1.10.6 Heart Sound Definition

Heart Sound Definition is a description of the discrete, audible vibrations produced within the heart and surrounding blood as a direct consequence of cardiac valve closure and the abrupt deceleration of blood flow that accompanies it. Heart sounds are transmitted through cardiac tissue, surrounding structures, and the chest wall, where they can be detected by auscultation, and their number, timing, and character provide a direct acoustic reflection of the mechanical events occurring within the cardiac cycle.

Physical Origin

Heart sounds are not produced by the valve leaflets striking one another but by the vibration of the entire cardiohemic system, the valves, chamber walls, and contained blood, set into motion by sudden changes in flow.

Sudden Flow Deceleration

When a valve closes, the column of blood that had been moving forward is abruptly stopped and briefly forced backward against the newly closed leaflets, generating a vibration that propagates through nearby tissues.

Transmission

These vibrations travel through the myocardium, pericardium, and chest wall to the body surface, where they can be perceived as sound using a stethoscope, a practice known as cardiac auscultation.

The Principal Heart Sounds

Two heart sounds are present in essentially all normal hearts, while additional sounds may appear under specific physiological or pathological conditions.

First Heart Sound

The first heart sound (S1) results from closure of the atrioventricular valves, the mitral and tricuspid valves, at the onset of ventricular systole. It is typically louder and slightly longer than the second heart sound.

Second Heart Sound

The second heart sound (S2) results from closure of the semilunar valves, the aortic and pulmonary valves, at the end of ventricular ejection. It is generally shorter and higher-pitched than S1.

Third and Fourth Heart Sounds

A third heart sound (S3) may occur during rapid ventricular filling, and a fourth heart sound (S4) may occur during atrial systole as blood strikes a stiffened or noncompliant ventricular wall; both are less commonly audible in healthy adults and often signal altered ventricular compliance when present.

Relationship to the Cardiac Cycle

Because heart sounds arise from specific valve closure events, they act as reliable temporal markers dividing the cardiac cycle into its major phases.

Marking Systole

S1 marks the beginning of ventricular systole, while S2 marks its end, such that the interval between S1 and S2 corresponds to the duration of systole.

Marking Diastole

The interval between S2 and the following S1 corresponds to the duration of diastole.

Diagrammatic Summary

Clinical Relevance

Alterations in the intensity, timing, splitting, or presence of extra heart sounds, as well as the appearance of murmurs between them, are among the most informative and accessible clinical signs of cardiac valve and chamber function, forming the basis of the physical examination technique of cardiac auscultation.