Splitting and bifurcation of heart sounds

Physiological splitting of the second heart sound

Normally, minor splitting of the second heart sound is closely related to the phases of respiration and is heard primarily at the base of the heart in the second intercostal space.

• Hemodynamic mechanism: During a deep inspiration, venous return to the right heart increases. The right ventricle must eject this increased blood volume, which physiologically prolongs its systole.
• Auscultatory picture: Due to the prolonged systole, the pulmonary valve closes slightly later. First, the aortic component is heard, followed shortly by the pulmonary component.

This phenomenon is best detected in young individuals directly over the pulmonary artery. However, splitting is not heard over the aorta itself, because the pulmonary sound component is too weak to be transmitted to that auscultation point.

Pathological variants of second sound splitting

The nature of pathological splitting depends on the state of the myocardial conduction system and hemodynamic features. Several key clinical scenarios are distinguished:

1. Delayed closure of the pulmonary valve. It occurs due to delayed blood ejection or impaired electrical impulse conduction. The main causes are right ventricular dilation (characteristic of pulmonary stenosis) and intraventricular blocks (e.g., right bundle branch block).
2. Fixed splitting of the second heart sound. The key feature of this phenomenon is the complete absence of respiratory variability. The interval between the sound components changes neither during inspiration nor expiration. This is a strictly pathognomonic diagnostic sign of an atrial septal defect (ASD). Due to a persistent pathological left-to-right shunt, a chronic volume overload of the pulmonary circulation and the right chambers of the heart develops.
3. Features in pulmonary hypertension. In chronic lung diseases, the right ventricle undergoes hypertrophy because it has to work against high pressure. Since pressure overload occurs rather than volume overload (unlike dilation), no significant prolongation of systole takes place. As a result, the splitting sounds less distinct.

Features of first sound splitting

Splitting of the first heart sound is less common and can be caused by both physiological factors and serious pathology. Graphically (on diagrams and phonocardiograms), this process appears as the division of the first sound into two separate components while the second sound remains completely intact.

• Normal variant. Normal splitting can be heard along the left sternal border, where the sound of tricuspid valve closure is well transmitted, and sometimes at the apex of the heart. In some cases, the auscultatory picture is supplemented by the presence of a fourth heart sound or an early systolic click, also known as an ejection sound.
• Pathological splitting. The main cause is intraventricular conduction disturbances, namely bundle branch blocks. In this case, the electrical signal is delayed in one of the ventricles, its systole is delayed, which naturally leads to asynchronous closure of the atrioventricular valves.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Pathogenesis of atrial septal defect (ASD)
• Changes in heart sounds during bundle branch blocks
• Mechanisms of early systolic click (ejection sound) formation
• Graphical representation of heart sounds on a phonocardiogram
• Pulmonary circulation hemodynamics in pulmonary hypertension

Other topics in the "Cardiovascular system" section

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