Written By: [Science Of Medicine]
Last Updated: September 7, 2026
(Note: For PDF File Swipe To The End Of Article)
Introduction
“Lub-dub” is one of the most familiar sounds in medicine. It is the characteristic sound produced by the human heart during each cardiac cycle. But what exactly causes the lub dub heart sound? Why does the heart make two sounds? What are S1 and S2 heart sounds? And when can additional sounds such as S3, S4, clicks, or murmurs indicate a cardiac problem?
Understanding heart sounds lub dub is an important part of learning cardiovascular physiology and performing a cardiovascular examination. Healthcare professionals use a stethoscope to listen to the heart and assess the timing, intensity, pitch, rhythm, and quality of its sounds.
The normal lub-dub corresponds primarily to two major heart sounds:
- S1 — the first heart sound (“lub”)
- S2 — the second heart sound (“dub”)
S1 is associated mainly with closure of the mitral and tricuspid valves, while S2 is associated with closure of the aortic and pulmonary valves. Together, they mark important transitions between systole and diastole.
This article explains heart sounds lub dub, the physiology behind S1 and S2, cardiac auscultation areas, normal and abnormal heart sounds, S3 and S4, heart murmurs, splitting of S2, and how healthcare professionals interpret heart sounds.
What Does “Lub Dub” Mean?
The phrase “lub dub” is a simple way of describing the two main sounds heard during a normal heartbeat.
The:
“Lub” = S1 = First heart sound
“Dub” = S2 = Second heart sound
A typical cardiac cycle can therefore be represented as:
Lub → Dub → Lub → Dub → Lub → Dub
The first sound occurs as the ventricles begin systole, while the second sound occurs as systole ends and diastole begins.
The sounds are generated primarily by vibrations associated with cardiac valve closure and surrounding cardiac structures rather than simply by blood “hitting” the valves.
What Causes the Lub Sound?
The lub is called S1, or the first heart sound.
S1 occurs primarily because the atrioventricular (AV) valves close at the beginning of ventricular systole.
The two important AV valves are:
- Mitral valve
- Tricuspid valve
When ventricular pressure rises above atrial pressure, the mitral and tricuspid valves close.
This prevents blood from flowing backward into the atria during ventricular contraction.
The resulting vibrations contribute to the sound known as S1 or “lub.”
S1 = “Lub”
Main valves involved:
- Mitral valve
- Tricuspid valve
Timing:
Beginning of ventricular systole
Clinical significance:
S1 helps identify the beginning of systole.
What Causes the Dub Sound?
The dub is called S2, or the second heart sound.
S2 occurs primarily because the semilunar valves close at the end of ventricular systole.
The two semilunar valves are:
- Aortic valve
- Pulmonary valve
When ventricular pressure falls below the pressure in the aorta and pulmonary artery, these valves close.
Their closure produces the components of S2.
S2 = “Dub”
Main valves involved:
- Aortic valve
- Pulmonary valve
Timing:
End of systole and beginning of diastole
The second heart sound therefore marks an important transition from ventricular contraction toward ventricular relaxation and filling.
S1 vs S2: What Is the Difference?
Understanding the difference between S1 and S2 heart sounds is fundamental to cardiovascular examination.
| Feature | S1 | S2 |
|---|---|---|
| Common name | Lub | Dub |
| Heart sound | First heart sound | Second heart sound |
| Main valves | Mitral + tricuspid | Aortic + pulmonary |
| Cardiac phase | Beginning of systole | Beginning of diastole |
| Ventricular state | Contraction begins | Relaxation begins |
| Main components | M1 + T1 | A2 + P2 |
S1 is generally lower-pitched than S2, and the timing between S1 and S2 helps clinicians determine which sound they are hearing.
Understanding the Cardiac Cycle
To understand lub dub heart sounds, it helps to understand the cardiac cycle.
The cardiac cycle consists broadly of:
- Ventricular systole
- Ventricular diastole
During systole, the ventricles contract and eject blood.
During diastole, the ventricles relax and fill with blood.
The major sequence is:
S1 → systole → S2 → diastole → S1
In simple terms:
S1 (“Lub”)
The ventricles begin contracting.
S2 (“Dub”)
The ventricles finish ejecting blood and begin relaxing.
Next S1
The next ventricular contraction begins.
This repetitive sequence creates the familiar lub-dub rhythm.
The Four Heart Valves
The human heart contains four major valves.
1. Mitral Valve
The mitral valve lies between:
Left atrium → Left ventricle
It prevents blood from flowing backward into the left atrium during ventricular systole.
Its closure contributes primarily to S1.
2. Tricuspid Valve
The tricuspid valve lies between:
Right atrium → Right ventricle
It prevents backward blood flow into the right atrium during ventricular contraction.
Its closure contributes to S1.
3. Aortic Valve
The aortic valve lies between:
Left ventricle → Aorta
It prevents blood from returning from the aorta into the left ventricle after ventricular ejection.
Its closure contributes to S2.
4. Pulmonary Valve
The pulmonary valve lies between:
Right ventricle → Pulmonary artery
It prevents blood from returning to the right ventricle after ventricular ejection.
Its closure contributes to S2.
Why Does the Heart Make Two Main Sounds?
A common question is:
“Why does the heart sound like lub-dub?”
The answer is that the heart has two major sets of valves that close at different points during the cardiac cycle.
First sound — S1
The AV valves close:
Mitral + Tricuspid → Lub
Second sound — S2
The semilunar valves close:
Aortic + Pulmonary → Dub
Therefore:
AV valve closure → S1 → “Lub”
Semilunar valve closure → S2 → “Dub”
This is the basic physiological explanation of the normal heart sounds lub dub.
S1: The First Heart Sound
The first heart sound is called S1.
It occurs at the beginning of ventricular systole and is primarily related to closure of the mitral and tricuspid valves.
S1 can have two components:
- M1 — mitral valve closure
- T1 — tricuspid valve closure
Normally, M1 occurs slightly before T1.
In healthy individuals, this separation is usually too small to be perceived as two distinct sounds.
Instead, they are generally perceived as one sound:
“Lub”
When Is S1 Loud?
The intensity of S1 can change depending on cardiac and valvular conditions.
A relatively loud S1 can occur in situations such as mitral stenosis, depending on valve mobility and other factors.
The position of the AV valves before ventricular contraction influences the intensity of S1.
When Is S1 Soft?
S1 may become softer in several circumstances.
Possible causes include:
- Certain conduction abnormalities
- Mitral regurgitation
- Reduced valve mobility
- Changes in ventricular contraction
- First-degree AV block
The clinical interpretation depends on the overall examination and patient context.
S2: The Second Heart Sound
The second heart sound is called S2.
It occurs at the end of ventricular systole and the beginning of diastole.
S2 consists primarily of:
- A2 — aortic valve closure
- P2 — pulmonary valve closure
Normally:
A2 occurs slightly before P2.
This difference becomes particularly important during respiration.
Physiological Splitting of S2
One of the most important concepts in learning heart sounds is splitting of S2.
During inspiration, more blood returns to the right side of the heart.
This increases right ventricular filling.
As a result, right ventricular ejection takes slightly longer, delaying pulmonary valve closure.
Therefore:
A2 → P2
may become more separated during inspiration.
This is known as physiological splitting of S2 and can be a normal finding.
What Does S2 Sound Like?
Normally, S2 is heard as:
“Dub”
However, when the components become separated, a clinician may hear something resembling:
“Lub-dub”
followed by a slightly separated second component during inspiration.
The ability to recognize S2 splitting is an important part of advanced cardiac auscultation.
Heart Auscultation: Where Do You Hear the Heart Sounds?
Healthcare professionals listen to the heart at several standardized locations.
The traditional five areas are:
- Aortic area
- Pulmonic area
- Erb’s point
- Tricuspid area
- Mitral area
These locations help clinicians identify different valve sounds and murmurs.
1. Aortic Area
Location:
Second right intercostal space at the right sternal border.
This area is useful for assessing sounds associated with the aortic valve.
2. Pulmonic Area
Location:
Second left intercostal space at the left sternal border.
This area is particularly useful for evaluating pulmonary valve sounds and the pulmonic component of S2.
3. Erb’s Point
Location:
Third left intercostal space near the sternal border.
Erb’s point is particularly useful for certain murmurs and assessment of S2.
4. Tricuspid Area
Location:
Approximately the fourth or fifth left intercostal space along the lower left sternal border.
This area is useful for evaluating sounds associated with the tricuspid valve.
5. Mitral Area
Location:
Approximately the fifth left intercostal space at the midclavicular line.
This corresponds approximately to the cardiac apex.
The mitral area is particularly important when evaluating:
- S1
- Mitral murmurs
- S3
- S4
The standard auscultation sequence and anatomical landmarks are described in clinical nursing and medical references.
The “APE To Man” Mnemonic
A popular mnemonic for remembering the major cardiac auscultation areas is:
A — Aortic
P — Pulmonic
E — Erb’s point
T — Tricuspid
M — Mitral
“APE To Man”
This can be particularly useful for medical, nursing, pharmacy, and allied-health students learning heart auscultation.
How to Listen to Heart Sounds With a Stethoscope
Proper technique is important when assessing S1 and S2 heart sounds.
The examination should ideally occur in a quiet environment.
The clinician listens systematically over the precordium and evaluates:
- Rate
- Rhythm
- S1
- S2
- Intensity
- Timing
- Splitting
- Additional sounds
- Murmurs
The diaphragm of the stethoscope is useful for higher-frequency sounds, while the bell is useful for lower-frequency sounds.
Patient Position for Heart Auscultation
Different positions can make certain cardiac sounds easier to hear.
A patient may be examined:
- Supine
- Sitting
- Leaning forward
- Left lateral decubitus position
For example, some low-frequency sounds such as S3 and S4 may be easier to detect at the apex with the patient in the left lateral position.
What Is S3 Heart Sound?
Not every heart sound is simply “lub-dub.”
A third heart sound is called:
S3
It occurs shortly after S2 during early ventricular filling.
The rhythm may sound like:
“Lub-dub-ta”
or:
S1 → S2 → S3
S3 is sometimes called a ventricular gallop.
Is S3 Always Abnormal?
No.
An S3 can be a normal physiological finding in some younger individuals and during pregnancy.
However, in many older adults, a new S3 may be clinically significant and can be associated with conditions such as ventricular dysfunction or volume overload.
Clinical interpretation depends heavily on age, symptoms, and the rest of the cardiovascular examination.
What Is S4 Heart Sound?
The fourth heart sound is called:
S4
It occurs immediately before S1.
The sequence is:
S4 → S1 → S2
It may sound approximately like:
“Ta-lub-dub”
S4 is sometimes called an atrial gallop.
S4 can occur when the ventricle has reduced compliance, making atrial contraction produce an additional low-frequency sound.
It may be associated with conditions involving a stiff ventricle, including certain forms of hypertensive or ischemic heart disease.
S3 vs S4 Heart Sounds
| Feature | S3 | S4 |
|---|---|---|
| Timing | After S2 | Before S1 |
| Phase | Early diastole | Late diastole |
| Nickname | Ventricular gallop | Atrial gallop |
| Rhythm | S1-S2-S3 | S4-S1-S2 |
| Typical significance | Volume overload/ventricular dysfunction in appropriate clinical settings | Reduced ventricular compliance |
| Best heard | Often at apex with bell | Often at apex with bell |
What Is a Heart Murmur?
A heart murmur is different from the normal “lub-dub.”
A murmur is usually described as a:
- Whooshing
- Swishing
- Blowing
- Harsh
sound produced by turbulent blood flow.
Murmurs can occur because of:
- Valve stenosis
- Valve regurgitation
- Congenital heart disease
- Increased blood flow
- Other structural or physiological conditions
Not every murmur automatically means serious heart disease, but a newly detected murmur should be appropriately evaluated.
Lub-Dub vs Heart Murmur
Normal heart sounds:
Lub → Dub
Possible murmur:
Lub → whoosh → Dub
or:
Lub → Dub → whoosh
The timing of a murmur in relation to S1 and S2 provides important diagnostic information.
Systolic vs Diastolic Murmurs
Heart murmurs can broadly be classified according to their timing.
Systolic Murmur
Occurs between:
S1 → S2
Diastolic Murmur
Occurs between:
S2 → S1
This makes accurate identification of S1 and S2 essential.
If you cannot determine where systole begins and ends, it becomes much more difficult to correctly characterize a murmur.
Why Is S1 Important for Identifying Systole?
S1 marks the beginning of ventricular systole.
Therefore:
S1 → S2 = systole
and:
S2 → next S1 = diastole
This is one reason medical students are taught to identify the lub before attempting to interpret murmurs.
Why Is S2 Important?
S2 marks the transition from ventricular systole to diastole.
It consists primarily of:
A2 + P2
The relationship between these components can provide important clinical clues.
For example, abnormal S2 splitting may be associated with different cardiac conditions depending on whether the split is:
- Wide
- Fixed
- Paradoxical
Wide Splitting of S2
A wide split means there is a greater-than-normal separation between A2 and P2.
Conditions that delay pulmonary valve closure can contribute to wide splitting.
Examples include:
- Right bundle branch block
- Pulmonary stenosis
The exact clinical interpretation depends on the complete cardiovascular examination.
Fixed Splitting of S2
In fixed splitting, the separation between A2 and P2 changes little with respiration.
A classic association is:
Atrial septal defect
This is an important concept for medical students studying cardiac auscultation.
Paradoxical Splitting of S2
Paradoxical, or reversed, splitting occurs when A2 is delayed.
Instead of the usual:
A2 → P2
the timing can become:
P2 → A2
Conditions that delay left ventricular ejection can cause this pattern.
Examples include:
- Left bundle branch block
- Aortic stenosis
Merck Manual describes paradoxical splitting as a reversed sequence of the components of S2.
Why Does Breathing Affect Heart Sounds?
Respiration changes intrathoracic pressure and venous return.
During inspiration:
- Intrathoracic pressure decreases.
- Venous return to the right heart increases.
- Right ventricular filling increases.
- Pulmonary valve closure may be delayed.
This explains why physiological splitting of S2 can become more noticeable during inspiration.
Can You Hear Your Own Heart “Lub Dub”?
Sometimes people can perceive their heartbeat without a stethoscope, particularly in quiet environments or after exercise.
However, the sounds heard externally are not necessarily identical to what a clinician hears through a stethoscope.
Heart sounds can be affected by:
- Body position
- Chest wall thickness
- Lung tissue
- Heart size
- Breathing
- Environmental noise
- Heart rate
Clinical auscultation provides a much more controlled assessment.
What Does a Normal Heart Sound Sound Like?
A normal heart generally produces a regular:
“Lub-Dub, Lub-Dub, Lub-Dub”
The rhythm should correspond to the cardiac cycle.
A clinician evaluates not only whether S1 and S2 are present but also:
- Whether the rhythm is regular
- Whether S1 and S2 have appropriate intensity
- Whether S2 splits normally
- Whether additional sounds are present
- Whether murmurs are present
Normal heart sounds are generally described as a steady two-beat “lub-dub” pattern.
When Are Heart Sounds Abnormal?
Heart sounds may be considered abnormal when there is:
- A new additional sound
- An unusual click
- A significant change in S1
- Abnormal S2 splitting
- S3 in an unexpected clinical setting
- S4
- A murmur
- A pericardial friction rub
- Marked irregularity
Abnormal sounds do not by themselves establish a diagnosis.
They need to be interpreted together with symptoms, physical examination findings, medical history, ECG, echocardiography, and other investigations when appropriate.
Pericardial Friction Rub
A pericardial friction rub is different from the normal heart sounds.
It can be described as a:
- Scratchy
- High-pitched
- Rough
sound.
It can occur when inflamed pericardial surfaces rub against one another, such as in pericarditis.
Heart Sounds and Heart Disease
Heart auscultation remains an important part of cardiovascular examination.
Abnormal heart sounds may provide clues to:
- Valvular heart disease
- Heart failure
- Cardiomyopathy
- Congenital heart disease
- Pericardial disease
- Abnormal ventricular compliance
- Conduction abnormalities
However, auscultation is only one component of diagnosis.
For example, if a clinician hears a murmur, further evaluation may include an echocardiogram to assess valve structure and blood flow.
Heart Sounds in Heart Failure
Certain additional heart sounds can be important in patients with heart failure.
An S3 may occur when the ventricle is receiving blood rapidly during early diastolic filling in the setting of increased volume or impaired ventricular function.
However, S3 should always be interpreted in context.
A clinician does not diagnose heart failure simply because someone reports hearing an unusual sound.
Heart Sounds in Valve Disease
Heart valve abnormalities can change normal cardiac sounds.
Examples include:
Mitral stenosis
May produce characteristic changes in S1 and additional diastolic sounds.
Aortic stenosis
May be associated with an ejection murmur and altered S2 characteristics.
Mitral regurgitation
May produce a systolic murmur and can affect S1 intensity.
Pulmonary stenosis
May produce a widened S2 and a characteristic systolic ejection murmur.
Heart Sounds in Children
Heart sounds can vary with age.
Children and young adults may have findings that would be considered abnormal in older adults but can be physiological in younger people.
For example, an S3 can be physiological in younger individuals.
Therefore, age matters when interpreting heart sounds.
A medical professional evaluates the sound together with:
- Age
- Symptoms
- Heart rate
- Blood pressure
- Physical examination
- Medical history
Heart Sounds During Exercise
Heart rate increases during exercise.
As the heart beats faster, the time between individual cardiac events becomes shorter.
After exercise, the heart rate gradually returns toward its resting level.
A faster heart rate can make it more difficult to distinguish certain sounds because the intervals between them become shorter.
Why Medical Students Learn “Lub-Dub”
The phrase lub-dub is more than a simple description.
It provides an entry point into understanding:
- Cardiac anatomy
- Valve function
- Cardiac physiology
- Systole
- Diastole
- Cardiac auscultation
- Heart murmurs
- Valvular disease
- Heart failure
- Abnormal cardiac rhythms
Learning to recognize S1 and S2 is therefore one of the fundamental skills in cardiovascular examination.
Easy Way to Remember S1 and S2
Use this simple rule:
S1 = AV valves close
Mitral + Tricuspid
“Lub”
S2 = Semilunar valves close
Aortic + Pulmonary
“Dub”
Therefore:
S1 = Lub = Mitral + Tricuspid
S2 = Dub = Aortic + Pulmonary
This is one of the most useful memory aids for learning normal heart sounds.
Heart Sounds Quick Revision
S1
First heart sound
“Lub”
Mitral + tricuspid valve closure.
Beginning of systole.
S2
Second heart sound
“Dub”
Aortic + pulmonary valve closure.
Beginning of diastole.
S3
Third heart sound
Occurs after S2.
May be physiological in some younger people but can indicate volume overload or ventricular dysfunction in appropriate clinical settings.
S4
Fourth heart sound
Occurs before S1.
Can be associated with reduced ventricular compliance.
Frequently Asked Questions About Heart Sounds Lub Dub
What are the two normal heart sounds?
The two primary normal heart sounds are S1 and S2.
S1 is the “lub,” and S2 is the “dub.”
What causes the lub sound?
The “lub” is primarily caused by closure of the mitral and tricuspid valves at the beginning of ventricular systole.
What causes the dub sound?
The “dub” is primarily caused by closure of the aortic and pulmonary valves at the end of ventricular systole.
Is lub dub a normal heart sound?
Yes. A regular S1-S2 sequence is the normal basic pattern of heart sounds.
However, the overall cardiac examination also considers rhythm, intensity, additional sounds, and murmurs.
What is S1 in heart sounds?
S1 is the first heart sound, commonly called “lub.”
It is primarily associated with closure of the mitral and tricuspid valves.
What is S2 in heart sounds?
S2 is the second heart sound, commonly called “dub.”
It is primarily associated with closure of the aortic and pulmonary valves.
What is the difference between S1 and S2?
S1 occurs at the beginning of systole and is primarily associated with AV valve closure.
S2 occurs at the beginning of diastole and is primarily associated with semilunar valve closure.
What is S3?
S3 is an additional heart sound occurring shortly after S2.
It can be physiological in some younger people but can have clinical significance in older adults or patients with appropriate symptoms.
What is S4?
S4 is an additional heart sound occurring before S1.
It is associated with atrial contraction against a less compliant ventricle.
What does a heart murmur sound like?
A murmur is commonly described as a whooshing, swishing, or blowing sound caused by turbulent blood flow.
Some murmurs are benign, while others are associated with structural heart disease.
Is an extra heart sound always dangerous?
No.
Some additional heart sounds can be physiological depending on age and circumstances.
However, a new or unexplained abnormal heart sound should be evaluated by an appropriately qualified healthcare professional.
When Should You Seek Medical Attention?
Hearing or feeling your heartbeat does not necessarily mean that something is wrong.
However, medical evaluation is appropriate if you experience symptoms such as:
- Chest pain
- Shortness of breath
- Fainting or near-fainting
- Persistent palpitations
- Unexplained swelling
- Severe dizziness
- New exercise intolerance
- A newly diagnosed heart murmur
Seek urgent medical attention for severe or sudden symptoms such as significant chest pain, severe breathing difficulty, fainting, or other symptoms suggestive of an acute cardiovascular emergency.
How Heart Rate Affects the Lub-Dub Sound
The heart rate affects how frequently the “lub-dub” sounds occur.
At a normal resting heart rate, there is enough time between S1 and S2 to distinguish the two sounds clearly.
When the heart rate increases, the cardiac cycle becomes shorter. The interval between S1 and S2 also becomes shorter, making the individual components of the cardiac cycle more difficult to distinguish.
For example:
Normal rate:
Lub → Dub → Lub → Dub
During tachycardia:
Lub-Dub-Lub-Dub-Lub-Dub
The sounds may seem closer together because the heart is completing each cardiac cycle more rapidly.
During bradycardia, the interval between heart sounds may become longer, making S1 and S2 easier to distinguish.
Heart rate should therefore always be considered when interpreting normal heart sounds and abnormal heart sounds.
The Relationship Between Heart Sounds and Blood Flow
Although heart sounds are traditionally taught in relation to valve closure, the actual sound is produced by vibrations within the cardiovascular system.
When a valve closes, the sudden changes in blood flow and pressure can create vibrations in the valve structures, blood, myocardium, and surrounding tissues.
These vibrations are transmitted through the chest wall and can be detected with a stethoscope.
This explains why heart sounds lub dub are not simply the sound of two pieces of tissue mechanically “slamming shut.”
The cardiovascular system is a dynamic pressure system.
During ventricular contraction, ventricular pressure rises. When it exceeds atrial pressure, the AV valves close, contributing to S1.
Later, when ventricular pressure falls below pressure in the aorta and pulmonary artery, the semilunar valves close, contributing to S2.
Therefore, pressure changes, blood movement, and vibrations all contribute to the sounds detected during cardiac auscultation.
Why S1 and S2 Sound Different
S1 and S2 are not identical sounds.
The first heart sound is generally:
- Lower in frequency
- Longer in duration
- Associated mainly with AV valve closure
The second heart sound is generally:
- Higher in frequency
- Shorter in duration
- Associated mainly with semilunar valve closure
The difference in pitch and duration can help clinicians identify the individual heart sounds.
Another useful clinical clue is the relationship between the sounds.
The interval:
S1 → S2
represents ventricular systole.
The interval:
S2 → next S1
represents ventricular diastole.
At normal heart rates, the S1-to-S2 interval is usually shorter than the S2-to-next-S1 interval, although this relationship changes with heart rate.
Learning this timing is particularly useful for medical students who are beginning to practice heart auscultation.
Heart Sounds and the Pulse: An Important Clinical Trick
One useful technique for identifying S1 is to correlate the heart sounds with the arterial pulse.
S1 occurs at the beginning of ventricular systole, while ventricular ejection follows shortly afterward.
Therefore, the sound occurring just before the carotid pulse is generally S1.
This can help students who have difficulty deciding which sound is “lub” and which is “dub.”
A simple approach is:
Listen to the heart → palpate the carotid pulse → identify the sound occurring immediately before the pulse.
That sound is generally S1.
The next major sound is S2.
This technique can be particularly helpful when learning to distinguish S1 and S2 heart sounds during clinical examination.
Why the Mitral Area Is Important for Hearing Heart Sounds
The mitral area, located near the cardiac apex, is an important location for cardiac auscultation.
At the apex, clinicians can assess:
- S1 intensity
- Mitral valve sounds
- S3
- S4
- Mitral murmurs
- Cardiac rhythm
S1 is often relatively prominent at the apex because the mitral component contributes significantly to the first heart sound.
The bell of the stethoscope can be particularly useful for detecting low-frequency sounds such as S3 and S4.
The patient may also be positioned in the left lateral decubitus position, which can bring the apex closer to the chest wall and make certain low-frequency sounds easier to appreciate.
A systematic examination of the apex is therefore an important part of learning cardiac auscultation.
Heart Sounds Lub Dub: What Medical Students Should Remember
For students learning cardiovascular examination, the most important concepts can be summarized very simply.
S1 = “Lub”
Mitral + Tricuspid
Beginning of ventricular systole
S2 = “Dub”
Aortic + Pulmonary
Beginning of ventricular diastole
S3
After S2
Early diastole
May be physiological in some individuals but can have clinical significance in appropriate settings.
S4
Before S1
Late diastole
Associated with atrial contraction against a relatively stiff ventricle.
S2 Splitting
Normally:
A2 → P2
The separation may become more noticeable during inspiration.
Heart Murmur
A murmur is an additional sound caused by turbulent blood flow and should be assessed according to its timing, location, radiation, intensity, pitch, and other characteristics.
The easiest memory rule remains:
S1 = Lub = AV valves close
S2 = Dub = Semilunar valves close
Once these fundamentals are mastered, students can progress to more advanced topics such as heart murmurs, valve disease, S3 and S4, abnormal S2 splitting, and other cardiac auscultation findings.
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Final Summary: Heart Sounds Lub Dub
The familiar “lub-dub” represents the two major normal heart sounds.
“Lub” = S1
Produced primarily by closure of the mitral and tricuspid valves at the beginning of ventricular systole.
“Dub” = S2
Produced primarily by closure of the aortic and pulmonary valves at the end of ventricular systole and beginning of diastole.
Understanding S1 and S2 heart sounds is essential for learning cardiovascular physiology and performing cardiac auscultation.
Beyond the normal lub-dub, clinicians may hear S3, S4, murmurs, clicks, or pericardial friction rubs. The timing, location, intensity, pitch, and relationship of these sounds to the cardiac cycle can provide valuable information about cardiovascular function.
For medical students, one of the simplest rules to remember is:
S1 = Lub = Mitral + Tricuspid valve closure
S2 = Dub = Aortic + Pulmonary valve closure
Once these two sounds are understood, interpreting more complex findings in cardiac auscultation becomes much easier.
