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

Asymmetrical mitral annular (MA) dilatation and papillary muscle dislocation are implicated in the pathogenesis of functional mitral regurgitation (MR).

To determine the mechanism by which annular and papillary muscle geometric alterations result in MR, we implanted radiopaque markers in the left ventricle, mitral annulus, anterior and posterior mitral leaflets, 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 topical 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% larger in the EXP group than in the CTL group (P<0.05 by ANOVA). This area increase resulted exclusively from intercommissural axis increase except in 1 heart with large (>1 cm) increases in both the intercommissural and septolateral 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.

MA dilatation in the intercommissural dimension with anterior papillary muscle tip displacement toward the annulus is insufficient to produce MR in sheep. Functional MR may require MA dilatation in the septolateral axis, as observed with proximal circumflex coronary occlusion.