Ultrasound For Abdominal Aortic Aneurysm
Ultrasound For Abdominal Aortic Aneurysm
Abdominal aortic aneurysms (AAA) are a leading cause of death in the United States. Abdominal aortic aneurysms occur when the walls of the aorta weaken due to atherosclerosis and hypertension. They are most common in males over the age of 50, especially those with a history of smoking or hypertension. Initially, AAAs are asymptomatic, but with expansion, they classically present as pain in the abdomen, flank, or back, and the risk of rupture is related to the size of the aneurysm. When it occurs, rupture results in the above symptoms or presents as syncope, dizziness, hypotension, pulseless electrical activity, or cardiac arrest. Emergent surgical repair is often the only possible life-saving measure, so rapid identification of AAAs is a critical skill for all emergency clinicians.
Physical examination is the fastest way to determine whether a patient has an AAA. Traditionally, the finding of a pulsatile abdominal mass is associated with an AAA; however, both the sensitivity and the specificity of the physical examination are relatively poor. In 1999, a meta-analysis of 15 studies was carried out to see how accurate clinicians are in detecting AAAs on physical examination.50 The study found that pooled sensitivities for AAA increased with the size of the aneurysm, from 29% for AAAs of 3.0 to 3.9 cm, to 50% for AAAs of 4.0 to 4.9 cm, and to 76% for AAAs of 5.0 cm or larger.
Review Of The Literature
In many ways, ultrasound is the ideal tool for diagnosing AAAs. (See Figures 6 and 7.) It offers the same benefits as for other applications, in that it is rapid, accurate, noninvasive, inexpensive, and reproducible; it involves nonionizing radiation; and it can be done at the patient’s bedside. This is particularly important in the unstable hypotensive patient who cannot travel to the CT scanner. The use of ultrasound to assess for AAAs was described in the literature as early as 1977. Goldberg described its use in detecting a normal aorta or an AAA and in defining the anatomy and extent of disease.51 Since then, there has been considerable research on the role of bedside ultrasound in the assessment of AAAs.
As for many applications of ultrasound, some reports have focused on the ability of emergency clinicians to accurately perform bedside ultrasound and to interpret the results. Dent et al described scans done by emergency physicians to assess for AAAs and found a sensitivity of 96% and a specificity of 100%.52 Costantino et al compared ultrasound performed by emergency physicians to a gold standard of CT, magnetic resonance imaging (MRI), or operative findings. In 238 patients, they found a sensitivity of 94% and a specificity of 100%.53 Similarly, in a study involving 104 patients, Knaut et al found good agreement between measurements of the aorta made with ultrasound by emergency physicians and those made with CT by radiology staff.54 The time needed to perform ultrasound was less than 5 minutes in the majority of cases. Tayal et al prospectively studied the accuracy of emergency physician–performed ultrasound compared with radiology staff–performed ultrasound, CT, MRI, or laparotomy over a 2-year period.55 They found that in 125 patients, the sensitivity and specificity were 98% and 100%, respectively, for emergency physician-performed studies.
As a screening tool, ultrasound is an effective way to identify AAAs that may require treatment. The Multicentre Aneurysm Screening Study (MASS) prospectively evaluated mortality and the cost-effectiveness of ultrasound screening in asymptomatic men in a large outpatient program. The study enrolled 67,800 males ages 65 to 74 who were randomized to receive or not receive a screening ultrasound. Subjects with positive results were followed up or referred for surgery. There was a 53% relative reduction in aneurysm-related death in the group that was screened when compared with the unscreened control group.56 In addition, short-term (30-day) mortality was 6% after elective aneurysm repair vs 37% after emergency surgery. A secondary analysis demonstrated that this screening was cost-effective.57 A 2007 Cochrane review of 4 studies confirmed a mortality reduction in men but stated that the evidence is not sufficient to prove the same in women.58
Few studies have examined the screening of asymptomatic patients in the ED. Moore et al studied 179 patients who presented to the ED with unrelated complaints.59 They found an incidence of AAA of 6.7%, and of the 12 patients with AAA, repair was recommended in 3. As in other studies, emergency physician-performed ultrasound was fast and accurate. Since mass screening has been shown to reduce mortality, some third-party payers have, over the past few years, approved reimbursement for screening at-risk patients. This practice has not yet been adopted in EDs, but theoretically could be if the appropriate equipment, documentation, and follow-up systems were in place.
Although ultrasound is an excellent tool for detecting AAA, its ability to differentiate between a ruptured and a nonruptured aneurysm is limited. Free fluid seen on a FAST examination certainly could represent blood from a ruptured aneurysm, but the majority of AAAs rupture into the retroperitoneum and thus are more challenging to detect on a FAST examination. Sensitivity may be improved with contrast-enhanced sonography. Ultrasound contrast solutions contain microbubbles that, when injected, flow within vessels and capillaries and better delineate their anatomy by “lighting up” the image. Recent reports have described the use of contrast-enhanced ultrasound in detecting aortic aneurysmal rupture.60,61 Evidence is limited to a case series, however, so the ability to definitively detect or rule out aortic aneurysm rupture is unclear.
Finally, ultrasound is also able to detect another aortic emergency: aortic dissection. When blood dissects into the tunica media and the tunica adventitia, an intimal flap can be seen moving within the aorta. Two case series of patients in the ED describe the use of ultrasound in detecting aortic dissection.62,63 While the exact sensitivity and specificity of emergency clinicianperformed echo for aortic dissection have not been studied, ultrasound is likely specific when a flap is seen, but not sensitive, as complete evaluation of the aorta requires multiple views and can be difficult.
Bedside ultrasound scanning for AAA is a high-yield application with high sensitivity and specificity that can be easily learned by emergency clinicians. Rapid diagnosis of an AAA in the unstable patient can be life-saving. Though this conclusion has not been confirmed in the literature, it would be difficult to perform a randomized controlled clinical trial to demonstrate it. Bedside ultrasound is not a sensitive test to differentiate between ruptured and unruptured aortic aneurysms, and in stable patients a CT scan is the test of choice.
James Q. Hwang; Heidi Harbison Kimberly; Andrew S. Liteplo; Dana Sajed
March 2, 2011