EARLY SYSTOLIC MITRAL LEAFLET LOITERING DURING ACUTE ISCHEMIC MITRAL REGURGITATION

Background: The mechanism by which incomplete mitral leaflet coaptatio n develops during ischemic mitral regurgitation is debated, with recen t studies suggesting that incomplete mitral leaflet coaptation may be due to apically displaced papillary muscle tips. Yet quantitative in v ivo three-dimensional mitral leaflet motion during ischemic mitral reg urgitation has never been described. Methods: Radiopaque markers (sutu red around the mitral anulus, to the central free mitral leaflet edges , and to both papillary muscle tips and bases) were imaged with the us e of biplane videofluoroscopy in six closed-chest, sedated sheep befor e (control) and during induction of acute ischemic mitral regurgitatio n. Leaflet coaptation was defined as the minimum distance measured bet ween edge markers during control conditions. Results: During control, leaflet coaptation occurred 23 +/- 7 msec (mean +/- standard error of the mean) after end-diastole, when left ventricular pressure was 27 +/ - 6 mm Hg, During ischemic mitral regurgitation, coaptation was delaye d to 115 +/- 19 msec after end-diastole (p less than or equal to 0.01 vs control [n = 4]) when left ventricular pressure was 88 +/- 4 mm Hg, At end-diastole during ischemic mitral regurgitation, the mitral anul us area was 14% +/- 2% larger than control (7.4 +/- 0.3 cm(2) vs 6.5 /- 0.2 cm(2), p less than or equal to 0.005) as the result of the leng thening of muscular annular regions (76.0 +/- 2.5 mm vs 70.5 +/- 1.4 m m, p less than or equal to 0.01). Mitral anulus shape (ratio of two di ameters) at end-diastole was more circular during ischemic mitral regu rgitation (0.79 +/- 0.01 vs 0.71 +/- 0,02, p < 0.01). At end-diastole during ischemic mitral regurgitation, the posterior papillary muscle t ip was displaced 1.5 +/- 0.5 mm laterally and 2.0 +/- 0.6 mm posterior ly (p less than or equal to 0.02 vs control), but there was no epical displacement of either papillary muscle tip, Conclusions: Incomplete m itral leaflet coaptation during acute ischemic mitral regurgitation oc curred early in systole, not at end-systole, and was due to ''loiterin g'' of the leaflets associated with posterior mitral anulus enlargemen t and circularization, as well as some posterolateral, but not apical, posterior papillary muscle tip displacement. These data suggest that early systolic mitral anulus dilatation and shape change and altered p osterior papillary muscle motion are the primary mechanisms by which i ncomplete mitral leaflet coaptation occurs during acute ischemic mitra l regurgitation.