STEPTRACKERCARDIOVASCULAR | PATHOLOGY + EMBRYOLOGY | HIGH-YIELD REVIEW

Congenital Heart Disease — Left-to-Right Shunts

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.
DEFECT
ANATOMY · EMBRYOLOGY
FINDINGS
CLUE · COURSE
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.
Fixed, wide split S2; systolic ejection murmur at LUSB (extra pulmonic flow); diastolic tricuspid rumble
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
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
PDA
Ductus arteriosus (6th aortic arch) stays open.
Continuous "machine-like" murmur below left clavicle; bounding pulses, wide pulse pressure
Prematurity, congenital rubella, high altitude · close with indomethacin/ibuprofen
Eisenmenger syndrome
Chronic L→R flow → pulmonary arterial remodeling (medial hypertrophy, intimal fibrosis, plexiform lesions) → shunt reverses.
Late cyanosis, clubbing, polycythemia, RVH
Irreversible; only heart-lung transplant · PDA → toes cyanosed, fingers pink
Ductus pharmacology
Prostaglandin E keeps the ductus open.
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
Fixed, wide split S2 → ASD
Continuous machine-like murmur + bounding pulses → PDA
Cataracts, deafness and a continuous murmur → congenital rubella with PDA
Premature infant with PDA → indomethacin or ibuprofen
Down syndrome with a heart defect → AV septal (endocardial cushion) defect
Young stroke patient, DVT, positive bubble study → PFO paradoxical embolism
Adult with new cyanosis and clubbing after a childhood murmur → Eisenmenger syndrome
Cyanosed toes with pink fingers → PDA with Eisenmenger (differential cyanosis)