What are the causes of a split S2 (second heart sound)?

Causes of Split S2 (Second Heart Sound)

The second heart sound (S2) can be split due to physiological or pathological causes, with the most common causes being atrial septal defect, right bundle branch block, and pulmonary stenosis. 1

Normal Physiological Splitting

• Respiratory Variation: Normal physiological splitting occurs during inspiration when:

  • Increased venous return to the right heart delays pulmonary valve closure (P2)
  • Decreased left heart filling slightly advances aortic valve closure (A2)
  • This creates a normal inspiratory splitting that narrows or disappears during expiration 2

• Characteristics:

  • Audible during inspiration
  • Narrows or disappears during expiration
  • Normal interval between A2 and P2 components (typically <30 ms)

Pathological Causes of S2 Splitting

Wide (Persistent) Splitting

1. Atrial Septal Defect (ASD)

   • Fixed splitting throughout respiratory cycle due to right ventricular volume overload
   • Characterized by pulmonic flow murmur and widely split S2 on physical examination 2
   • Often accompanied by right axis deviation and right bundle branch block on ECG

2. Right Bundle Branch Block (RBBB)

   • Delayed right ventricular activation leads to delayed pulmonary valve closure
   • Results in wide splitting of S2 that persists throughout respiratory cycle 1

3. Pulmonary Stenosis (PS)

   • Increased resistance to right ventricular outflow delays pulmonary valve closure
   • Often accompanied by an ejection sound heard only in the pulmonic area and left sternal border during expiration 2
   • May present with widely split S2 3

4. Right Ventricular Failure

   • Delayed right ventricular contraction and relaxation
   • Can be caused by right ventricular volume or pressure overload

Paradoxical (Reversed) Splitting

1. Left Bundle Branch Block (LBBB)

   • Delayed left ventricular activation causes delayed aortic valve closure
   • Results in P2 occurring before A2, creating paradoxical splitting 4
   • S2 is single during inspiration and split during expiration (opposite of normal)

2. Aortic Stenosis (AS)

   • Increased resistance to left ventricular outflow delays aortic valve closure
   • May present with soft or absent A2 or reversed splitting of S2 2
   • Often accompanied by a systolic ejection murmur

3. Left Ventricular Volume Overload

   • Conditions like aortic regurgitation or ventricular septal defect
   • Delayed left ventricular emptying leads to delayed aortic valve closure 4

4. Pacemaker-Induced

   • Right ventricular pacing can cause a pattern similar to LBBB
   • Results in paradoxical splitting of S2 4

Fixed Splitting

1. Atrial Septal Defect

   • Fixed splitting throughout respiratory cycle regardless of inspiration/expiration
   • Associated with right ventricular volume overload 2, 3

2. Severe Right Ventricular Failure

   • Right ventricular dysfunction prevents normal respiratory variation

Diagnostic Approach

1. Cardiac Auscultation:

   • Listen at the pulmonic area (left upper sternal border)
   • Assess splitting during different phases of respiration
   • Evaluate response to maneuvers (Valsalva, standing, squatting) 2

2. Electrocardiogram (ECG):

   • Identify conduction abnormalities (RBBB, LBBB)
   • Look for chamber enlargement patterns
   • Check for signs of ASD (right axis deviation, incomplete RBBB)

3. Echocardiography:

   • Gold standard for confirming underlying causes
   • Visualizes valve morphology and function
   • Assesses chamber size and function
   • Identifies congenital anomalies 1

Clinical Pearls

• A fixed splitting of S2 during both inspiration and expiration strongly suggests ASD 2
• Paradoxical splitting is most commonly caused by LBBB but can also occur in severe aortic stenosis 4
• Split S2 must be differentiated from S1 + ejection sound, S1 + S4, and early systolic click 1
• The time interval between A2 and P2 components can be used to quantify interventricular dyssynchrony 5
• Advanced techniques like wavelet transform analysis can help detect subtle abnormalities in S2 splitting 6, 7