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).

Physiology

It occurs at the beginning of diastole

Diastole

Diastole is the period of time when the heart fills with blood after systole . Ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing...

 approximately 0.12 to 0.18 seconds after S₂. This produces a rhythm classically compared to the cadence of the word "Kentucky

Kentucky

The Commonwealth of Kentucky is a state located in the East Central United States of America. As classified by the United States Census Bureau, Kentucky is a Southern state, more specifically in the East South Central region. Kentucky is one of four U.S. states constituted as a commonwealth...

" 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

Stethoscope

The stethoscope is an acoustic medical device for auscultation, or listening to the internal sounds of an animal body. It is often used to listen to lung and heart sounds. It is also used to listen to intestines and blood flow in arteries and veins...

 of the stethoscope

Stethoscope

The stethoscope is an acoustic medical device for auscultation, or listening to the internal sounds of an animal body. It is often used to listen to lung and heart sounds. It is also used to listen to intestines and blood flow in arteries and veins...

.

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

Gallop rhythm

A gallop rhythm refers to a rhythm of the heart on auscultation. It includes three or four sounds, thus resembling the sounds of a gallop....

 by virtue of having an extra sound; the other gallop rhythm is called S₄

Fourth heart sound

The fourth heart sound or S4 is a rare extra heart sound that occurs immediately before the normal two "lub-dub" heart sounds . It occurs just after atrial contraction and immediately before the systolic S1...

. The two are quite different, but they may sometimes occur together forming a quadruple gallop. If the heart rate

Heart rate

Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....

 is also very fast (tachycardia

Tachycardia

Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...

), 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

Chordae tendineae

The chordae tendineae, or heart strings, are cord-like tendons that connect the papillary muscles to the tricuspid valve and the mitral valve in the heart....

 during rapid filling and expansion of the ventricle.

Rapid ventricular filling

• Mitral regurgitation
  Mitral regurgitation
  Mitral regurgitation , mitral insufficiency or mitral incompetence is a disorder of the heart in which the mitral valve does not close properly when the heart pumps out blood. It is the abnormal leaking of blood from the left ventricle, through the mitral valve, and into the left atrium, when...
  
   - this is when one of the mitral valve leaflets that usually stop blood flowing from the left ventricle to the left atria fails, allowing blood into the atria during systole
  Systole (medicine)
  Systole is the contraction of the heart. Used alone, it usually means the contraction of the left ventricle.In all mammals, the heart has 4 chambers. The left and right ventricles pump together. The atria and ventricles pump in sequence...
  
  . This means that the left atria will be overfilled, leading to rapid ventricular filling when the mitral valve opens.
• Elevated left atrial and left ventricular filling pressures, usually a result of a stiffened and dilated left ventricle
• Ventricular septal defect
  Ventricular septal defect
  A ventricular septal defect is a defect in the ventricular septum, the wall dividing the left and right ventricles of the heart.The ventricular septum consists of an inferior muscular and superior membranous portion and is extensively innervated with conducting cardiomyocytes.The membranous...
  
   - 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
  Myocardial infarction
  Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
  
   - the death of tissue in the ventricular wall due to loss of blood supply causes wall areas which do not move as well as normal (hypokinesia
  Hypokinesia
  Hypokinesia refers to decreased bodily movement. It is associated with basal ganglia diseases , mental health disorders and prolonged inactivity due to illness, amongst other diseases.Hypokinesia describes a spectrum of disorders:...
  
  ), or not at all (akinesia), meaning they relax more slowly, so the ventricular filling is relatively too rapid.
• Dilated cardiomyopathy
  Dilated cardiomyopathy
  Dilated cardiomyopathy or DCM is a condition in which the heart becomes weakened and enlarged and cannot pump blood efficiently. The decreased heart function can affect the lungs, liver, and other body systems....
  
   - the ventricular walls are abnormal for a variety of reasons, and become thin and stiff so do not relax well.

S3 can also be due to tricuspid regurgitation, and could indicate hypertensive heart disease.

In conditions affecting the pericardium

Pericardium

The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...

 or diseases that primarily affect the heart muscle (restrictive cardiomyopathies

Restrictive cardiomyopathy

Restrictive cardiomyopathy is a form of cardiomyopathy in which the walls are rigid, and the heart is restricted from stretching and filling with blood properly....

) 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