Undersizing Mitral Annuloplasty Acutely Eliminates Functional Mitral Regurgitation but Does Not Relieve Tethering Forces on the Mitral Valve Chordae Tendineae

Introduction: Functional mitral regurgitation (FMR) develops secondary to mitral valve (MV) geometric distortions such as isolated annular dilatation (AD), asymmetric tethering (Asym), and symmetric tethering (Sym). Undersizing mitral annuloplasty (UMA) has been religiously used to repair FMR with the notion that it can restore coaptation and relieve tethering forces, which has not been tested. Hypothesis: UMA does not relieve MV chordal tension in the setting of FMR from AD, Asym and Sym despite correcting regurgitation. Methods: In an ex vivo MV simulator ( Fig. A ), fresh pig MVs were mounted onto an adjustable annulus and two papillary muscle displacing rods ( Fig. B ), which are used to induce AD, Asym and Sym MV configurations. A 28mm annuloplasty ring was implanted ( Fig. C ) after the three MV geometries were established. MV hemodynamics were measured using electromagnetic flowmeters, leaflet kinematics were measured by 2D echocardiography, and chordal forces were measured using miniature C-ring force transducers. Results: Regurgitation fraction was significantly reduced in the Asym and Sym geometries after 28mm annuloplasty (Asym: 5.3±3.7% vs 1.6±1.2%, p=0.024; Sym: 12.8±5.6% vs 5.6±3.1%, p=0.006) ( Fig. D ). Tenting height was not significantly reduced after annuloplasty in all valve geometries, indicating persistent tethering despite annular downsizing ( Fig. E ). Neither anterior nor posterior leaflet mobility was improved after annuloplasty ( Fig. F-G ). The chordal forces in the anterior strut and posterior strut chordae tendineae were depicted in ( Fig. H1-4 ). The peak forces in the chordae slightly reduced after UMA, but the reduction in the peak forces did not reach statistical significance. Conclusions: UMA corrects FMR by forcing the leaflets together in the mitral valve orifice but does not improve leaflet mobility or relieve the tethering forces on the leaflets, which explains the mechanistic basis for high rates of recurrent FMR after UMA.