The Mitral Apparatus

This review deals with the functional anatomy of the six components of the mitral apparatus, namely, the left atrial wall, annulus, leaflets, chordae tendineae, papillary muscles, and left ventricular wall. Each component is considered individually, in the context of the apparatus as a whole, in relation to the mode of closure of the normal mitral valve, and in the light of many acquired and congenital disorders that disturb the harmony of the finely coordinated mitral mechanism and render it incompetent. The left atrium is related to mitral valve competence in terms of contraction and relaxation and in terms of dilatation of its posterior wall. The annulus not only serves as a fulcrum for the leaflets but exhibits sphincteric contraction in systole that decreases the size of the orifice. The two leaflets differ in shape but are nearly identical in area, and together are about two and one half times the area of the orifice that they are required to close. Leaflet abnormalities causing acquired or congenital mitral regurgitation result from deficient leaflet tissue, excessive leaflet tissue, or restricted leaflet mobility. Chordae tendineae are considered according to their leaflet attachments, ventricular attachments, thicknesses, lengths, and arborization patterns. Mitral regurgitation due to chordal abnormalities results from chordae that are abnormally long, abnormally short, ectopically inserted, or ruptured. In this context, systolic clicks and late systolic murmurs are discussed, and severe acute mitral regurgitation is contrasted with severe chronic mitral regurgitation. The papillary muscles and the left ventricular wall represent the two muscular components of the mitral apparatus. An appraisal of papillary muscle dysfunction includes dysfunction with loss in continuity (rupture) and dysfunction without loss in continuity (fibrosis, ischemia, replacement). Finally, the role of altered left ventricular shape is discussed in the context of mitral regurgitation, and the effect of dilatation is ascribed chiefly to alterations in the position of papillary muscles and their directional axes of tension.