The aortic valve is best imaged in the parasternal views.222 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,223 while doming of the leaflets suggests congenital aortic stenosis and is usually encountered in younger patients (B+-B; Fig. 13-59).Echocardiography can distinguish valvular from sub- and supravalvular AS, can accurately identify bicuspid valves, and can delineate the presence of LV hypertrophy.225,226 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 (B+;Si Fig. 13-60). 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.223 Therefore, attempts to measure valve area by transthoracic planimetry have been unsuccessful, although multiplane TEE has been of greater value!33 Fig. 13-61). 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.227 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.228 (See also Chap. 56).
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 catheterization117 119 Fig. 13
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. 56).
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 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.111-115 As both AS jet velocity and aortic annulus 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 LV hypertrophy 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 aortic valve 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.155,229 Cardiac catheterization is still necessary for the delineation of coronary anatomy.
In contrast to AS, the aortic valve leaflets are often anatomically normal by echocardiography in patients with aortic regurgitation (AR).230,231 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 mitral valve leaflet, aortic root enlargement, and incomplete coaptation of the aortic valve leaflets.232,233 The important M-mode finding of premature diastolic closure of the mitral valve prior to the onset of systole due to LV filling by the regurgitant jet signifies acute, severe AR234 (Fig. 13-63) and the need for surgery (Chap. 56).
Perhaps the most important contribution of echocardiographic tissue imaging to the assessment of AR is in identifying the etiology.235 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 aortic valve Fig. 13-60). 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 views236 (B+Si Fig. 13-64A B, and Q+iB C, Plate 60). 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 apex237 Fig. 13-62). 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.238 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 (B-H0i Fig. 13-65).237 240 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 (B+-B- Fig. 13-65). Severe, acute AR can also cause diastolic MR (H-»-0- Fig. 1364 C, Plate 60). 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 aortic regurgitation as assessed by invasive methods.240 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 compliance of the left ventricle. 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.236 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.241 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.236 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).440 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.110,242-244 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.245 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. 13-66).
Determination of the optimal timing of surgical intervention in patients with AR remains a difficult problem in clinical medicine (see also Chap. 56). Several criteria derived from echocardiographic recordings have been proposed to guide this decision.246-249 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.250 Considerable debate continues regarding this issue, however, and no universally accepted echocardiographic criteria exist by which to determine the optimal role for surgical treatment.
The thoracic aorta is best visualized from the left and right parasternal positions and from the suprasternal notch.254 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.252 Although 2D imaging is used most commonly, M-mode recordings of the aortic root facilitate precise measurement of its dimensions.
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