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Chapter 13: THE ECHOCARDIOGRAM DISEASES OF THE MITRAL VALVE Mitral Stenosis

Detection of mitral stenosis (MS) was one of the earliest clinical applications of echocardiography274 (see Chap. 57). In most individuals, the mitral valve leaflets are easily visualized and yield thin linear echoes that exhibit wide bipeaked excursions as they open in early and late diastole.24 The characteristic 2D ultrasound findings of MS are seen clearly in nearly all patients with this disorder.275 The mitral valve leaflets are thickened and often present bright high-intensity reflections indicating calcification. The process may involve thickening and shortening of the chordal apparatus as well. There are varying degrees of commissural fusion restricting mitral leaflet separation, especially at the distal tips.276,277 This leads to diastolic "doming" or a right-angle bend of the anterior mitral valve leaflet, as high LA pressure creates a bulge in the leaflet's midportion (which is generally more pliable than the distal portion) (B->;B; Fig. 13-73). The posterior leaflet actually may be pulled anteriorly during diastole because of commissural fusion with the longer anterior leaflet.278 Mitral doming may also occur in congenital valvular disease, but it is not seen when mitral leaflet opening is reduced due to low-flow states32 or AR jets. The LA is nearly always enlarged with MS.

The effects of stenosis upon mitral valve motion are often best demonstrated by M-mode recordings Fig. 13-74). In addition to leaflet thickening and reduced excursion, M-mode tracings also depict a characteristic decrease in the reclosure rate of the anterior mitral leaflet in early diastole (reduced E-F slope) due to a persistent LA-LV pressure gradient and a slow rate of LV filling.276-279 The decrease of the E-F slope has been found to correlate grossly with the severity of mitral stenosis. This finding is not specific for MS, however, and may occur whenever early diastolic filling is reduced.24,280 Attempts to calculate mitral valve orifice area using the E-F slope have proved unsatisfactory.28!

The entire perimeter of the mitral valve orifice can be visualized in the 2D parasternal short-axis view, and mitral leaflet excursion normally approaches the endocardial borders of the LV at the mitral tip level. In the setting of MS, the thickened leaflets form a fish-mouth orifice, which occupies only a small portion of the cross-sectional area of the left ventricle (see also Chap. 57).275,282 Measurements of mitral valve area may be obtained by planimetry of the orifice visualized in the parasternal short-axis view and correlate well with those obtained by cardiac catheterization Fig. 13-73).282-284 Since the shape of the mitral valve resembles a funnel, it is crucial to identify the smallest cross-sectional area and obtain recordings with orthogonal beam orientation at that point in order to avoid overestimation. Optimal gain settings must be employed to avoid encroachment of tissue signals upon the orifice.285

Doppler examination provides additional quantitation of MS.286,287 Interrogation of mitral inflow with either PW or CW modes (depending on velocity and Nyquist limit) reveals elevated diastolic velocities, with a reduction in the rate of deceleration in early diastole yielding a pattern similar to decreased E-F slope seen with M-mode in MS (B-hB; Fig. 13-75). In a fashion similar to that of AS, the maximal gradient across the mitral valve can be calculated from the peak diastolic velocity utilizing the Bernoulli equation.286,288 But since the maximal transmitral gradient is very sensitive to changes in heart rate and loading, the mean transmitral gradient obtained as the average of a number of individual gradients derived throughout diastole is customarily utilized to assess the severity of MS.288 In addition, Doppler technique may provide estimates of mitral valve area (MVA) by means of the calculation of the pressure half-time.284,287 The pressure half-time represents the interval required for transmitral velocity to decelerate from its highest point (E) to a velocity that yields one-half of the pressure equivalent (B+;B; Fig. 13-75). As the severity of MS increases, the rate of deceleration decreases, prolonging the pressure half-time. Further, dividing an empiric constant of 220289 by the pressure half-time yields an estimate of MVA, which correlates with values obtained during cardiac catheterization. Since Doppler estimates of mitral valve area are indirect and involve the use of empiric constants, they are considered less accurate than direct measurements of MVA derived by planimetry of the mitral valve orifice.290 The pressure half-time method is inaccurate immediately following mitral commissurotomy.291,292

Echocardiography can help assess the feasibility and appropriateness of percutaneous catheter balloon mitral commissurotomy (CBMC) to treat individual patients with MS293,294,294a (Chap. 37). An echocardiographic scoring system based on evaluation of mitral valvular thickening, calcification, mobility, and subvalvular involvement has been devised. Each variable is assigned a grade of 1 (minimal involvement) to 4 (severe), with a maximal score of 16. Although the prognostic capability of this method is limited, the outcome of balloon valvuloplasty in patients with higher scores, particularly greater than 12, is less satisfactory and involves a higher risk of complications than in patients with lower scores.293,294,294b Therefore, echocardiographic analysis is an important part of the decision-making process prior to CBMC. Preprocedural TEE is also often performed to detect left atrial thrombi, which can embolize during transseptal catheterization.295,296 Following CBMC, echocardiography can identify complications including mitral regurgitation297 and atrial septal defect.298

Mitral Regurgitation

Although echocardiography is extremely accurate in the detection of mitral (and aortic) regurgitation, quantitation is more difficult. 2D imaging alone does not provide direct evidence of mitral regurgitation (MR) but usually reveals the etiology of the lesion.299 Thus, 2D echocardiography reveals thickened, restricted leaflets in rheumatic disease, vegetations in infective endocarditis, flail mitral leaflets with torn chordae, and redundant leaflets with abnormal coaptation in mitral valve prolapse.300 2D echocardiography can also detect LA and LV abnormalities associated with MR, such as myxoma, papillary muscle dysfunction, and dilated cardiomyopathy. In addition, enlargement of these chambers offers indirect evidence of MR severity. In cases of chronic, severe MR, 2D echocardiography can also discern the presence of depressed LV function and decreased ejection fraction (see also Chap. 57).

Doppler echocardiography is the primary method for the detection and evaluation of MR301-303 and reveals a disturbed flow jet in the LA during systole. Spectral Doppler recordings provide several indexes of severity which are of semiquantitative value. Since the intensity of the Doppler signal is a function of the number of RBCs in the sample volume, the videodensity of the jet correlates in a general way with regurgitant volume.304 Similarly, an increase in transmitral filling velocities reflects increased forward flow and suggests a large regurgitant volume.304^ Measurements obtainable from the envelope of the CW Doppler recording of the MR jet include a slow rate of acceleration, indicative of a diminished LV dP/dt305 (&»■□■ Fig. 13-76). Early peaking followed by rapid deceleration of the MR jet suggests a large V wave, increased left atrial pressure, and usually acute severe MR.306

As in the case of AR, volumetric calculations of LV inflow and outflow by combined pulsed Doppler and 2D-echocardiographic imaging techniques can be used to derive measurements of regurgitant volume.307,308 In the case of MR, transmitral filling represents both systemic and regurgitant volume, while aortic outflow represents only systemic flow. Therefore, mitral filling should exceed left ventricular ejection, and the difference will be regurgitant volume.

The most commonly applied method for evaluation of MR is assessment of jet size by CFD.303,309,310 Imaging of the left atrium in systole reveals a turbulent, mosaic jet of varying direction, size, and configuration (Fig. 13-77A and B, Plate 64). Previous studies have demonstrated that a mitral regurgitant jet whose absolute area exceeds 8 cm2 303,310 or fills at least 40 percent of the area of the LA309 is predictive of finding 3+ to 4+ MR by LV angiography. Unfortunately, neither jet size nor angiographic grade correlates closely with measurements of actual regurgitant volume.310 The lack of correlation between CFD jet area and regurgitant volume is attributable to the additional variables that influence the distribution of the flow disturbance, such as pressure gradient and the volume and compliance of the LA, as well as technical limitations. The Coanda effect is of particular significance in regard to MR, since jets into the left atrium are often eccentric (for example, in cases of mitral valve prolapse and torn chordae tendineae). Due to differential frictional forces and resistance to flow, eccentric MR jets are drawn along the walls of the LA, resulting in cross-sectional jet areas that are smaller than centrally directed flow disturbances of comparable regurgitant volume (Figs. 13-77 and Q-hB; 1378). This effect can lead to underestimation of severity of regurgitation.311312

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