Atrial septal defect (ASD) classically presents with which auscultatory finding and mechanism?

• A. Fixed splitting of S2 due to constant left-to-right shunt from LA to RA
• B. Variable S2 splitting that varies with respiration
• C. Single S2 with absent P2
• D. Loud P2 with highly audible S2 splitting

Answer: (A)

Atrial septal defect causes a left-to-right shunt that increases right-heart and pulmonary blood flow. This volume overload delays the closure of the pulmonic valve, so the second heart sound components (A2 and P2) are separated by a wider interval, and because the shunt is constant, this delay is present regardless of breathing. The result is a fixed, wide splitting of S2. A fixed split is the hallmark, while additional flow across the pulmonary valve can produce a systolic ejection murmur at the left upper sternal border. The other patterns—splitting that varies with respiration, a single S2 with no P2, or loud P2 with variable splitting—do not fit ASD.