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Diseases of the Aortic Valve and Aorta
Aortic Stenosis The aortic valve is best imaged in the parasternal views.138 The leaflets are thin, linear structures. All three can be visualized in the short-axis view and produce a triangular orifice during systolic opening. The long-axis view exhibits the right and usually the noncoronary leaflets, which normally open to the walls of the aorta. Mild thickening and reduction of mobility is often observed in the elderly (aortic sclerosis) and is associated with an increased risk of CAD. In older adults, acquired aortic stenosis (AS) is manifested by markedly thickened, often calcified, immobile aortic valve leaflets,139 while doming of the leaflets suggests congenital aortic stenosis and is usually encountered in younger patients (Fig. 15–59). Echocardiography can distinguish valvular from sub- and supravalvular AS, can accurately identify bicuspid valves, and can delineate the presence of LVH.140 Subaortic stenosis may be caused by asymmetrical septal hypertrophy with systolic anterior mitral motion, a subaortic membrane, or (less commonly) a subaortic tunnel. Bicuspid valves exhibit an oval rather than triangular orifice (Fig. 15–60). Although the severity of stenosis can be assessed semiquantitatively by 2D and M-mode image echocardiography, valvular calcification may shadow the leaflets or produce reverberations and obscure their motion.139 Therefore, attempts to measure valve area by transthoracic planimetry have been unsuccessful, although multiplane TEE has been of greater value109 (Fig. 15–61). Thus, 2D-echocardiographic imaging accurately detects the presence and etiology of AS but not the severity. Likewise, CFD demonstrates turbulent flow through the aortic valve and may guide continuous wave interrogation but provides little quantitative data. The use of Doppler echocardiography and the modified Bernoulli and continuity equations have now made noninvasive calculation of aortic gradients and valve area routine and have affected utilization of cardiac catheterization in AS patients (Chap. 66).
The cornerstone of the ultrasound evaluation of AS is CW Doppler interrogation through the aortic valve. The calculated gradient using the peak Doppler velocity [4(AS velocity)2] correlates closely with the peak instantaneous gradient measured at catheterization87,88 (Fig. 15–62). In interpreting echocardiographic studies, it is important to distinguish between the peak instantaneous pressure gradient, the mean gradient, and the peak-to-peak gradient. The first two physiologic parameters represent simultaneous pressure differences between LV and aorta and can be measured accurately by Doppler echocardiography. The peak-to-peak gradient, commonly used in the catheterization laboratory, compares the highest pressures reached in the LV and aorta (even though not simultaneous) and is uniformly lower than the peak instantaneous gradient recorded by Doppler. Therefore, the maximal Doppler gradient does not correlate with the peak-to-peak catheterization gradient, and comparisons between the two should be avoided (Chap. 66).
A number of potential sources of error exist in the estimation of the transvalvular aortic gradient by CW Doppler recordings. It is imperative that Doppler signals from the stenotic jet be obtained with an angle of incidence of less than 20 degrees. Since the direction of the jet rarely can be known with precision from 2D techniques, each examination must employ all possible windows and angulations, including apical, parasternal, and suprasternal transducer positions. Also, one must be careful to account for the proximal flow velocity in the Bernoulli equation if it is 1.5 m/s or greater. Finally, since some degree of pressure recovery occurs distal to the aortic valve leaflets, it is important to record continuous wave signals as close to these structures as possible. Values for the aortic valve area can be calculated using the continuity equation by measuring the velocity of the jet across the aortic valve with CW Doppler, the velocity in the LV outflow tract just proximal to the valve with PW Doppler, and by deriving the area of the outflow tract from the diameter of the aortic annulus. Results from the continuity equation have been found to correlate well with the area calculations based on catheterization data and the Gorlin formula.85,86 CW Doppler can occasionally overestimate peak systolic pressure gradients, especially in patients with narrow aortic roots. As both AS jet velocity and aortic annular radius are squared in the continuity equation, accurate determination of these parameters is essential for reliable measurements. When atrial fibrillation is present, the peak Doppler velocity still correlates with peak instantaneous gradient through the aortic valve, but calculations of valve area may be problematic, as the outflow tract and peak aortic velocities are not measured simultaneously. In summary, a comprehensive echocardiographic examination in a patient with AS should establish both the presence and severity of disease. Echocardiographic imaging should identify the structural abnormality involving either the subvalvular, valvular, or supravalvular area; distinguish congenital from acquired etiologies; and evaluate the state of LVH and function. CW Doppler recordings should provide accurate measurements of instantaneous and mean transaortic valvular gradients, and the continuity equation should provide reliable estimates of AoV area. In cases where the relative roles of orifice stenosis and LV dysfunction are uncertain, TEE imaging or Doppler recordings during inotropic stimulation with dobutamine may be of value.109,141 In addition, dobutamine echocardiography is helpful in distinguishing high-risk patients with aortic stenosis and severe LV dysfunction. Cardiac catheterization is still necessary for the delineation of coronary anatomy. Aortic Regurgitation In contrast to AS, the aortic valve leaflets are often anatomically normal by echocardiography in patients with aortic AR.142 2D and M-mode echocardiography often provide indirect evidence of the presence of AR, including signs of LV volume overload, diastolic fluttering of the anterior MV leaflet, aortic root enlargement, and incomplete coaptation of the aortic valve leaflets.143 The important M-mode finding of premature diastolic closure of the MV prior to the onset of systole due to LV filling by the regurgitant jet signifies acute, severe AR (Fig. 15–63) and the need for surgery (Chap. 66).
Perhaps the most important contribution of echocardiographic tissue imaging to the assessment of AR is in identifying the etiology. Thus, thickened leaflets that are restricted in movement are observed in patients with acquired AS, while oval doming of two functional leaflets will be observed in the presence of a bicuspid AoV (Fig. 15–60). AR due to infectious endocarditis can be identified by the presence of valvular vegetations, while regurgitation due to diseases of the aorta are manifest by anatomic changes of the vessel. Less common etiologies of AR, such as those associated with subvalvular pathology or ventricular septal defect, may also be recognized by echocardiographic imaging. Although the findings yielded by echocardiographic imaging are useful, Doppler interrogation is necessary to obtain direct evidence of the presence and severity of AR. Screening with CFD demonstrates turbulent flow in the LV outflow tract during diastole in virtually all views144 (Fig. 15–64A,B, and C). The jet is typically elliptical and may be located anywhere in the LV outflow tract. CW Doppler spectral recordings from this jet yield a high-velocity diastolic signal directed toward the apex (Fig. 15–62). Since AR jet velocity accurately reflects the diastolic pressure gradient between aorta and LV, it is maximum at the point of valve closure and decreases throughout diastole.145 The flow pattern of AR may be readily distinguished from mitral inflow in that it is higher in velocity, begins immediately after aortic valve closure, generally has a much slower deceleration, and does not have an increased velocity following atrial contraction.
Several approaches exist for the quantitation of AR by echocardiography. Conventional echocardiographic imaging can provide evidence of the presence and extent of LV volume overload. More direct evidence of the severity of AR can be derived from the deceleration rate of the jet recorded by CW Doppler (Fig. 15–65).145 In the presence of mild degrees of AR, the transvalvular pressure gradient will be maintained throughout diastole, creating a high-velocity jet with a minimal deceleration rate. Conversely, severe AR reduces aortic pressures and increases LV pressures in diastole, eliminating the pressure gradient and creating a rapid jet deceleration to a low velocity (Fig. 15–65). Severe, acute AR can also cause diastolic MR (Fig. 15–64C). The most common approach to assessing the deceleration rate of the AR jet is by calculating the time required for the velocity to fall to one-half of the maximal pressure equivalent, a technique similar to the pressure half-time measurements performed in the quantitation of mitral stenosis (MS). Previous studies have demonstrated that a pressure half-time of less than 250 ms reliably identifies patients with severe degrees of AR as assessed by invasive methods. Application of the pressure half-time approach to quantifying AR must take into account that, since the deceleration rate is a reflection of pressure gradient, it is determined by both the volume of AR and the LV compliance. Accordingly, ventricles that vary greatly in stiffness or distensibility will yield different AR deceleration rates for the same regurgitant volume.
The estimate of severity most commonly derived from echocardiography is the size of the AR jet by CFD.144 Conceptually, jets that are distributed over a small area of the LV outflow tract represent lesser degrees of AR than jets that penetrate widely and to the level of the papillary muscles. Some studies have demonstrated a general correlation between jet length and severity of AR.146 The optimal results have been obtained when the width of the AR jet just proximal to the valve was expressed as a percentage of the width of the LV outflow tract; a jet occupying 50 percent or more of the outflow tract correlates with severe regurgitation by angiography.144 Quantitation of AR based upon the size of the flow disturbance is subject to errors induced by the other factors that influence jet area: transvalvular pressure gradient, volume and compliance of the receiving chamber, regurgitant orifice, the Coanda effect (wall effect), and technical factors relating to the operator and instrument settings. In addition, entrainment and displacement of RBCs in the LV outflow tract also influence the size of the regurgitant jet. Finally, convergence of AR with normal transmitral filling may obscure the flow disturbance. Therefore, assessment of the severity of AR by analysis of the size and shape of the flow disturbance is at best semiquantitative. The AR volume can be estimated by comparing volumetric measurements of LV inflow and LV outflow calculated from annular velocity and cross-sectional area (derived from pulsed Doppler and 2D images respectively).84 This method is contingent upon the absence of valvular stenosis and of other regurgitant lesions. In the setting of AR, the volume ejected through the aortic annulus represents both systemic flow and regurgitant volume, while the volume coursing through the mitral annulus represents only systemic flow. Thereby, LV outflow will exceed LV inflow by the amount of the regurgitant volume.84,147 This technique can provide useful estimates of regurgitant volume, but with any flow volume calculation by echocardiography, errors in technique and the assumptions involved in volume calculation can result in significant errors. An alternate quantitative approach derives estimates of regurgitant fraction from reverse diastolic flow in the aorta.148 Assuming a constant cross-sectional aortic area, comparison of integrated flow velocities during forward systolic flow and retrograde diastolic flow should yield an estimate of regurgitant fraction. Although this is somewhat imprecise, the presence of a significant flow reversal in the aorta visualized by color or spectral Doppler is a reliable marker of severe AR (Fig. 15–66).
Determination of the optimal timing of surgical intervention in patients with AR remains a difficult problem in clinical medicine (Chap. 66). Several criteria derived from echocardiographic recordings have been proposed to guide this decision. Most prominently, an LV end-systolic dimension of 55 mm or greater with a shortening fraction of 25 percent or less have been advocated as sufficient criteria for surgical intervention in the absence of symptoms. However, no universally accepted echocardiographic criteria exist by which to determine the optimal role for surgical treatment. Diseases of the Aorta The thoracic aorta is best visualized from the left and right parasternal positions and from the suprasternal notch. The descending aorta may also be imaged from subcostal and modified apical views. Normally, short-axis images of the aortic root yield a circular structure, while long-axis images exhibit two parallel linear walls with a maximal diameter of 35 mm.149 Although 2D imaging is used most commonly, M-mode recordings of the aortic root facilitate precise measurement of its dimensions. Aortic Dissection Echocardiography has dramatically changed the diagnostic approach to aortic dissection. TTE is a convenient screening test (Fig. 15–67) and often enables accurate detection of ascending aortic dissection. The diagnostic findings include a dilated aorta with a mobile intimal flap that presents as a thin, linear signal within the lumen. Transthoracic imaging is unreliable for detection of descending aortic dissection, although it occasionally visualizes the complete length of the thoracic aorta (Chap. 98).
Although several noninvasive methods exist to diagnose aortic dissection, TEE has become the procedure of choice in many hospitals because of its accuracy, portability, rapid procedural time, and ability to provide data regarding valvular regurgitation and LV function.99,111,150,151 Except for a short portion of the proximal aortic arch, which is obscured by the bronchus, multiplane TEE provides excellent visualization of the entire thoracic aorta and high accuracy in detecting aortic enlargement, intimal tears, and false lumen thrombus (Fig. 15–68). CFD may reveal communications between true and false channels (Fig. 15–51; Fig. 15–69). TEE also appears useful for the diagnosis of aortic intramural hematoma, an increasingly recognized disorder which has a clinical prognosis similar to that of classic dissection. In this disorder, hemorrhage occurs within the aortic media, but an intimal tear (and a dissection flap) is absent. The finding of a curvilinear, asymmetric density within the aortic wall in a patient with typical symptoms of dissection strongly suggests a diagnosis of aortic intramural hematoma.152
Aortic Aneurysm Aneurysms of the aorta may be saccular or fusiform and are recognized as localized or circumferential areas of aortic enlargement, often with thin walls. TTE is especially useful in detecting ascending aortic dilatation but can also visualize descending thoracic and abdominal aortic aneurysms.149,153 Echocardiography has been used extensively to assess aortic pathology in patients with Marfan's syndrome.154 The nature of the lesion is relatively specific in that there is symmetrical dilatation of the annulus, sinuses of Valsalva, and aortic root (Fig. 15–70A). Aortic leaflet coaptation may be compromised leading to AR. Echocardiography is helpful in determining prognosis and optimal timing of aortic root replacement.
Sinus of Valsalva aneurysms are also well visualized by both TTE and TEE.155 These lesions cause asymmetric dilatation of the aortic root and seem to affect the right coronary sinus most frequently. They are prone to rupture, often into the right heart (Fig. 15–70B). Doppler echocardiography in such settings demonstrates fluttering of the TV, a color jet crossing from the aortic root into the right heart, and occasionally diastolic opening of the PV. Congenital aortic disease, such as supravalvular aortic stenosis (SAS), aortic coarctation, patent ductus arteriosus, and truncus arteriosus also can be detected with echocardiography (see "Congenital Heart Disease," below, as well as Chaps. 63 and 64).140 In these conditions, suprasternal and transesophageal imaging are often helpful. SAS is recognized as an "hourglass" narrowing or a discrete fibrous ridge just distal to the leaflets, while coarctation presents a more localized, abrupt luminal reduction in the descending aorta or distal portion of the aortic arch. Patent ductus arteriosus and truncus arteriosus are often best identified by virtue of the accompanying flow disturbance on CFD.156 Aortic Atherosclerosis As mentioned in the section on TEE, recent studies suggest that aortic atherosclerosis is an important cause of stroke and embolic events.113 Mobile and protruding intimal plaques have been detected by TEE (Fig. 15–53) in patients with stroke with a prevalence greater than in controls, a finding not previously appreciated by other imaging techniques. Optimal treatment for extensive aortic atherosclerosis is currently unknown, although warfarin appears useful.115,157 It appears that the presence of aortic arch plaques and atherosclerosis increase the risk of perioperative stroke.158 Given this, discovery of such plaques prior to cardiopulmonary bypass should prompt adjustment of cannula placement to avoid dislodging the aortic debris. Penetrating aortic ulceration, which affects the descending aorta and mimics the clinical syndrome of acute aortic dissection, may also be diagnosed by TEE (Fig. 15–71). The diagnosis is based on visualization of a localized defect with protrusion of the ulcer into the vessel wall (in the absence of dissection). This disease entity, which occurs in the setting of atherosclerosis, warrants urgent surgery to avoid aortic rupture. Aortic tears induced by trauma are also accurately detected by TEE (Fig. 15–72).
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