What is S2 Splitting and Why Does It Occur?
S2 splitting refers to the audible separation between the aortic valve closure (A2) and pulmonic valve closure (P2) components of the second heart sound, occurring because the right and left ventricles do not finish ejecting blood simultaneously.
Mechanism of S2 Splitting
The split interval represents the time difference between closure of the aortic and pulmonic valves. Normally, A2 precedes P2 because:
- Left ventricular ejection completes first - The left ventricle typically finishes systole before the right ventricle
- Pulmonary vascular impedance changes - The major contributor to normal splitting is actually the impedance characteristics of the pulmonary vascular bed, not simply prolonged right ventricular ejection 1
Normal Physiologic Splitting
During inspiration, the A2-P2 interval widens (averaging 27.2 msec in normal subjects) 1. This occurs primarily through:
- Decreased pulmonary vascular impedance (accounts for ~45% of inspiratory widening) - This delays P2 by prolonging the interval from end of RV systole to valve closure 1
- Prolonged RV electromechanical systole (accounts for ~28% of widening) 1
- Shortened left ventricular ejection (accounts for ~27% of widening) 1
The traditional explanation that increased venous return prolongs RV systole is actually NOT the primary mechanism - research demonstrates that decreased pulmonary vascular impedance during inspiration is the dominant factor 1.
During expiration, the split narrows or becomes single as these hemodynamic changes reverse.
Clinical Patterns and Their Significance
Fixed Splitting
Fixed splitting during both inspiration and expiration strongly suggests atrial septal defect 2, 3. This occurs when there is persistent equalization of ventricular filling regardless of respiratory phase.
Reversed (Paradoxical) Splitting
When P2 precedes A2, consider:
- Severe aortic stenosis - Prolonged left ventricular ejection delays A2 2, 3
- Hypertrophic obstructive cardiomyopathy (resting type) - 69% show paradoxical splitting with mean interval of -23.8 msec 4
- Left bundle branch block - Delays left ventricular activation
Wide Splitting
- Hypertrophic cardiomyopathy (non-obstructive) - Mean split interval of 41.0 msec during expiration, with 67% showing abnormal respiratory variation 4
- Right bundle branch block - Delays right ventricular activation
- Pulmonic stenosis - Prolongs right ventricular ejection
Narrow or Single S2
- Severe aortic regurgitation - 86% of patients have inspiratory augmentation <0.02 sec, with prolonged Q-A2 interval eliminating normal asynchrony 5
- Pulmonary hypertension - When normal pulmonary vascular impedance is lost, splitting depends primarily on changes in RV systolic duration rather than impedance changes 1
- Hypertension with LV hypertrophy - Mean interval of 14.5 msec, significantly narrower than normal 4
Key Clinical Pitfalls
- Don't assume increased venous return is the primary cause of inspiratory splitting - impedance changes dominate
- A single S2 in a young patient warrants investigation - should normally split with inspiration
- Fixed splitting is pathognomonic for ASD until proven otherwise 2, 3
- In severe aortic stenosis, absence of A2 or reversed splitting indicates advanced disease 2, 3
The split interval can be quantified using advanced techniques (S-transform amplitude ridge tracking algorithm shows accuracy within 5 msec) 6, though clinical auscultation remains the standard bedside assessment.