Third heart sound (S3)

The third heart sound or S3 is a rare extra heart sound that occurs soon after the normal two "lub-dub" heart sounds (S1 and S2).

Segmentation of heart sound with S3. [Source]

A double apical impulse can represent an auscultated third heart sound (S3). Just as with the displaced point of maximal impulse, a third heart sound is not sensitive (24 percent) for heart failure, but it is highly specific (99 percent).


Physiology

It occurs at the beginning of diastole approximately 0.12 to 0.18 seconds after S2. This produces a rhythm classically compared to the cadence of the word "Kentucky" with the final syllable ("-ky") representing S3.

S3 may be normal in people under 40 years of age and some trained athletes but should disappear before middle age. Re-emergence of this sound late in life is abnormal and may indicate serious problems like heart failure. The sound of S3 is lower in pitch than the normal sounds, usually faint, and best heard with the bell of the stethoscope.

It has also been termed a ventricular gallop or a protodiastolic gallop because of its place in early diastole. It is a type of gallop rhythm by virtue of having an extra sound; the other gallop rhythm is called S4. The two are quite different, but they may sometimes occur together forming a quadruple gallop. If the heart rate is also very fast (tachycardia), it can become difficult to distinguish between S3 and S4 thus producing a single sound called a summation gallop. S3 is a dull, low-pitched sound best heard with the bell placed over the cardiac apex with the patient lying in the left lateral decubitus position. This heart sound when present in a child or young adult implies the presence of a supple ventricle that can undergo rapid filling. Conversely, when heard in a middle-aged or older adult, an S3 is often a sign of disease, indicating increased ventricular filling due to congestive heart failure or severe mitral or tricuspid regurgitation.


Causes

S3 is thought to be caused by the oscillation of blood back and forth between the walls of the ventricles initiated by the inflow of blood from the atria. The reason the third heart sound does not occur until the middle third of diastole is probably because during the early part of diastole, the ventricles are not filled sufficiently to create enough tension for reverberation. It may also be a result of tensing of the chordae tendineae during rapid filling and expansion of the ventricle.


Further reading:

1. Davie AP, Francis CM, Caruana L, Sutherland GR, McMurray JJ. Assessing diagnosis in heart failure: which features are any use? Q J Med 1997;90:
   335-9.