How to differentiate between A2 (aortic component of the second heart sound) and P2 (pulmonic component of the second heart sound) when S2 (second heart sound) is single?

Differentiating A2 from P2 When S2 is Single

When S2 is single, the component heard is most likely A2, as P2 is typically inaudible in this situation due to A2 being louder and masking P2.

Anatomical and Physiological Basis

The second heart sound (S2) normally consists of two components:

• A2: Aortic valve closure component
• P2: Pulmonic valve closure component

When S2 appears single, several factors help determine whether you're hearing A2 or P2:

Key Differentiating Features

1. Intensity and Location:

   • A2 is typically louder than P2 in normal individuals
   • A2 is best heard at the right second intercostal space (aortic area)
   • P2 is best heard at the left second intercostal space (pulmonic area)

2. When S2 is Single:

   • The component heard is almost always A2, as it's normally louder and masks P2 1
   • A single S2 due to inaudible A2 is rare and suggests severe aortic stenosis 2, 1

Diagnostic Approach

1. Respiratory Maneuvers

• During normal respiration, S2 should split during inspiration and become single during expiration
• If S2 remains single throughout respiration:
  • Most likely A2 is dominant and P2 is inaudible
  • Alternatively, P2 may be absent or delayed significantly

2. Auscultation Locations

• Listen at both aortic and pulmonic areas
• A2 is normally louder at the aortic area
• If you hear a component only at the pulmonic area and not at the aortic area, it's likely P2

3. Associated Clinical Findings

• If A2 is the predominant component:

  • Normal finding in healthy individuals
  • May indicate aortic valve disease if abnormally loud or soft

• If P2 is the predominant component (rare when S2 is single):

  • May indicate pulmonary hypertension 3, 4
  • P2 hyperphonesis (abnormally loud P2) suggests elevated pulmonary artery pressure

4. Pathological Considerations

• A single S2 due to inaudible A2 suggests severe aortic stenosis 2, 1
• A single S2 due to inaudible P2 is common in normal individuals
• The American College of Cardiology notes that "the only physical examination finding that reliably excludes severe AS is a normally split second heart sound" 2, 1

Special Techniques

1. Dynamic Auscultation:

   • Have the patient perform Valsalva maneuver or stand up
   • These maneuvers can sometimes accentuate subtle splitting

2. Electronic Stethoscope Analysis:

   • Modern electronic stethoscopes with phonocardiography can help identify subtle splitting not audible to the human ear 5, 6
   • Time-frequency analysis can separate A2 and P2 components even when they appear single to the ear 7

Common Pitfalls

• Mistaking S1 for S2: Ensure you're correctly identifying the heart sounds by correlating with the carotid pulse
• Failing to auscultate in different positions: Sometimes splitting becomes more apparent when the patient leans forward or lies in the left lateral position
• Not considering pathological causes: A single S2 can be normal, but could also indicate conditions like severe aortic stenosis or pulmonary hypertension

In most clinical scenarios, when S2 is single, you are hearing A2, as it is the louder component that typically masks P2 in normal physiological conditions.