Heart Sounds S1–S4 & Splitting: Step 1 Guide

Creator: Progress Note
Published: 2026-05-02
Keywords: heart sounds clinical, heart sounds, s1 s2 s3 s4, split s2, physiologic splitting, fixed splitting atrial septal defect, paradoxical splitting, s3 gallop heart failure, s4 gallop stiff ventricle, opening snap, ejection click, auscultation areas, cardiac cycle

bolt The 60-second version

Heart sounds are the vibrations of valves closing and blood decelerating in the ventricles. Each sound lines up with a moment in the cardiac cycle, so reading them is really reading physiology: when the valves close, how pressures change with breathing, and how stiff or overloaded the ventricles are.

check_circleS1 = mitral and tricuspid closure at the start of systole; S2 = aortic (A2) and pulmonic (P2) closure at the end of systole.
check_circlePhysiologic splitting: inspiration ↑ venous return to the right heart, delaying P2 — the split widens on inspiration.
check_circleFixed splitting = atrial septal defect; paradoxical splitting (P2 before A2) = delayed aortic closure (aortic stenosis, LBBB).
check_circleWide splitting that still varies with breathing = delayed RV emptying (RBBB, pulmonic stenosis).
check_circleS3 = early diastolic rapid filling into a dilated, volume-overloaded ventricle (heart failure; normal in the young and pregnancy).
check_circleS4 = late diastolic atrial kick into a stiff ventricle (LVH, hypertension, acute MI); never heard in atrial fibrillation.

At a glance

The four heart sounds side by side — start here, then read the detail below.

Sound	Timing	Mechanism	Best heard	What it signals
S1	Start of systole	Mitral + tricuspid valve closure	Apex	Loud in mitral stenosis; soft in severe MR or first-degree AV block
S2	End of systole	Aortic + pulmonic valve closure	Base	Fixed split → ASD; paradoxical split → LBBB or severe aortic stenosis
S3	Early diastole	Rapid filling of a volume-loaded ventricle	Apex, with the bell	CHF, dilated cardiomyopathy, MR (pathologic in adults over 40)
S4	Late diastole	Atrial contraction into a stiff, non-compliant ventricle	Apex, with the bell	Hypertension, LVH, HCM, aortic stenosis

target What you'll be able to do · aligned to USMLE Step 1 · Cardiovascular Physiology

PHYSRelate S1, S2, S3, and S4 to valve closure and ventricular filling in the cardiac cycle.
PHYSExplain physiologic splitting of S2 and how respiration changes right- and left-sided filling.
CARDIODistinguish wide, fixed, and paradoxical splitting and identify their causes.
CARDIOInterpret S3, S4, and extra sounds such as opening snaps and ejection clicks.

How heart sounds are tested on Step 1

Expect a vignette describing a split S2 that changes (or does not change) with respiration, or an extra sound in early or late diastole, and a question about the underlying lesion or physiology.

workspace_premium Think valves, then pressures

Every sound is a valve closing (S1, S2) or blood hitting a ventricle wall (S3, S4). Ask which valve, at what point in the cycle, and what pressure change made it early or late.

The cardiac cycle and the four sounds

S1 and S2 bracket systole. Diastole is longer and contains the two filling sounds.

Cardiac cycle. S3 follows S2 closely ("Ken-TUCK-y"); S4 comes just before S1 ("TEN-nes-see").

Sound	Cause	Best heard	Significance
S1	Mitral + tricuspid closure	Apex	Loud in mitral stenosis and short PR; soft in long PR
S2	Aortic + pulmonic closure	Left upper sternal border	Loud P2 in pulmonary hypertension
S3	Rapid passive filling of a dilated ventricle	Apex, left lateral decubitus, bell	Volume overload, HFrEF; normal < 40 years and in pregnancy
S4	Atrial contraction into a stiff ventricle	Apex, left lateral decubitus, bell	LVH, HFpEF, hypertension, acute ischemia

S3 and S4 are low-pitched, so they are heard with the bell pressed lightly.

Splitting of S2

Inspiration lowers intrathoracic pressure, drawing more blood into the right heart and pooling blood in the pulmonary vessels. The right ventricle ejects longer, so P2 comes later; the left ventricle receives slightly less, so A2 comes a little earlier.

Pattern	What you hear	Mechanism	Causes
Physiologic	Split on inspiration, single on expiration	Inspiratory delay of P2	Normal
Wide	Split in both phases, wider on inspiration	Delayed RV emptying	RBBB, pulmonic stenosis
Fixed	Same split in both phases	Left-to-right shunt keeps RV volume high regardless of breathing	Atrial septal defect
Paradoxical (reversed)	Split on expiration, single on inspiration	A2 delayed after P2	Aortic stenosis, LBBB, HOCM, RV pacing

Paradoxical splitting is the only pattern in which P2 precedes A2.

Fixed vs paradoxical. Both are abnormal; the respiratory behavior tells them apart.

Extra sounds and where to listen

Several other sounds point to specific lesions.

Extra sounds. A shorter S2-to-opening-snap interval means more severe mitral stenosis (higher left atrial pressure).

Area	Location	Listen for
Aortic	2nd right intercostal space	Aortic stenosis, flow murmurs
Pulmonic	2nd left intercostal space	S2 splitting, pulmonic stenosis, ASD flow
Tricuspid	Left lower sternal border	Tricuspid regurgitation, VSD
Mitral	Apex (5th ICS, midclavicular)	S3, S4, mitral regurgitation, mitral stenosis

Mnemonic for the order from upper right: "All Physicians Take Money" — aortic, pulmonic, tricuspid, mitral.

Clinical application

Connect a sound to pressure physiology.

functions Worked example — the split that will not change

A 28-year-old woman has exertional dyspnea and a widely split S2 that does not vary with breathing. ECG shows right axis deviation and incomplete RBBB. What lesion is present, and why is the split fixed?

A split that is the same in inspiration and expiration is fixed splitting.
An atrial septal defect lets blood flow from left atrium to right atrium.
During inspiration more systemic venous blood returns; during expiration more shunted blood crosses — either way the right ventricle stays volume-loaded.
P2 is therefore delayed equally in both phases.

Answer. Atrial septal defect (most often secundum). The left-to-right shunt keeps RV filling constant, fixing the split.

Next in the roadmap

Heart Sounds

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2 real questions on Heart Sounds

Straight from the Progress Note question bank, each with a cited explanation. No account needed.

A 55-year-old man with blunt chest trauma develops jugular venous distension, hypotension, and muffled heart sounds. Pulsus paradoxus (>10 mmHg inspiratory drop in systolic blood pressure) is noted on exam. Echocardiograph is shown below. What is the physiologic basis of the pulsus paradoxus observed in this patient?

Image for question 1 — Ben Smith https://www.ultrasoundoftheweek.com, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

A 56-year-old man presents with episodic facial flushing, watery diarrhea, and wheezing. A new holosystolic murmur at the left lower sternal border increases with inspiration, and echocardiography shows thickened, retracted tricuspid valve leaflets with regurgitation. CT shows a small ileal mass with hepatic metastases; biopsy, photomicrograph shown below, shows nests of uniform cells with 'salt-and-pepper' chromatin, positive for chromogranin A. The valvular heart disease observed in this patient develops through which of the following mechanisms?

Image for question 2 — CoRus13, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Frequently asked questions

What causes S1 and S2?expand_more

S1 is closure of the mitral and tricuspid valves at the start of ventricular systole. S2 is closure of the aortic and pulmonic valves at the end of systole.

What does an S3 mean?expand_more

An S3 is an early diastolic sound from rapid filling of a dilated ventricle. It suggests volume overload or systolic heart failure in older adults but can be normal in children, young adults, and pregnancy.

What does an S4 mean?expand_more

An S4 is a late diastolic sound from the atrium contracting against a stiff ventricle, as in left ventricular hypertrophy, hypertension, or acute ischemia. It is usually abnormal in adults (though it can be heard in some healthy older adults and trained athletes) and is absent in atrial fibrillation.

What causes paradoxical splitting of S2?expand_more

Anything that delays aortic valve closure so that A2 follows P2: aortic stenosis, left bundle branch block, hypertrophic cardiomyopathy, or right ventricular pacing.

References & further reading

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Heart Sounds: The USMLE Step 1 Guide

At a glance

How heart sounds are tested on Step 1

The cardiac cycle and the four sounds

Splitting of S2

Extra sounds and where to listen

Clinical application

2 real questions on Heart Sounds

Your next questions are ready.

Frequently asked questions

References & further reading

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Turn this guide into questions you’ll actually remember

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