Hemodynamics
What causes the murmur? When blood is leaking back to the left atrium, turbulence is created. The difference (pressure gradient) during ventricular contraction between the left ventricle and the left atrium will indicate when the murmur will occur. In  chronic Mitral Regurgitation the pressure gradient is important throughout systole, and we will detect a holosystolic murmur. As with Acute Mitral Regurgitation, left atrial pressure will increase more rapidly, and the murmur will peak earlier in systole then drop in intensity towards the end.

In Mitral Valve Prolapse, the valve does not regurgitate until the prolapse of the valve has occurred. The murmur will then not be present until late in systole.