This heart sound represents atrial contraction (the late filling phase of diastole). It becomes more forceful and therefore more easily audible when left ventricular filling is impaired. Causes of impairment include aortic stenosis, hypertension, myocardial ischemia, and almost all instances of acute myocardial infarction. The S4 is common, especially in people over the age of 60.
The S4 is often confused with a split S,. The following points favor an S4:
The S4 tends to be softer and of lower frequency than a split S,.
The S4 of left-sided origin is best heard at the cardiac apex and is accentuated by expiration.
When the S4 derives from increased right ventricular stiffness,‚ it is accentuated on inspiration.
Obvious splitting at the apex suggests an S4, whereas splitting of S, is best heard at the lower sternal border.
The S4 is best heard, as are all low-frequency sounds, with the stethoscope bell applied with very light pressure to the chest wall.