LLSB: Fourth Heart Sound (S₄)

The fourth heart sound is a low frequency sound heard just before the first heart sound. It is most commonly known as S₄.

The S₄ is a diastolic sound which occurs during the late diastolic filling phase at the time when the atria contract. When the ventricles have a decreased compliance or are receiving an increased diastolic volume, they generate a low frequency vibration, the S₄.

The S₄ of right ventricular origin is best heard at the LLSB. It is usually accenttuated with inspiration and may be due to pulmonary valve obstruction, pulmonary stenosis, pulmonary hypertension, or right ventricular myocardial infarction. It is best heard with the belly lightly held.

The sound you heard is an S₄. The fourth sound is removed and added again.