It is now recognised that the mitral valve is an integral part of the structure of the left ventricle (LV). The mitral valve annulus and papillary muscles contribute significantly to LV function. The reverse is also true and significant impairment of LV function in patients with advanced dilated LV failure can significantly impair mitral valve function such that mitral regurgitation occurs. Dilatation of the LV not only results in dilatation of the mitral annulus but also tethering of the papillary muscles as the ventricular wall dilates. Both of these lesions result in impaired coaptation of the mitral leaflets with resulting central mitral regurgitation. The anatomy of the mitral valve apparatus with overlap of a large portion of the leaflet surface area in a normal patient means that the left ventricle is often very significantly dilated before functional mitral regurgitation occurs.

The physiological basis for mitral valve annuloplasty in patients with significant mitral regurgitation due to advanced left ventricular failure is to stop the vicious cycle whereby mitral regurgitation begets more mitral regurgitation through volume overload and geometric distortion. Restoring the competency of the mitral valve restores forward flow of blood through the left ventricle and hence increases cardiac output. Both preload and afterload are also reduced as a result. There is also emerging evidence that restoring the size and shape of the mitral annulus in these patients also restores the volume and geometry of the dilated left ventricle. This concept was first proposed by Bolling who suggested that reducing the size of the mitral annulus not only reduces the volume of the dilated left ventricle, but also draws the base of the left ventricle inwards, resulting in the long axis of the left ventricle becoming more ellipsoid from base to apex [2]. This restores the spherical dilated ventricle to a more normal elliptical shape [1, 2]. A reduction in left ventricular volume would reduce left ventricular wall stress and hence improve sub-endocardial perfusion and oxygenation. Restoring the left ventricle to a more ellipsoid shape would restore the orientation of myofibrils to a more oblique direction optimising its efficiency during ventricular systole.

The technique of mitral annuloplasty has been described in a separate chapter. The mitral annulus is sized and an annuloplasty ring that is at least one size smaller than that measured is implanted. Undersizing is done to achieve leaflet coaptation and to restore the elliptical shape of the left ventricle. An important principle in this type of surgery is to use a rigid complete ring and not a flexible band. This is because the mitral annulus has been shown to dilate in both the anterior and posterior annulus. A smaller ring also achieves better long term results. However, care must be taken to place sufficient number of sutures and of sufficient depth as the sutures and annuloplasty ring are at increased tension due to the undersizing.