Fixed split S2 on auscultation is most commonly associated with which condition?

• A. Atrial septal defect
• B. Pulmonary hypertension
• C. Aortic regurgitation
• D. Mitral stenosis

Answer: (A)

Fixed splitting of the second heart sound means the interval between A2 and P2 stays consistently wide, regardless of breathing. This pattern points to a left-to-right shunt that sustains delayed pulmonic valve closure. The classic scenario is an atrial septal defect. With ASD, increased flow through the right heart and pulmonary circulation keeps the pulmonic component delayed throughout the cardiac cycle, so inspiration cannot widen the split further. A systolic ejection murmur at the left upper sternal border from increased pulmonary flow often accompanies this finding.

Other conditions don’t produce a fixed split. Pulmonary hypertension tends to make P2 prominent and can alter splitting, but the variation with respiration isn’t fixed in the same way. Aortic regurgitation and mitral stenosis produce other characteristic auscultatory findings (like different S2 abnormalities or opening snap with diastolic murmur) rather than a persistently split S2.