[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Heart](https://durgical.com/en/general-pathology/areas/heart/) › Left-to-right shunts: ASD, VSD, PDA, AV canal # 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. Updated Nov 12, 2026 ## Key points - ASD secundum (about 90 %): defect at the fossa ovalis; primum ASD is a partial AV septal defect; sinus venosus ASD near the venae cavae. - Patent foramen ovale: about one in four adults; permits paradoxical embolism when right atrial pressure rises. - VSD: most are membranous; small ones close spontaneously; large ones cause heart failure and Eisenmenger syndrome. - PDA: continuous ‘machinery’ murmur; associated with prematurity, hypoxia and congenital rubella; prostaglandin keeps it open. - AV septal defect: Down syndrome; ostium primum ASD with cleft mitral leaflet, or complete common AV valve. ## 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](https://durgical.com/en/conditions/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](https://durgical.com/en/conditions/ventricular-septal-defect/) | Interventricular septum, mostly membranous | LV and RV overload, pulmonary hypertension | Holosystolic murmur at the left sternal border | | [Patent ductus arteriosus](https://durgical.com/en/conditions/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. ## Memory aid ASD in the attic, VSD in the ventricle, PDA in the pipes between aorta and lung; all pink at first, all can turn blue later. ## 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. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/general-pathology/left-to-right-shunts-asd-vsd-pda-av-canal/) [More Pathology questions →](https://durgical.com/en/practice/?f=pathology) ## Elsewhere in DurgiPedia - [Atrial septal defect and patent foramen ovale](https://durgical.com/en/embryology/atrial-septal-defect-and-patent-foramen-ovale/) - [Ventricular septal defect](https://durgical.com/en/embryology/ventricular-septal-defect/) - [Patent ductus arteriosus](https://durgical.com/en/embryology/patent-ductus-arteriosus/) - [Atrioventricular septal defect (endocardial cushion defect)](https://durgical.com/en/embryology/atrioventricular-septal-defect/) - [Atrial septation: septum primum, septum secundum and foramen ovale](https://durgical.com/en/embryology/atrial-septation-septum-primum-septum-secundum-and-foramen-ovale/) ## Connections ## Related 4 - [Congenital heart disease: overview](https://durgical.com/en/general-pathology/congenital-heart-disease-overview/) - [Right-to-left shunts: tetralogy, transposition and others](https://durgical.com/en/general-pathology/right-to-left-shunts-tetralogy-transposition-and-others/) - [Systemic (arterial) thromboembolism](https://durgical.com/en/general-pathology/systemic-thromboembolism/) - [Pulmonary hypertension](https://durgical.com/en/general-pathology/pulmonary-hypertension/) ## Related conditions 5 - [Atrial septal defect](https://durgical.com/en/conditions/atrial-septal-defect/) - [Ventricular septal defect](https://durgical.com/en/conditions/ventricular-septal-defect/) - [Patent ductus arteriosus](https://durgical.com/en/conditions/patent-ductus-arteriosus/) - [Eisenmenger syndrome](https://durgical.com/en/conditions/eisenmenger-syndrome/) - [Down syndrome (trisomy 21)](https://durgical.com/en/conditions/down-syndrome/) [← All of general pathology](https://durgical.com/en/general-pathology/) 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](https://www.ncbi.nlm.nih.gov/books/NBK535440/) (StatPearls (NCBI Bookshelf), 2025); [Ventricular Septal Defect](https://www.ncbi.nlm.nih.gov/books/NBK470330/) (StatPearls (NCBI Bookshelf), 2025); [Patent Ductus Arteriosus](https://www.ncbi.nlm.nih.gov/books/NBK430758/) (StatPearls (NCBI Bookshelf), 2025). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/general-pathology/left-to-right-shunts-asd-vsd-pda-av-canal/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/left-to-right-shunts-asd-vsd-pda-av-canal/