Third heart sound Classification and external resources
ICD-9	427.9

Diagram of the heart.

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

S₃ is normal in people under 40 years of age and some trained athletes but normally disappears 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 is 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.

Causes

S₃ is thought to be caused by the oscillation of blood back and forth between the walls of the ventricles initiated by inrushing 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.

Associations

It is associated with heart failure, caused by conditions which have:

Rapid ventricular filling

• Mitral regurgitation - this is when one of the heart valves that usually stops blood going from the left ventricle to the left atria fails, allowing blood into the atria during systole. This means they will be overfilled when they come to contract, leading to the rapid ventricular filling.
• Elevated left atrial and left ventricular filling pressures, usually a result of a stiffened and dilated left ventricle
• Ventricular septal defect - this is a hole in the wall between the two ventricles, which allows rapid filling from the other ventricle.

Poor Left Ventricular Function

• Post-MI - the death of tissue in the ventricular wall due to loss of blood supply causes areas which do not move as well, if at all (hypokinetic and akinetic), meaning they do not relax quickly enough so the ventricular filling is relatively too quick.
• Dilated cardiomyopathy - the ventricular walls are abnormal for a variety of reasons, and become thin and stiff so do not relax well.

In conditions affecting the pericardium or diseases that primarily affect the heart muscle (restrictive cardiomyopathies) a similar sound can be heard, but is usually more high-pitched and is called a 'pericardial knock'.

The S3 can also be confused with a widely split S2, or a mitral opening snap, but these sounds are typically of much higher pitch and occur closer to the onset of S2.

Treatment

The condition itself does not need to be treated, but rather the underlying cause requires correction. Depending on the aetiology the gallop rhythm may resolve spontaneously.

External links

• chfpatients.com - suitable for non-medical people, but requires some knowledge of medical terms
• familypractice.com - requires knowledge of medical terms
• KUO PT, SCHNABEL TG, BLAKEMORE WS, WHEREAT AF (1957). "Diastolic gallop sounds, the mechanism of production". J. Clin. Invest. 36 (7): 1035–42. PMID 13449156.