What is the cause of the second heart sound (S2)?

The Second Heart Sound (S2) is Caused by Closure of the Aortic and Pulmonic Valves

The second heart sound (S2) is produced by the closure of the aortic valve (A2) and the pulmonic valve (P2), with the aortic valve typically closing before the pulmonic valve, creating a physiological split. 1

Normal S2 Physiology

• S2 consists of two distinct components:

  • A2 (aortic component): Results from aortic valve closure
  • P2 (pulmonic component): Results from pulmonic valve closure

• The normal physiological split of S2:

  • Widens during inspiration (5-30 ms) due to increased venous return to the right heart delaying pulmonic valve closure
  • Narrows or disappears during expiration 1

• Auscultation characteristics:

  • S2 is typically louder than S1 at the base of the heart
  • Best heard using the diaphragm of the stethoscope for high-frequency components
  • Having the patient breathe slowly helps appreciate respiratory variation 1

Abnormal S2 Split Patterns

Fixed Split

• Occurs when the split remains constant throughout the respiratory cycle
• Pathognomonic for atrial septal defect (ASD)
• Caused by right ventricular volume overload preventing normal respiratory variation 1, 2
• Often accompanied by a pulmonic flow murmur

Wide Split

• Characterized by a delayed P2 component
• Common in pulmonic stenosis
• Results from prolonged right ventricular ejection 1

Paradoxical Split

• Occurs when P2 closes before A2 (reverse of normal)
• Single sound during inspiration, split during expiration
• Most commonly caused by left bundle branch block
• Also seen in severe aortic stenosis 1, 3

Clinical Significance and Evaluation

• Careful auscultation of S2 can provide important diagnostic clues:

  • Normal split virtually excludes severe aortic stenosis
  • Single S2 (due to inaudible A2) suggests severe aortic stenosis
  • Fixed split strongly suggests ASD 1

• Diagnostic evaluation:

  • Echocardiography is essential for confirming underlying causes
  • Can visualize valve morphology and function
  • Assesses chamber size and function
  • Identifies congenital anomalies 1

Common Pitfalls in S2 Assessment

• Failure to have the patient breathe properly during auscultation
• Not distinguishing between physiological and pathological splits
• Overlooking subtle splits that may indicate significant pathology
• Confusing S2 with other heart sounds or clicks

Advanced Assessment Techniques

Recent research has developed methods to quantitatively measure S2 splitting using:

• Wavelet transform analysis 2, 3
• Instantaneous frequency analysis 4
• Signal decomposition techniques 5

These techniques can detect splits as small as 10-20 ms and help identify subtle abnormalities that might be missed on routine auscultation.