Equine Aortic Insufficiency Murmur

A decrescendo murmur starting at the time of S2 is appreciated in this equine heart sound; the murmur gradually tapers from the initial peak as it extends through diastole (decrescendo in shape). 

The most common cause of a decrescendo diastolic murmur in older horses is degeneration of the aortic valve, leading to aortic insufficiency.