Guidelines and standards
Yüklə 9,02 Mb. Pdf görüntüsü
|
|
aorta and its branches can be displayed in multiple planar views, which permits more accurate diameter measurements than axial im- aging. In addition, both modalities can provide a reconstructed, surface-shaded 3D display of the aorta, which is helpful in demon- strating the anatomic relations of the aorta and its branch vessels. In contrast, TEE is not generally preferred for routine aortic imaging, because it is semiinvasive, is relatively unpleasant for the patient, does not provide full visualization of the arch vessels, and does not permit easy identification of landmarks when comparing serial exam- inations to assess aortic changes over time. When the region of clinical interest is specifically the aortic root, such as in screening for or following Marfan syndrome, TTE may be preferred, because the aortic root is generally well visualized and easily measured, whereas on conventional nongated CTA, the aortic root may be poorly visualized because of its angulation and significant motion artifact produced by the beating heart. On the other hand, echocardiography is less consistently able to image the distal ascending aorta, aortic arch, and descending thoracic aorta. To image these segments, CTA and MRA are preferred. Another consideration in selecting an imaging modality is the previous modality used. When following a patient with an enlarging aortic aneurysm, it is best to use the same imaging modality for future imaging, so that a comparison of one study with the next is comparing apples to apples rather than ap- ples to oranges. For imaging of suspected AAS, the primary consideration should be the accuracy of the imaging modality, given the serious Figure 25 ‘‘Real-world’’ sensitivity of imaging modalities in eval- uating suspected aortic dissection in a sample of 618 patients in the IRAD. Ó Massachusetts General Hospital Thoracic Aortic Center; reproduced with permission. Table 4 Comparison of five imaging modalities for diagnostic features of AAS Diagnostic performance CTA TTE
TEE MRA Angiography Sensitivity +++ ++
+++ +++ ++ Specificity +++ ++ +++ +++ +++ Ability to detect IMH +++ +
++ +++
À Site of intimal tear +++ À
+++ ++ Presence of AR À +++ +++ ++ +++ Coronary artery involvement + À
+ +++
Presence of pericardial effusion
++ +++ +++ ++ À Branch vessel involvement ++ À + ++ +++
CTA, Computed tomographic angiography; +++, very positive; ++, positive; +, fair; À, no. Adapted from Cigarroa et al. 182 and Isselbacher. 243 Table 5
Practical assessment of five imaging modalities in the evaluation of suspected AAS Advantages of modality CTA
TTE TEE
MRA Angiography Readily available +++
+++ ++ + + Quickly performed +++ +++
++ + + Performed at bedside À +++ +++ À À Noninvasive +++
+++ + +++ À No iodinated contrast À +++
+++ +++
À No ionizing radiation À +++
+++ +++
À Cost
++ + ++ ++ +++
CTA, Computed tomographic angiography; +++, very positive; ++, positive; +, fair; À, no. Adapted from Cigarroa et al. 182 and Isselbacher. 419 Table 6
Benign conditions or findings that can mimic AAS on the basis of imaging studies Aortitis Atheromatous plaque Prior surgery of aorta Pericardial recess Remnant of a nonpatent PDA Artifacts on CT (streak and motion) Reverberation artifacts in ascending aorta on TEE Innominate vein Periaortic fat and hemiazygos sheath may mimic IMH
Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 137 consequences of false-positive and particularly of false-negative re- sults. There have been a number of studies carried out over the past two decades comparing CTA, MRA, TEE, and aortography for the diagnosis of aortic dissection, and a recent meta-analysis by Shiga et al. 122
showed that CTA, MRA, and TEE are all outstanding, with sensitivities of 98% to 100%, as shown in Figure 23 . On the
other hand, aortography has a sensitivity of only 88%, perhaps reflect- ing the fact that IMH often goes undetected with this technique. In the same meta-analysis, the specificity of the four imaging modalities was roughly equivalent at 94% to 98%, as shown in Figure 24 . Therefore CTA, MRA, and TEE are all reasonable first-line imaging studies to choose for this purpose. It is important to note, however, that the research studies that evaluate the accuracy of imaging modalities are usually performed at centers of excellence and interpreted by designated experts in aortic imaging, and it is therefore reasonable to suspect that accuracy may be lower when the same imaging modalities perform in the ‘‘real-world’’ setting. Indeed, a report from the International Registry of Acute Aortic Dissection (IRAD) examined this very ques- tion, and the results are shown in Figure 25 . 120
The real-world sensi- tivity of both CTA and TEE is lower than in the above meta-analysis, probably reflecting a lesser degree of expertise among the readers. Interestingly, the real-world sensitivity of MRA remained at 100%, which may reflect the fact that MR angiograms tend to be read by specialists (e.g., vascular radiologists) rather than general radiologists (e.g., emergency department radiologists). The diagnostic and prac- tical features of each of the five common imaging modalities are summarized in Tables 4 and 5 . III. ACUTE AORTIC SYNDROMES A. Introduction The term AAS 128 refers to the spectrum of aortic pathologies, including classic aortic dissection, IMH, penetrating aortic ulcer (PAU), and aortic aneurysm rupture (contained or not contained). Although the pathophysiology of these heterogeneous conditions dif- fers, they are grouped because they share common features: (1) similar clinical presentation (‘‘aortic pain’’), (2) impaired integrity of the aortic wall, and (3) potential danger of aortic rupture requiring emergency attention. 128-133
Moreover, some of these conditions may represent stages in the evolution of the same process. We have elected not to include, as some authors do, aortitis and traumatic aortic rupture, because they have totally distinct clinical and pathophysiologic profiles. 128
Clinical databases, such as the IRAD, have contributed tremendously to our knowledge of these acute aortic pathologies. 134
Because of the life-threatening nature of these conditions, prompt and accurate diagnosis is paramount. Misdiagnosis of these condi- tions, usually because of confusion with myocardial ischemia, can lead to untimely deaths. Table 6 lists some less urgent conditions that can potentially mimic AAS. The noninvasive imaging techniques that play a fundamental role in the diagnosis and management of patients with AAS include CTA, TTE, TEE and MRI. Some patients may require more than one noninvasive imaging study and, in rare instances, invasive aortog- raphy may be necessary. Imaging is used to confirm or exclude the diagnosis, determine the site(s) of involvement, delineate exten- sion, and detect complications to plan the most appropriate manage- ment approach. B. Aortic Dissection 1. Classification of Aortic Dissection. Accurate classification of aortic dissection is important because significant differences in clin- ical presentation, prognosis, and management depend on the loca- tion and extent of the dissection. Figure 26 illustrates the two commonly used classifications: the DeBakey system (types I, II, and III) 124,125
and the Stanford system (types A and B). 126
Dissections involving the aortic arch without involving the ascending aorta are classified as type B in the Stanford system. The majority of dissections, whether type A or type B, propagate beyond the diaphragm to the iliac arteries. The appropriate management of aortic dissections depends not only on the location of the dissection but also on the time that has elapsed between onset of the process and the patient’s presentation. Although the adjectives acute, subacute, and chronic are often applied, there is no standard definition for these time periods. 135-138
There is a 24-hour ‘‘hyperacute’’ period during which dissections involving the ascending aorta carry a risk for rupture approaching 1% per hour. Studies have shown that 75% of aortic dissection–related deaths occur in the initial 2 weeks. At the opposite extreme are ‘‘old dissec- tions’’ encountered incidentally during aortic imaging or surgery. These are clearly ‘‘chronic.’’ Hirst et al., Levinson et al., and DeBakey considered an aortic dissection to be ‘‘acute’’ when the onset of Figure 26 Diagram illustrating the two commonly used classifi- cation systems for aortic dissection. In the older of the two, the DeBakey system, type I dissection originates in the ascending aorta and propagates distally to includes at least the arch and typically the descending aorta. Type II dissection, not shown (the least common type) originates in and is confined to the ascending aorta. Type III dissection originates in the descending thoracic aorta (usually just distal to the left subclavian artery) and propagates distally, usually to below the diaphragm. The Stan- ford system, in a simpler scheme, divides dissections into two categories: those that involve the ascending aorta, regardless of the site of origin, are classified as type A, and those beginning beyond the arch vessels are classified as type B. The majority of dissections, whether type A or type B, propagate beyond the diaphragm to the iliac arteries. 138 Goldstein et al Journal of the American Society of Echocardiography February 2015
symptoms was <2 weeks in duration at the time of diagnosis. 139-143
The subsequent 2-month period was designated ‘‘subacute,’’ and beyond the second month, an aortic dissection was termed ‘‘chronic’’. We endorse this classification, as it has some basis in pathologic obser- vations. The extremely high initial death rate declines after 2 weeks. Moreover, friable aortic tissue extends beyond 2 weeks. By 6 to 8 weeks, the outer aortic wall has largely ‘‘healed,’’ in that it has devel- oped scar, a reasonable marker for the beginning of the chronic stage.
144 It must be acknowledged that any time-based division of ‘‘acute’’ from ‘‘subacute’’ and ‘‘subacute’’ from ‘‘chronic’’ is arbitrary. Nevertheless, such distinctions are necessary for analyzing outcomes. Moreover, some imaging features of acute and chronic dissections are different. In chronic dissection, the dissection flap tends to be thicker, more echodense, and relatively immobile (as distinct from the oscil- lating flaps seen in acute dissection). 2. Echocardiography (TTE and TEE). The sensitivity of 2D TTE by fundamental imaging was previously reported to be only 70% to 80% for the detection of type A dissection. However, because of new transducers with improved resolution, harmonic imaging, and Figure 27 Acute aortic dissection evaluated by TTE. (Top left) Parasternal long-axis view showing a flap in the proximal ascending aorta (Ao) (arrow) and a flap in the thoracic descending aorta (arrow). (Top right) Parasternal short-axis view of the proximal ascending aorta showing the presence of a typical true lumen (TL) and false lumen (FL) divided by a flap. (Bottom left) Five-chamber apical view showing the presence of a flap in the proximal ascending aorta (arrow). (Bottom right) Subcostal view of the descending abdominal aorta with a clear flap inside the aortic lumen. LV, Left ventricle; RV, right ventricle. Figure 28 Proximal descending thoracic aorta visualized from supraclavicular view. Use of contrast echo illustrates entry tear (arrow) by showing contrast emanating from true lumen (TL) to false lumen (FL). Figure 29 Transesophageal echocardiographic longitudinal view of the aortic root and ascending aorta (ASC’G AO) illus- trating a folded, convoluted dissection flap (arrow) that had marked oscillation in real-time. LA, Left atrium. Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 139 contrast enhancement, the sensitivity of TTE has improved to approximately 85% on the basis of recent data from Cecconi et al. 145 and Evangelista et al. 146 Therefore, TTE may be of some use as the initial imaging modality, especially in the emergency room ( Figure 27 ). In addition, TTE provides assessment of left ventricular contractility, pericardial effusion, aortic valve function, right ventricular size and function, and pulmonary artery pressure, which may facilitate the diagnosis of chest pain due to myocardial ischemia and/or infarction, pulmonary embolism, or pericardial disease and may identify dissection complications such as aortic regurgitation (AR) in an early fashion. Moreover, the use of contrast agents may further improve the accuracy, as illustrated in Figure 28 . 146
Nevertheless, because of the potential catastrophic nature of type A aortic dissection, negative results on TTE should not be considered definitive, and further imaging should follow. Moreover, TTE is less sensitive for the diagnosis of type B dissec- tion, because the descending thoracic aorta (located farther from the transducer) is imaged less easily and accurately. Therefore, although TTE may be diagnostic in many instances, its role is predom- inantly that of a screening procedure. TEE, on the other hand, is highly accurate for establishing the diagnosis of both type A and type B acute aortic dissection. Since the landmark multicenter European Cooperative Study, 119 several additional studies have demonstrated the high accuracy of TEE, with sensitivity approaching 100%. 42,147-150 a. Echocardiographic Findings.–The diagnostic hallmark of aortic dissection is a mobile dissection flap that separates the true and false lumens ( Figure 29 ). Important features of the dissection flap include oscillation or motion that is independent of the aorta itself, visualiza- tion in more than one view, and clear distinction from reverberations from other structures, such as a calcified aortic wall, catheter in the right ventricular outflow tract, pacemaker wire, or pericardial fluid in the transverse oblique sinus. The true and false lumens can almost always be differentiated. In the descending thoracic aorta, the false lumen is usually larger than the true lumen. The dissection flap typically moves toward the false lumen in systole (systolic expansion of the true lumen) and toward the true lumen in diastole (diastolic expansion of the false lumen), Table 7
Role of echocardiography in detecting evidence of aortic dissection and echocardiographic definitions of main findings Diagnostic goals Definition by echocardiography Identify presence of a dissection flap Flap dividing two lumens Define extension of aortic dissection Extension of the flap and true/false lumens in the aortic root(ascending/arch/ descending abdominal aorta) Identify true lumen Systolic expansion, diastolic collapse, systolic jet directed away from the lumen, absence of spontaneous contrast, forward systolic flow) Identify false lumen Diastolic diameter increase, spontaneous contrast and or thrombus formation, reverse/delayed or absent flow Identify presence of false luminal thrombosis Mass separated from the intimal flap and aortic wall inside the false lumen Localize entry tear Disruption of the flap continuity with fluttering or ruptured intimal borders; color Doppler shows flow through the tear Assess presence, severity and mechanisms of AR Anatomic definition of the valve (bicuspid, degenerated, normal with/without prolapse of one cusp); dilation of different segments of the aorta; flap invagination into the valve; severity by classic echocardiographic criteria Assess coronary artery involvement Flap invaginated into the coronary ostium; flap obstructing the ostium; absence of coronary flow; new regional wall motion abnormalities Assess side-branch involvement Flap invaginated into the aortic branches Detect pericardial and/or pleural effusion Echo-free space in the pericardium/pleura Detect signs of cardiac tamponade Classic echocardiographic and Doppler signs of tamponade Figure 30 Three-dimensional TEE showing the entry tear of a type B aortic dissection located in the proximal descending aorta. (Left) Live 3D image showing a large entry tear (asterisk). (Right) Maximum orthogonal diameters (D2 and D1) are 17 and 11 mm, and area measured by full volume is 1.5 cm 2 .
Journal of the American Society of Echocardiography February 2015 sometimes causing compression of the true lumen. Moreover, the characteristics of blood flow vary in the true and false lumens. In the true lumen, antegrade systolic flow is rapid enough to create brighter shades of red or blue on color Doppler. In contrast, flow in the false lumen is generally slower, producing duller colors. In fact, flow in the false lumen may be absent or in the opposite direction (retrograde) to that of the true lumen. The sluggish flow in the false lumen may result in the presence of spontaneous echo contrast, sometimes referred to as ‘‘smoke.’’ The false lumen may also contain variable degrees of thrombus. Additional findings in patients with aortic dissection include dilatation of the aorta, compression of the left atrium, AR, pericardial and/or pleural effusion, and involvement of the coronary arteries. Table 7
summarizes the main echocardio- graphic findings in aortic dissection. Three-dimensional TEE may provide information beyond what can be obtained with 2D TEE. 151
For example, the size of the entry tear size and its relationship to surrounding structures may be shown in greater detail, allowing better morphologic and dynamic evaluation of aortic dissection ( Figure 30 ). Such information may be particularly helpful when the flap spirals around the long axis of the aorta. Moreover, 3D TEE dem- onstrates the dissection flap not as a linear structure but as a sheet of tissue of variable thickness in the long, short, or oblique axis. This may make it possible to distinguish a true dissection flap from an artifact when it is relatively immobile. In addition, multiplane 3D TEE provides a more rapid and accurate evaluation of the aortic arch than 2D TEE. b. Detection of Complications.–AR occurs in approximately 50% of patients with type A aortic dissection. The presence, severity, and mechanism(s) of AR may influence surgical decision making and aid the surgeon in deciding whether to spare, repair, or replace the aortic valve. 148,152,153 The mechanisms of AR are listed in Table 8
, and several of these are illustrated in Figure 31 . These mechanisms will be discussed in greater detail in section III.B.6, ‘‘Use of TEE to Guide Surgery for Type A Aortic Dissection.’’ A pericardial effusion in an ascending aortic dissection is an indica- tor of poor prognosis and suggests rupture of the false lumen in the pericardium. Echocardiography is the best diagnostic technique for estimating the presence and severity of tamponade. Periaortic hema- toma and pleural effusion are best diagnosed by CT. The presence of periaortic hematoma has also been related to increased mortal- ity. 154,155
TEE is capable of imaging the ostia and proximal segments of the coronary arteries in nearly all patients and may demonstrate coronary involvement due to dissection (flap invagination into the coronary ostium and origin of coronary ostium from the false lumen). 148 Color Doppler is useful for verifying normal or abnormal or absent flow into the proximal coronary arteries. Detection of segmental wall motion abnormalities of the left ventricle by TTE or TEE may also help identify this complication. Color Doppler also reveals reentry sites (often multiple, as in Figure 32 ), which explain why the false lumen often remains patent over time.
with aortic dissection are few but deserve mention. Interposition of the trachea between the ascending aorta and the esophagus limits visualization of the distal ascending aorta and proximal arch. In a small Yüklə 9,02 Mb. Dostları ilə paylaş:
|