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blood pressure. Aortic distensibility and the stiffness index can be determined from the changes in the aortic diameter from systole to diastole and from changes in the arterial pressure using the following formula: Distensibility À 10
$mm Hg À1 Á ¼ Area
systole À Area
diastole Area
diastole $Pulse pressure $1; 000: For these calculations, the pulse pressure should be measured ideally at the same level of the aorta at which the aortic diameter is Figure 9
Diagram illustrating the potential pitfall of obtaining an oblique cut resulting in an ‘‘ellipsoid’’ cross-section that overes- timates the true diameter. This is especially a problem when the descending aorta is tortuous. Figure 8 Computed tomographic scan image of aortic root illus- trates that the mean difference of the aortic root diameter is about 2 mm larger measured by the anteroposterior diameter (sinus-sinus) shown by red arrow than by the sinus- commissure diameter (black arrow). Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 127
measured. In clinical practice, however, brachial artery pressure can be used, even though the pulse pressure obtained from the brachial artery may be slightly higher than that obtained from the aorta because of the amplification phenomenon, which is more apparent in young individuals. 31 2. Regional Indices of Aortic Stiffness: Pulsewave Velocity (PWV). PWV is defined as the longitudinal speed of the pulsewave in the aorta. PWV is inversely related to aortic elasticity. Hence, a stiffer aorta will conduct the pulsewave faster than a more compliant aorta. Central pressure, at the level of the ascending aorta, is produced as a combination of the antegrade wave from the left ventricle and the retrograde ‘‘reflective’’ waves from the pe- riphery. In young individuals, because the aorta is more elastic, the pulsewave speed is low, so the retrograde flow arrives in the prox- imal aorta during diastole. As a result of aortic stiffening, the PWV increases, and the retrograde flow arrives in the proximal aorta earlier in systole, leading to a greater LV afterload and decreased diastolic pressure. Reported normal values for invasively determined PWV measure- ments in middle-aged humans are 4.4 6 0.4 m/sec in the aortic root, 5.3 6 0.2 m/sec in the proximal descending thoracic aorta, 5.7 6 0.4 m/sec in the distal thoracic descending aorta, 5.7 6 0.4 m/sec in the suprarenal abdominal aorta, and 9.2 6 0.5 m/sec in the infrarenal aorta. 32 Carotid-femoral PWV is considered to be the gold-standard measure of arterial stiffness, especially because it is simple to obtain and because multiple epidemiologic studies have demonstrated its predictive value for cardiovascular events. However, the ability of a given individual’s PWV value to predict aortic events has not been previously evaluated. 33 A recent expert consensus adjusted this threshold value to 10 m/sec by using the direct carotid-to-femoral dis- tance.
34 The main limitation of PWV interpretation is that it is signif- icantly influenced by arterial blood pressure. Multimodality imaging techniques provide a unique opportunity to assess aortic PWV by the formula: Aortic PWV ðm=secÞ ¼ Distance
ðmmÞ Transit time ðmsecÞ : Echocardiography can accurately estimate the transit time be- tween aortic levels by the subtraction of the time between a fixed reference in the QRS complex and the beginning of the flow at the two levels studied. Distance can be grossly estimated externally with a tape. MRI can measure the PWV using the transit time of the flow curves from a phase-contrast acquisition. Transit time can be calculated by the upslope approach, which has been described previously and cor- relates more with age and aortic stiffness indices than point-to-point approaches such as foot-to-foot and half-maximum methods. 35 Distance can accurately be measured at the centerline of the aorta between aortic levels studied. A normal-size aorta may be functionally abnormal. Thus, determi- nation of aortic function may help define the nature of the underly- ing disease and give prognostic information in some diseases. This was emphasized by Vriz et al., 4 who stated that aortic stiffness should be taken into account when increases in aortic diameter are de- tected.
In summary, aortic biophysical properties can be easily and reli- ably assessed by imaging techniques, particularly Doppler echocardi- ography and phase-contrast MRI. This evaluation may provide Table 3
Plain CXR findings in aortic dissection 1. Mediastinal widening 2. Abnormalities in region of aortic knob 1. Enlargement (expansion of aortic diameter) 2. Presence of double density (due to enlargement of false lumen)
3. Irregular contour 4. Blurred aortic knob (indistinct aortic margin) 3. Displacement of intimal calcium 4. Discrepancy in diameters of ascending and descending aorta 5. Displacement of trachea, left main bronchus, or esophagus 6. Pleural effusion (more common on the left) Figure 10 Sites for measurements of the aortic root and ascending aorta. This diagram illustrates the four sites at which measurements are recommended: 1 = aortic valve annulus (hinge point of aortic leaflets), 2 = aortic root at sinuses of Val- salva (maximal diameter, usually midpoint), 3 = STJ, 4 = prox- imal tubular portion of the ascending aorta. Ao, Aorta; LA, left atrium; LV, left ventricle. Figure 11 Transthoracic echocardiographic suprasternal notch view of the distal ascending aorta (Asc Ao), aortic arch, supra- aortic vessels (arrows), and proximal descending thoracic aorta (Desc Ao). 128 Goldstein et al Journal of the American Society of Echocardiography February 2015 important pathophysiologic and prognostic information that may have clinical implications both in disease states and in the general population. II. IMAGING TECHNIQUES A. Chest X-Ray (CXR) Most articles describing the use of imaging to evaluate patients with suspected AAS have focused on the role of CT, MRI, echocardiog- raphy, and aortography. Although routine CXR rarely provides a definitive diagnosis, it can provide several important diagnostic clues to aortic diseases that prompt further evaluation. Table 3 lists some of the common and uncommon plain CXR findings of aortic diseases. Moreover, in cases of asymptomatic or chronic aortic diseases, CXRs may actually provide the first clue to aortic pathology. Importantly, CXR can identify other chest disorders that may contribute to a patient’s illness (e.g., pneumonia, pneumothorax, rib fracture). Nevertheless, although CXR may be valuable, it is neither sensitive nor specific for AAS. 36-38 Moreover, normal results on CXR with respect to the aorta should never prevent or delay the further diagnostic evaluation of a patient with a suspected AAS. In summary, although useful in drawing attention to the possibility of aortic disease, its low sensitivity, specificity, and interobserver agreement limit the role of the CXR. B. TTE
The thoracic aorta should be routinely evaluated by TTE, 39-42
which provides good images of the aortic root, adequate images of the ascending aorta and aortic arch in most patients, adequate images of the descending thoracic aorta in some patients, and good images of the proximal abdominal aorta. New advances in imaging quality and harmonic imaging have significantly improved the assessment of the aorta by TTE. The aortic root and proximal ascending aorta are best imaged in the left parasternal long-axis view. The left lung and sternum often limit imaging of the more distal portion of the ascending aorta from this transducer position. In some patients, especially those with aortic dilatation, the right parasternal long-axis view can provide supplemental information. The ascending aorta may also be visual- ized in the apical long-axis (apical three-chamber) and apical five- chamber views and (especially in children) in modified subcostal views.
There is usually no clear echocardiographic delineation between the sinus and tubular portion of the ascending aorta, but occasion- ally a fibrotic or sclerotic ridge, located at the STJ, is imaged. This ridge may be prominent and should not be confused with vegeta- tion, abscess, mass, atherosclerotic plaque, dissection flap, or supra- aortic stenosis. The maximum diameter of the aorta is normally in the root (sinus portion), which is immediately distal to the aortic valve.
Echocardiographic measurements of the aortic root will vary in an individual patient at different levels ( Figure 10 ). 3,43,44 The aortic diameter is smallest at the annulus and largest at the mid–sinuses of Valsalva. The tubular portion of the ascending aorta is typically about 10% smaller than the diameter at the sinus level. 45 The aortic arch is usually easily visualized from the suprasternal view. Portions of the ascending and descending aorta can be visualized simulta- neously. One or more of the three arch branches can usually be imaged: the left carotid and left subclavian arteries are identified in >90% of cases and the brachiocephalic (inniminate) artery in up to 90% (
Figure 11 ). Just distal to the left subclavian artery is the level of the ligamentum arteriosum, which is a common site of atheroscle- rosis, and a shelf or indentation (a ductus diverticulum) is sometimes imaged in this region. The descending thoracic aorta is often incompletely imaged by TTE. A cross-sectional view of the descending thoracic aorta may be seen in the parasternal long-axis view, as it passes posteriorly to the left atrium near the atrioventricular groove. It can also be seen in short axis in the apical four-chamber view. By rotating the trans- ducer 90
, a long-axis view of the midportion of the descending thoracic aorta may be obtained. A portion of the descending thoracic aorta can also be imaged from a suprasternal view. In patients with left pleural effusion, scanning from the back may also provide satis- factory views of the descending thoracic aorta. However, the distal descending thoracic aorta frequently cannot be imaged clearly because of reduced resolution in the far field. Moreover, physical characteristics of some patients exceed the limit of ultrasound pene- tration.
The normal descending thoracic aorta is smaller than both the aortic root and ascending aorta. As it descends, its diameter progres- sively narrows from 2.5 to 2.0 cm. Larger dimensions are reported in patients with hypertension, aortic valve disease, and coronary athero- sclerosis. 46 The aorta is consistently about 2 mm smaller in female than it is in male subjects. 47 A substantial portion of the upper abdominal aorta can be easily imaged in subcostal views, to the left of the inferior vena cava. This should be routinely performed as a part of a 2D echocardiographic study. 16
artery can also be imaged. When present, aneurysmal dilatation, external compression, intra-aortic thrombi, protruding atheromas, and dissection flaps can be imaged, and flow patterns in the abdom- inal aorta can be assessed. The infrarenal abdominal aorta is best imaged as part of an abdominal ultrasound examination by use of a linear array probe. In summary, to reliably evaluate patients with suspected aortic dis- ease, the entire thoracic aorta must be imaged well. This is possible in some, but not all, patients on systematic TTE. TTE is particularly useful Figure 12 Transesophageal echocardiographic deep transgas- tric view, which illustrates the aortic root (Ao R), the entire ascending aorta (AA), and the proximal arch (not labeled). The left coronary artery is also imaged (black arrow). Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 129
for evaluating the aortic root, and the ascending aorta and arch may also be adequately visualized in patients with good acoustic windows. TTE is less helpful for evaluating the descending thoracic aorta. However, TTE is an excellent screening tool for detecting aneurysms of the upper abdominal aorta. C. TEE
TEE, introduced clinically in the late 1980s, has had a major impact on the evaluation of numerous diseases involving the aorta. TEE has two main advantages over TTE. First, superior image quality can be obtained from the use of higher frequency transducers than are possible with TTE. Second, because of the close proximity of the esophagus to the thoracic aorta, TEE provides high-quality imaging of nearly all of the ascending and descending thoracic aorta. 48-50
TEE incorporates all the functionality of TTE, including 3D imaging, which can reliably interrogate cardiovascular anatomy, function, hemodynamics, and blood flow. The current multiplane TEE transducer consists of a single array of crystals that can be rotated electronically or mechanically around the long axis of the ultrasound beam in an arc of 180
. With rotation of the transducer array, multiplane TEE produces a continuum of transverse and longitudinal image planes. 1. Imaging of the Aorta. As mentioned, the anatomic proximity of the thoracic aorta and the esophagus allows superb visualization of the aorta using TEE. The multiplane transesophageal echocardio- graphic examination of the aorta is conducted as follows 51 : with the tip of the transesophageal echocardiographic probe in the esophagus, the ascending aorta is best visualized from a 100
to 140
view: the image is analogous to the transthoracic echocardiographic parasternal long-axis view (but ‘‘flipped’’ upside down if the transesophageal echocardiographic probe ‘‘bang’’ is at the top). This view can be opti- mized by carefully rotating the transducer between 100
and 140
. Short-axis views of the aortic root and ascending aorta can be obtained from the 45
to 60 angle, usually with an anteflexed probe. From the midesophagus at 0
, the probe needs to be rotated posteri- orly to obtain short-axis images of the descending thoracic aorta. Figure 13 IVUS documentation of an IMH evolving to full dissection on (left) with corresponding contrast aortogram showing only luminal compression (right). Figure 14 IVUS evaluation of an aortic dissection extending across the level of the left renal artery (left frame, arrow); the IVUS in the true lumen identifies the renal artery ostium and the reentry point (right frame, arrow) at the orifice level. FL, False lumen; TL, true lumen.
130 Goldstein et al Journal of the American Society of Echocardiography February 2015
While keeping the thoracic aorta in view, the probe can be withdrawn to image upper thoracic levels of the descending aorta or advanced to sequentially image the lower thoracic and upper abdominal aorta. With the transducer array at 90
, a longitudinal view of the aorta can be obtained. By advancing the probe into the stomach, the proximal portion of the abdominal aorta and the celiac trunk can be seen. The mid and distal abdominal aorta are usually not seen because of difficulty main- taining good contact with the mucosa of the stomach. To obtain images of the arch, the transesophageal echocardiographic probe needs to be facing posteriorly and withdrawn from the midesopha- gus. With the transducer array at 90
, a short-axis view of the trans- verse arch can be obtained. It is usually possible to visualize the takeoff of the left subclavian artery, but the left common carotid and brachiocephalic arteries can be difficult or impossible to image and usually require careful clockwise rotation of the probe. A portion of the distal ascending aorta and proximal aortic arch may not be visible because of interposition of the trachea. This ‘‘blind spot’’ can be partially resolved with longitudinal views. An additional view, the deep transgastric view, can sometimes image the entire ascending aorta and often the proximal arch ( Figure 12 ). D. Three-Dimensional Echocardiography Real-time 3D TEE, a relatively new technology, appears to offer some advantages over 2D TEE in a growing number of clinical applica- tions. 52-56
However, as of this writing, there is limited information regarding the clinical application of this novel technology to the thoracic aorta. 57 Moreover, 3D TEE has some limitations. Like 2D TEE, it often fails to adequately visualize the distal ascending aorta and the aortic arch and its branches, because of interposition of the trachea. In addition, spatial imaging of the thoracic aorta is limited because of the 90
im- age sector, which is too narrow to include long segments of the thoracic aorta and therefore limits topographic orientation. In sum- mary, recent advances in 3D TEE provide an opportunity to recon- sider the role of TEE for diagnosing and monitoring patients with aortic diseases. Future experience will be required to verify its benefits and establish its value relative to CT and MRI. E. Intravascular Ultrasound (IVUS) IVUS is performed by introducing a miniature, high-frequency (10–30 MHz) ultrasound transducer mounted on the tip of a disposable catheter, through a large arterial (usually femoral) sheath, and advanced over conventional guidewires using fluoro- scopic guidance. Less commonly, the IVUS imaging catheter can be inserted into the femoral vein, navigated into the inferior vena cava, and aimed at the adjacent aorta. IVUS produces an axial view that is a 360
real-time image. Consecutive axial images can be obtained during a ‘‘pullback’’ of the ultrasound catheter. This procedure can be safely performed in a few minutes. 58 Because of its intraluminal position, IVUS permits visualization of the aortic wall from the inside. This intraluminal perspective can pro- vide information that supplements the other imaging modalities. 59-62 Using the pullback technique, luminal diameter, cross-sectional area, and wall thickness can be measured. In addition to providing mea- surements, IVUS also provides qualitative information on nearly all aortic pathologies, including aortic aneurysms, aortic dissections, atherosclerosis, penetrating ulcers, and traumatic lesions ( Figures 13 and 14
). Unlike TEE, IVUS can also determine the dissection characteristics in the abdominal aorta. 1. Limitations. The normal aorta appears on IVUS as a circular cross- sectional image with an intact wall and a clear lumen. The ultrasound catheter and the guidewire are seen within the lumen. In some in- stances, it can be difficult to obtain complete cross-sectional images of the aorta within a single frame of the image display at the arch and locations where the aorta is significantly dilated, because of diffi- culty maintaining the ultrasound catheter in a central and coaxial orien- tation and because of the limited penetration with high-frequency transducers. This limitation can be partially overcome by periodic reor- ientation of the ultrasound catheters. There are also concerns with IVUS measurements. Off-center measurements or those taken in tortuous portions of the aorta (tangential measurements on a curve) do not reflect a true centerline diameter, may provide an oblique slice, and are less accurate than centerline computed tomographic measure- ments.
63 Another major limitation of IVUS is that it lacks Doppler capabilities (color Doppler can detect flow into small arteries, false luminal flow, and endoleaks). Last, the high cost of the disposable trans- ducers and invasive nature of the technique limit IVUS for most clinical applications other than guidance of endovascular procedures. F. CT Multidetector computed tomographic scanners ( $64 detector rows) are the currently preferred technology for aortic imaging. Computed tomographic aortography (CTA) remains one of the most frequently Figure 15 Volume-rendered image from an electrocardiograph- ically gated thoracic computed tomographic aortogram in the presurgical study of a patient with ascending aortic aneurysm. Note the excellent quality of both the aortic and coronary ves- sels, with calcified atheromatous plaques of the coronary ar- teries.
Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 131
used imaging techniques for diagnosis and follow-up of aortic condi- tions in acute as well as chronic presentations. This popularity reflects its widespread availability, accuracy, and applicability, even for critically Yüklə 9,02 Mb. Dostları ilə paylaş:
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