What is the Difference Between First and Second Heart Sound

Heart is a muscular structure that has four chambers and pumps and receives blood throughout the human body. The sounds produced or generated by the beating heart during each cardiac cycle are known as heart sounds. There are four heart sounds: first heart sound (S1), second heart sound (S2), third heart sound (S3) and fourth heart sound (S4).

The key difference between first and second heart sound is the location they generate. The first heart sound occurs at the start of the ventricular systole, while the second heart sound occurs at the end of the ventricular systole or at the start of the ventricular diastole.

CONTENTS

1. Overview and Key Difference
2. What is First Heart Sound
3. What is Second Heart Sound
4. Similarities – First and Second Heart Sound
5. First vs Second Heart Sound in Tabular Form
6. Summary – First vs Second Heart Sound
7. FAQ – First and Second Heart Sound

What is First Heart Sound?

The first heart sound (S1) occurs at the start of the ventricular systole, and it is produced by the vibration given by the sudden closing of the AV valves, known as mitral and tricuspid, at the beginning of the ventricular systole. The first heart sound is soft, long, and low-pitched. Moreover, it is also known as the “lub” sound.

The frequency of the first heart sound is 25 to 45 Hz (30 to 80 cps). Its duration is 0.14 seconds. First heart sounds are audible over the mitral and tricuspid sites. Furthermore, the first heart sound is important because it signals the start of ventricular systole, reveals the state of the myocardium, and shows how well the atrioventricular valves are working.

Figure 01: First Heart Sound– Mitral (M) and Tricuspid (T). Second Heart Sound  – Aortic (A) and Pulmonary/Pulmonic (P).

What is Second Heart Sound?

The second heart sound (S2) occurs at the end of the ventricular systole or at the start of the ventricular diastole. It is generated by the closing of semilunar valves such as aortic and pulmonary valves. The second heart sound is loud, short, and high-pitched. Moreover, it is also known as the “dub” sound.

Figure 02: Wiggers Diagram

The frequency of the first heart sound is 50 Hz (150 to 200 cps), while its duration is 0.11 seconds. The second heart sound is audible over the aortic and pulmonary areas.  Furthermore, the second heart sound indicates the end of systole and the start of diastole and shows the compliance of semilunar valves.

Similarities Between First and Second Heart Sound

• First and second heart sounds are two different types of heart sounds.
• They are generated by the closure of valves.
• Both are very helpful in identifying the proper functioning of the heart.
• Both are very helpful in diagnosing heart-related diseases.

Difference Between First and Second Heart Sound

Definition

1. The first heart sound (S1) occurs at the start of the ventricular systole, and it is produced by the vibration of the sudden closing of the AV valves, which are known as mitral and tricuspid.
2. The second heart sound (S2) occurs at the end of the ventricular systole or at the start of the ventricular diastole, and it is generated by the closing of semilunar valves, such as the aortic and pulmonary valves.

Other Names

1. The first heart sound is often referred to as the “lub” sound.
2. The second heart sound is often referred to as the “dub” sound.

Pitch

1. The first heart sound is lower-pitched, duller, and longer.
2. The second heart sound is loud, short, and high-pitched.

Frequency

1. The frequency of the first heart sound ranges from 25 to 45 Hz (30 to 80 cps).
2. The frequency of the second heart sound is around 50 Hz (150 to 200 cps).

Duration

1. The duration of the first heart sound is approximately 0.14 seconds.
2. The duration of the second heart sound is around 0.11 seconds.

Significance

1. The first heart sound marks the start of ventricular systole and provides information about the condition of the myocardium and the function of the atrioventricular valves.
2. The second heart sound marks the end of systole and the beginning of diastole and provides information about the function of the semilunar valves.

The below infographic  summarizes the difference between first and second heart sound.

Summary – First vs Second Heart Sound

Heart sounds specifically represent the turbulence created when the heart valves snap shut. First and second heart sounds are two different types of heart sounds. The first heart sound (S1) occurs at the start of the ventricular systole, while the second heart sound (S2) occurs at the end of the ventricular systole or at the start of the ventricular diastole. Furthermore, the first heart sound is produced by the vibration given by the sudden closing of mitral and tricuspid whereas the second heart sound is generated by the closing of semilunar valves such as aortic and pulmonary valves. This is the summary of the difference between first and second heart sound.

FAQ: First and Second Heart Sound

1. Is the first heart sound lub or dub?

• During each cardiac cycle, two prominent sounds are generated- lub and dub. The first heart sound is lub or s1, which is associated with the closure of the tricuspid and bicuspid valves. The second heart sound is dub or s2, which is associated with the closure of the semilunar valves.

2. What is the best description of S1 heart sound?

• S 1 sound is soft, low pitched and long, occurring at the start of the ventricular systole. The frequency is 25 to 45 Hz (30 to 80 cps). It is best heard over the mitral and tricuspid sites.

3. What is the second heart sound caused by?

• The second heart sound, or S2, is produced immediately following the closure of the aortic and pulmonic valves. The vibrations of the second heart sound occur at the end of ventricular contraction and the onset of ventricular diastole.

4. What are the 1st, 2nd, 3rd and 4th heart sounds?

• First heart sound or “lub” is caused by the closing of the AV valves; second heart sound or “dub” is caused by the closing of semilunar valves; third heart sound is linked with the flow of blood into the ventricles and fourth heart sound is linked with atrial contraction.

5. What is the significance of heart sounds?

• The heart sounds are the cornerstone of physical medical examination and a valuable first line to evaluate a patient. Some sounds are very characteristics of significant pathological lesions.

Reference:

1 “Cardiac Second Sounds.” Stanford Medicine.
2. “Heart Sounds.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “Gray1216 modern locations” By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body -Modifications made by Huckfinne. (Public Domain) via Commons Wikimedia

2. “Wiggers Diagram 2” By adh30 revised work by DanielChangMD who revised original work of DestinyQx; Redrawn as SVG by xavax – Wikimedia Commons: Wiggers Diagram.svg (CC BY-SA 4.0) via Commons Wikimedia

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