Characterization of 3-dimensional papillary muscle displacement in in vivo ovine models of ischemic/functional mitral regurgitation

Objective: Papillary muscle (PM) displacement contributes to ischemic/functional mitral regurgitation (IMR/FMR). The displaced PMs pull the mitral leaflets into the left ventricle (ie, toward the apex) thus hampering leaflet coaptation. Intuitively apical leaflet tethering results from apical PM displacement. The 3-dimensional directions of PM displacement are, however, incompletely characterized. Methods: Data from in vivo ovine models of IMR (6-8 weeks of posterolateral infarction, n = 12) and FMR (9-21 days of rapid left ventricular pacing, n = 11) were analyzed. All sheep had radiopaque markers implanted on the anterior and posterior PM (PPM) tips, around the mitral annulus, and on the left ventricular apex. To explore 3-dimensional PM displacement directions, differences in marker coordinates were calculated at end-systole before and during IMR/FMR using a right-handed coordinate system centered on the mitral annular “saddle horn” with the y-axis passing through the apical marker. Results: No apical PM displacement was observed during either IMR or FMR. The anterior PM displaced laterally during FMR. Posterolateral PPM displacement was observed during IMR and FMR. Conclusions: Experimental in vivo ovine models suggest posterolateral PPM displacement as a predominant pathomechanism leading to apical leaflet tethering during IMR/FMR.