Left-to-right shunts: ASD, VSD, PDA, AV canal
- Lesion
Holes that let oxygenated blood flow back to the right heart and lungs. Babies are not blue at first, but the lungs are overloaded.
What it is
Left-to-right shunts send already oxygenated blood back through the pulmonary circulation. Atrial septal defects (ASD) raise right-sided volume and are often found in adulthood. A patent foramen ovale is a flap-valve opening present in a quarter of adults, important for paradoxical emboli. Ventricular septal defects (VSD) are the commonest congenital lesion; large ones cause early failure and pulmonary hypertension. A patent ductus arteriosus (PDA) connects aorta and pulmonary artery. Atrioventricular septal defects come from failure of the endocardial cushions.
Structure
- Shunt jets cause fibrous thickening of the endocardium where they strike.
- Lungs of long-standing shunts: medial hypertrophy, intimal fibrosis and, in late Eisenmenger, plexiform lesions of pulmonary arteries.
The five left-to-right shunts
| Lesion | Where the hole is | Consequence | Clue |
|---|---|---|---|
| Atrial septal defect | Interatrial septum (secundum at the fossa ovalis) | Right atrial and ventricular volume overload | Fixed split S2; often found in adults |
| Patent foramen ovale | Unsealed flap of septum primum and secundum | Paradoxical emboli when right pressure rises | Stroke in a young adult with a venous clot |
| Ventricular septal defect | Interventricular septum, mostly membranous | LV and RV overload, pulmonary hypertension | Holosystolic murmur at the left sternal border |
| Patent ductus arteriosus | Aorta to pulmonary artery | Lung overcirculation, LV volume overload | Continuous machinery murmur; prematurity, rubella |
| Atrioventricular septal defect | Endocardial cushions (primum ASD, inlet VSD, abnormal AV valves) | Shunt plus AV valve regurgitation | Down syndrome |
Clinical notes
- Closure before pulmonary vascular disease is fixed prevents Eisenmenger syndrome.
- In premature infants, PDA can be closed with prostaglandin synthesis inhibitors; in some cyanotic lesions prostaglandin is given to keep it open.
Test yourself
Think of the answer, then tap the question to see it.
What is the most common type of atrial septal defect?
The secundum ASD at the fossa ovalis (about 90 %).
Why does a patent foramen ovale matter in a young adult with a stroke?
It allows a venous thrombus to cross into the systemic circulation (paradoxical embolism) when right atrial pressure transiently rises.
Which murmur suggests a patent ductus arteriosus?
A continuous, ‘machinery-like’ murmur.
Elsewhere in DurgiPedia
- Atrial septal defect and patent foramen ovale
- Ventricular septal defect
- Patent ductus arteriosus
- Atrioventricular septal defect (endocardial cushion defect)
- Atrial septation: septum primum, septum secundum and foramen ovale
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Atrial Septal Defect (StatPearls (NCBI Bookshelf), 2025); Ventricular Septal Defect (StatPearls (NCBI Bookshelf), 2025); Patent Ductus Arteriosus (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.