This laboratory study tested the hypothesis that left ventricular function following mitral valve replacement may be better preserved by retaining the tethering effect of the mitral valve apparatus in chronic mitral regurgitation. A chronic model of mitral regurgitation has been developed an extensively studies in sheep. Due to the progressive enlargement of an anterior mitral leaflet defect, there were significant increases in left ventricular mass, wall thickness, LV mass to body weight ratio, end-diastolic volume, stroke volume and ejection fraction over a 7-8 month period compared to controls. Mitral annular - papillary muscle discontinuity following mitral valve replacement in chronic, volume overloaded ventricles, led to a significant increase in the end-diastolic volume thus creating an increased "effective" preload. This resulted in an increased stroke volume secondary to increased equatorial, minor axis shortening. Despite this increase in forward flow, the reserve of the left ventricle with chronic mitral regurgitation to perform pressure-volume work was severely impaired with dramatic, significant decreases in maximum systolic blood pressure and max dp/dt at matched heart rates and preload after all chordae tendinae were severed. These data show that maintenance of mitral annular-papillary muscle continuity following mitral valve replacement preserves left ventricular function in setting of chronic volume overload secondary to mitral regurgitation.