What is the primary reason for S1 (first heart sound) and S2 (second heart sound) in cardiac auscultation?

Primary Reason for S1 and S2 Heart Sounds

S1 (first heart sound) is produced by closure of the atrioventricular valves (mitral and tricuspid) at the beginning of ventricular systole, while S2 (second heart sound) is produced by closure of the semilunar valves (aortic and pulmonic) at the end of ventricular systole.

Physiologic Mechanism of S1

• S1 marks the onset of ventricular contraction when rising ventricular pressure exceeds atrial pressure, causing the mitral and tricuspid valves to close 1
• The mitral component (M1) typically occurs slightly before the tricuspid component (T1), though this split is usually not audible in normal individuals 2, 3
• S1 is best heard at the cardiac apex and corresponds with the beginning of the carotid pulse 1

Physiologic Mechanism of S2

• S2 marks the end of ventricular ejection when ventricular pressure falls below aortic and pulmonary artery pressures, causing the aortic (A2) and pulmonic (P2) valves to close 2, 3
• The aortic valve normally closes before the pulmonary valve, creating the A2 and P2 components of S2 2, 3
• This time delay between A2 and P2 is called "physiologic splitting" and varies with respiration 3

Clinical Significance of S2 Splitting

• Normal (physiologic) splitting increases with inspiration because increased venous return to the right ventricle delays pulmonic valve closure 1, 3
• Fixed splitting of S2 during both inspiration and expiration suggests atrial septal defect and requires echocardiographic evaluation 1, 3
• Reversed splitting (P2 before A2) or absent/soft A2 may indicate severe aortic stenosis 1
• Wide splitting can occur with right bundle branch block or pulmonic stenosis 3

Timing in the Cardiac Cycle

• The precise timing of S1 and S2 depends on when physiologically important pressure differences between chambers occur during the cardiac cycle 1
• S1 occurs when ventricular pressure exceeds atrial pressure (beginning of systole) 1
• S2 occurs when aortic and pulmonary artery pressures exceed ventricular pressures (end of systole) 2, 3

Clinical Pitfalls

• In elderly patients with severe aortic stenosis, the typical characteristics of S2 may be altered or absent due to vascular aging effects 1
• Electronic stethoscopes and phonocardiography can help visualize and measure the components of S1 and S2, particularly the A2-P2 split, which may be difficult to appreciate with traditional auscultation 2, 4
• Sensor placement matters when recording heart sounds—different chest locations yield different timing relationships between S1, S2, and ECG findings 5