Echocardiographic Valve Anatomy

Standard two-dimensional (2D) transthoracic echocardiography (TTE) in multiple image planes identifies involved valve and often allows precise definition of the etiology of valve disease, on the basis of the typical anatomic features of each disease process. When TTE image quality is suboptimal, transesophageal echocardiography (TEE) may be appropriate. Three-dimensional (3D) imaging is increasingly important in evaluation of valve disease, particularly for TEE evaluation of myxomatous mitral valve disease and for guidance of transcatheter valve interventions.3–5

Mitral stenosis most often is due to rheumatic disease with the pathognomonic features commissural fusion, thickening of the leaflet tips, and chordal thickening, fusion, and shortening, all of which are easily recognized on 2D and 3D imaging ( Figures 6-1 and 6-2). In contrast, the occasional elderly patient with functional mitral stenosis due to extension of mitral annular calcification onto the valve leaflets has thin, mobile leaflet tips, with calcification and thickening at the leaflet bases. The specific anatomic features of the rheumatic mitral valve, including severity of leaflet calcification, extent of chordal fusion, and asymmetry in commisural calcification, are important factors in predicting prognosis and in decision making for percutaneous or surgical intervention, discussed in Chapter 17.

Although aortic valve stenosis of any cause is characterized by thickened, stiff leaflets with reduced systolic opening, calcific aortic valve disease is typified by increased echogenicity and thickness in the body of the leaflets without evidence of commissural fusion, resulting in a stellate orifice in systole ( Figure 6-3). It may be difficult to separate calcific changes superimposed on a bicuspid aortic valve from calcification of a trileaflet valve, although 3D TEE may help with this distinction. Rheumatic aortic valve disease is characterized by commissural fusion with increased thickening and echogenicity along the leaflet closure lines and is invariably associated with rheumatic mitral valve disease. Congenital aortic stenosis, seen in young adults, is characterized by a deformed (often unicuspid) valve that “domes” in systole with a restrictive orifice.

Evaluation of the etiology of a regurgitant lesion by echocardiography is more challenging, given the wide range of abnormalities that can lead to valvular incompetence. Mitral regurgitation may be due to abnormalities of the mitral annulus, leaflets, subvalvular apparatus, papillary muscle, or regional or global LV dysfunction ( Figure 6-4). Echocardiographic imaging allows assessment of each of these valve components, so that the etiology of the regurgitant lesion can be discerned in many cases, as discussed in detail in Chapters 18 and 19. Selection of patients for surgical and percutaneous mitral valve interventions is discussed in Chapters 17, 21 and 22. In adults with secondary “functional” mitral regurgitation due to ischemic disease or dilated cardiomyopathy, imaging allows evaluation of both valve anatomy and the left ventricle. Quantitative evaluation of regurgitant severity also may be helpful in determining whether mitral regurgitation is the cause or consequence of ventricular dysfunction.

Aortic regurgitation may be due to abnormalities of the valve leaflets (such as a bicuspid valve and endocarditis), inadequate support of the valve structures (for example, a subaortic ventricular septal defect), or aortic root dilation (such as in Marfan syndrome or annuloaortic ectasia) ( Figure 6-5). Echocardiographic imaging provides accurate measurements of aortic root dimensions and allows detailed evaluation of valve anatomy and dynamics. A bicuspid valve is diagnosed on the basis of the typical appearance in systole of two open leaflets with two commissures; the closed valve in diastole may mimic a trileaflet valve if there is a raphe in one leaflet. Other recognized abnormalities of the valve leaflets that correspond to a specific etiology include valvular vegetations in endocarditis, redundant leaflets in myxomatous disease, and commissural thickening and associated mitral valve involvement in rheumatic disease, all of which can be recognized on echocardiographic imaging.

With aortic root disease, the specific pattern of root dilation and associated features may indicate a specific etiology, such as the “water balloon” appearance of the root in Marfan syndrome with loss of the normal tapering at the sinotubular junction and associated mitral valve abnormalities. In other cases, the pattern of root dilation is nonspecific, so incorporation of other clinical information is needed to determine the etiology of disease. For example, aortic root dilation in a patient with a systemic immune-mediated process (such as rheumatoid arthritis) is probably due to this systemic disease process. In contrast, dilation of the ascending aorta in a patient with a bicuspid aortic valve is likely related to bicuspid aortic valve disease.6,7

Right-sided valve abnormalities in adults most likely are due to residual congenital heart disease (e.g., congenital pulmonic stenosis, Ebstein anomaly of the tricuspid valve) or are secondary to left-sided heart disease (e.g., tricuspid annular dilation due to pulmonary hypertension in a patient with mitral stenosis). Again, 2D imaging usually allows determination of the valve anatomy and etiology of the valvular lesion, particularly when other aspects of the examination and clinical features are incorporated in the echocardiographic interpretation.

Transthoracic versus Transesophageal Echocardiographic Imaging

TTE provides diagnostic images in the vast majority of patients with valvular heart disease and is the standard approach both for initial evaluation and for follow-up studies. However, TTE image quality may be suboptimal in patients in whom ultrasound access is poor because of body habitus, hyperexpanded lungs, or the postoperative state. Even when TTE images are adequate, TEE provides better image resolution for posterior structures, including the mitral valve, left atrium (LA), and atrial appendage. 3D TEE imaging provides a “surgical” view of the mitral valve from the perspective of the LA, demonstrating the presence, location, and severity of prolapse; identification of chordal rupture; and evaluation of the valve commissures ( Figure 6-6).^8,9 TEE or intracardiac echocardiography is essential for excluding left atrial thrombus in candidates for balloon mitral valvotomy.

Other indications for TEE in patients with valvular disease include assessment of regurgitant severity when TTE images are nondiagnostic or when a prosthetic mitral valve is present, monitoring of surgical and transcatheter valve repair procedures (see Chapters 17 and 22), measurements for valve sizing with transcatheter valve procedures ( Figures 6-7 and 6-8), and determining the exact level of obstruction in a patient with a differential diagnosis of valvular or subvalvular obstruction. Rarely, TEE is needed for evaluation of stenosis severity when TTE data are not diagnostic.

Velocity Data and Pressure Gradients

The fluid dynamics of a stenotic valve are characterized by a high-velocity jet in the narrowed orifice; laminar, normal velocity flow proximal to the stenosis; and a flow disturbance distal to the obstruction.10,11 The pressure gradient across the valve (ΔP) is related to the high-velocity jet (V_max) in the stenosis, the proximal velocity (V_prox), and the mass density of blood (ρ), as stated in the Bernoulli equation, which includes terms for conversion of potential to kinetic energy (convective acceleration), the effects of local acceleration, and viscous (v) losses:

where (dv/dt)dx is the time varying velocity at each distance along the flow stream; and R is a constant describing the viscous losses for that fluid and orifice.

In clinical practice, the terms for acceleration and viscous losses are ignored, so that the following equation is used:

The accuracy of the simplified Bernoulli equation in measuring transvalvular pressure gradients has been shown in in vitro studies, animal models, and clinical studies of patients with valvular disease ( Table 6-2). However, accuracy depends on optimal data acquisition; specifically, care is needed to obtain a parallel intercept angle between the continuous-wave (CW) Doppler beam and the direction of blood flow in order to avoid underestimation of the velocity and, hence, pressure gradient across the valve. The high velocities encountered in aortic and pulmonic stenosis mandate the use of CW Doppler echocardiography to avoid signal aliasing. A dedicated small dual-crystal CW Doppler transducer is recommended. Pulsed or high pulse repetition frequency Doppler echocardiography can be used for evaluation of the lower velocities seen in mitral and tricuspid stenosis with the advantage of a better signal-to-noise ratio and clearer definition of the diastolic deceleration slope than with CW Doppler echocardiography. Other potential technical sources of error in measuring transvalvular velocities include poor acoustic access with an inadequate flow signal, incorrect identification of the flow signal (e.g., mistaking the mitral regurgitation signal for aortic stenosis), respiratory motion, and measurement variability. In addition, physiologic sources of error include beat-to-beat variability with irregular rhythms and interim changes in volume flow rates leading to changes in velocity and pressure gradient.

For aortic stenosis, the maximum velocity across the stenotic valve provides the most important diagnostic and prognostic information. As indicated by the Bernoulli equation, there is a consistent relationship between maximum velocity and maximum pressure gradient. In addition, there is a consistent relationship between maximum velocity and mean gradient in native aortic valve stenosis, so that maximum velocity, maximum gradient, and mean gradient all convey the same information about the degree of valve narrowing. Increasingly, clinicians rely on velocity data alone in clinical decision making, without the intermediate step of converting velocities to pressure gradients.

Valve Area Concept and Measurement

Pressure gradients and velocities depend on the volume flow rate across the valve as well as the degree of valve narrowing. Valve area (or the size of the stenotic orifice) is a useful measure of stenosis severity that, at least in theory, more closely reflects valve anatomy independent of the flow rate across the valve. Valve area can be calculated from invasive data as discussed in Chapter 7 or noninvasively from 2D and Doppler echocardiographic data using the continuity equation.

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