Abstract
Chronic diseases of the aorta, especially aneurysms, are typically asymptomatic and are often only diagnosed when detected incidentally on imaging studies, such as a transthoracic echocardiogram (TTE), ordered for other indications. It is therefore essential that the sonographer performing a TTE make an effort to image each segment of the aorta that can be visualized by ultrasound. It is also imperative the echocardiographer interpreting the TTE examines these aortic segments, measures diameters carefully, and highlights any abnormalities in the TTE report. Aneurysms of the aortic root and/or ascending aorta can lead to leaflet tethering and incomplete aortic valve closure that, in turn, can result in significant aortic insufficiency (AI). Identifying aortic dilatation as the cause of AI can help cardiologists and cardiac surgeons better predict the ability to correct the AI with aortic replacement surgery (with a prosthetic tube graft with sparing of the valve rather than with aortic valve replacement). When patients present with symptoms or signs that raise suspicion of an acute aortic syndrome, transesophageal echocardiography (TEE) is highly sensitive in diagnosing aortic dissection and intramural hematoma. Moreover, in the setting of an acute aortic dissection, TEE can also diagnose important complications, such as aortic insufficiency, coronary ostial involvement, and the presence of hemopericarium. Finally, in patients who suffer distal arterial embolization, TEE can identify the presence of protruding atheromas with mobile components, aortic mural thrombus, and, rarely, intraluminal aortic tumors.
Keywords
acute aortic syndromes, aortic aneurysm, aortic dissection, echocardiography, intramural hematoma, penetrating aortic ulcer
Anatomy, Nomenclature, and Dimensions
The thoracic aorta is divided into four segments ( Fig. 34.1 ), both because of anatomical distinctions and because they are affected differentially by conditions that affect the aorta. The aortic root is the most proximal segment, extending from the annulus of the aortic valve and extending to the sinotubular junction (STJ); and the aortic root is composed of the right, left, and noncoronary sinuses of Valsalva. The ascending thoracic aorta is tubular and extends from the STJ to the ostium of the innominate (brachiocephalic) artery. Many cardiologists and radiologists refer to the entire proximal aorta, both the root and ascending aorta, as the “aortic root.” However, this is a misnomer, as only the segment below the STJ is truly the “root,” and using the correct nomenclature is critical to communicate accurately and effectively regarding a given patient’s aortic pathology. Both the aortic root and ascending aorta lie within the pericardial space, which means that the ascending aorta can be surrounded by pericardial fluid in the setting of an effusion; it also means that rupture of the ascending thoracic aorta can cause cardiac tamponade. The aortic arch extends from the proximal ostium of the innominate artery to just beyond the left subclavian artery at the ligamentum arteriosum. The normal aortic arch gives rise to the innominate (also known as brachiocephalic) artery, left common carotid, and left subclavian arteries. In a minority of patients, the innominate and left common carotid arteries arise as a common trunk in a conformation known as a “bovine arch.” The descending thoracic aorta begins just distal to the origin of the left subclavian artery, courses distally under the pleura and just to the left of the vertebral column, and extends to the crux of the diaphragm. The abdominal aorta extends from the diaphragm to the aortic bifurcation; its proximal and distal portions are referred to as the suprarenal and infrarenal abdominal aorta, respectively.
Aortic Measurements and Dimensions
The diameter of the aorta decreases as it moves distally, and therefore each segment has a different range of normal diameters. Because the aorta is not a straight tube running from cephalad to caudal, axial imaging (computed tomography [CT] and magnetic resonance imaging [MRI]) often cuts the aorta obliquely, leading to an overestimate of its true diameter ( Fig. 34.2 ). Even on echocardiography, the aorta can be imaged obliquely, so one must be careful to ensure that the aorta is measured along the axis perpendicular to its long axis (the axis of blood flow).
The aortic root diameter is typically measured in the parasternal long-axis (PLAX) view from the right-coronary sinus to the opposite sinus of Valsalva (usually the noncoronary sinus). It is occasionally difficult to obtain a long-axis image that shows both sinuses of Valsalva, in which case, the root may be measured more accurately in the parasternal short-axis view. However, there is not universal agreement about the optimal landmarks from which to measure the root diameter in short axis. Some experts advocate taking the diameter from the right coronary sinus to the opposite commissure (between the left- and noncoronary sinuses), whereas others advocate measuring to the more posterior of the opposite sinuses ( Fig. 34.3 ); that latter method generally leads to a diameter measurement about 2 mm larger than the former. We prefer the sinus-to-sinus approach, as it most closely approximates the diameter measurement made in the parasternal view, and it reflects the true maximal diameter that dictates maximal aortic wall stress according to the law of Laplace.
Measurements of the aortic root and ascending aorta are typically performed at end diastole, as this phase of the cardiac cycle demonstrates the resting aortic diameter; since the aorta is elastic, measurement at end systole can be several millimeters larger, especially in young people, as the aorta is actively distended by peak systolic pressure. In contrast, the aortic annulus is typically measured in mid-systole.
Historically, aortic root measurements had been made using M-mode echocardiography, and over several decades, multiple clinical and epidemiologic studies have used the M-mode leading-edge to leading-edge method. Multiple guidelines have reported normal limits based on this method and, consequently, the American Society of Echocardiography has also recommended using the leading-edge to leading-edge approach for measuring the aortic root. However, measurements using two-dimensional images are preferred over M-mode images, as the latter may be off-axis and are subject to aortic motion during the cardiac cycle that may produce erroneous measurements. Moreover, harmonic imaging has improved the ability to visualize the blood-tissue interface, permitting more accurate inner-edge to inner-edge measurements ( Fig. 34.4 ). It should be noted, however, that measurements made using the leading-edge to leading-edge method are approximately 2 mm larger than those made using the inner-edge to inner-edge method. Regardless of the technique employed, echocardiography laboratories should use the same method consistently to allow accurate reporting of changes in aortic diameters over time. Complicating the issue further is the fact that in the interpretation of CT and MRI, many radiologists measure the aorta from outer-edge to outer-edge, which results in a diameter about 1–2 mm larger than that obtained by echocardiography.
In adults, aortic dimensions are strongly correlated with age and body size. Because of the differential in body size, the aorta is on average 2 mm smaller in women compared to men; and the upper limit of normal for the diameter of the aorta is defined as 2 standard deviations greater than the mean predicted diameter, as shown in Table 34.1 . More refined stratification by age and body surface area have been published by society consensus.
| Men (cm) | Women (cm) | Indexed For BSA (cm/m 2 ) For Both Men and Women | |
|---|---|---|---|
| Sinus of Valsalva | 4.0 | 3.6 | 2.1 |
| Sinotubular junction | 3.6 | 3.2 | 1.9 |
| Proximal ascending aorta | 3.8 | 3.5 | 2.1 |
For the more distal aortic segments, the upper limit of normal is approximately 3.6 cm for the aortic arch, 3.0 cm for the proximal descending thoracic aorta, and 2.0 cm for the distal descending thoracic aorta.
Echocardiographic Views
Transthoracic Echocardiography
Transthoracic echocardiography (TTE) can visualize the aortic root, proximal ascending aorta, aortic arch, and a short segment of the descending aorta. The aortic root and the proximal 4 cm of the ascending aorta are typically well seen on the PLAX view. However, positioning the transducer one rib space higher than the usual PLAX view may provide better visualization of the proximal ascending aorta. Moreover, in some patients, especially in the setting of ascending aorta dilatation, the right parasternal view ( Fig. 34.5 ) may better image the mid and distal ascending thoracic aorta. The aortic arch and its branches are imaged from the suprasternal notch view. The proximal descending thoracic aorta can also be seen from the suprasternal notch view, but the mid-descending thoracic aorta is best imaged in short axis from the PLAX and apical four-chamber views and in long axis from the apical two-chamber view. From the subcostal view, the distal descending thoracic aorta and suprarenal abdominal aorta are usually well visualized.
Transesophageal Echocardiography
The quality of TTE images of the aorta is limited by both the distance of the aorta from the transducer as well as acoustic interference by both ribs and lung. A clear advantage of transesophageal echocardiography (TEE) is the close proximity of the esophagus to the aorta, such that the transducer is within several centimeters of the aorta and there is no acoustic interference, except for a “blind spot” where the interposition of the air-filled trachea and right main bronchus obstruct the view of the distal ascending thoracic aorta ( Fig. 34.6 ).
From the mid-esophageal transducer position, the aortic root and proximal ascending aorta are well visualized in both short axis and long axis, typically at omniplane angles of 0–45 degrees and 90–135 degrees, respectively ( Fig. 34.7A–D ). Slight withdrawal of the probe then permits visualization of the mid-ascending thoracic aorta in both short and long axes, but further withdrawal of the probe leads to the blind spot, in which the airway obscures the distal ascending thoracic aorta.
The descending aorta is readily visualized from the left subclavian artery to the celiac trunk, because the esophagus is immediately adjacent to this portion of the aorta. However, the probe’s proximity is sometimes a limitation in that it can be difficult to capture both side walls of the aorta within the one field of view because of the limited angle of beam (see Fig. 34.7E and F ). The typical TEE exam of the descending thoracic aorta begins with imaging the most distal visible segment, followed by progressive withdrawal of the probe to image the mid- and then proximal segments.
The aortic arch is most easily visualized by slowly withdrawing the probe with the omniplane at 0 degrees after imaging the most proximal segment of the descending aorta. When the probe is withdrawn to most proximal segment of the descending aorta, the distal arch appears to the left of the screen, after which, rotating the probe clockwise and advancing slightly will reveal a long-axis view of the mid and distal arch. Following this, rotating the imaging plane to 90 degrees provides a short-axis view of the mid-arch. The ostia of the left common carotid and left subclavian arteries can usually be identified, but the proximal arch and ostium of the innominate artery are typically obscured by the blind spot and are thus not visible.
Doppler Flows in the Aorta
To document flow profiles and velocities on TTE, Doppler interrogation is typically performed in the suprasternal or subcostal views. Normal antegrade systolic aortic flow has a brisk upstroke, with a peak velocity of approximately 1 m/s. Even in normal patients, the wave of antegrade systolic flow is typically followed by a brief and low-velocity wave of retrograde flow in early diastole ( Fig. 34.8A ) that is thought to result from reflection of the pressure wave by the distal arterial circulation.
Abnormal flow patterns may be seen in the setting of significant aortic valve or thoracic aortic disease. For example, holodiastolic flow reversal with an end-diastolic velocity of greater than 20 cm/s occurs in the setting of severe aortic regurgitation (see Fig. 34.8B ). A flow profile in the descending thoracic aorta that demonstrates a slow systolic upstroke followed by persistent antegrade diastolic flow occurs in the setting of significant coarctation of the aorta (see later discussion).
Aortic Aneurysms
Thoracic aortic aneurysms are typically asymptomatic, and most produce no findings on physical exam. Consequently, the large majority of thoracic aortic aneurysms go undetected until discovered incidentally on imaging studies ordered for other indications. Aneurysms tend to involve primarily one segment of the aorta, such as the aortic root, ascending aorta, arch, or descending aorta, although some will extend from one segment into the next. Rarely, the entire thoracic aorta can be aneurysmal.
The standard definition of an “aneurysm” is a vessel that is dilated to a diameter of 50% greater than an adjacent normal segment or the vessel’s expected diameter for age and body size, and that definition applies well to most arteries, including the descending thoracic and abdominal aorta. However, that definition has not been used routinely for aneurysms of the aortic root and ascending thoracic aorta, because it would require that the aorta reach a diameter of close to 6.0 cm before being called an aneurysm, which makes little sense when the threshold for surgical repair is a diameter of just 5.0–5.5 cm. Consequently, most cardiologists and cardiac surgeons consider a dilated root or ascending thoracic aorta to be an “aneurysm” when it reaches a diameter of 4.5 cm or larger.
Most aortic aneurysms are fusiform , a shape in which the walls of the aorta bulge outward symmetrically ( Fig. 34.9 ). Less common are saccular aneurysms, in which one wall of the aorta protrudes asymmetrically ( Fig. 34.10 ). Pseudoaneurysms are rare, and unlike true aneurysms, they result from a focal rupture of the aortic wall that is contained either by the remaining adventitia or by surrounding mediastinal structures. Most pseudoaneurysms arise at sites of penetrating atherosclerotic ulcers (see later Fig. 34.17 ), surgical anastomoses, or sites of prior surgical cannulation.
Of thoracic aortic aneurysms, those involving the aortic root or ascending aorta are most common (60%), followed by the descending aorta (40%), the aortic arch (10%), or the thoracoabdominal aorta (10%); the percentages add up to greater than 100% because, although most aneurysms are localized, some extend across more than one segment, such as ascending aortic aneurysms that extend into the arch. Moreover, patients with aneurysms in one segment of the aorta may develop discrete aneurysms elsewhere in the aorta; consequently, when an aortic aneurysm is first detected, one should image the remainder of the thoracic and abdominal aorta to see that there are no unrecognized aneurysms elsewhere. However, given the limitations of echocardiographic imaging, surveillance of the entire aorta is best performed using either CT or MRI.
The most common known causes of aneurysms of the aortic root and/or ascending aorta are bicuspid aortic valve disease, familial thoracic aortic aneurysm syndrome, Marfan syndrome, and Turner syndrome. Less common connective tissue disorders, such as Loey-Dietz syndrome and Ehlers-Danlos syndrome type IV (vascular type), or systemic arteritides such as giant cell arteritis can cause aneurysms as well. However, the majority of ascending thoracic aortic aneurysms remain idiopathic. Marfan syndrome affects primarily the aortic root with relative sparing of the ascending thoracic aorta ( Fig. 34.11A ), but there is sometimes effacement of the STJ that produces pear-shaped aortic dilatation that is often referred to as annuloaortic ectasia (see Fig. 34.11B ). In patients with bicuspid aortic valves, most often the ascending thoracic aorta alone is dilated (see Fig. 34.11C ), less often the root alone is dilated, and in some cases, both the root and ascending aorta are dilated (see Fig. 34.11D ).
