What causes a loud A2 heart sound?

Causes of Loud A2 Heart Sound

A loud A2 heart sound is primarily caused by systemic hypertension, but can also result from conditions that increase aortic pressure or alter the position of the aortic valve, including pulmonary hypertension, congenitally corrected transposition, and certain aortic valve pathologies.

Pathophysiological Mechanisms

The aortic component of the second heart sound (A2) is produced when the aortic valve closes. Several factors can lead to increased intensity of this sound:

Primary Causes

• Systemic Hypertension: Increased arterial pressure leads to more forceful closure of the aortic valve 1
• Aortic Dilation: A dilated aorta can transmit the closure sound more effectively 2
• Thin Chest Wall: Allows better sound transmission to the chest surface 2

Specific Cardiac Conditions

• Congenitally Corrected Transposition: The anterior position of the aortic valve makes A2 more audible 3
• Early Stages of Aortic Stenosis: Before valve calcification becomes severe 2, 1
• Aortic Regurgitation: Can lead to a louder A2 due to increased stroke volume 4

Clinical Significance and Examination

Associated Findings

• Location: A2 is best heard at the right second intercostal space (aortic area) 1
• Timing: In normal individuals, A2 is the dominant component of S2 and may mask P2 1
• Respiratory Variation: Normally, A2 becomes more prominent during expiration 2

Diagnostic Clues

• A loud A2 with a late-peaking systolic ejection murmur may indicate early aortic stenosis 1
• A loud A2 with a diastolic murmur may suggest aortic regurgitation 4
• A loud A2 with a single S2 during both phases of respiration is common in severe aortic regurgitation (86% of cases) 4

Differential Considerations

Loud P2 vs. Loud A2

• Loud P2: Typically indicates pulmonary hypertension and is best heard at the left upper sternal border 1
• Loud A2: Usually associated with systemic hypertension and is best heard at the right upper sternal border 1

S2 Splitting Patterns

• Normal Splitting: Widens during inspiration, narrows during expiration 1
• Fixed Splitting: Suggests atrial septal defect 2, 1
• Paradoxical Splitting: A2 occurs after P2, suggesting severe aortic stenosis, LBBB, or other causes of delayed LV ejection 5

Important Distinction

• In severe aortic stenosis, A2 typically becomes soft or absent rather than loud 2, 1
• Therefore, a loud A2 generally excludes severe aortic stenosis 1

Clinical Pearls

• The intensity of A2 correlates with aortic pressure at the time of valve closure 6
• A loud A2 in children should raise suspicion for pulmonary hypertension or congenital heart defects 3
• The timing between aortic valve closure on echocardiography and the onset of A2 is typically 10-15 milliseconds 7
• Echocardiography remains essential for confirming the underlying cause of a loud A2 1

Remember that while a loud A2 is often a benign finding related to normal variants or mild hypertension, it can also be an important clue to underlying cardiovascular pathology that warrants further investigation.