Why does an atrial septal defect produce both a systolic ejection murmur and a diastolic rumbling murmur?

Why ASD Produces Both Systolic and Diastolic Murmurs

An atrial septal defect produces a systolic ejection murmur from increased flow across the normal pulmonary valve and a diastolic rumbling murmur from increased flow across the normal tricuspid valve—both are flow murmurs caused by right ventricular volume overload, not from the defect itself.

Mechanism of the Systolic Murmur

The systolic murmur in ASD is not generated by flow through the atrial septal defect itself. Instead:

• The left-to-right shunt causes right ventricular volume overload, resulting in increased stroke volume ejected through the pulmonary valve during systole 1
• This creates a systolic pulmonary flow murmur heard best at the left upper sternal border, typically grade 2-3/6 in intensity 1
• The murmur represents flow across a normal pulmonary valve that is handling an abnormally large volume of blood 1
• This is a relative pulmonary stenosis—the valve is structurally normal but functionally inadequate for the increased flow volume 1

Mechanism of the Diastolic Murmur

The diastolic rumble occurs only when the shunt is hemodynamically significant:

• With large shunts, the increased blood volume returning to the right atrium must pass through the tricuspid valve during diastolic filling 1
• This creates a middiastolic flow rumble across the normal tricuspid valve, heard best at the lower left sternal border 1
• The murmur represents a relative tricuspid stenosis—again, a structurally normal valve handling excessive flow 1
• This diastolic rumble is analogous to the flow murmurs seen across the mitral valve in VSD or across the tricuspid valve in severe tricuspid regurgitation 1

The Pathophysiology Behind Both Murmurs

The fundamental mechanism is ventricular compliance mismatch:

• The right ventricle has higher compliance than the left ventricle, creating the pressure gradient that drives left-to-right shunting 1
• This shunt volume depends on RV/LV compliance ratio, defect size, and atrial pressures 1
• The resulting RV volume overload is the key hemodynamic finding and best characterizes defect severity 1
• Both murmurs intensify with conditions that increase left atrial pressure (hypertension, ischemic heart disease, LV dysfunction), which increases the shunt volume 1

Critical Distinguishing Features on Examination

Fixed splitting of S2 is the pathognomonic finding:

• The increased RV stroke volume delays pulmonic valve closure throughout the respiratory cycle 1
• This fixed splitting, combined with the systolic flow murmur, should immediately suggest ASD 1
• The diastolic rumble may be absent in small defects but becomes audible when Qp:Qs exceeds approximately 2:1 1

Common Pitfalls to Avoid

• Do not mistake the systolic murmur for pulmonary stenosis—the valve itself is normal; echocardiography will show RV volume overload without valve pathology 1
• Do not confuse the diastolic rumble with tricuspid stenosis—there is no opening snap, and the tricuspid valve is structurally normal 1
• In elderly patients with new-onset atrial fibrillation and these murmurs, always investigate for previously undiagnosed ASD, as decreased LV compliance with age increases shunting 1
• The murmurs may become softer or disappear if severe pulmonary hypertension develops and the shunt decreases or reverses 1