Asian Cardiovasc Thorac Ann 1998;6:153-157
© 1998 Asia Publishing EXchange Pte Ltd
Recent Advances In Echocardiography of Aortic Disorders
Ajay Kanojia, MD,
Ravi R Kasliwal, DM
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Department of Noninvasive Cardiology Escorts Heart Institute and Research Centre New Delhi, India
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Ravi R Kasliwal, DM Department of Noninvasive Cardiology Escorts Heart Institute and Research Centre Okhla Road New Delhi 110025, India Tel: 91 11 684 4820 Fax: 91 11 683 2605 E-mail: ajaykanojia{at}hotmail.com
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ABSTRACT
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Pathological processes affecting the thoracic aorta are a source of significant morbidity and mortality. It is not surprising therefore that many imaging modalities have evolved for comprehensive assessment of this vital structure. Currently available techniques include chest radiography, computed tomography, magnetic resonance imaging, echocardiography, and aortography. Compared with other modalities, echocardiography is the most attractive because of its advantages of speed, portability, immediate availability of results, and low cost. With recent advances in echocardiographic instrumentation, technology, and newer approaches such as multiplane transesophageal echocardiography it has evolved as the most promising imaging technique for diagnosis of aortic disorders, providing comprehensive information of the site and extent of aortic pathology and associated complications.
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INTRODUCTION
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Echocardiography has revolutionized the diagnosis and management of aortic disorders. Morphologically and functionally, the aorta is a simple structure and clinical manifestations of its disorders are limited but they can be caused by a large number of disease processes. Weakness of its wall may result in aneurysm, dissection, or rupture. Narrowing of the main trunk may occur or obstruction of the origin of its main branches (Table 1
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Echocardiographic examination using M-mode, two-dimensional echocardiography, spectral Doppler, and color Doppler modalities can provide accurate and comprehensive assessment of aortic pathology.1–3 Although surface echocardiography provides a limited but clinically useful evaluation of various aortic disorders, recent developments in transesophageal echocardiography have brought echocardiography to the forefront of comprehensive diagnostic imaging of the entire thoracic aorta.4–12
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ECHOCARDIOGRAPHIC TECHNIQUES FOR EVALUATION OF AORTIC DISEASE
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It is possible to examine almost the entire aorta using various echocardiographic modalities: transthoracic echocardiography, transesophageal echocardiography, epiaortic echocardiography, intravascular ultrasound, three-dimensional echocardiography. Successful transthoracic echocardiographic evaluation of the thoracic aorta requires multiple acoustic windows and manipulation of the interrogation beam through multiple angles. In addition to the left parasternal, apical, and subcostal views commonly used with standard examination, images are always necessary from the right parasternal, suprasternal, and right and left supraclavicular windows. During transthoracic echocardiographic examination, the root of the aorta and the proximal portion of the ascending aorta may be visualized in parasternal views (Figure 1) and the modified parasternal long-axis view allows visualization of the thoracic part of the descending aorta posterior to the left atrium and left ventricle. The suprasternal approach provides visualization of the arch of the aorta and the proximal portion of the descending aorta. The abdominal aorta can be visualized by placing a transducer in the subcostal position and over the abdomen.
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Figure 1. Two-dimensional echocardiography of an aortic aneurysm.AO = aorta, LA = left atrium, LV = left ventricle.
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Transesophageal echocardiography (TEE) has greatly improved echocardiographic examination of the aorta. Enhanced resolution due to high-frequency transducers, an unobstructed window, and multiplane echocardiographic examination coupled with spectral Doppler and color flow examination provide the most useful and definitive anatomical and functional information for clinical decision-making.4 Compared to the earlier single-plane and biplane transesophageal echocardiographic techniques, currently available multiplane transesophageal echocardiography is capable of providing multidimensional tomographic images in multiple orientations with enhanced ease of use and less discomfort to the patient (Table 2).
Epiaortic echocardiography is performed using a 5-MHz to 7.5-MHz probe applied directly over the surface of the aorta. It was earlier used mainly for intraoperative identifications of aortic atheroma but with the introduction of multiplane transesophageal echocardiography, this procedure has now lost its charm.5 Intravascular ultrasound is emerging as a potentially useful invasive tool for the diagnosis of aortic dissection, which can delineate the extent of dissection as well as branch-vessel involvement, especially when transesophageal echocardiography is contraindicated or undesirable.6 Advances in computer technology have introduced three-dimensional echocardiography as a developing modality in cardiac imaging. It has the potential to strengthen our understanding of structural relationships and it can give detailed morphological information about aortic disorders. It opens up the possibility of quantifying lesions such as an atheromatous plaque (Figure 2) or a coarctation in its true dimensions and could aid in interventional management of these disorders.7
AORTIC DISSECTION
Transesophageal echocardiography is emerging as the procedure of choice for early bedside diagnosis of aortic dissection. Versatility, easy portability of the ultrasound equipment, and immediate availability of results have greatly contributed to the expanding use of echocardiography in the diagnosis of aortic dissection (Figure 3). The most important diagnostic goals in acute aortic dissection are summarized in Table 3. It is very important to achieve all the diagnostic goals promptly and accurately for management of an acutely ill patient. A comprehensive transesophageal echocardiographic study provides almost all the necessary information.8
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Figure 3. Echocardiographic three-dimensional perspectives of aortic dissection. Note the clear visualization of an intimal tear and easy identification of the true and false lumens.
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The limitations of TEE for evaluation of aortic dissection are few. An earlier limitation due to a tracheal blind spot has been eliminated with the introduction of multiplane transesophageal echocardiography. Although complete evaluation of cerebral vessels and the abdominal aorta is still a limitation of transesophageal echocardiography, this can be easily overcome by using other echocardiographic modalities such as duplex carotid scanning and abdominal ultrasound. False negative TEE findings in cases of aortic dissection usually occur because of a missed dissection flap located only in the ascending aorta. False positive TEE findings usually result from reverberant artifacts in the ascending aorta mimicking intimal flaps. Visualization of the true intimal flap in M-mode and more than one plane in two-dimensional echocardiography, documenting Doppler flow signals in both lumens, and differential color flow in both lumens may improve the specificity of the diagnosis of aortic dissection by TEE.9,10
Other currently available diagnostic modalities for aortic dissection include contrast-enhanced computed tomography and magnetic resonance imaging. Both have certain advantages and disadvantages with respect to accuracy, speed, convenience, risk, and cost but neither is appropriate in all situations.11–17 Nienaber and colleagues11 compared magnetic resonance imaging, contrast-enhanced computed tomography, and echocardiography (transthoracic and single-plane TEE) for reliability in confirming aortic dissection confirmed by operative findings, autopsy, or contrast angiography (in 62, 7, and 64 patients respectively). Patients were prospectively studied and underwent at least two different procedures. Studies were assessed in a blinded fashion. The sensitivity and specificity of single-plane TEE were 97.7% and 76.9% respectively, compared with 98.3% and 97.8% for magnetic resonance imaging and 93.8% and 87.1% for computed tomography. Six false positive results were found among 26 patients studied by TEE. The rather low specificity of TEE found in this study contrasts with the 98% specificity reported by Erbel and colleagues8 and 100% specificity reported by Ballal and colleagues12 for TEE. Comprehensive echocardiographic examination may provide all the necessary morphological and functional information in an acutely ill patient, at the bedside, in a very short time, with excellent sensitivity and specificity, and immediate availability of results (Table 4).
AORTIC ANEURYSM
The role of echocardiography for evaluation of aortic aneurysm is currently evolving.18 Echocardiographically, an aneurysm is said to be present when the diameter of a segment is increased by 50% or more of an adjacent segment. When visible, an aneurysm can be serially followed in a simple and noninvasive manner using echocardiography. The major goals of echocardiographic imaging in aortic aneurysm include: confirmation of the diagnosis; measurement of the maximum longitudinal and transverse extent of the aneurysm; determination of any involvement of the aortic valve and arch vessels; detection of mural thrombus; and differentiation from aortic dissection. Multiplane transesophageal echocardiography has overcome the problem of a tracheal blind spot and it has been reported to be an extremely sensitive and accurate technique for diagnosis of thoracic aortic aneurysm.19 Tortuosity in the aorta can lead to inaccurate measurements when the imaging plane is oblique but attention to overall diameter and shape may help in correct evaluation.20
AORTIC ATHEROMATOUS DISEASE
The most frequently encountered abnormality of the aorta is involvement in the atherosclerotic process. Routine methods of evaluating the ascending aorta and aortic arch for atheromatous disease in cardiac surgical patients have consisted of palpation by the surgeon or detection of calcification at angiography or chest radiography. Several authors have demonstrated that epiaortic echocardiographic imaging during surgery was able to detect 50% more lesions than surgical palpation.21 Several studies have demonstrated that severe atheromatous disease of the aorta is associated with a higher risk of perioperative stroke.21–24 Transesophageal echocardiography provides a clear image of atheromatous disease in the aorta in almost all patients. Thus, assessment of site, total extent, surface morphology, and detection of a mobile element become simple. This mapping of aortic atheromatous pathology may aid the surgeon in modifying plans for aortic cannulation, cross-clamping, and operative technique in order to reduce the risk of stroke and peripheral embolization.24 Multiplane transesophageal echocardio-graphy is particularly helpful in the evaluation of athero-matous disease of the aorta. It facilitates examination of the ascending aorta, the arch, and the thoracic part of the descending aorta, which are areas that are manipulated and cannulated during cardiopulmonary bypass.
TRAUMATIC AORTIC INJURY
Blunt chest trauma, especially when resulting from a decelerating injury such as in a motor vehicle accident can result in rupture of the thoracic aorta. Although most patients die before reaching hospital, approximately 15% will survive long enough to receive medical attention. Prompt diagnosis is essential since nearly all such patients will die without surgical repair.25 Usually, aortography is recommended in patients with blunt chest trauma to demonstrate more than 8-cm mediastinal widening that is found to be a very sensitive indicator of aortic rupture. Unfortunately, mediastinal widening has only 10% spe-cificity for the diagnosis of traumatic aortic rupture. Recently, several studies have reported excellent diagnostic accuracy with transesophageal echocardiography in the recognition of traumatic aortic injury.26
To conclude, echocardiography is a noninvasive, rapid, accurate, highly sensitive, and specific imaging modality for the diagnosis of aortic disorders. The advent of multiplane transesophageal echocardiography has recently extended the use of echocardiography for more com-prehensive evaluation of these disorders. Detailed deli-neation of the exact site, total extent, surface morphology, associated complications and their functional significance makes echocardiography an ideal imaging tool for the evaluation of all types of aortic disorders.
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ABSTRACT
INTRODUCTION
