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Aortic component of the second heart sound (commonly denoted as A2)

The aortic component of the second heart sound (commonly denoted as A2) is the primary and louder part of the second heart sound (S2), which we hear as the "dub" in the classic "lub-dub" rhythm of the heartbeat.

What causes it?
S2 is produced by the closure of the semilunar valves at the end of ventricular systole (when the ventricles finish pumping blood out):
  • The aortic valve (between the left ventricle and aorta) closes first → this generates the A2 component.
  • The pulmonic valve (between the right ventricle and pulmonary artery) closes slightly later → this generates the P2 component.

A2 occurs earlier because:
  • The pressure in the aorta (systemic circulation) is much higher (~80 mmHg diastolic) compared to the pulmonary artery (~10 mmHg diastolic).
  • The left ventricle is more muscular and empties faster. This results in the aortic valve snapping shut sooner than the pulmonic valve.

Key characteristics of the aortic component (A2)
  • It is louder and higher-pitched than P2 (due to the higher systemic pressures and thicker aortic root structures).
  • It radiates widely across the precordium — best heard at the aortic area (2nd right intercostal space, right upper sternal border), but also audible at the apex and other areas.
  • In normal physiology, A2 precedes P2, leading to physiological splitting of S2 (wider on inspiration, narrower/single on expiration).

Clinical significance:
  • Normal → A2 is prominent and the dominant component of S2 in most listening areas.
  • Soft or absent A2 → Suggests issues like severe aortic stenosis (calcified, immobile valve leaflets), low aortic pressure, or significant aortic regurgitation.
  • Accentuated/loud A2 → Can occur in systemic hypertension (high aortic pressure) or conditions with a dilated aortic root.
  • Paradoxical (reversed) splitting → A2 is delayed (P2 comes first), so splitting occurs on expiration and becomes single on inspiration — seen in aortic stenosis, left bundle branch block, hypertrophic cardiomyopathy, etc.

In summary, the aortic component (A2) is the main contributor to what we perceive as the second heart sound in everyday auscultation, reflecting normal left-sided cardiac dynamics and providing important clues about aortic valve function and systemic pressures.