Acute mitral regurgitation is distinct from chronic mitral regurgitation. The chronic state represents compensatory changes, including dilation of the left ventricle, which preserves cardiac output through increasing stroke volume. In acute mitral regurgitation however, the left ventricle is typically normal in size, and the regurgitant streams shoots backward, leaving the ventricular volume pathologically low. Forward flow is diminished, and these patients present with shock. The retrograde flow causes pulmonary edema.
The classic teaching is that an acute murmur of mitral regurgitation suggests disruption or rupture of a leaflet, usually either from endocarditis or papillary muscle infarction. But the murmur is different. Sometimes it can be loud, but typically it is quieter. The regurgitant blood quickly raises atrial pressure to equalize with the left ventricle. Thus, regurgitation stops earlier and the murmur is earlier, shorter, and quieter (Stout, Circulation, 2009).
One other finding might help. The pulmonary pressure is acute high, so P2 happens later. Meanwhile the left ventricular volume is lower, so A2 happens earlier. Thus, wide splitting of S2 is present, though this does not appear to have been well-studied recently.
The main point is that if the patient has shock and pulmonary edema without an explanation, look for signs of mitral regurgitation. A new early systolic murmur or wide splitting of S2 is a sign that your patient may be heading to the operating room.
Take home points:
The murmur of acute mitral regurgitation may be subtle
Consider this diagnosis in unexplained shock and pulmonary edema
Listen for wide splitting of S2