In humans, each heartbeat produces two distinct sounds known as the first and second heart sounds. These two, the loudest sounds of the heart, are easily detected clinically by a stethoscope.
Two other heart sounds have also been described that cannot be clearly detected with the help of a stethoscope. These two sounds are the third and fourth heart sounds and can only be detected by graphical records.
The heart sound is often referred to as the “lub-dub” sound of the heart. It provides valuable information about the heart’s functioning and is clinically very important to diagnose any underlying cardiovascular problems.
Definition of Heart Sound
The sounds that are produced in different phases of a cardiac cycle are called heart sounds.
Types of Heart Sound
During a cardiac cycle, the heart produces four main sounds. These are S1 (first heart sound), S2 (second heart sound), S3 (third heart sound), and S4 (fourth heart sound).
The first and second heart sounds are close to each other. There is a long pause after the second heart sound. Then the first heart sound is heard again.
S1 (First Heart Sound)
The first heart sound is produced at the beginning of the ventricular systole. It is a long, soft, low-pitched sound of about 25 to 45 Hz.
The S1 is heard like the word LUB, and hence it is referred to as the “Lub” sound. It’s duration is about 0.10-0.17 seconds.
Causes
- The sudden closure of the two AV (atrioventricular) valves and the vibration in the valve leaflets due to increased intraventricular pressure.
- Ejection of ventricular blood and the vibration flow to the walls of the aorta and pulmonary artery.
- Contraction of thick ventricular muscle.
Significance
- The first heart sound indicates the beginning of the ventricular systole of the heart.
- The condition of the myocardium is known from the duration and intensity of the first sound. If the myocardium is weak, the duration and intensity will be short.
- A clear first heart sound indicates that the two AV valves are closing properly.
S2 (Second Heart Sound)
The second heart sound is produced during the protodiastole phase. It is a short, loud, high-pitched sound of about 50 Hz.
The S2 is heard as the word DUB, so it is called the “Dub” sound. The duration of this sound is 0.10-0.14 seconds.
Causes
- The second heart sound is caused by the sudden closing of the two SL (semilunar) valves in the aorta and pulmonary artery. The vibrations that occur in the leaflets of the SL valves are heard as the second sound.
Significance
- The second heart sound indicates the end of systole and the beginning of diastole in the heart.
- The intensity of this sound is directly proportional to the blood pressure.
- A clear second heart sound indicates that the two SL valves are properly closing.
S3 (Third Heart Sound)
The third sound occurs during the rapid ventricular filling phase of the cardiac cycle, just after the second sound. It is a short, weak, low-pitched sound.
The S3 is rarely heard in normal conditions in adults (it can be detected in 60% of normal people). When heard, it is called the “Lub-Dub-Ta” sound and is often an indication of some issue in the heart.
The duration of S3 is about 0.04 seconds.
Causes
- The third heart sound is caused by the rapid rush of blood from the atria into the ventricles when the AV valves open.
Significance
- The third sound indicates the beginning of ventricular filling.
S4 (Fourth Heart Sound)
The fourth heart sound is produced during the atrial systole, just before the first sound. It is considered a physiologic heart sound (as it is caused by the contraction of the atria) and is also called the atrial sound.
The S4 is a short, low-pitched sound (duration of about 0.10 seconds) and is very difficult to detect clinically. It is also rarely heard in normal conditions. When heard, it is described as the “Ta-Lub-Dub” sound and is usually an indication of a problem in the heart.
Causes
- The fourth heart sound is caused by the contraction of the atria and the rapid flow of atrial blood into the ventricles.
Significance
- The fourth sound indicates the end of the ventricular filling.
Table: Heart sounds Timing and Causes
| Heart Sound | Timing | Causes |
| First heart sound (Dull and prolonged) | It occurs during ventricular systole. | Sudden closure of the two AV valves. |
| Second heart sound (Sharp and short) | It is produced at the end of the protodiastole. | Sudden, loud closing of the two SL valves. |
| Third heart sound (Very weak) | It takes place during the inflow of blood from the atria into the ventricles. | Due to the rapid rush of atrial blood into the ventricles. |
| Fourth heart sound (Very weak) | It occurs at the end of ventricular diastole, just before the first sound. | Due to atrial contraction and the flow of blood. |
Difference between First and Second Heart Sounds
| First heart sound (S1) | Second heart sound (S2) |
| It occurs when the AV valves close. | It occurs when the semilunar valves become closed. |
| S1 is low-pitched and long. | S2 is high-pitched and short. |
| This sound is heard like the word lub. | It is heard like the word dub. |
| Its duration is about 0.15 seconds. | The duration of this sound is usually 0.1 second. |
