Heart Sounds: S1 S2 S3 S4 Explained with Clinical Pearls

Creator: Progress Note
Published: 2026-07-22
Keywords: heart sounds clinical, S3 S4 heart sounds, heart murmur auscultation, cardiac auscultation USMLE, heart sounds mnemonic

The 60-second version

S1 = mitral + tricuspid closure (start of systole) — best at apex
S2 = aortic + pulmonic closure (end of systole) — fixed splitting = ASD
S3 = early diastole, volume overload — 'Ken-tuck-Y' — normal <40, pathological >40
S4 = late diastole, stiff ventricle — 'Ten-nes-see' — always pathological
Physiologic splitting widens on inspiration; fixed splitting suggests ASD

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

S1 — The 'LUB'

S1 is produced by closure of the mitral (M1) and tricuspid (T1) valves at the beginning of systole. M1 closes slightly before T1, but they're heard as one sound in most patients. S1 is best heard at the apex (mitral area). Loudest in: tachycardia, fever, thin chest wall, mitral stenosis. Soft in: first-degree AV block, severe mitral regurgitation (valve can't close fully).

S2 — The 'DUB'

S2 results from closure of the aortic (A2) and pulmonic (P2) valves at the end of systole. Normal physiologic splitting: A2 precedes P2, and the gap widens on inspiration (increased venous return delays P2). Fixed splitting = ASD. Paradoxical splitting (P2 before A2, widens on expiration) = LBBB or severe aortic stenosis.

S3 — The 'Kentucky' Sound

S3 is a low-pitched early diastolic sound heard just after S2. Mnemonic: 'Ken-tuck-Y' (lub-dub-S3). It's caused by rapid ventricular filling hitting a non-compliant or volume-overloaded ventricle. Pathological in adults over 40: think CHF, dilated cardiomyopathy, mitral regurgitation. Normal in young adults, athletes, and pregnancy.

S4 — The 'Tennessee' Sound

S4 is a low-pitched late diastolic sound heard just before S1. Mnemonic: 'Ten-nes-see' (S4-lub-dub). Caused by atrial contraction forcing blood into a stiff, non-compliant ventricle. Think: hypertension (most common), hypertrophic cardiomyopathy, aortic stenosis, post-MI. Always pathological — there's no normal S4.

Test yourself

Quick check — 3 questions

1. An S3 gallop in a 65-year-old with dyspnea on exertion most likely indicates:

2. Fixed splitting of S2 that does not change with respiration is classically associated with:

3. An S4 gallop is heard in a 72-year-old hypertensive patient. The mechanism is:

Frequently asked questions

What causes a split S2?

Physiologic split S2 occurs because the aortic valve (A2) closes slightly before the pulmonic valve (P2). On inspiration, increased venous return to the right heart further delays P2, widening the split. Fixed split S2 (doesn't change with breathing) classically indicates ASD, where the left-to-right shunt keeps right heart filling constant regardless of respiration.

When is an S3 normal vs. pathological?

S3 can be normal in children, young adults under 40, athletes, and pregnant women — where it reflects rapid, vigorous ventricular filling. In adults over 40, an S3 ('ventricular gallop') is pathological and suggests volume overload states: CHF, dilated cardiomyopathy, or significant mitral or tricuspid regurgitation.

How do you remember which gallop is which?

Use state mnemonics: S3 = 'Ken-TUCK-y' (the extra sound falls after S2: lub-DUB-ta). S4 = 'TEN-nes-see' (the extra sound falls before S1: da-LUB-dub). Practice by saying them in rhythm. S3 is associated with volume overload (CHF); S4 with pressure/stiffness (hypertension, AS, HCM).

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