Introduction

An abdominal aortic aneurysm (AAA) can be a challenging diagnosis to make and a deadly diagnosis to miss. Patients may be asymptomatic until rupture, or they may have vague complaints such as chronic abdominal or back pain. Frequently, these symptoms are misdiagnosed as less deadly processes, such as renal colic.^1,2 Once ruptured, time to diagnosis is the biggest determinant of survival.³ Commonly relied on methods of diagnosis, such as computed tomography (CT), may delay care and result in patient decompensation. Bedside ultrasound has emerged as a powerful tool in the diagnosis and disposition of patients with a suspected AAA or rupture of an AAA. When used properly, bedside ultrasound improves the time to diagnosis and survival.⁴

What We are Looking for

Because the signs and symptoms of an AAA may be vague and nonspecific, bedside ultrasound of the aorta is indicated in the following clinical scenarios: abdominal pain, back pain, flank pain, unexplained hypotension, syncope, cardiac arrest, or known history of an aneurysm.^5,6 The goal of bedside ultrasound is to measure the size of the abdominal aorta and exclude the presence of an AAA. When rupture of an AAA is suspected, the peritoneum should also be evaluated with focused abdominal sonography for trauma (FAST) to search for free fluid.

Supporting Evidence

A large amount of research has confirmed bedside ultrasound to be a useful and lifesaving tool in the emergency department (ED). Tayal et al. conducted a prospective study of the accuracy and outcome of bedside ultrasound for the diagnosis of AAA by emergency physicians. They evaluated 125 patients suspected of having an AAA with bedside ultrasound. Twenty-nine patients were found to be positive for AAA, for a positive predictive value of 93% and a negative predictive value of 100%. The sensitivity was 100% with a specificity of 98% for 10 of the 27 patients found to have an AAA, and disposition was immediate to the operating room without a confirmatory study.^3,7

Another emergency medicine study performed in 2005 evaluated 238 patients who arrived at the ED with symptoms suggestive of a ruptured AAA. Third-year emergency medical residents, trained according to guidelines from the American College of Emergency Physicians, performed all ultrasound examinations. Thirty-six aortic abnormalities were diagnosed with a sensitivity of 100% and specificity of 100% for this end point in comparison with “gold standard” diagnostic testing.⁸

Knaut et al. assessed the accuracy of measurements taken by emergency physicians via ultrasound versus measurements taken by CT. They found that in all cases, their measurements approximated those found on CT within 1.41 cm or less.⁹

Plummer et al. randomized patients to ultrasound versus standard-of-care diagnostics and compared time to diagnosis and to the operating room. Ultrasound improved time to diagnosis (5.4 versus 83 minutes) and improved time to disposition for patients requiring operative intervention (90 versus 12 minutes).⁴

Scanning Protocols

Bedside ultrasound of the aorta can be completed in 2 to 3 minutes. To rule out the presence of an aneurysm, the entire length of the abdominal aorta from the diaphragmatic hiatus to the aortic bifurcation should be evaluated in both the longitudinal and transverse planes. A low-frequency transducer should be selected for this examination. A 2- to 5-MHz curvilinear transducer will be the best choice for most patents because it offers adequate depth and resolution. A phased-array or microconvex transducer can also be used.

The examination begins just below the patient’s xiphoid process. With the indicator pointing toward the patient’s right, the transducer should be facing straight down to the patient’s back (Fig. 67.1). Although the aorta may be identified immediately, some effort is frequently required to orient the sonographer to the anatomy. The easiest landmark to identify initially is the vertebral body. It is seen as a hyperechoic (white) arc casting a dark acoustic shadow, usually near the bottom of the screen. Just above the vertebral body, two circular, anechoic (black) vessels should be seen (Fig. 67.2