Early systolic mitral leaflet "loitering" during acute ischemic mitral regurgitation.

BACKGROUND: The mechanism by which incomplete mitral leaflet coaptation develops during ischemic mitral regurgitation is debated, with recent studies suggesting that incomplete mitral leaflet coaptation may be due to apically displaced papillary muscle tips. Yet quantitative in vivo three-dimensional mitral leaflet motion during ischemic mitral regurgitation has never been described.
METHODS: Radiopaque markers (sutured around the mitral anulus, to the central free mitral leaflet edges, and to both papillary muscle tips and bases) were imaged with the use of biplane videofluoroscopy in six closed-chest, sedated sheep before (control) and during induction of acute ischemic mitral regurgitation. Leaflet coaptation was defined as the minimum distance measured between 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 delayed to 115 +/- 19 msec after end-diastole (p < or = 0.01 vs control [n = 4]) when left ventricular pressure was 88 +/- 4 mm Hg. At end-diastole during ischemic mitral regurgitation, the mitral anulus area was 14% +/- 2% larger than control (7.4 +/- 0.3 cm2 vs 6.5 +/- 0.2 cm2, p < or = 0.005) as the result of the lengthening of muscular annular regions (76.0 +/- 2.5 mm vs 70.5 +/- 1.4 mm, p < or = 0.01). Mitral anulus shape (ratio of two diameters) at end-diastole was more circular during ischemic mitral regurgitation (0.79 +/- 0.01 vs 0.71 +/- 0.02, p < 0.01). At end-diastole during ischemic mitral regurgitation, the posterior papillary muscle tip was displaced 1.5 +/- 0.5 mm laterally and 2.0 +/- 0.6 mm posteriorly (p < or = 0.02 vs control), but there was no apical displacement of either papillary muscle tip.
CONCLUSIONS: Incomplete mitral leaflet coaptation during acute ischemic mitral regurgitation occurred early in systole, not at end-systole, and was due to "loitering" of the leaflets associated with posterior mitral anulus enlargement 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 posterior papillary muscle motion are the primary mechanisms by which incomplete mitral leaflet coaptation occurs during acute ischemic mitral regurgitation.