Second Heart Sound Definition

Second Heart Sound Definition is a description of the higher-pitched, relatively shorter sound produced by the closure of the semilunar valves, the aortic and pulmonary valves, at the end of ventricular ejection. Commonly abbreviated as S2, the second heart sound marks the moment ventricular pressure falls below the pressure in the aorta and pulmonary artery, sealing the semilunar valves and beginning the isovolumetric relaxation phase of the cardiac cycle.

Mechanism of Production

S2 arises from the abrupt deceleration and brief reversal of blood flow that occurs as the aortic and pulmonary valves close.

Valve Closure Event

As ventricular contraction wanes toward the end of ejection, falling ventricular pressure drops below arterial pressure, and the cusps of the aortic and pulmonary valves are pushed closed by the pressure gradient, their pocket-like shape trapping the small volume of blood attempting to flow backward and reinforcing complete closure.

Vibration and Sound Generation

The sudden arrest of retrograde flow against the closing cusps sets the elastic walls of the great vessels and adjacent cardiac structures into vibration, producing the audible sound.

Components of S2

Because the aortic and pulmonary valves do not close at exactly the same instant, S2 is composed of two components.

Aortic Component

The aortic valve closes first, owing to the higher pressure in the systemic circulation compared to the pulmonary circulation, producing the earlier and typically dominant component of S2.

Pulmonary Component

The pulmonary valve closes slightly later, contributing a second component; the separation between these two components, known as splitting of S2, normally widens during inspiration due to increased venous return to the right heart.

Position Within the Cardiac Cycle

S2 occupies a fixed and predictable position relative to other cardiac events.

End of Systole

S2 marks the end of ventricular ejection and the beginning of isovolumetric relaxation, during which all four cardiac valves are once again closed.

Relationship to Arterial Pressure

The timing of S2 corresponds closely to the dicrotic notch observed on the arterial pressure waveform, both reflecting the moment of semilunar valve closure.

Diagrammatic Summary

Clinical Relevance

The character and splitting pattern of S2 provide important diagnostic information about pulmonary and systemic arterial pressures as well as semilunar valve function; abnormal splitting, a single loud component, or a delayed pulmonary component can indicate conditions such as pulmonary hypertension, aortic stenosis, or conduction abnormalities affecting right ventricular activation.