 Mitral annulus

The mitral annulus is the supporting attachment region around the mitral valve between the left atrium and left ventricle. It supplies a reference for the valve leaflets but is not adequately described as a rigid, flat circle. Its spatial shape and motion form part of the coupled valve apparatus.

A primary left-heart simulator study compared flat and saddle-shaped annular configurations in eleven human valve specimens. The investigators measured chordae tendineae forces in selected experiments and found a redistribution of those forces with annular shape.^[ref] This tests a geometrical variable directly rather than assuming that one diameter captures the complete attachment boundary.

An original circular approximation illustrates why fractional diameter and area changes differ. If a hypothetical ring's diameter changes from $D$ to $0.9D$, its planar area changes from $\pi D^2/4$ to $0.81\pi D^2/4$. A $10\%$ diameter decrease gives a $19\%$ area decrease in this model. The equation is not a claim that the native mitral annulus is circular or planar.

Motion adds another geometrical contribution. In a rigid-piston model, translation of a boundary with area $A$ by distance $\Delta z$ changes the represented volume by $\Delta V=A\Delta z$. Stipulated values 10 cm${}^2$ and 0.4 cm give 4 mL. Actual mitral annular excursion occurs in a deforming heart, so this does not assign all stroke volume to one translating plane.

Annular area, shape, position and leaflet coaptation are therefore related but distinct observations. Comparisons need consistent landmarks and phases, while models must specify which parts of the surrounding apparatus are allowed to move.