The initial vibrations of the first heart sound.

The major component of the normal first heart sound is that due to valvular closure. Preceding this there occur low frequency, low amplitude vibrations commonly attributed to auricular systole. It has been shown in this study that such vibrations appear invariably even though the auricles are fibrillating, and in heart block when auricular systole is widely seperated from ventricular systole. It is concluded that the onset of ventricular contraction itself causes these vibrations through mechanisms unrelated to auricular contraction, and prior to the valvular component.