Acyanotic defects that overload the lungs — until pulmonary pressures climb high enough to reverse the shunt.
FOCUS ON
Know the murmur and the O₂ step-up site for each shunt. VSD: holosystolic at LLSB, RV step-up. ASD: fixed split S2, RA step-up. PDA: continuous "machine" murmur, PA step-up. Late cyanosis means Eisenmenger syndrome. Indomethacin closes a PDA; PGE1 keeps it open.
PICTURE
DEFECT
ANATOMY · EMBRYOLOGY
FINDINGS
CLUE · COURSE
How to read: atria on top, ventricles below (RA/RV on the left, LA/LV on the right). Blue tube = pulmonary artery from RV; pink tube = aorta from LV, joined at the top by the ductus. Red = the defect; teal arrow = shunt direction.
VSD
most common CHD
Usually membranous septum.
Harsh holosystolic murmur at left lower sternal border; large defects → HF in infancy
Small ones often close spontaneously · Down syndrome, fetal alcohol
ASD — secundum
most common ASD
Deficient septum primum or secundum at the fossa ovalis.
Often found in adulthood: atrial arrhythmias, paradoxical emboli
ASD — primum / AV septal defect
Endocardial cushion defect; complete form adds VSD + common AV valve.
ASD findings ± VSD murmur, mitral cleft
Down syndrome
ASD — sinus venosus
Defect near the SVC (or IVC) entry, outside the fossa ovalis.
ASD physiology; often with anomalous right pulmonary venous return
Missed on routine echo → TEE or MRI
Septal embryology
Septum primum grows down, ostium secundum opens, septum secundum covers it, leaving the foramen ovale; IV septum = muscular part + membranous part (endocardial cushions + aorticopulmonary septum).
Membranous VSD = failed fusion of cushions, IV septum and AP septum
Foramen ovale closes when LA pressure rises at first breath
Patent foramen ovale
Septum primum and secundum fail to fuse (~25% of adults).
Usually silent; not a true ASD
Stroke in a young patient with DVT → paradoxical embolism
Indomethacin, ibuprofen (block PG synthesis) → close · alprostadil (PGE1) → keep open
PGE1 for ductal-dependent lesions (TGA, HLHS, critical coarctation)
Mechanism — from shunt to Eisenmenger
Left pressure > right
→
L→R shuntAcyanotic
→
↑ Pulmonary flowVolume load
→
Vascular remodelingPulmonary HTN
→
RV pressure > LVR→L, late cyanosis
Where the oxygen steps up
DEFECT
O₂ STEP-UP
MURMUR
ASSOCIATION
ASD
Right atrium
SEM at LUSB + fixed split S2
Primum → Down
VSD
Right ventricle
Holosystolic at LLSB
Down, fetal alcohol
PDA
Pulmonary artery
Continuous machine-like
Rubella, prematurity
AV septal defect
RA and RV
Mixed
Down syndrome
How it's tested
COMMON CONFUSIONThe ASD murmur is not blood crossing the septum. It is extra flow across the pulmonic valve, which is why it's a systolic ejection murmur at the upper left sternal border.
THE TRAPA cyanotic newborn whose oxygenation depends on the ductus needs PGE1. Indomethacin in that baby would be fatal.
YOUR APPROACHMurmur + S2 pattern → defect → step-up site → association (Down, rubella, prematurity) → late complication (Eisenmenger).
If you see… think…
Infant with harsh holosystolic murmur at LLSB → VSD