PAPILLARY-MUSCLE DISCOORDINATION RATHER THAN INCREASED ANNULAR AREA FACILITATES MITRAL REGURGITATION AFTER ACUTE POSTERIOR MYOCARDIAL-INFARCTION

Background Acute posterior myocardial infarction that produces immedia te mitral regurgitation alters the mitral annulus and its spatial rela tionship with both papillary muscles. The precise deformations that ca use valve insufficiency are not understood and impair efforts to repai r the valve. Methods and Results In six Dorsett hybrid sheep, sonomicr ometry transducers were placed around the mitral annulus (6) and at th e tips and bases of both papillary muscles (4). Two weeks later, three branches of the circumflex coronary artery were occluded to infarct a pproximately 32% of the posterior left ventricle. This infarction prod uced acute 2 to 3+ mitral regurgitation in all animals, as determined by color flow Doppler velocity mapping. Before and after infarction, d istance measurements between sonomicrometry transducers were used to p roduce the three-dimensional coordinates of each transducer every 5 ms . After infarction, the area of the annulus increased only 9.2+/-6.3% at end systole (ES). In addition, the normal shortening of the posteri or papillary muscle was obliterated to allow its tip to move 1.4+/-0.6 mm closer to the centroid of the annulus at ES. After infarction, the anterior papillary muscle continued to shorten normally, but at ES, i ts tip and base were 0.9+/-0.7 mm and 1.3+/-0.7 mm farther from the ce ntroid, respectively. Conclusions These deformations tend to produce a relative prolapse of leaflet tissue attached to the posterior papilla ry muscle and restriction of leaflet tissue attached to the anterior p apillary muscle. This papillary muscle discoordination with minimal an nular dilatation distorts leaflet coaptation sufficiently to produce s evere mitral regurgitation.