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Diseases of the Mitral Valve
Mitral Stenosis Detection of mitral stenosis (MS) was one of the earliest clinical applications of echocardiography159 (see Chap. 67). In most individuals, the MV leaflets are easily visualized and yield thin linear echoes that exhibit wide bipeaked excursions as they open in early and late diastole.26 The characteristic 2D ultrasound findings of MS are seen clearly in nearly all patients with this disorder.160 The MV 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. This leads to diastolic "doming" or a right-angle bend of the anterior MV leaflet as high LA pressure creates a bulge in the leaflet's midportion (which is generally more pliable than the distal portion) (Fig. 15–73). The posterior leaflet actually may be pulled anteriorly during diastole because of commissural fusion with the longer anterior leaflet. Mitral doming may also occur in congenital valvular disease, but it is not seen when mitral leaflet opening is reduced due to low-flow states34 or AR jets. The LA is nearly always enlarged with MS.
The effects of stenosis upon MV motion are often best demonstrated by M-mode recordings (Fig. 15–74). 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. The decrease of the E-F slope has been found to correlate grossly with the severity of MS. This finding is not specific for MS, however, and may occur whenever early diastolic filling is reduced.26 Attempts to calculate the area of the MV orifice using the E-F slope have proved unsatisfactory.
The entire perimeter of the MV 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 (Chap. 67).160 Measurements of the area of the MV orifice 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. 15–73). Since the shape of the MV resembles that of 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.161 Doppler examination provides additional quantitation of MS.162 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 the decreased E-F slope seen with M-mode in MS (Fig. 15–75). In a fashion similar to that of AS, the maximal gradient across the MV can be calculated from the peak diastolic velocity utilizing the Bernoulli equation. 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. In addition, Doppler technique may provide estimates of MV area (MVA) by means of the calculation of the pressure half-time.162 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 (Fig. 15–75). As the severity of MS increases, the rate of deceleration decreases, prolonging the pressure half-time. Further, dividing an empiric constant of 220163 by the pressure half-time yields an estimate of MVA, which correlates with values obtained during cardiac catheterization. Since Doppler estimates of MVA are indirect and involve the use of empiric constants, they are considered less accurate than direct measurements of MVA derived by planimetry of the MV orifice. The pressure half-time method is inaccurate immediately following mitral commissurotomy.
Echocardiography can help assess the feasibility and appropriateness of percutaneous catheter balloon mitral commissurotomy (CBMC) to treat individual patients with MS164 (Chap. 46). 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.164 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.165 Following CBMC, echocardiography can identify complications including MR and atrial septal defect. 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 MR but usually reveals the etiology of the lesion.166 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 MV prolapse. 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 the severity of MR. In cases of chronic, severe MR, 2D echocardiography can also discern the presence of depressed LV function and decreased ejection fraction (Chap. 67). Doppler echocardiography is the primary method for the detection and evaluation of MR167–169 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 red blood cells (RBCs) in the sample volume, the videodensity of the jet correlates in a general way with regurgitant volume. Similarly, an increase in transmitral filling velocities reflects increased forward flow and suggests a large regurgitant volume.167 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/dt (Fig. 15–76). Early peaking followed by rapid deceleration of the MR jet suggests a large V wave, increased left atrial pressure, and usually acute severe MR.
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.168 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 the evaluation of MR is assessment of jet size by CFD.169 Imaging of the LA in systole reveals a turbulent, mosaic jet of varying direction, size, and configuration (Fig. 15–77A and B). Previous studies have demonstrated that a mitral regurgitant jet whose absolute area exceeds 8 cm2 or that fills at least 40 percent of the area of the LA 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.169 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 the 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 LA are often eccentric (for example, in cases of MV 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. 15–77 and 15–78). This effect can lead to underestimation of the severity of regurgitation.
TEE is also useful for the assessment of MR, as the close proximity of the probe and its higher-frequency interrogating beam permit imaging of regurgitant jets in greater detail than with TTE.170 Eccentric jets and mitral valvular anatomy are well visualized (Fig. 15–78A and B) and rightward bulging of the interatrial septum with severe MR is also sometimes apparent. As the regurgitant jets often appear larger with TEE than with TTE, one must avoid overestimation of MR severity.108 TEE often yields Doppler interrogation of the pulmonary veins that is superior to that of TTE, and several recent studies have shown that systolic reversal of flow into the pulmonary veins is a reliable sign of severe MR106 (Fig. 15–79).
Another color Doppler method of flow quantitation involves measurement of the zone of flow convergence proximal to the regurgitant orifice [or the proximal isovelocity surface area (PISA)]. The mechanism for this phenomenon is derived from the hydrodynamic principle that blood flow accelerates before passing through a small orifice under high pressure. If this increase in flow velocity exceeds the Nyquist limit, color aliasing occurs and the velocity aliasing border is equal to the Nyquist limit (Fig. 15–31 and Fig. 15–80A and B). If one assumes that the aliasing border conforms to the geometry of a hemisphere around the mitral orifice, then the instantaneous flow rate of blood through the orifice can be calculated as:
where r is the radius of the hemisphere shell (distance from alias border to orifice) and Vr is the velocity of blood at distance r (the Nyquist limit velocity). If the maximal calculated flow rate is divided by the peak regurgitant flow velocity (measured with CW Doppler), the regurgitant orifice area is then obtained.171 The product of regurgitant orifice area and integrated velocity of the MR jet by CW yields regurgitant volume. The PISA method avoids the variables associated with jet size and the assumptions and technical limitations of volumetric calculations. Numerous studies have shown a correlation between both flow rate and regurgitant orifice area calculated by PISA and the severity of MR assessed by standard methods.171 In addition, flow convergence calculations have been applied to other valvular lesions, including AR and MS (Fig. 15–81), ventricular septal defect, and prosthetic heart valves.172 The proximal flow convergence assumes a hemispheric geometry for the PISA signal and that the plane of the mitral leaflets is flat, two sources of potential error.173 The method also assumes that regurgitant blood is flowing through only one orifice, an assumption often untrue in MR. Despite these limitations, the method can be useful in selected cases of valvular regurgitation.
Mitral Valve Prolapse As is true of so many aspects of MV prolapse (MVP), the echocardiographic findings in this disorder have been controversial for many years.174 Recent insights into the anatomy of the mitral annulus and the significance of abnormal leaflet structure have established a central role for echocardiography in the diagnosis and prognosis of MVP.175 The classic echocardiographic findings in overt MVP syndrome consists of mid- to late-systolic bulging of one or both mitral leaflets across the plane of the MV annulus into the LA (Fig. 15–82A to C). The leaflets are often observed to be structurally abnormal, with thickening, elongation, and hooding. Mid- to late-systolic MR is sometimes present, often eccentric, and generally directed away from the prolapsing leaflet. The chordae tendineae may be thickened and elongated, the aortic root may be dilated, and the TV leaflets may prolapse as well. LV function is usually normal, although the LA and LV may be enlarged if MR is significant. The greater temporal resolution of M-mode over 2D echocardiography often yields striking evidence of abrupt midsystolic posterosuperior motion of the MV leaflets in prolapse patients (Fig. 15–82C). Although such M-mode findings, which resemble a question mark on its side, are specific for MV prolapse, patients with classic MVP occasionally may demonstrate diagnostic findings only with 2D imaging (Chap. 68).
Although the diagnosis of classic, fully expressed MVP is straightforward by echocardiography, identification of mild prolapse is more difficult, and no absolute diagnostic criteria currently exist. This is largely related to the absence of any "gold standard" with which to validate findings, including auscultation, angiography, and even pathology. For prolapse to be present, the MV leaflets must cross the plane of the MV annulus after initial systolic coaptation. Recent studies have established that the MV annulus is not flat but rather saddle-shaped. The annulus reaches its nadir in the apical four-chamber view, and even normally coapting MV leaflets may appear to prolapse in this projection. Therefore, current criteria require that MVP be diagnosed only when one or both of the mitral leaflets clearly bulge past the plane of the MV annulus in the parasternal long-axis view.175 Unfortunately, the degree to which the mitral leaflets must break the plane of the annulus is unclear. The greater the portion of the MV leaflets entering the LA, the more likely the existence of signs and symptoms related to this disorder; a peak distance behind the annulus of 2 mm almost invariably establishes the presence of MVP. The diagnosis of mild MVP may be assisted by examination of the structure of the leaflets and chordae tendineae, since it has been demonstrated that patients with redundant or thickening valve leaflets (greater than 5 mm in midleaflet) are at increased risk of complications, including severe MR and infective endocarditis (Chap. 78). Torn Chordae Tendineae Rupture of chordae tendineae may occur spontaneously or in conjunction with MVP or endocarditis. This can result in a flail mitral leaflet and severe MR. Although TTE often detects these lesions, TEE is especially sensitive and accurate and often demonstrates free motion of the leaflet and ruptured chord into the LA even when the TTE is equivocal (Fig. 15–83A and B). As with MVP, the MR jet in this condition is usually eccentric and directed away from the affected leaflet, often "hugging" the adjacent left atrial wall (Coanda effect). Therefore, the jet's cross-sectional area may be misleadingly small. The findings of mitral valvular anatomy on TEE may also be helpful in predicting the feasibility and success of valve repair surgery.176
In the setting of ischemic heart disease, both LV enlargement and papillary muscle dysfunction (from infarction or transient ischemia) may cause MR. Both the MR and the contractile abnormality responsible for it are usually well visualized by 2D echocardiography. In rare cases, papillary muscle rupture (partial or complete) occurs in the postinfarction period. Rapid echocardiographic diagnosis often requires TEE and may be lifesaving in these cases. Mitral Annular Calcification The finding of mitral annular calcification (MAC) is fairly common in adults and occurs more frequently with advancing age. Although ultrasound cannot discern histology, calcification typically appears as thickened, extremely high-intensity ("bright") signals (Fig. 15–84). The posterior portion of the mitral annulus is affected much more commonly than the anterior segment, and calcification often extends into the posterior mitral leaflet, sometimes restricting its motion. The abnormality, best visualized in the parasternal long- and short-axis views, is seen as a bright calcific density at the junction of the posterior mitral leaflet and the annulus. In the short-axis view, the posterior band of calcification often appears crescentic. Rarely, the calcification is extensive enough to cause marked valvular thickening and clinically significant MS. MAC has been associated with cardiogenic embolization, stroke, and cardiovascular events, although causality has not been established.177,178
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