What is the significance and treatment of a split S2 (second heart sound)?

Split S2: Significance and Treatment

A split S2 (second heart sound) is primarily a diagnostic finding that can indicate normal physiological variation or underlying cardiac pathology, with treatment directed at the underlying cause rather than the split itself. 1

Normal vs. Abnormal S2 Split

Normal Physiological Split

• A normal physiological split of S2 widens during inspiration (5-30 ms) and narrows or disappears during expiration due to increased venous return to the right heart delaying pulmonic valve closure 1
• This normal variation occurs because inspiration increases blood return to the right ventricle, delaying pulmonic valve closure while aortic closure remains relatively constant

Abnormal S2 Split Patterns

1. Wide Fixed Split

   • Characterized by two clearly separated components heard during both inspiration and expiration 1
   • Most commonly associated with atrial septal defect (ASD) due to right ventricular volume overload preventing normal respiratory variation 1
   • Also seen in right ventricular dysfunction or delayed right ventricular activation

2. Paradoxical Split

   • Occurs when P2 (pulmonic component) closes before A2 (aortic component)
   • Most commonly caused by left bundle branch block (LBBB) 1
   • Also seen in severe aortic stenosis, causing delayed left ventricular ejection 1
   • Presents as a single sound during inspiration and split during expiration 2

3. Wide Split

   • Characteristic finding in pulmonic stenosis, with delayed P2 component 1
   • Can also occur in right bundle branch block

Diagnostic Approach

Auscultation Technique

• Use the diaphragm of the stethoscope for high-frequency S2 components 1
• Have the patient breathe slowly to appreciate respiratory variation 1
• Listen at the base of the heart (2nd intercostal space) where S2 is typically louder than S1 1
• Compare the split during held inspiration versus expiration to determine if it narrows appropriately 1

Further Evaluation

• Electrocardiogram (ECG) to identify conduction abnormalities (e.g., bundle branch blocks) and chamber enlargement patterns 1

  • ECG has inadequate sensitivity (55%) and specificity (70%) to be a screening tool for pulmonary hypertension 3
  • Right ventricular hypertrophy on ECG is present in 87% and right axis deviation in 79% of patients with idiopathic pulmonary arterial hypertension 3

• Echocardiography is essential for:

  • Confirming underlying causes 1
  • Visualizing valve morphology and function 1
  • Assessing chamber size and function 1
  • Identifying congenital anomalies like ASD 1
  • Evaluating severity of conditions like pulmonic stenosis 1

• Chest Radiograph may show:

  • Central pulmonary arterial dilatation with peripheral vessel pruning in pulmonary hypertension 3
  • Right atrial and ventricular enlargement in advanced cases 3

Clinical Significance and Treatment

Atrial Septal Defect (ASD)

• Significance: Fixed splitting of S2 with pulmonic flow murmur 1
• Treatment: Surgical or percutaneous closure of the defect, which addresses the underlying cause of the split S2 4
• Caveat: Not all ASD cases present with the classic fixed split; variable splitting can occur in some patients 4

Pulmonary Stenosis

• Significance: Wide splitting of S2 with delayed P2 component 1
• Severity Assessment:
  • Mild: Peak gradient <36 mmHg (peak velocity <3 m/s)
  • Moderate: Peak gradient 36-64 mmHg (peak velocity 3-4 m/s)
  • Severe: Peak gradient >64 mmHg (peak velocity >4 m/s) 1
• Treatment: Balloon valvuloplasty or surgical valvotomy for moderate to severe cases

Left Bundle Branch Block (LBBB)

• Significance: Paradoxical splitting of S2 1
• Treatment: Address underlying cause; cardiac resynchronization therapy may be indicated in some cases

Severe Aortic Stenosis

• Significance: Paradoxical splitting or single S2 due to inaudible A2 1
• Associated Findings: Loud, late-peaking systolic ejection murmur radiating to the carotids; delayed and diminished carotid upstroke 1
• Treatment: Aortic valve replacement (surgical or transcatheter) for severe symptomatic cases

Pulmonary Hypertension

• Significance: Accentuated pulmonary component of S2 3
• Associated Findings: Left parasternal lift, pansystolic murmur of tricuspid regurgitation, right ventricular S3 3
• Treatment: Targeted therapy based on etiology (vasodilators for PAH, treatment of underlying lung disease, etc.)

Clinical Pitfalls and Caveats

1. A normal ECG does not exclude the presence of severe pulmonary hypertension 3

2. The absence of a wide and persistent splitting of S2 does not exclude the diagnosis of uncomplicated ASD when ECG and x-ray findings suggest the disease 4

3. A single S2 due to inaudible A2 suggests severe aortic stenosis, while a normally split second heart sound reliably excludes severe AS 1

4. Conditions with high cardiac output (anemia, thyrotoxicosis, pregnancy) may cause abnormal S1-to-S2 intensity ratios that can confuse auscultation findings 1

5. Advanced signal processing techniques like wavelet transform analysis are being developed to better quantify and characterize S2 splits, which may improve diagnostic accuracy in the future 5, 6, 7