Why is mitral valve prolapse effective?

Mitral valve prolaps: I will assume you did not mean effective but harmful :-) when the valve leaflets "prolapse" they bend back and no longer seal, resulting in significant leakage of the valve. This increases the amount of work the heart has to perform, resulting in decrease in exercise capacity and more shortness of breath. Of course, the leakage can be a little with few symptoms or severe...

Not sure of quest: Mitral valve prolapse is a description of how the leaflets come together. This may cause the valve to leak. In most people prolapse is a relatively benign process. In some, however, it can lead to severe mitral leak which would need surgery to repair.