During which phase of the respiratory cycle does the second heart sound (S2) split?

S2 Splitting During Inspiration

The second heart sound (S2) physiologically splits during inspiration, when the aortic valve (A2) closes before the pulmonic valve (P2), creating an audible separation between these two components. 1

Physiologic Mechanism

During inspiration, venous return to the right heart increases, which prolongs right ventricular ejection time and delays pulmonic valve closure (P2). 1 Simultaneously, the aortic component (A2) may occur slightly earlier due to decreased left ventricular filling from pooling of blood in the pulmonary vasculature. 1 This creates the characteristic split that widens during inspiration and narrows or becomes single during expiration. 2

Clinical Significance of Normal Splitting

• A normally split S2 reliably excludes severe aortic stenosis, making it a valuable bedside finding. 3
• The split typically measures 10-70 milliseconds during inspiration in healthy individuals. 4
• During expiration, the two components merge into a single sound or very closely split sound. 5

Pathologic Splitting Patterns

Fixed Splitting

Fixed splitting (present during both inspiration and expiration) suggests atrial septal defect. 2 The right ventricular volume overload from the left-to-right shunt causes persistent delay in pulmonic valve closure regardless of respiratory phase. 6

Paradoxical (Reversed) Splitting

Paradoxical splitting occurs when P2 precedes A2, causing the split to appear during expiration and disappear during inspiration. 7 This results from delayed left ventricular ejection due to:

• Left bundle branch block 7
• Severe aortic stenosis 7
• Left ventricular volume overload 7

Absent or Single S2

A soft or absent A2 component, resulting in a single S2, indicates severe aortic stenosis with valve calcification preventing normal leaflet closure. 2, 3 In severe aortic regurgitation, prolonged left ventricular ejection causes the split to disappear during both respiratory phases. 5

Common Pitfall

Do not confuse the normal respiratory variation in S2 splitting with pathologic fixed splitting. Normal splitting should widen appreciably during inspiration (>20 ms augmentation) and narrow or become single during expiration. 5 Fixed splitting maintains the same interval throughout the respiratory cycle and warrants echocardiographic evaluation for structural heart disease, particularly atrial septal defect. 2