Quantitative echocardiography of the mitral complex in dilated cardiomyopathy: the mechanism of functional mitral regurgitation.

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Abstract
The mechanism of mitral regurgitation (MR) in dilated cardiomyopathy (DCM) was investigated. Quantitative 2-dimensional echocardiographic examinations were performed in 27 patients, 18 with DCM (9 with MR on physical examination, 9 without MR) and 9 without underlying heart disease. The MR and no MR patients were clinically comparable. Spatial reconstructions from multiple apical cross sections were used to estimate the mitral leaflet area needed to occlude the orifice for a given midsystolic coaptation configuration (LEAF), as well as mitral annular area index, left ventricular volume and left atrial volume. Similarly, reconstructions from parasternal short-axis views were used to estimate central chordae tendinae length and angulation. From selective parasternal views papillary muscle (PM) length and contraction and the tethering length from the PM base to the annular plane were measured. The MR group was characterized by markedly enlarged occlusional leaflet area (LEAF 19.8 .+-. 3.1 in MR vs. 13.8 .+-. 2.8 in the no MR group vs. 6.3 .+-. 0.9 cm2 in the normal group; P < 0.01), striking mitral annular dilatation (midsystolic annular area index 7.5 .+-. 0.8 in MR vs. 4.6 .+-. 0.9 in the no MR group vs. 2.9 .+-. 0.4 cm2/m2 in the normal group; P < 0.01), and left atrial enlargement (end-systolic left atrial volume 129 .+-. 39 in MR vs. 73 .+-. 14 in the no MR group vs. 29 .+-. 5 ml in the normal group; (P < 0.01). Chordal length and angulation, PM length, contraction, and tethering length, and left ventricular volume were not significantly different in the MR vs. the no MR group. Noncoaptation of the mitral leaflets at their free margins was not observed in any MR patient. With the use of stepwise linear regression LEAF was determined chiefly by annular size (R2 0.868), with left ventricular size having little additional influence (R2 increment 0.071). Thus, DCM is associated with enlargement of the mitral anulus, which is more pronounced in those patients with MR. Based on the quantitative estimates of occlusional leaflet area, it was postulated that mitral leaflet tissue can stretch somewhat to accommodate dilatation of the mitral complex, but as the requirement for occlusional leaflet area increases less tissue is available for coaptation. Thus, although coaptation continues to occur, the valvular seal becomes ineffective once a critical LEAF is reached. The chief determinant of LEAF is the mitral annular size, while left ventricular size is a less important factor.