S2 splitting and heart sounds S1–S4, explained
Heart sounds are valves closing. Read S2 splitting with two questions: which valve is late, and does the gap move with breathing?
Chapters (11)
Explain the valve events behind S1–S4 and interpret physiological, wide, fixed and paradoxical splitting of S2.
The short version
- S1 is mitral and tricuspid valve closure; S2 is aortic (A2) and pulmonic (P2) valve closure.
- Physiological S2 splitting is split on inspiration and single on expiration.
- Wide splitting varies with breathing; fixed splitting never does and classically means an atrial septal defect.
- Paradoxical splitting is delayed aortic closure: split on expiration, single on inspiration, the reverse of normal.
- S3 is rapid ventricular filling and can be normal when young; S4 is a stiff ventricle and is considered abnormal.
An extra heart sound: normal or a warning?
A 19-year-old long-distance runner has a resting heart rate of 52. The examiner hears an extra sound early in diastole, and she is healthy. The same kind of sound can be harmless in one patient and a warning in another.
This lesson links each of the heart sounds S1 to S4 to its valve, then shows how to read S2 splitting: normal, wide, fixed or paradoxical. It builds on the Wiggers diagram from the previous lesson, where valves open and close because of pressure gradients. Heart sounds are the sound of valves snapping shut.[1][2]
S1 and S2: the valves behind the heart sounds
S1 comes when the mitral and tricuspid valves close, as the ventricles start to contract. It marks the start of systole.[1][2][3]
S2 comes when the aortic and pulmonic valves close, as systole ends. It marks the start of diastole.[1][2]
| Sound | Valves closing | Timing |
|---|---|---|
| S1 | Mitral and tricuspid | Start of systole |
| S2 | Aortic (A2) and pulmonic (P2) | End of systole, start of diastole |
S2 has two parts. A2 is the aortic valve closing and P2 is the pulmonic valve closing. Whether you hear one sound or two depends on the gap between them.[1][2]
Physiological splitting of S2: breathing and the right heart
Normally the aortic valve closes first, then the pulmonic, and breathing changes the gap.[3][1]
- Breathe inPressure inside the chest falls and more blood returns to the right heart.
- The right ventricle takes longer to emptyP2 is delayed, so you hear two sounds: A2 then P2.
- Breathe outThe gap closes and S2 sounds single (the usual exam convention; some healthy adults keep a faint split).
That is physiological splitting: split on inspiration, single on expiration.[3][1][2][4]
Wide, fixed and paradoxical S2 splitting
Wide splitting. P2 is late, so the gap is wider, but it still varies with breathing. Right bundle branch block and pulmonic stenosis do this.[2][4][1]
Fixed splitting. The gap is wide, and it does not change at all with breathing. The classic cause is an atrial septal defect.[3][1][2][4]
Paradoxical splitting. Now the aortic valve is the late one, so A2 comes after P2. On inspiration, P2 is delayed and lands on top of the late A2, so you hear one sound. On expiration, you hear two.[1][2][4]
The causes delay aortic valve closure: left bundle branch block, aortic stenosis or hypertrophic obstructive cardiomyopathy. Paradoxical means split on expiration, single on inspiration, the exact reverse of normal.[1][2][4]
| Pattern | What you hear | Typical causes |
|---|---|---|
| Physiological | Split on inspiration, single on expiration | Normal |
| Wide | Wide split that still varies with breathing | Right bundle branch block, pulmonic stenosis |
| Fixed | Wide split that never changes with breathing | Atrial septal defect |
| Paradoxical | Split on expiration, single on inspiration | Left bundle branch block, aortic stenosis, hypertrophic obstructive cardiomyopathy |
S3 and S4: the diastolic heart sounds
S3 comes in early diastole, as blood rushes into the ventricle after the mitral valve opens. It can be normal in children, athletes and pregnancy, like the runner above, but in an older adult it usually means a failing ventricle.[1][2][3]
S4 comes in late diastole, when the atria contract against a stiff ventricle. It is considered abnormal.[1][2][3]
| Sound | Timing | Mechanism | Meaning |
|---|---|---|---|
| S3 | Early diastole | Rapid ventricular filling, too fast or too full | Can be normal when young; usually a failing ventricle in an older adult |
| S4 | Late diastole | Atrial contraction against a stiff ventricle | Abnormal |
In short: S3 is filling that is too fast or too full. S4 is a stiff ventricle.
Practice question
Original · USMLE styleA 27-year-old woman has had breathlessness on exertion for 6 months. Heart rate is 78/min and blood pressure is 110/70 mm Hg. On auscultation, S2 is widely split, and the interval between the two components is the same in expiration and inspiration. A soft systolic flow murmur is heard at the upper left sternal border.
Common questions
What causes S1 and S2 heart sounds?
S1 is closure of the mitral and tricuspid valves at the start of systole. S2 is closure of the aortic (A2) and pulmonic (P2) valves as systole ends.
What is the difference between fixed and wide splitting of S2?
Both have a wide gap. In wide splitting the gap still varies with breathing (right bundle branch block, pulmonic stenosis). In fixed splitting it never changes, the classic sign of an atrial septal defect.
What is paradoxical splitting of S2?
Aortic valve closure is delayed, so A2 follows P2. S2 is split on expiration and single on inspiration, the reverse of normal. Left bundle branch block, aortic stenosis and hypertrophic obstructive cardiomyopathy cause it.
Is an S3 heart sound always abnormal?
No. S3 can be normal in children, athletes and pregnancy. In an older adult it usually means a failing ventricle.
What is the difference between S3 and S4?
S3 comes in early diastole from rapid ventricular filling. S4 comes in late diastole when the atria contract against a stiff ventricle, and it is considered abnormal.
Transcript
Read the full narration, by chapter717 words
0:00A runner with an extra sound: normal or a warning?
A 19-year-old long-distance runner has a resting heart rate of 52. Her doctor hears an extra sound early in diastole, and she is healthy. An extra heart sound: sometimes normal, sometimes a warning. How do you tell? Today: link each heart sound to its valve, and read S two splitting: normal, wide, fixed or paradoxical. Last time, on the Wiggers diagram, valves opened and closed by pressure gradients.
0:30S1 and S2: heart sounds are valves snapping shut
Now we listen: heart sounds are valves snapping shut. S one comes when the mitral and tricuspid valves close, as the ventricles start to contract. S two comes when the aortic and pulmonic valves close, as systole ends. S one starts systole and S two starts diastole.
0:50S2 has two parts: A2 and P2
S two has two parts: A two, the aortic valve, and P two, the pulmonic valve. Normally the aortic valve closes first, then the pulmonic, and breathing changes the
1:04Physiological splitting: breathing and the right heart
gap. When you breathe in, pressure inside the chest falls, and more blood returns to the right heart. The right ventricle takes longer to empty, so P two is delayed and you hear two sounds. When you breathe out, the gap closes, and S two sounds single. That is physiological splitting: split on inspiration, single on expiration.
1:29Two questions to ask, and wide splitting
Every abnormal pattern is a change to this rule. Ask two questions: which valve is late, and does the gap move with breathing? First, wide splitting. Here P two is late, so the gap is wider, but it still varies with breathing. Right bundle branch block and pulmonic stenosis do this.
1:51Fixed splitting and atrial septal defect
Second, fixed splitting. The gap is wide, and it does not change at all with breathing. The classic cause is an atrial septal defect. Wide and moves is delay on the right. Wide and never moves is an atrial septal defect.
2:08Paradoxical splitting: the reverse of normal
Third, paradoxical splitting. Now it is the aortic valve that is late, so A two comes after P two. On inspiration, P two is delayed and lands on top of the late A two, so you hear one sound. On expiration, you hear two. The causes delay left ventricular emptying: left bundle branch block, aortic stenosis, or hypertrophic cardiomyopathy. So paradoxical means split on expiration, single on inspiration. That is the exact reverse of normal.
2:43S3 and S4: the diastolic sounds
Now the diastolic sounds. S three comes in early diastole, as blood rushes into the ventricle after the mitral valve opens. It can be normal in children, athletes and pregnancy, like our runner, but in an older adult it usually means a failing ventricle. S four comes in late diastole, when the atria contract against a stiff ventricle. S three is filling that is too fast or too full. S four is a stiff ventricle.
3:15Practice question
Your turn. A 27-year-old woman has had breathlessness on exertion for six months. Her heart rate is 78, and blood pressure is 110 over 70. On auscultation, S two is widely split. The split is the same in expiration and inspiration. A soft systolic flow murmur is heard at the upper left sternal border. Which best explains the splitting of S two? A, normal physiological splitting. B, left bundle branch block with delayed aortic valve closure. C, right bundle branch block with delayed pulmonic valve closure. D, an atrial septal defect with right ventricular volume overload. E, severe aortic stenosis with prolonged left ventricular emptying. Pause here and decide.
4:11Why each option is right or wrong
A is out: normal splitting comes and goes with breathing, and here it never changes. B is out: left bundle branch block gives paradoxical splitting, split on expiration, single on inspiration. C is out: right bundle branch block does give a wide split, but it still varies with breathing. E is out: aortic stenosis delays A two, so that is paradoxical, not fixed. So the answer is D: a wide split that does not move with breathing is fixed splitting, from an atrial septal defect.
4:47Recap and next lesson
Three things to keep. One: S one is mitral and tricuspid closure, S two is aortic and pulmonic closure, and normally S two splits only on inspiration. Two: wide splitting varies with breathing, fixed splitting never does and means atrial septal defect, and paradoxical splitting is delayed aortic closure. Three: S three is rapid filling and can be normal when young, S four is a stiff ventricle and is not. Next up: murmurs, and how breathing and posture change them.
Sources
Numbers in the text, e.g. [1], point here- MSD Manual Professional: Cardiac Auscultation. msdmanuals.com/professional/cardiovascular-disorders/approach-to-the-cardiac-patient/cardiac-auscultation
- Healio, Learn the Heart: Heart Sounds; S2 Heart Sound. healio.com/cardiology/learn-the-heart/cardiology-review/topic-reviews/heart-sounds · healio.com/cardiology/learn-the-heart/cardiology-review/topic-reviews/s2-heart-sound
- UTMB Pediatric Cardiology Core Curriculum: Cardiology, heart sounds. utmb.edu/pedi_ed/CoreV2/Cardiology/cardiologyV2/cardiologyV23.html
- FPnotebook: S2 Heart Sound. fpnotebook.com/CV/Exam/SHrtSnd1.htm
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): stethoscope, lungs. "heart-valves" by Servier via Bioicons (bioicons.com), CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/), adapted (recoloured, outlined). Timing diagrams, charts and all handwriting are drawn in code (original). No AI-generated anatomy. Previous: The Cardiac Cycle: Reading the Wiggers Diagram (CARD-005) · Next: Murmurs and Dynamic Manoeuvres (EP 02)
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-CARD-010.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01S2 splitting and heart sounds S1–S4, explained5:26
- EP 02Heart murmurs and dynamic manoeuvres, from preload and afterload5:15
- EP 03HOCM vs aortic stenosis murmur: telling them apart at the bedside5:31
- EP 04Ventricular action potential: phases 0 to 4 and their ion currents5:15
- EP 05SA node pacemaker potential: phase 4 sets the heart rate5:17
- EP 06ECG waves and intervals: what P, PR, QRS, ST and QT mean5:29