By Michelle Hunter-Behrend, MD and Laleh Gharahbaghian MD, FACEP
Abdominal aortic aneurysms (AAA) are relatively common amongst older men, with a prevalence of 12.5% for men 75-84 years old, decreasing in prevalence with decreased age, and are uncommon in women.1 When ruptured, the mortality rate is high, between 50-80% for those who survive to the hospital making the diagnosis of ruptured AAA time sensitive. Early detection and diagnosis in the emergency department can decrease mortality from 75% to 35%.2 Although the gold standard for diagnosis is CT angiogram, it is not appropriate for the unstable patient. Unfortunately, with physical exam alone, a ruptured AAA is misdiagnosed in 32% of patients, usually as ureteral colic and myocardial infarction, with only 61% of patients presenting with the classic triad of abdominal pain, hypotension, and a pulsatile mass.3
Fortunately, the use of point-of-care ultrasound (POCUS) quickly identifies AAA with sensitivity and specificity approaching 99% and 98% respectively.1 Furthermore, emergency department POCUS may also identify aortic dissection (AD). When combining transthoracic and transabdominal POCUS to assess for an intimal flap, POCUS has a sensitivity of 67-80% and specificity of 99-100% for aortic dissection.4 Using transthoracic echocardiography views, sections of the thoracic aorta can be visualized including the aortic outflow tract (parasternal long view), and parts of the descending thoracic aorta (parasternal long and short, apical views).
Here we will review how to optimize acquisition and interpret views of the thoracic and abdominal aorta to aid in diagnosis of AAA and AD.
- Evaluate the entire aorta. While you should begin your aortic ultrasound just below the subxiphoid process, evaluation of the suprarenal aorta is not a complete exam. The suprarenal aorta may be normal in caliber, while a large AAA exists at the infrarenal aorta (Figure 1 and 2). A complete exam includes evaluation of both the suprarenal and infrarenal aorta down to the bifurcation into the iliac arteries.
Clip 1: https://youtu.be/3Grz0c0s0mA
- Get the gas out of the way. The most frequent difficulty encountered in obtaining abdominal aortic views is navigating past the bowel gas. Using a probe with a wider footprint such as the curvilinear probe may help with displacement of bowel gas. In addition, applying firm, slow, direct pressure by pressing down with the probe on the patient’s abdomen helps displace the bowel to allow visualization of the aorta.
Clip 2 - https://youtu.be/4_dEOZBIwc8
- Know your IVC. The inferior vena cava (IVC) and aorta run parallel to each other, thus the IVC may be mistaken as the aorta. The aorta will lie to the patient’s left of the IVC, just anterior to the vertebral bodies. Ensure that you are evaluating the correct vessel by verifying the screen dot and probe indicator are oriented in the correct location. (Figure 1)
- Measure outer to outer wall. One common pitfall is measurement of the inner lumen of the aorta. Unfortunately, a clot within the lumen may look like the aortic wall, and thus the inner measurement of the wall can underestimate the correct aortic size. A correct measurement is obtained by placing calibers from outer wall to outer wall of the aorta. Abdominal aortic aneurysm is considered a measurement greater than 3 cm whereas iliac artery aneurysms are greater than 1.5 cm. (Figure 3)
- The Right Upper Quadrant (RUQ) view of the aorta. If all attempts at visualizing the aorta from the patient’s midline have failed, consider using the RUQ view of the FAST scan to visualize part of the aorta. Place the probe in the right mid-axillary line using the liver as your acoustic window to visualize a longitudinal view of the aorta. Unfortunately, only the superior aspect of the aorta is seen in this view. (Figure 4)
Clip 3 - https://youtu.be/W1psN3yF3js
- Don’t just look for size; look for the flap. When performing an assessment for AAA, do not conclude the exam after determining that no AAA exists. In addition to evaluating aortic size, the lumen of the aorta should also be evaluated for a dissection flap. (Figure 5)
In assessing for aortic dissection, thoracic views of the aorta should be obtained in addition to abdominal views. Thoracic views should include the traditional transthoracic echocardiography views of the heart. In the parasternal long view of the heart, the aortic outflow tract may be visualized to assess for a mobile intimal flap as well as aortic size (Figure 6). Measurement of the ascending aorta should be done during diastole with the calibers at the level of the sinuses of Valsalva. An ascending aortic aneurysm at this level is considered with measurements greater than 4.5 cm.5 Remember, a pericardial effusion should also heighten suspicion of an ascending aortic dissection in the appropriate clinical scenario. In addition, the descending thoracic aorta may also be visualized and measured in the parasternal long view, seen just posterior to the mitral valve. Similar to the abdominal aorta, a descending thoracic aorta measurement of greater than 3 cm is considered aneurysmal.
Clip 4 - https://youtu.be/GAoEKrW8LgI
In conclusion, ruptured AAA and ADs require prompt diagnosis and treatment to reduce the high morbidity and mortality associated with these conditions. Emergency medicine physicians should be familiar with performing these studies in the appropriate patients.