Distortions of the Mitral Valve in Acute Ischemic Mitral Regurgitation

Background. In the absence of papillary muscle rupture, the precise deformations that cause acute postinfarction mitral valve regurgitation are not understood and impair reparative efforts. Methods. In 6 Dorsett hybrid sheep, sonomicrometry transducers were placed around the mitral annulus (n = 6) and at the tips and bases of both papillary muscles (n = 4). Later, specific circumflex coronary arteries were occluded to infarct approximately 32% of the posterior left ventricle and produce acute 2 to 3+ mitral regurgitation. Before and after infarction, distance measurements between sonomicrometry transducers produced three-dimensional coordinates of each transducer every 5 ms. Results. After infarction, the annulus dilated asymmetrically orthogonal to the line of leaflet coaptation, but the annular area increased only 9.2% ± 6.3% (p = 0.02). At end-systole, posterior papillary muscle length increased 2.3 ± 0.9 mm (p = 0.005); the posterior papillary muscle tip moved closer to the annular plane and centroid, and the anterior papillary muscle tip moved away. Conclusions. Small deformations in mitral valvular spatial geometry after large posterior infarctions are sufficient to produce moderate to severe mitral regurgitation. The most important changes are asymmetric annular dilatation, prolapse of leaflet tissue tethered by the posterior papillary muscle, and restriction of leaflet tissue attached to the anterior papillary muscle.