This is an example of commotio cordis as heard at the mitral valve area.
This condition is caused by blunt force trauma to the chest such as a baseball batter being hit in the chest by a pitch. Severe damage to the right and left ventricles and mitral and tricuspid valves may result.
In the example we are showing, the trauma is limited to the mitral valve leaflets. Rupture of a chordae tendinae has occurred resulting in a systolic murmur.
The first half of the murmur is rectangular. It is followed by a decrescendo late systolic component. This is caused by rapid filling of the left atrium due to torrential mitral regurgitation.