S1 is the first heart sound that doctors can hear using a stethoscope. The vibrations that occur when the mitral and tricuspid valves in the heart close produce the S1 sound.

There are two common heart sounds and other less common sounds doctors can hear using a stethoscope. S1 is one of the sounds that the heart produces as blood passes through the chambers when the valves open and close. Doctors may refer to the S1 as “lub” and the second heart sound (S2) as “dub.”

Medical professionals use a stethoscope to examine heart sounds and check if a person has heart murmurs. Electrical devices may also be helpful, as they can help record the sounds produced by the heart and blood.

In this article, we examine the S1 sound in detail and its clinical significance. We also look at the other heart sounds and the treatments for sound deviations.

To discover more evidence-based information and resources for cardiovascular health, visit our dedicated hub.

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The S1 sound develops when ventricular pressure rises and closes mitral and tricuspid valves at the beginning of the systole. The systole is the phase where the heart muscle contracts and pumps blood to the arteries from the chambers. The increase in ventricular pressure also opens the aortic and pulmonary valves.

The closure of the mitral valve is louder than the tricuspid valve. However, the closure of both valves is necessary to produce the S1 sound.

The cardiac cycle refers to a series of pressure changes that occur in the heart. These happen when blood flows through different heart chambers.

The cardiac cycle involves two events:

  • Diastole: This is when the heart muscles relax and ventricles fill with blood from the atria. Diastole begins when the aortic and pulmonic valves close and ends when the mitral and tricuspid valves close.
  • Systole: Systole is the period of heart contraction when blood leaves the ventricles through the aortic and pulmonic valves. Systole immediately follows diastole. It begins with the closure of the mitral and tricuspid valves and ends with the closure of the aortic and mitral valves.

Learn more about the difference between diastole and systole.

The heart produces four sounds:

  • S1: This is the first heart sound that doctors associate with the closure of the mitral and tricuspid valves. It usually sounds like a single noise because both valves’ closure occurs almost simultaneously.
  • S2: This occurs when the aortic and pulmonic valves close when the heart muscles relax.
  • S3: The third heart sound has low-frequency components and produces a short vibrating sound. It occurs after S2, and medical professionals may refer to it as ventricular gallop. S3 occurrence is normal in young people and athletes, but can also indicate heart failure.
  • S4: The fourth heart sound is low-pitched and quiet. Doctors may refer to it as atrial gallop and presystolic gallop. It may occur in people with stiffened left ventricles, indicating that they may have hypertension or aortic stenosis, among other health problems.

Learn more about heart sounds.

A person may experience a loud or soft S1 sound, indicating they may have an underlying health problem.

Loud S1 sound

The first heart sound may be loud if individuals have mitral stenosis or exaggerated contractions of the heart, such as in tachycardia.

Mitral stenosis

An older publication states that an individual may have mitral stenosis if the first heart sound is loud. The loud S1 sound comes from the stiffened mitral valves.

Mitral stenosis is the narrowing of the heart’s mitral valve, allowing less blood supply to flow from the left atrium to the left ventricle.

This condition can cause tiredness and shortness of breath. Also, more blood accumulates in the left atrium, leading to fluid buildup in the lungs.


Tachycardia refers to a fast heart rate. It causes an increase in ventricular pressure, which may also lead to a loud S1 sound.

A person has tachycardia if they have a heart rate of more than 100 beats per minute.

Tachycardia does not always cause symptoms, but some individuals may experience:

Anemia and hyperthyroidism can also cause exaggerated heart contractions.

Soft or muffled S1 sound

Certain forms of mitral regurgitation and severe congestive heart failure may cause a soft or muffled S1 heart sound.

Mitral regurgitation

Mitral valve regurgitation is a problem that occurs when there is a valve leak, leading to blood flowing back into the left atrium. The incomplete valve closure may produce a softer S1 sound.

Symptoms that may arise in people with this valve problem include:

  • dizziness
  • breathlessness
  • chest pain

Without treatment, mitral valve regurgitation can lead to atrial fibrillation, heart failure, or pulmonary hypertension.

Severe congestive heart failure

The heart may produce a soft S1 sound in individuals with severe congestive heart failure in which the heart’s contraction is reduced.

This is a condition in which fluid builds up in the heart, preventing it from pumping enough blood throughout the body.

It can develop at any age but is more common in people 65 years and older.

Symptoms of severe congestive heart failure may include:

  • breathlessness after exercising or resting
  • lightheadedness
  • swelling of the ankles
  • persistent cough

Medical professionals may prescribe medications to help with problems that cause loud or soft S1 sounds. However, the treatment depends on the condition causing the sound.

For example, vasodilators may be beneficial for those with congestive heart failure. These medications widen the blood vessels so that blood can pass through and offload pressure on the heart.

Diuretics help eliminate excess fluid in the body and treat hypertension and other conditions that lead to volume overload.

Doctors often prescribe blood thinners to prevent stroke in people who have both mitral valve stenosis and atrial fibrillation. These drugs prevent blood clot formation and growth. Without treatment, blood clots can cause strokes or heart attacks.

Tachycardia does not often require treatment. In cases where it is necessary, treatment depends on the underlying cause. A doctor may suggest making dietary and lifestyle changes, such as limiting caffeine intake and quitting smoking, to help reduce the heart rate.

The S1 sound is a term for the first heart sound. Doctors may also refer to it as “lub.”

Some people may have a loud S1 sound if they develop mitral stenosis or tachycardia. S1 may also be muffled or soft if someone has a leaky valve or congestive heart failure.

Doctors can provide a suitable treatment plan for an individual, depending on the heart problem they develop.