4.2.1 Aortic Dissection
Aortic dissection occurs generally in patients with risk factors: age, hypertension, disease of fibroelastic tissue (Marfan syndrome), congenital disease (bicuspid valve, coarctation) or pregnancy. Iatrogenic or traumatic causes are less frequent. The dissecting process has generally an anterograde longitudinal extension, but retrograde dissection is possible. Circular extension involves partial or complete wall circumference. Two clas-
sifications may be used. De Bakey classification presents three types. In type I, the ascending segment is at least dissected with variable extension to horizontal and descending parts. In type II, dissection is only localized in the ascending aorta. Type III consists in an involvement of the descending thoracic aorta (IIIa) with possible extension to the abdominal aorta (IIIb). Stanford classification is now currently used. Type A defines dissections with, at least, the involvement of the ascending aorta, whatever the extension or the site of intimal tear. Type B dissection respects the ascending aorta. It concerns essentially the descending aorta with possible antero-grade extension to the abdominal segment or retrograde to the horizontal aorta [5, 7].
184.108.40.206 Intimal Flap
Dissection consists in a cleavage of the medial layer of the aortic wall. It leads to formation of the intimal flap, which really corresponds to the association of the inti-mal layer and two thirds of media. The intimal flap represents the first echocardiographic sign of aortic dissection (Fig. 4.1) . On TEE it appears as a linear intra-luminal, thin and mobile echo that divides the aortic lumen in two parts: true and false lumens. There is a correlation between flap mobility and mortality.
The intimal tear corresponds to the beginning of the dissecting process. Its detection is determinant to choose the time and type of treatment. From this point, dissection has generally an anterograde progression but retrograde extension is possible. The tear appears as a discontinuity of the intimal flap. A multiplane TEE probe provides a direct image of the tear and allows its measurement (Fig. 4.2). Color Doppler imaging enhances detection of the tear and can reveal the presence of other multiple small communications between the two lumens, especially in the descending aorta. They appear as thin color flows without a direct image of the tear (Figs. 4.3, 4.4). Anatomical controls showed that these images might correspond to the origin of intercostal arteries . Intravenous infusion of an echographic contrast agent can detect them with good accuracy (Fig. 4.5). There is a positive relation between flap mobility and the site of the tear. Increased mobility is observed around the tear and helps to localize it. Pulsed Doppler imaging flow velocities through the intimal tear reflect the pressure gradient between the two lumens (Fig. 4.6). During a cardiac cycle, the velocity profile may have a mono-, bi-, tri- or quadriphasic aspect .
Fig. 4.3. Dissection of the descending aorta. Color Doppler flow through two intimal tears. a Towards the false lumen (FL) in the systole. b Towards the true lumen (TL) in the diastole
Fig. 4.4. Dissection of the descending aorta. Color Doppler flow of small communications (arrows) between the true lumen (TL) and the false iumen (FL) that might correspond to ostia of intercostal arteries
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