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The Speed of Sound

The Speed of Sound

Ultrasound's Power to Evaluate Complications of Dissection

Beran, David DO; Harrell, Lauren MD; Butts, Christine MD

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doi: 10.1097/01.EEM.0000466615.93370.f4
    Figure. C
    Figure. C:
    T angiogram demonstrating dissection at the arch of the aorta.
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    Figure

    If you'll remember, last month we discussed a 69-year-old woman who presented with extremity weakness in her left arm and right leg, and was subsequently found to be hypotensive. Initial evaluation in the ED revealed a normal head CT but a widened mediastinum on chest x-ray. Bedside ultrasound demonstrated that an intimal flap was present to the aorta. When we left the case last month, the patient was en route to radiology for an emergent CT angiogram of the aorta.

    The patient's CTA revealed a complex type A aortic dissection. The intimal flap extended into her left carotid, left subclavian, and right innominate arteries, and traveled distally through the abdominal aorta to occlude her right common femoral artery. Once these findings were discovered, she was immediately consented for emergent surgical repair.

    This case demonstrates some of the pitfalls in diagnosing acute aortic dissection. Classically, acute aortic dissection is described as a sudden tearing or ripping chest pain that is greatest at onset. Most patients are described as being hypertensive and having pulse deficits on presentation, but few patients actually present with this classic picture. Aortic dissection can be a great mimicker of other conditions depending on the anatomic distribution of the vessels affected. Patients can present with syncope, abdominal or flank pain, shortness of breath, neurologic symptoms resembling an acute CVA, or findings consistent with an acute ST segment myocardial infarction.

    Hypotension is an atypical finding in patients with aortic dissection, and is a red flag for complications. This patient's case provides us with the opportunity to review the possible etiologies of hypotension in dissection.

    Beginning proximally, a type A dissection can cause acute aortic regurgitation by involving the aortic valve. Acute aortic regurgitation causes an abrupt increase in left ventricular end-diastolic volume and subsequently causes acute heart failure, pulmonary edema, and cardiogenic shock. If the dissection involves the pericardium, pericardial effusion and resulting tamponade can cause profound hypotension. A dissection involving the ostia of the coronary arteries can also lead to acute myocardial ischemia and acute heart failure. Rupture is another serious and life-threatening cause of hypotension in aortic dissection. Increased shearing forces can lead to dissection through the adventitia and result in exsanguinations into the thoracic and abdominal cavities.

    Figure. Vie
    Figure. Vie:
    w of the aortic valve demonstrating regurgitation.
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    Figure:
    Reconstructed CT angiogram demonstrating the extensive nature of the dissection.
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    Figure:
    Parasternal long axis view demonstrating presence of a pericardial effusion. The free wall of the right ventricle (arrow) can be seen to be bowed inward, indicating mass effect and possible tamponade.

    Diagnosing acute aortic dissection remains challenging in the ED because its presentation is often atypical. CT angiography is considered the test of choice, but bedside ultrasound can be used, as it was in this case to help confirm the diagnosis once it is considered. The abdominal aorta can be visualized from the xiphoid process to its bifurcation at approximately the umbilicus. It should be less than 3 cm with a smooth inner wall. The thoracic aorta can be more difficult to evaluate by ultrasound because of the surrounding lung, but it can be examined at its root (using the parasternal long axis cardiac view), at its arch (using the suprasternal view), and in a portion of its descent (in the apical cardiac view). These views should be examined for enlargement of the aorta and the presence of an intimal flap.

    Bedside ultrasound also has great power to evaluate for complications of dissection, most notably pericardial effusion and acute aortic regurgitation. An effusion can be quickly identified using standard cardiac windows (subxiphoid or parasternal views). Signs of tamponade include right atrial or ventricular collapse during diastole and IVC dilation from elevated right atrial pressure. Using the parasternal long axis or apical views, color Doppler can be applied to the aortic valve to assess for regurgitation.

    The patient in this case underwent an extensive repair of her aorta. She was transferred to the ICU in critical condition, and was weaned off mechanical ventilation over the next several weeks. She did well, and was discharged to a rehab facility.

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