Diseases of the Great Arteries

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Diseases of the Great Arteries



Echocardiographic evaluation of the aorta and main pulmonary artery is a routine part of the standard echocardiographic examination. For descriptive purposes, the aorta is divided into segments, beginning at the aortic valve, including the:



The term “aortic root” has a variable definition, which can lead to errors in communication. In studies of genetic connective tissue disorders, aortic root usually refers to the aortic sinuses, from the valve level to the sinotubular junction. However, a surgical aortic root replacement typically extends from the aortic annulus to the mid-ascending aorta. Thus, when the aorta is abnormal, more specific terms describing anatomic location are preferred.


Evaluation of the aortic annulus and sinuses is a routine component of an echocardiographic study. In addition, further evaluation of the ascending aorta, arch, and descending aorta can be performed when disease is suspected clinically. Transthoracic (TTE) images often are suboptimal because of overlying or adjacent air-filled structures, so transesophageal echocardiography (TEE) imaging greatly enhances the diagnostic utility of echocardiography for diseases of the aorta.



Basic Principles


Aortic abnormalities include:



The most common abnormality of the aorta is dilation, or an increase in diameter greater than expected for age and body size. When aortic dilation exceeds the expected diameter by 50% or more (over 5.0 cm for the ascending aorta), the term aneurysm is used. Aneurysms can involve one or more segments of the aorta (ascending, arch, and descending) and may be tubular or saccular in configuration.


Aortic dilation associated with hypertension or atherosclerosis is characterized by normal contours of the sinuses of Valsalva and narrowing at the sinotubular junction, with enlargement primarily in the ascending aorta. A bicuspid aortic valve often is accompanied by dilation of the aortic sinuses or ascending aorta, but some narrowing at the sinotubular junction usually is preserved. Inherited connective tissue disorders that cause aneurysms of the ascending aorta, such as Marfan syndrome and Loeys-Dietz syndrome, are characterized by effacement of the sinotubular junction and enlargement of the sinuses of Valsalva, in addition to dilation of the ascending aorta, resulting in a “water balloon” appearance of the proximal aorta (Fig. 16-1). Systemic inflammatory diseases, such as ankylosing spondylitis, are associated with aortic aneurysms and often also affect the valve tissue. Aneurysms also may be seen in tertiary syphilis (with a characteristic pattern of calcification), aortic arteritis (such as Takayasu arteritis and giant cell arteritis), and as a result of blunt or penetrating chest trauma. Large aortic aneurysms are prone to rupture, so prophylactic repair often is recommended.



An aortic dissection is a life-threatening situation in which an intimal tear in the aortic wall allows passage of blood into a “false” channel between the intima and the media (Fig. 16-2). This false channel may be localized or may propagate downstream, often in a spiral fashion, because of the pressure of blood flow in the channel. Complications related to the false lumen include:




An accurate, rapid diagnosis of the presence or absence of aortic dissection and the site of the entry tear is crucial in the treatment of patients with chest pain and suspected dissection. The most prevalent risk factors for aortic dissection are hypertension and atherosclerosis. However, patient groups with the highest risk of aortic dissection include those with Marfan syndrome and other inherited connective tissue disorders, Turner’s syndrome, aortitis, and vascular-type Ehlers-Danlos syndrome. There also is a fivefold increased risk of aortic dissection in patients with a congenital bicuspid or unicuspid aortic valve. Dissection is more likely in patients with a preexisting aneurysm, although dissection can occur in the absence of dilation in patients with Marfan syndrome or other connective tissue disorders.


A pseudoaneurysm is a blood collection outside the aorta, in contrast to an aneurysm, which involves all of the layers of the aortic wall. Pseudoaneurysms may be due to a contained rupture after complex aortic valve or aortic surgery, as a complication of endocarditis, or following an aortic dissection.


Sinus of Valsalva aneurysms (Fig. 16-3) may be congenital or may be due to infection, Marfan syndrome, or previous surgical procedures. A sinus of Valsalva aneurysm protrudes into adjacent chambers and may be associated with a fistula. Specifically, an aneurysm of the right coronary sinus protrudes into the right ventricular (RV) outflow tract, the left coronary sinus into the left atrium (LA), and the noncoronary sinus into the right atrium (RA).



Atherosclerosis of the aorta may lead to dilation, aneurysm, or dissection. In addition, the presence of atheroma may be important as a marker for coexisting coronary artery disease and as a potential source of embolic cerebrovascular events.



Echocardiographic Approach



Transthoracic Approach



Echocardiographic Imaging


On TTE imaging, the proximal ascending aorta is well seen in parasternal long- and short-axis views (Fig. 16-4). Depending on ultrasound penetration, images of additional segments of the ascending aorta may be obtained by moving the transducer cephalad one or more interspaces. Image quality is enhanced by positioning the patient in a steep left lateral decubitus position, bringing the aorta in contact with the anterior chest wall (Figs. 16-5 and 16-6). In all adults, aortic diameter measurements should be made:






When aortic dimensions are above normal or when the shape of the aortic sinuses and sinotubular junction is abnormal, additional measurements should be made as described below (Tables 16-1 and 16-2). Aortic sinus diameter also can be measured with an M-mode tracing at the level of the valve leaflets when a perpendicular orientation between the ultrasound beam and the aortic sinuses can be obtained. With M-mode tracings a leading edge-to-leading edge measurement convention is used. M-mode tracings may allow for more accurate identification of the aortic wall because of the high temporal sampling rate.




The aortic arch is imaged from a suprasternal notch or supraclavicular approach with the patient in a supine position with the neck extended. Both longitudinal (Fig. 16-7) and transverse views of the arch are obtainable in nearly all individuals. Usually only a short segment of the ascending aorta is visible from the suprasternal notch window, but this is variable among patients. Also note that the descending aorta appears to taper because of an oblique image plane with respect to its curvature (i.e., the descending aorta is only partially in the image plane).



The descending thoracic aorta is seen in cross section posterior to the LA in the parasternal long-axis view. A longitudinal section of this segment of the descending aorta may be obtained by clockwise rotation and lateral angulation of the transducer. From the suprasternal notch approach, a small portion of the descending thoracic aorta is seen. From the apical two-chamber view, a longitudinal section of a segment of the descending aorta is seen by lateral angulation and clockwise rotation of the transducer (Fig. 16-8).



From the subcostal approach, the distal thoracic and proximal abdominal aorta is seen as it traverses the diaphragm. In patients with a left pleural effusion, images of the aorta also may be obtained by imaging through the fluid from the left posterior chest (paraspinal) with the patient in a right lateral decubitus position.



Doppler Flows


Color Doppler interrogation of the ascending aorta from the parasternal approach allows for evaluation of the flow pattern in the proximal aorta and assessment of aortic regurgitation severity, with evaluation of the cause of aortic regurgitation and measurement of vena contracta width. The ascending aorta also may be imaged from the apical approach in an anteriorly angulated four-chamber view and in an apical long-axis view. While 2D image quality may be limited at the depth of the ascending aorta from the apical approach, this view does allow a parallel intercept angle between the Doppler beam and the direction of blood flow for spectral Doppler flow velocity measurements.


Pulsed or continuous-wave (CW) Doppler recordings of descending aortic flow from the suprasternal notch show systolic flow away from the transducer at a velocity of approximately 1 m/s. Normal flow in the descending aorta shows:



The use of low wall filter settings is needed to appreciate this normal flow pattern (Fig. 16-9). Flow abnormalities may be related to aortic disease (e.g., coarctation), shunts (e.g., patent ductus arteriosus), or aortic valve disease (e.g., regurgitation). Flow patterns in the proximal abdominal aorta are similar to those in the descending thoracic aorta and are easily recorded from the subcostal approach (Fig. 16-10).





Limitations of Transthoracic Imaging of the Aorta


The major limitations of the TTE approach to ultrasound evaluation of the aorta are acoustic access and image quality. In many individuals, acoustic access is suboptimal or minimal from one or more of the windows needed for full evaluation of the aorta, leaving “gaps” in the echocardiographic examination. Even when acoustic access is adequate, image quality often is poor because of beam width at the depth of the aorta—particularly the descending thoracic aorta from apical and parasternal windows. Beam-width artifact, noise, and poor lateral resolution make differentiation of intraluminal defects from artifacts difficult. Because of these limitations, evaluation by TEE or other imaging modalities is appropriate in many patients with acute or chronic aortic disease.



Transesophageal Approach



Echocardiographic Imaging


From a high TEE probe position with the image plane rotated to approximately 45°, the aortic valve and sinuses of Valsalva are seen in short-axis. Slight withdrawal of the probe in the esophagus may allow short-axis imaging of the proximal ascending aorta, but more distal segments are obscured by interposition of the air-filled trachea between the esophagus (and transducer) and the ascending aorta. The long-axis view of the aortic valve, sinuses of Valsalva, and ascending aorta is obtained by rotating the image plane to approximately 120° (Fig. 16-11). In the long-axis plane, more cephalad segments of the ascending aorta may be seen by slowly moving the transducer to a higher esophageal position.



The aortic arch is best imaged from a high esophageal position. Starting with a short-axis view of the descending thoracic aorta, the probe is withdrawn to the level of the arch and then the entire probe is turned toward the patient’s right and angulated inferiorly to obtain a long-axis view of the arch. In some patients, images of the aortic arch may be suboptimal because of the positions of the trachea and bronchi. In many cases, transthoracic suprasternal notch views of the arch provide superior image quality.


The descending thoracic aorta and the proximal abdominal aorta are well seen by the TEE approach. The descending thoracic aorta lies immediately lateral and slightly posterior to the esophagus, so posterior rotation of the probe provides excellent images in either a cross-sectional (transverse plane at 0°) or long-axis (at 90° to 120°) view (Figs. 16-12 and 16-13). Slight turning of the TEE probe is needed at different levels as the aorta curves relative to the esophagus. From a transgastric position, the proximal abdominal aorta is seen posterior to the stomach. Many examiners prefer to examine the length of the aorta in sequential cross-sectional views as the probe is slowly withdrawn from the stomach and esophagus, with imaging of the aortic arch just before probe removal. Any areas of abnormality can then be further examined in long-axis views.




Three-dimensional (3D) TEE imaging of the aorta is not routine but may be helpful when aortic pathology is present, for example, when defining the anatomy and extent of an aortic dissection flap, identifying the location and size of the dissection entry site, or quantitating total aortic atherosclerotic burden.



Doppler Flows


TEE color flow imaging of the aorta shows the normal antegrade flow pattern in the ascending aorta, arch, and descending aorta and is helpful in the evaluation of abnormal blood flow patterns when aortic dissection is present. However, quantitative evaluation of aortic flow velocities is challenging from the TEE approach. The ascending aorta sometimes can be evaluated from a transgastric long-axis view, although the intercept angle may not be parallel to flow, which results in velocity underestimation, particularly when high velocities are present. For the descending aorta, the direction of blood flow is nearly perpendicular to the direction of the ultrasound beam, limiting spectral Doppler velocity measurement, although flow signals may be obtained in a long-axis plane with the Doppler beam aligned at the distal end of the aorta. This approach likely underestimates velocity because of the nonparallel intercept angle, but it is useful to evaluate flow patterns, such as holodiastolic flow reversal with aortic regurgitation.


In addition, color Doppler evaluation of the aortic valve is a standard component of the examination, because aortic valve regurgitation may be the result of several types of aortic disease, including:



Evaluation of aortic regurgitation includes the measurement of vena contracta width, identification of the origin and etiology of regurgitation, and evaluation of left ventricle (LV) size and systolic function (see Chapter 12).



Aortic Dilation and Aneurysm


Aortic dilation often is first recognized on the chest radiograph or on an echocardiographic examination requested for other reasons. In specific clinical settings, such as Marfan syndrome, aortic dilation is an expected consequence of a systemic disease (Table 16-3). In these cases, echocardiography is requested to assess the presence and degree of aortic abnormality. In patients with a bicuspid aortic valve, aortic dimension should be routinely measured (Fig. 16-14).



Jul 10, 2018 | Posted by in CARDIOLOGY | Comments Off on Diseases of the Great Arteries
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