Guidelines and standards
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Presence and severity of AR Sensitivity not sufficient distal to aortic root Descending thoracic aorta imaged less easily and accurately Misses IMH and PAU MRI
Third-line 3D multiplanar, and high resolution Very high diagnostic accuracy Does not require ionizing radiation or iodinated contrast Appropriate for serial imaging over many years Less widely available Difficult monitoring critically ill patients Not feasible in emergent or unstable clinical situa- tions
Longer examination time Caution with use of gadolinium in renal failure Angiography Fourth-line Rarely necessary Often misses IMH (up to 10%–20% of ADs) Long diagnostic time Requires ICM Morbidity Less sensitivity than CT, TEE, and MRI AD, Aortic dissection; ECG, electrocardiographic; ER, emergency room; ICM, iodinated contrast media; IMH, intramural hematoma; LV, left ven- tricular; OR, operating room; PA, pulmonary artery; PAU, penetrating atherosclerotic ulcer; RV, right ventricular; SMA, superior mesenteric artery. *In IRAD. Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 145 despite significant disruption of the aortic valve. Alternatively, a circumferential dissection of the ascending aorta can tear away and produce a tubular proximal dissection flap that prolapses the aortic valve in diastole, essentially akin to ‘‘intussusception,’’ producing severe AR. 196
These patients may not require repair or replacement of the aortic valve. Some patients with aortic dissection have more than one of these anatomic processes occurring simultaneously. Fortunately, most of these anatomic causes of AR are correctable during surgery, so in- forming the surgeon in detail about the anatomic findings and mech- anisms of AR may permit successful repair rather than replacement of the aortic valve. Type A aortic dissection can sometimes compromise flow to one of the coronary arteries, the right coronary artery more often than the left. Although coronary involvement may be evident preoperatively with ischemic changes on electrocardiography, the process may be dynamic, so the echocardiographer should examine both coronary ostia to determine if they are compromised ( Figure 37 ). Color Doppler is useful to document normal or disturbed or absent flow in each coronary artery. The emergent surgical treatment of type A dissection is limited to proximal aortic segments in the majority of patients. However, when the dissection extends into the abdominal aorta, patients are at risk for malperfusion, which occurs from either of two mechanisms: static obstruction occurs when the dissection flap extends into a branch ar- tery and limits antegrade arterial flow, and dynamic obstruction occurs because of marked compression of the true lumen by a distended false lumen, resulting in impaired forward flow through the true lumen to feed the otherwise patent branch arteries. Because TEE is usually un- able to visualize the abdominal branch arteries themselves, the pres- ence of static obstruction cannot be readily assessed. However, TEE can identify true luminal compression in the distal descending thoracic aorta and confirm impaired systolic flow by Doppler. Although such findings do not necessarily indicate clinical malperfusion, at the very least they represent the substrate for dynamic malperfusion, and it is therefore important to bring this to the attention of the surgeon. On occasion, distension of the false lumen will compress the true lumen and produce malperfusion of organs or limbs. 197 Usually, stan- dard surgical repair of the ascending aorta restores flow to the true lumen partially or fully. If the true lumen remains compressed and is associated with malperfusion, further intervention (e.g., endovascu- lar stent grafting or percutaneous fenestration of the dissection flap to decompress the false lumen) may be necessary. 198
On occasion, in pa- tients with organ malperfusion, endovascular stent grafts may be placed before repair of the ascending aorta. Preoperative TEE should also evaluate the pericardial space for the presence of an associated pericardial effusion. In some cases, aortic dissection may be accompanied by a small serous effusion, but more often, the presence of an effusion is due to bleeding into the pericardial space. In such cases, the blood pooling acutely in the peri- cardial space will typically clot and appear echocardiographically as a mass sliding back and forth within a layer of pericardial fluid. This finding of a clot within the pericardial fluid heralds potential cata- strophic aortic rupture and should therefore be communicated promptly to the surgeon. After the repair of a type A aortic dissection, the echocardiog- rapher should systematically reexamine the anatomic features of the aortic valve and proximal aorta to make sure that the surgical correction has been adequate (including exclusion of the entry tear and exclusion of all proximal communications) and that the aortic valve is competent. In addition, when the dissection has extended to the distal aorta, the echocardiographer should reexamine the de- scending thoracic aorta to determine the presence of adequate flow through the true lumen. 7. Use of Imaging Procedures to Guide Endovascular Therapy.
The success of TEVAR is critically dependent on high-quality, accurate imaging before, during, and after stent-graft placement. 199
Although invasive
catheter-based angiography ( Figure 14 ) is the method of first choice for the guidance of aortic stent-graft placement, 42 TEE offers definite advantages in the hands of an experienced examiner, 59,60,62,200,201 TEE is particularly useful in the operating room and provides contributions at various phases of the procedure. In patients with type B aortic dissection, guidewire advancement and positioning can be guided by both fluoroscopy and TEE. However, unlike fluoroscopy, TEE can differentiate between true and false lumens and can confirm correct guidewire placement in the true lumen and prevent misplacement of a catheter or wire before deploying any device. In atherosclerotic aneurysms, protruding aortic
plaques at the Figure 37 Transesophageal echocardiogram from a patient with type A aortic dissection that illustrates the dissection flap (arrow) entering the ostium of the right coronary artery (RCA). LA, Left atrium. Table 10
Prevalence of IMH (as percentage of aortic dissection or nontraumatic AAS) Author Year
n % Source Mohr-Kahaly 1994
27/114 23%
420 Nienaber
1995 25/195
12.8% 421
Keren 1996
10/49 20%
422 Harris
1997 19/84
23% 423
Vilacosta 1997
15/88 17%
229 Nishigami 2000 59/130
45% 424
Ganaha 2002
66/725 9% 425 Evangelista 2003
68/302 22%
154 Attia (meta-analysis) 2009 —
426 Totals
289/1,687 17%
146 Goldstein et al Journal of the American Society of Echocardiography February 2015
proximal neck may impede tight adhesion between the stent-graft and aortic wall, leading to dangerous proximal leaks. These plaques are easily detected by TEE and not by angiography or fluoroscopy. Therefore, just before proximal stent-graft deployment, TEE is essen- tial for selecting an aortic wall segment without protruding plaques and confirming selection of the stent-graft diameter. 59,60,62 Orientation and navigation as guided by TEE can be comple- mented by the use of IVUS (usually with 10-MHz transducers) over a guidewire, thereby confirming or correcting navigation in the true lumen even at the level of the abdominal aorta and iliac arteries. In addition, intraprocedural IVUS may clarify the mechanism of branch vessel compromise when malperfusion is suspected (e.g., dynamic vs static obstruction of a branch vessel). 125,126 Thrombus formation within the false lumen can also be visualized by spontaneous echo contrast, and IMH is easily depicted as crescent-shaped or circular wall thickening. Device sizing can be very challenging with aortic dis- sections because of the possibility of compromising the true lumen. After endovascular stent graft implantation, IVUS also enables dy- namic evaluation of the success of the procedure. 200,202-204 Angiography, TEE, and IVUS are used for evaluating the expansion of stent grafts, verification of branch anastomosis and the beginning of false lumen thrombosis, and reevaluation of improved malperfusion. During a procedure, TEE may be superior for assessing retrograde type A dissection and can provide immediate information on left ven- tricular function. With the use of color Doppler, TEE is superior to angiography, and especially to IVUS, in the detection of endoleaks after stent graft implantation. 59,62,125 In several studies, TEE provided decisive additional information to angiography and fluoroscopy, leading to successful procedural changes in up to 40% to 50% of patients. 59,60,62 After stent-graft deployment, color Doppler TEE is highly useful for detecting persistent leaks that can be promptly resolved by balloon dilatation or further stent-graft im- plantations. 205
Most of these leaks are not visible on angiography. To maximize sensitivity for persistent leaks, reduced Doppler scale (25 cm/sec) can improve color signal detection. However, by itself, this can lead to false-positive diagnoses of leaks, because immediately after implantation, Dacron porosity can create temporary low blood flow through the stent (and seen with low-velocity color flow Doppler), especially when systolic blood pressure is >120 mm Hg. To prevent false-positive diagnosis of leaks, pulsed Doppler velocity assessment permits distinction between Dacron porosity (usually with velocity <50 cm/sec) and the faster flow of true persistent leaks (usually >100 cm/sec) with higher sensitivity than angiography. 126
In aortic dissection, TEE is also useful for detecting small distal reentry tears not visible on angiography; thoracic reentry tears can subse- quently be resolved by additional stent-graft deployment. 59,62,125 TEE is partially limited for visualizing the brachiocephalic and left common carotid artery ostia, and this information may be crucial to proximal positioning of the stent graft. It should be noted that TEE is useful when a Dacron stent graft is used, whereas it is not useful with polytetrafluoroethylene or Gore-Tex prostheses because polyte- trafluoroethylene acts as a barrier to ultrasound. In a recent small study, intraluminal phased-array ultrasound imag- ing proved to be superior to IVUS and to TEE in detecting communi- cations between the true and false lumens of aortic dissection. 200 However, IVUS and intraluminal phased-array ultrasound imaging catheters are disposable and therefore more expensive than TEE and cannot be performed simultaneously with stent-graft placement, whereas TEE is suited to parallel imaging and intraprocedural moni- toring.
In summary, TEE and IVUS are particularly useful for guiding endovascular procedures requiring hybrid monitoring techniques, such as a combination of stent-graft placement and open visceral bypass grafting. 59,62,206 TEE is crucial for selecting and monitoring surgical treatment and detecting complications that may require intervention. Thus, intraoperative TEE should be considered mandatory. TEE may also be useful during endovascular procedures in patients with descending aortic dissections by differentiating true and false lumens, permitting correct guidewire placement in the true lumen, helping guide correct stent-graft positioning, and identifying suboptimal results and presence of leaks. 8. Serial Follow-Up of Aortic Dissection (Choice of Tests). After the diagnosis and management of acute aortic dissec- tion, imaging techniques play a major role in prognosis assessment and
in the
diagnosis of complications during follow-up. Morphologic and dynamic information may be useful for predicting aortic dissection evolution and identifying the subgroup of patients with a greater tendency to severe aortic enlargement. Regular assess- ment of the aorta should be made 1, 3, 6 and 12 months after the acute event, followed by yearly examinations. After discharge, variables related to greater aortic dilatation were entry tear size, maximum descending aorta diameter in the subacute phase, and the high-pressure pattern in false lumen. Maximum aortic Figure 38 (A) Diagram of classic aortic dissection on the left illustrating a dissection flap separating a true lumen (TL) from a false lumen (FL). (B) An IMH lacks a dissection flap and true and false lumens and instead appears as a thickened aortic wall, typically with crescentic thickening as in this diagram. Notice that the aortic lumen is preserved (remains round and smooth walled). Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 147
diameter in the subacute phase was a significant predictor of progres- sive dilatation because, according to the law of Laplace, larger aortic diameters are associated with increased wall stress. TEE provides prognostic information in acute type A dissection beyond that provided by clinical risk variables. A flap confined to ascending aorta or a completely thrombosed false lumen has proved to have a protective role. 207
Finally, increased false luminal pressure is another important factor predictive of future false luminal enlarge- ment. In the majority of cases, high false luminal pressure relates to a large entry tear without distal emptying flow or reentry site of similar size. It may be difficult to identify the distal reentry communication; thus, in the presence of a large entry tear, indirect signs of high false luminal pressure such as true luminal compression, partial false luminal thrombosis, or the velocity pattern of the echocardiographic contrast in the false lumen should be considered. CT is the technique most frequently used for serial follow-up of aortic dissection. The large field of view of CT permits identification of anatomic landmarks that allow measurements to be obtained at identical levels as previous measurements. CT has excellent reproduc- ibility for aortic size measurement, has excellent accuracy for identi- fying entry tears and distal reentry sites, and allows the assessment of vessel malperfusion. MRI appears to be an excellent alternative technique for following patients treated medically or surgically in AAS. MRI avoids exposure to ionizing radiation and the nephrotoxic contrast agents used for computed tomographic angiography and is less invasive than TEE. Furthermore, the integrated study of anatomy and physiology of blood flow can provide information that may explain
the mechanism(s) responsible for
aortic dilatation. Time-resolved MRA can provide additional dynamic information on blood flow in entry tears. Velocity-encoded cine MR sequences have a promising role in the functional assessment of aortic dissection by virtue of quantification of flow in both lumens and the possibility of identifying hemodynamic patterns of progressive dilatation risk. For planning surgery or endovascular repair, it is very useful to demon- strate the course of the flap, entry tear location, false luminal throm- bosis, aortic diameter, and main arterial trunk involvement. Both computed tomographic angiography and MRA take advantage of postprocessing software capabilities that allow multiplane reconstruc- tions, maximum-intensity projection (MIP) and volume-rendering re- constructions. 9. Predictors of Complications by Imaging Techniques. a.
acute phase is a major predictor of complications during follow-up. Both CT and MRI are superior to TEE for measuring the aortic size distal to the aortic root. Aneurysmal dilatation of the dissected aorta will occur in 25% to 40% of patients surviving acute type B aortic dissection. Secondary dilatation of the aorta during follow-up of aortic dissection has been considered a significant predictor of aortic rupture. A descending thoracic aortic diameter > 45 mm after the acute phase and the presence of a patent false lumen have been related to aneurysm development of the false lumen (>60 mm) and surgical reintervention. A diameter > 60 mm or annual growth > 5 mm implies a high risk for aortic rupture. 208 Other studies have shown maximum false luminal diameter in the proximal part of descending aorta to be a predictor of complications. 209 However, this diameter has low reproducibility, mainly due to move- ment of the intimal flap. b. Patent False Lumen.–In addition to aortic diameter, a consistent predictor of outcomes in acute type B aortic dissection has been the hemodynamic status of the false lumen, classically divided into either a thrombosed false lumen or a patent false lumen. Persistence of pat- ent false lumen in the descending aorta is common in both dissection types and has been strongly associated with poor prognosis. Total thrombosis of the false lumen, considered a precursor of spontaneous healing, is a rare event, even after surgical repair of a type A aortic dissection. A persistently patent false lumen can be found in most type B aortic dissections during follow-up and in >70% of type A Figure 39 Transesophageal echocardiogram of a cross- sectional view of the descending thoracic aorta at 35 cm from the incisors illustrates a crescentic-shaped IMH. Table 11 Imaging features of IMH 1. Focal aortic wall thickening (crescentic > concentric) 2. Preserved luminal shape with smooth luminal border 3. Absence of dissection flap and false lumen 4. Echolucent regions may be present in the aortic wall 5. Central displacement of intimal calcium Figure 40 Transesophageal echocardiogram of a cross- sectional view of the descending thoracic aorta (Ao) illustrating a concentric IMH. There is a small right pleural effusion. 148 Goldstein et al Journal of the American Society of Echocardiography February 2015 aortic dissections after surgical repair. 197
After type A dissection repair, patent false lumen in the descending aorta is linked to survival at 5 years. Thus, use of intraoperative TEE to direct elimination of the entry tear, not just repairing the ascending aorta, is of great impor- tance.
completely thrombosed false lumens have improved outcomes, whereas patent false lumens carry an increased risk for aortic expan- sion and death. 197,210 However, in the IRAD series, partial thrombosis of the false lumen, defined as the concurrent presence of both flow and thrombus and present in a third of patients, was the strongest independent predictor of follow-up mortality, with a 2.7-fold increased risk for death compared to patients with patent false lumen without thrombus formation. 199
Prospective studies us- ing CT or MR for assessing the whole aorta are required to confirm these results.
uated by Evangelista et al., 211 who documented that a large entry tear is a strong predictor of late mortality and of the need for aortic surgical treatment. An entry tear size $ 10 mm was an optimal cutoff value for predicting dissection-related adverse events, with sensitivity of 85% and specificity of 87%. TEE and CT are superior to MRI in the assess- ment of entry tear size and location. Recently it has been shown that agreement between entry tear area by 3D TEE and CT is excellent. 151
When the entry tear is small, the flow volume that enters the false lumen is low, and thus the false luminal pressures will be low. Therefore, the combination of a large entry tear and indirect signs of high pressure of the false lumen, distinguishable by imaging tech- niques, should be considered a predictor of aortic enlargement and adverse events and warrants close follow-up. e. True Luminal Compression.–True luminal compression is an indirect sign of high false luminal pressure. However, true luminal compression assessment may be limited by intimal flap movement during the cardiac cycle, as well as local factors such as in spiral dissec- tion, that may reduce reproducibility of this finding. Patients with clear overall true luminal compression have a higher risk for rapid false luminal enlargement and further aortic complications. Yüklə 9,02 Mb. Dostları ilə paylaş:
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