1st heart sound (systolic)
The first heart sound is one of the loudest and longest. It occurs during the transition from ventricular relaxation to active contraction.
Phase of the cardiac cycle: The sound is generated during the phase of isometric ventricular contraction and extends into the very beginning of the subsequent rapid ventricular ejection phase.
Main mechanism of formation: The fundamental cause of the 1st sound is the closure of the atrioventricular (AV) valves (mitral and tricuspid). When pressure in the ventricles begins to rise rapidly, the valves between the atria and ventricles snap shut.
The sound of the first heart sound is a complex acoustic phenomenon consisting of several components:
- Vibrations of the tightly taut leaflets of the AV valves.
- Vibration of the chordae tendineae attached to the edges of the valve leaflets, preventing them from everting into the atrial cavity.
- Valvular component: vibrations of the semilunar valves (aortic and pulmonary valves) at the moment they open to eject blood are added to the sound of AV valve closure.
2nd heart sound (diastolic)
The second sound marks the end of systole and the transition of the heart into diastole.
Phase of the cardiac cycle: This acoustic phenomenon is strictly tied to the phase of isometric ventricular relaxation. At this moment, pressure in the ventricular cavities drops.
Main mechanism of formation: The key event here is the sharp snapping shut of the semilunar valves — the aortic and pulmonary valves. Blood from the great vessels tends to flow back into the relaxing ventricles, fills the cusps of the semilunar valves, and forces them tightly closed.
The source of the sound is the intense vibrations of the valve leaflets themselves after they snap together.
3rd and 4th heart sounds (additional)
Unlike the 1st and 2nd sounds, which are primarily of valvular origin, the 3rd and 4th sounds are associated with hemodynamic impacts of blood against the muscular walls of the heart.
3rd heart sound
- Cycle phase: Rapid ventricular filling phase.
- Mechanism: After the opening of the atrioventricular valves, blood rushes passively and very rapidly from the atria into the ventricles. The impact of this blood mass against the ventricular muscular walls causes low-frequency vibrations, which are recorded as the third heart sound.
4th heart sound
- Cycle phase: Atrial systole.
- Mechanism: At the very end of ventricular diastole, the atria actively contract, expelling the remaining portion of blood. This supplementary filling of the ventricles with blood causes their walls to vibrate once again, forming the fourth heart sound.
Briefly: answers to topical questions
Where are the normal auscultation points for the 1st heart sound?
The auscultation points for the 1st heart sound correspond to the areas where the valves involved in its formation (mitral and tricuspid) are best heard.
- Apex of the heart (mitral valve) — usually the fifth intercostal space 1–1.5 cm medial to the left midclavicular line (linea medioclavicularis sinistra).
- Lower third of the sternal body (tricuspid valve) — base of the xiphoid process.
Where is the 2nd heart sound best auscultated (auscultation points)?
The 2nd heart sound is best heard over the projections of the valves whose closure forms it (aortic and pulmonary valves).
- Aortic valve — in the second intercostal space to the right at the sternal border.
- Pulmonary valve — in the second intercostal space to the left at the sternal border.
Additionally, Botkin-Erb's point in the third intercostal space to the left at the sternal border is used to evaluate the aortic valve.
The phases are easy to remember by key events: 1st sound — AV valves close (tension before ejection), 2nd sound — semilunar valves snap shut (relaxation after ejection).
Class and exam questions
During which phase of the cardiac cycle is the first (1st) heart sound generated?
The 1st sound occurs during the phase of isometric ventricular contraction and continues into the very beginning of the rapid ventricular ejection phase.
What is the main source of sound during the formation of the 2nd heart sound?
The main source of sound for the second sound is the vibration of the closing leaflets of the semilunar valves (aortic and pulmonary valves).
What is the appearance of the 3rd and 4th heart sounds associated with?
Both sounds are associated with vibrations of the ventricular muscular walls as they fill with blood: the 3rd sound during rapid passive filling, and the 4th sound during supplemental filling during atrial systole.
What else to study on this topic
- Detailed analysis of the isometric contraction phase
- Physiology of the rapid ventricular ejection phase
- Hemodynamics of the isometric ventricular relaxation phase
- Mechanics of atrioventricular valves and chordae tendineae
- Atrial systole and its contribution to ventricular filling
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