% to 95%. In fact, mortality increases by 1% with each subsequent minute, mandating prompt diagnosis and intervention. Nearly 30% of ruptured AAAs are misdiagnosed on initial presentation. Physical examination has poor sensitivity of less than 65%. Moreover, less than 25% of patients present with the characteristic triad of hypotension, abdominal pain, and a pulsatile abdominal mass.
Prompt diagnosis by utilizing point of care ultrasound has demonstrated sensitivities of 94% to 99%. Costantino et al. confirmed that bedside ultrasound is accurate within 4 millimeters of CT measurements with respect to AAA. Bedside ultrasound is a safe and effective diagnostic imaging modality that can be performed in under 5 minutes. More significantly, its use has decreased mortality by 20% to 60%.
Anatomy and Physiology
The retroperitoneal abdominal aorta enters the abdomen via the aortic hiatus caudal to the xiphoid process. It rests anterior to the vertebral body and parallel to the inferior vena cava. Extending about 1 to 2 centimeters below the umbilicus, the aorta divides into the common iliac arteries at the level of L4. The aorta diminishes in size as it descends through the abdominal cavity, moving more superficially as well. As it moves caudally, the aorta has consecutive arterial branches: the celiac, superior mesenteric, renal, gonadal, and the inferior mesenteric.
An aneurysm is classified as a focal dilatation greater than 50% of a vessel’s normal diameter. A diameter greater than 3 centimeters demarcates an AAA. Two categories of AAAs exist: fusiform and saccular. The majority are fusiform. Fusiform aneurysms expand circumferentially. Whereas saccular aneurysms are localized outpouchings, often secondary to an infectious etiology.
Approximately 90% of AAAs occur infrarenal, although the renal vessels are often difficult to image with a point of care ultrasound. Scan to where the aorta bifurcates to confirm visualization of the aorta in its entirety.
Who to Scan?
It is important to remember that less than 25% of individuals present with the classic triad of hypotension, abdominal pain, and a pulsatile abdominal mass.
Consider sonographic assessment of the abdominal aorta in the following instances:
- Greater than 50 years old with one of the following: chest, abdominal, flank, groin, or back pain; renal colic; hematuria; or hydronephrosis
- Cardiac Arrest
- Thromboembolic events to the lower extremities
- Neurologic deficit of the lower extremities
Expert consensus regarding sonographic screening for AAAs in asymptomatic individuals includes the following.
The United States Preventative Services Task Force (USPSTF) & the American Academy of Family Physicians (AAFP) recommendations:
- Men greater than 65 years who ever smoked
Society for Vascular Surgery recommendations:
- All men age greater than 65
- Men greater than 55 with a family history of AAA
- Women greater than 65 with family history of AAA or who have ever smoked
- Greater than 50 years old
- Family history of AAA
- Coronary artery disease
- Diabetes mellitus
- Peripheral arterial disease
To begin, place the transducer below the xiphoid process in the transverse alignment. Perform a complete scan through the bifurcation of the aorta around the level of the umbilicus. Several consecutive videos may be necessary to image the entire aorta. Measure the abdominal aorta at its maximal diameter to include each outer wall. Dimensions are most precise with the probe positioned completely perpendicular to the aorta. Perform a similar scan in the sagittal view with the transducer position towards the patient’s head.
Be sure to include any visible thrombus. Thrombus will appear as an echogenic substance within the aorta. However, it is easily overlooked. Generally, it is best visualized along the anterolateral wall and may create a false lumen that underestimates the actual extent of an aortic aneurysm.
In unstable patients, providers should routinely complete a right upper quadrant ultrasound to evaluate for pathologic fluid. The majority of AAAs rupture in the retroperitoneum (70% to 90%), where ultrasound cannot assess routinely. Nevertheless, one study established a sensitivity of 97% for identifying ruptured AAAs when point of care ultrasound was combined with clinical gestalt.
Lastly, sonographic assessment of the aorta includes assessing for the occurrence of an undulating intimal flap, the pathognomonic finding that is 100% specific for aortic dissections. Strict blood pressure management and emergent surgical consultation are mandated.