The fourth heart sound or S4 is a rare extra heart sound that occurs immediately before the normal two "lub-dub" heart sounds (S1 and S2). It occurs just after atrial contraction and immediately before the systolic S1. It is best conceptualized as hearing a "split S1." This produces a rhythm classically compared to the cadence of the word "Tennessee" with the first syllable ("Ten-") representing S4. One can also use the phrase "a-STIFF-wall" to help with the cadence (a S4, stiff S1, wall S2), as well as the pathology of the S4 sound.
Read more about Fourth Heart Sound: Physiology, Causes, Associations, Treatment
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