Mitral annular dilatation and papillary muscle dislocation without mitral regurgitation in sheep

Background-Asymmetrical mitral annular (MA) dilatation and papillary muscle dislocation are implicated in the pathogenesis of functional mitral regurg itation (MR), Methods and Results-To determine the mechanism by which annular and papilla ry muscle geometric alterations result in MR, we implanted radiopaque marke rs in the left ventricle, mitral annulus, anterior and posterior mitral lea flets, and papillary muscle tips and bases in 2 groups of sheep. One group served as controls (CTL, n=7); an experimental group (EXP, n=9) underwent t opical phenol application to obliterate anterior annular and leaflet muscle (confirmed histologically ex vivo). After 1 week of recovery, markers were imaged with biplane videofluoroscopy, and hemodynamic data were recorded. MA area (computed from 3-dimensional marker coordinates) was 11% to 13% lar ger in the EXP group than in the CTL group (P<0.05 by ANOVA), This area inc rease resulted exclusively from intercommissural axis increase except in 1 heart with large (>I cm) increases in both the intercommissural and septola teral annular axes. The anterior papillary muscle tip in EXP was displaced from CTL by 2.9+/-0.23 mm toward the anterolateral left ventricle and 2.5+/ -0.12. mm toward the mitral annulus at end systole; the posterior papillary muscle geometry was unchanged. Transthoracic echocardiography revealed MR only in the heart exhibiting biaxial annular enlargement. Conclusions-MA dilatation in the intercommissural dimension with anterior p apillary muscle tip displacement toward the annulus is insufficient to prod uce MR in sheep. Functional MR may require MA dilatation in the septolatera l axis, as observed with proximal circumflex coronary occlusion.