Atrial septal defect: why does it cause a murmur?
Young dog with a soft grade 2/6 systolic murmur at the left base. Echocardiography shows an ostium secundum atrial septal defect with left-to-right flow and dilation of the right atrium and ventricle.
What is the origin of the murmur in an atrial septal defect?
chevron_right Show answer and explanation
A. Turbulent flow through the atrial defect itself
The pressure difference between the atria is very small, so flow through the defect is slow and laminar and usually inaudible.
B. Mitral regurgitation associated with the defect
Some atrioventricular septal (AV canal) defects include a mitral cleft, but in secundum ASD the murmur is not due to mitral regurgitation, which would be heard at the left apex.
C. Relative pulmonic stenosis due to excess flow through a normal pulmonic valve
Correct. The shunt volume-overloads the right chambers, and the large volume ejected by the right ventricle through a normal pulmonic valve produces a systolic ejection murmur at the left base. It may be accompanied by fixed splitting of the second heart sound.
D. Continuous flow similar to patent ductus arteriosus
A continuous murmur requires a pressure gradient sustained in systole and diastole between two circulations, as between the aorta and pulmonary artery in PDA. This does not occur in ASD.
lightbulb Key takeaway
ASD produces few signs: a soft left basilar murmur and right-sided volume overload. Many go unnoticed until echocardiography. Large defects can cause right-sided heart failure or pulmonary hypertension with shunt reversal. It is described in Boxers, Samoyeds and Dobermans, and in cats often as part of an atrioventricular canal defect.
Test yourself with 10 random questions
Cardiology · 100 questions, a different test every time
Related questions
Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.