Chordal geometry determines the shape and extent of systolic anterior mitral motion: in vitro studies

Cardiovascular Fluid Mechanics Laboratory, School of Chemical Engineering, Georgia Institute of Technology, Atlanta.

In patients with hypertrophic cardiomyopathy, the mitral valve moves anteriorly and assumes a unique shape, with mitral-septal contact centrally and preserved valve orifice area laterally. This shape is not clearly predicted by the Venturi mechanism, which stresses flow above the valve as opposed to changes intrinsic to the valve. On the other hand, it has been suggested that displacement of the papillary muscles anteriorly and toward one another, as observed in this disease, can promote anterior mitral valve motion and produce this unusual shape. The purpose of this in vitro study was to test the hypotheses that anterior motion of a membrane in a flow field can be generated by altering the distribution or effectiveness of chordal tension tethering the membrane, and that the shape achieved by this membrane depends on the geometry of chordal tension. Accordingly, a horizontal leaflet mounted in a flow chamber was attached by chords at its distal end to a series of upstream screws. Chordal tension could be varied by turning the screws or redirected by shifting the screws anteriorly. Anterior leaflet motion having the same unusual configuration seen in patients was reproduced by decreasing central chordal restraint while tension on the leaflet edges was maintained. Directing chordal tension anteriorly caused greater degrees of anterior motion at earlier stages in the release of chordal restraint; increased flow rate had a similar but less marked effect. These studies suggest that primary geometric alterations in the papillary-mitral apparatus can play an important role in determining the presence and geometry of systolic anterior mitral motion. The nature of these alterations suggests a role for anterior and inward papillary muscle displacement in promoting such motion. The geometric factors embodied in this model can explain many observed features of this motion not adequately explained by the Venturi effect, such as early systolic onset and the importance of a distal residual leaflet. Finally, flow visualization studies emphasize the importance in this process of drag forces caused by interposing the leaflet into the flow stream, and of geometric factors that enhance such forces.

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