Normal Heart Sounds

Overview

• Normal heart sounds are described as "lub, dub" heard on auscultation with a stethoscope.
• They arise from vibrations of valves, blood, and cardiac walls transmitted to the chest wall.
• The two primary normal sounds are S1 ("lub") and S2 ("dub").

S1 — First Heart Sound ("lub")

• Cause: Closure of the atrioventricular (AV) valves — mitral (bicuspid) and tricuspid valves — at the onset of ventricular systole.
• Mechanism: Ventricular contraction causes backflow of blood against the AV valves → valves close and bulge toward the atria → chordae tendineae abruptly halt the bulging → elastic recoil causes blood and ventricular walls to vibrate → sound is transmitted to the chest.
• Quality: Described as "lub" — relatively low-pitched and longer in duration (~0.14 seconds).
• Pitch: Lower frequency than S2 because the AV valves are less taut and the ventricles are less elastic.
• Best heard: At the mitral area (cardiac apex / 5th intercostal space, midclavicular line) and tricuspid area (lower left sternal border).

S2 — Second Heart Sound ("dub")

• Cause: Closure of the semilunar valves — aortic and pulmonary valves — at the end of ventricular systole.
• Mechanism: After ejection, semilunar valves close and bulge back toward the ventricles → elastic recoil sends blood back into the arteries → blood reverberates between arterial walls and valves → vibrations travel along the arterial walls to the chest wall.
• Quality: Described as "dub" — relatively higher-pitched and shorter in duration (~0.11 seconds).
• Pitch: Higher than S1 because semilunar valves are tauter and arterial walls are more elastic than ventricular walls.
• Best heard: At the aortic area (2nd right intercostal space) and pulmonic area (2nd left intercostal space).

S3 — Third Heart Sound (Physiological)

• Timing: Occurs at the beginning of the middle third of diastole (early-to-mid diastole).
• Cause: Rapid ventricular filling from the atria — blood rushes in and reverberates against ventricular walls, causing low-frequency vibration.
• Quality: Weak, low-frequency rumbling — usually below the audible threshold for the ear but detectable on phonocardiography.
• Normal in: Children, adolescents, and young adults.
• Abnormal if present in older adults — suggests systolic heart failure (S3 gallop).

S4 — Fourth Heart Sound (Atrial Sound)

• Timing: Occurs at atrial contraction (late diastole / presystole).
• Cause: Atrial contraction forces blood into the ventricles → inrush initiates low-frequency ventricular vibration.
• Quality: Very low frequency (~20 Hz or less) — rarely audible with a stethoscope; recorded only on phonocardiography in normal individuals.
• Abnormal if audible (S4 gallop): Common in left ventricular hypertrophy or decreased ventricular compliance, where atrial kick is important for filling.

Duration and Pitch Summary

Phonocardiogram — S1 and S2

Auscultation Areas on the Chest

• Aortic area: 2nd right intercostal space (sound transmitted up the aorta)
• Pulmonic area: 2nd left intercostal space (along the pulmonary artery)
• Tricuspid area: Lower left sternal border (over the right ventricle)
• Mitral area: Cardiac apex / 5th ICS midclavicular line (over the left ventricle)

Key Clinical Points

• The interval between S1 and S2 (systole) is shorter than the interval between S2 and next S1 (diastole) at normal heart rates.
• S1 marks the beginning of ventricular systole; S2 marks the end of ventricular systole.
• The audible frequency range for heart sounds is ~40–520 cycles/sec; large portions of the vibration energy are below audible range and only detectable by electronic phonocardiography.
• Splitting of S2 (A2 before P2) is physiologically normal during inspiration due to delayed closure of the pulmonary valve.