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Mitral Valve Force Balance: The Left Ventricular Tug of War

Morten Ølgaard Jegstrup Jensen


     The mitral valve apparatus is one of the most complex cardiac structures. It consists of the annulus, where two main leaflets, anterior and posterior leaflet, are attached. The annulus is identified as the posterior, commissural, and the anterior segments. The leaflets are anchored to the inner wall of the left ventricle through the chordae tendinae at the papillary muscles. Increasing mitral valve repair durability requires successful restoration and support of the valvular and subvalvular apparatus, reestablishing the natural force balance of the mitral valve. The stress distribution in mitral valve repair devices indirectly determines the outcome of the repair. This involves ensuring the correct load of the subvalvular apparatus as well as restoration and support of the annulus with annuloplasty rings.
     This thesis summarizes the development of in vitro and in vivo force measurement equipment for the mitral valve apparatus and repair devices to identify the forces in the valular anchoring points to the left ventricle. Insight into the torque and bending forces in the mitral valve annulus and the total papillary muscle force elucidate the biomechanical coupling between the annular and the subvalvular apparatus.
     The overall hypothesis was that the force transmission at the annular and subvalvular anchoring points of the mitral valve to the left ventricle could be assessed by strain gauge based transducer systems dedicated for in vitro and in vivo force measurements at the annulus and the papillary muscles. In substudy 1, the forces on the subvalvular apparatus and how these forces are distributed throughout the individual chordae tendineae of the mitral valve were investigated. The apically directed forces in the mitral valve annulus were identified in substudy 2.
     In substudy 1, fresh porcine mitral valves were excised and attached to a physiologic annular ring. Mitral valve function was studied in vitro with a rigid transparent left heart model. Experiments were carried out with fresh valves under physiologic conditions. Geometrical measurements and performance characteristics were obtained from hydrodynamic pressure and flow data, ultrasound, and three dimensional papillary muscle force measurements. In substudy 2, an experimental porcine animal model was used to obtain the mitral annulus 3D dynamic geometry with sonomicrometry crystal array location. Dynamic force measurements perpendicular to the annulus plane (apical direction) was assessed by mounting strain gauges on dedicated D-shaped rigid flat annuloplasty rings.
     The absolute magnitude of the force on the papillary muscles from the in vitro investigations was found to be 4.3 ± 0.9N. This force is mainly carried by the valvular/ventricular interacting basal chordae tendineae, while the marginal chordae tendineae at the tip of the leaflets govern leaflet motion during valvular closure. The in vivo experiments identified a saddle shaped annulus in systole and a more flat annulus in diastole. Force accumulation was seen from the anterior (0.7 ± 0.4 N) and commissural (1.4 ± 1.0 N) annular segments. Both were acting in opposite directions. This in turn supports the hypothesis that the mitral valve annulus and its attached valvular and subvalvular structures apply torque onto the flat annuloplasty ring, in an attempt to conform it into the saddle-shaped configuration.
     The in vitro and the in vivo data complement each other. This indicates that the different experimental setups can indeed be combined to further investigate the force balance of the left ventricle. These detailed investigations of the force distribution in the mitral valve annulus and the subvalvular apparatus establish a biomechanical approach that will not only provide a rational basis for innovative repair device designs, but also increase the present understanding of the force balance of the complete mitral valve, leading to better repair techniques. Eventually we will understand the most important factors in the cause of improper mitral valve function and be able to completely solve the mitral valve tug-of-war mystery, which can ultimately improve the lives of people with mitral valve deficiencies.