Identifying a thoracic aortic aneurysm using ultrasound

Learn how to identify the landmarks and criteria for diagnosing a thoracic aortic aneurysm.

David Mackenzie MDCM FRCPC FACEP
David Mackenzie MDCM FRCPC FACEP
5th Jul 2018 • 4m read

Learn how to evaluate vascular structures and make diagnoses using ultrasound. In this video, from our POCUS Masterclass, you'll learn how to identify the landmarks and criteria for diagnosing a thoracic aortic aneurysm.

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Video Transcript

[00:00:00] We already know about the evaluation of the abdominal aorta for aneurysm. In this lesson, we'll turn our attention to the thoracic aorta and see how we can use ultrasound to evaluate for dilation or aneurysm. Let's say you're working in a small emergency department, in the middle of the night. You obtain a chest x-ray for a patient with shortness of breath and history of sweats, maybe a fever, and you see this. Is it a pneumonia or something else? What's your next move? On the differential diagnosis

[00:00:30] for this chest x-ray, is a thoracic aortic aneurysm. You want to know if this is indeed an aneurysm, as it could entirely change how you manage the patient, if they have an enlarged aorta. In this lesson, we'll see how to make the diagnosis. We can use ultrasound to answer a targeted question to determine if there is an aneurysm or with the knowledge of how to recognize the appearance of an aneurysm, we can be ready to spot a thoracic aortic aneurysm, if we perform an echo for another reason. So, let's see how we go about evaluating the thoracic

[00:01:00] aorta. We typically perform our evaluation using the cardiac transducer. The evaluation of the thoracic aorta typically starts with the parasternal long-axis view of the heart. Recall that in a parasternal long-axis view, we expect to see about 1:1:1 ratio of the right ventricle, aortic root, and left atrium as we see here. A change in this ratio can be a sign that you have an abnormal size of a chamber or the aorta. Here we see the aortic root is not in a 1:1 ratio with the right

[00:01:30] ventricle and left atrium. Sometimes, this may be an incidental tip-off that there is a thoracic aortic aneurysm. Here, you can see in the clip, that there is an enlargement of the aortic root. In order to diagnose thoracic aortic dilation or aneurysm, the first measurement should be made at the aortic root, where the aorta leaves the heart. There are different structural levels at which we can measure the aortic root, in the parasternal long-axis. There's the level of the aortic annulus seen here, the sinus of Valsalva, the sinotubular

[00:02:00] junction and the ascending aorta. Complete evaluation will include a measurement at each of these levels. You can see in this patient, that the diameter varies with the level you're measuring. Notice how we have different measurements that are obtained depending on where we sample and obtain our measurement. The cutoffs that we used to diagnose aortic dilation or aneurysm also vary with patient's sex and body size. In normal anatomy, the values are largest at the sinus of Valsalva and taper slightly toward the sinotubular

[00:02:30] junction. But in general, anything greater than 4 cm is abnormally dilated and above 4.5 cm is consistent with an aneurysm. These are the key numbers to keep in mind. The measurement technique is as follows: we measure the aortic root in the parasternal long-axis window, using a leading edge to leading edge approach as you see here. This is different from the outer wall to outer wall that we use for the abdominal aorta. This is a measure taken at the level of the sinuses of Valsalva.

[00:03:00] Here's a real live example. This is a parasternal long-axis image. Notice how the aortic root appears larger than the right ventricle or the left atrium. It's a little bit greater than that 1:1:1 ratio that's normal. Now, we've gone ahead and measured the length and we see that it's 5.1 cm. This is diagnostic of a thoracic aortic aneurysm. We can refer the patient for appropriate follow-up care. There are other approaches you can use to look at the aorta. For instance, don't forget to look at the descending

[00:03:30] thoracic aorta in your parasternal long-axis view. As you see here, this can be the level where you'll notice the dilation. Here's another example. Notice that descending thoracic aorta. It's enlarged and also has some mural thrombus in it. Again, this is consistent with a thoracic aortic aneurysm. In addition to evaluating the descending thoracic aorta in the parasternal long-axis window, we can also use the suprasternal approach to measure the diameter and evaluate for thoracic aortic aneurysm. By looking in this window, you may

[00:04:00] see an aneurysm or enlargement that's not evident in the other windows. But our core view is going to be the look at the root, as it exists the heart in the parasternal long-view. Think of it as the exit in your evaluation of the other E's: effusion right ventricle size and ejection fraction. Remember that point of care echo is not perfect. Other modalities such as computed tomography or magnetic resonance imaging, along with transesophageal echo, are more sensitive for thoracic aortic aneurysm.

[00:04:30] But POCUS is very specific for thoracic aneurysm. If you see it, you can factor this into your differential diagnosis for the presenting complaint or counsel the patient and get them to the appropriate follow-up.