An aortic dissection begins with a tear in the intimal layer of the aorta. The tear allows blood to enter and separate the intima from the media or adventitia, creating a “flap” of tissue separating the true aortic lumen from the false lumen. Thoracic aortic dissections are commonly classified by either the Stanford or DeBakey systems (Fig. 17.2). While the DeBakey system is based upon the location and extent of the tear, the Stanford system is based upon whether the ascending aorta either is (type A) or is not (type B) involved. Aortography was historically considered the “gold standard” for diagnosis of aortic dissection, but it is very rarely used today due to its invasiveness and lower sensitivity than other modalities. Over the past 20 years, computerized tomography (CT) has become the primary imaging modality for diagnosis, being the first-line choice in over 70% of cases in the past 5 years (5). Even if surgical intervention is initiated based upon CT findings, intraoperative TEE still plays an essential role in confirming the diagnosis and helping to guide surgical repair.
Important aspects for the intraoperative echocardiographer to address during the examination are summarized in Table 17.1 (6).

The hallmark finding of an acute aortic dissection on TEE is the visualization of the intimal flap. It is usually very irregularly shaped and highly mobile (Video 17.4). Visualizing the flap in two separate imaging planes (Fig. 17.3) is important in order to distinguish artifacts from true dissection flaps. One such confounder is the presence of a pulmonary artery catheter, which can create reverberation artifacts mimicking a flap (Fig. 17.4). When in doubt, this can be remedied by pulling the catheter back and seeing if the “flap” remains. Other sources of linear artifacts include calcifications on the aortic valve or root, and
atherosclerosis of the ascending aorta (7). They can typically be distinguished from a true dissection by their lack of rapid, oscillatory movement, their tendency to cross known anatomic boundaries, and their indistinct structural borders.

Once the dissection flap is visualized, it is useful to determine where the break in the intima (aka the “tear” or “entry point”) has occurred, since one of the primary aims of surgical treatment is to excise this region.
Color flow Doppler is often used to visualize a turbulent jet flowing from the true lumen into the false lumen (Fig. 17.5). This separation, or “margination,” of flow can also be helpful in distinguishing a true dissection from an artifact which has no margination of color flow Doppler. The true lumen can usually be differentiated from the false lumen due to its expansion during systole (Video 17.5). Additionally, the false lumen is often larger, and frequently has spontaneous echo contrast or thrombus within it, particularly in the descending aorta (8).

While the initiating event for the classical aortic dissection is a break in the intimal layer, the underlying cause of an IMH is thought to be a rupture of vasa vasorum in the medial layer (9). This blood accumulation causes a thickening of the medial layer which may progress to intimal fracture, and subsequent flap formation like a classic aortic dissection, or frank aortic rupture. The mortality from IMH is dependent on the location (i.e., ascending or descending aorta) and is similar to that of classic aortic dissection (10). As such, the Stanford classification system is applied, with type A considered a surgical emergency.

Unlike the classic aortic dissection, there is no intimal flap. On TEE imaging, IMH appears as a thickening of the medial layer of the aorta (Fig. 17.6 and Video 17.6). This typically measures 7 ± 2 mm in type A cases, and 15 ± 6 mm in type B (11). Other features may include echolucency within the medial layer, and medial displacement of calcium in the intimal layer. It is important to note that atherosclerotic plaques appear above the intimal layer, while IMHs occur below the intimal layer.

Of the acute aortic syndromes, penetrating atherosclerotic ulcer (PAU) is the least common, accounting for somewhere between 2% and 7% of cases (12). While most atherosclerotic disease protrudes above the intimal layer into the vessel lumen, PAU consists of plaque that has eroded through the internal elastic lamina into the aortic media. No dissection flap is created, but the localized erosion of the medial wall can lead to aneurysm formation or even aortic rupture. Like IMHs, PAUs of the ascending aorta are often considered for emergent surgical intervention due to the risk of rupture (13). However, most PAUs occur in the descending aorta, where there is controversy among advocates of stent placement, frequent monitoring, or open repair (14,15).

No dissection flap or false lumen is visualized on TEE in cases of PAU. Instead, there is a crater-like ulcer with jagged edges and possible echolucent areas within the plaque (Fig. 17.7). The erosion into the medial layer can make it difficult to clearly see the exact borders of the aortic lumen. There are typically extensive surrounding areas of atherosclerotic disease, and adjacent effusions or fluid collections are common.

Following repair of an ascending aortic dissection, efforts should be made to identify both the proximal and distal ends of the graft. Most synthetic grafts are made from either a polyester fiber (e.g., Dacron) or polytetrafluoroethylene (PTFE) (16). On TEE, these materials can usually be distinguished from native tissue by their striated appearance (Fig. 17.8). Interrogation of the aortic valve should be performed as per usual practice following any valvular intervention. It should be noted, however, that prosthetic valves or resuspended native valves are typically sewn within the aortic graft.

Dilatation of the thoracic aorta can occur due to a variety of clinical conditions including altered flow dynamics, underlying connective tissue disorders, and atherosclerosis. It is therefore useful to provide certain measurements of the aortic root and ascending aorta as part of a complete examination, which are illustrated in Figure 17.9. Typical values for these measurements are provided in Table 17.2 (17,18,19), although the size of a “normal” aorta will depend upon age, sex, and overall body surface area (20). Aortic measurements exceeding the values in Table 17.2 by more than 50% are considered aneurysmal. Aneurysms can be broadly divided into either saccular, meaning a focal outpouching, or fusiform, meaning cylindrical and affecting the entire circumference of the aorta.