Atrial & Ventricular Septation

Rapid Review·Cardiovascular

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EMBRYOLOGY

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FA P284-286

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Two septa sliding past each other above, one septum closing below, and the defect each failure leaves behind.

Key takeaways

Atrial septation

Two septa slide past each other, and the overlap is the foramen ovale

follow the sequence and every atrial defect becomes a named failure of one step.

What this shows

Atrial septation in five steps. (1) Septum primum grows down toward the dorsal endocardial cushion, narrowing the ostium primum. (2) Ostium secundum opens in the septum primum by cell death as the ostium primum closes. (3) Septum secundum develops to the right of septum primum. (4) Septum secundum covers most of the ostium secundum, and the residual opening is the foramen ovale. (5) The remaining septum primum degenerates into the one-way valve that closes the foramen at birth.

What this shows

Atrial septation seen in the heart itself, four panels. Septum primum (green) descends toward the endocardial cushions leaving the ostium primum; the ostium secundum then perforates it; septum secundum (blue) grows down to its right and overlaps the opening, leaving the foramen ovale as the residual channel between the two septa.

Septum primum grows toward the endocardial cushions

narrowing the ostium primum.

Ostium secundum forms within septum primum by cell death

opening as the ostium primum regresses, so the right-to-left flow is never interrupted.

Septum secundum develops to the RIGHT of septum primum

it expands and covers most of the ostium secundum, and the residual opening is the foramen ovale. The remaining portion of septum primum becomes the foramen's one-way valve.

At birth the two septa are pressed together

septum primum closes against septum secundum and seals the foramen ovale, because LA pressure rises and RA pressure falls once the lungs inflate. The two fuse during infancy and early childhood to form the definitive atrial septum.

PFO is a failure to FUSE, not a hole

present in 25% of the population, and most are asymptomatic and never detected.

It can cause paradoxical emboli, meaning a venous thromboembolus crossing a right-to-left shunt into the systemic circulation and causing stroke, exactly as an ASD can.
A PFO produces no abnormal heart sound, whereas an ASD gives wide fixed splitting of S2, which is the fastest way to separate them at the bedside.

Patent foramen ovale vs Atrial septal defect (Mehlman: "PFO = ASD on USMLE")
Feature	Patent foramen ovale	Atrial septal defect
Mechanism	Failed fusion of septum primum and secundum	True defect in the interatrial septum
Prevalence	25% of population	Less common
Heart sound	None	Wide fixed splitting of S2
Ostium secundum type	Not applicable	Most common, usually isolated
Ostium primum type	Not applicable	Rarer; occurs with other anomalies; Down syndrome
Shared risk	Paradoxical emboli	Paradoxical emboli

Ventricular septation

What this shows

Ventricular septation in three steps. (1) The muscular interventricular septum grows up from the apex, leaving the interventricular foramen open beside the atrioventricular canals. (2) The aorticopulmonary septum descends. (3) It fuses with the muscular septum to form the membranous interventricular septum, closing the foramen; failure here is the membranous VSD.

The muscular septum grows UP from the apex

leaving the interventricular foramen open at its top.

The aorticopulmonary septum rotates down and fuses with it

forming the membranous interventricular septum and closing the foramen. That junction is the weak point, and it is why the membranous septum is where VSDs occur.

Endocardial cushion growth separates atria from ventricles

and contributes to both the atrial septum and the membranous interventricular septum, which is precisely why cushion defects produce combined lesions rather than isolated ones.

The two septal contributions
Portion	Formed by	Defect
Muscular interventricular septum	Grows up from the apex	Muscular VSD, most self-resolve
Membranous interventricular septum	Aorticopulmonary septum rotates and fuses with muscular septum	Membranous VSD, the most common site

The defects

Ventricular septal defect is the MOST COMMON congenital cardiac anomaly

usually in the membranous septum. See Congenital Heart Disease: Left-To-Right Shunts.

Atrioventricular septal defect is incomplete fusion of the endocardial cushions

also called an endocardial cushion or AV canal defect, giving defects in the septa and in the AV valves.

Complete: a common AV valve with both ASD and VSD.
Partial: a primum ASD with a cleft mitral valve, causing mitral regurgitation, and no VSD.
Associated with Down syndrome. Mehlman notes that "endocardial cushion defect" is also used loosely to describe the ASD and VSD seen in Down syndrome.

Complete vs Partial AVSD
Feature	Complete AVSD	Partial AVSD
Valve	Common AV valve	Cleft mitral valve
Septal defects	ASD and VSD	Primum ASD only, no VSD
Consequence	Mixing at both levels	Mitral regurgitation
Association	Down syndrome	Down syndrome

A patent foramen ovale is not a hole in the septum

The distinction sounds pedantic and decides questions. In a PFO both septa are fully formed; they simply never fused, so the flap of septum primum lies unsealed against septum secundum. At rest the higher left atrial pressure holds it shut, and there is no shunt, no murmur and no split S2. It opens only when right atrial pressure transiently exceeds left, during a Valsalva, a cough, or straining, and that is the moment a venous clot can cross into the systemic circulation as a paradoxical embolus. In an ASD there is a genuine tissue defect, so the shunt is continuous and left-to-right, which increases right-sided volume, delays pulmonic valve closure, and produces the wide fixed splitting of S2 that is fixed precisely because the shunt equalizes respiratory variation. So a cryptogenic stroke in a young patient after straining, with a normal cardiac examination, is the PFO stem, while a fixed split S2 is the ASD stem.

A neonate with trisomy 21 has a single large valve orifice between the atria and ventricles on echocardiography, with a defect in the lower interatrial septum and the upper interventricular septum. A second infant with trisomy 21 has a defect in the lower interatrial septum, mitral regurgitation, and an intact ventricular septum. Name each lesion and the single embryologic structure that failed.

Both are atrioventricular septal defects, from failed fusion of the endocardial cushions, and that one structure explains the whole picture. The endocardial cushions build three things: the lower (primum) part of the atrial septum, the membranous part of the interventricular septum, and the AV valves. That is why cushion failure never produces an isolated lesion. The first infant has a complete AVSD, with a common AV valve plus both a primum ASD and a VSD, so there is mixing at the atrial and ventricular levels. The second has a partial AVSD, meaning a primum ASD with a cleft mitral valve producing mitral regurgitation, and crucially no VSD. Note what makes the primum location itself informative: an ostium secundum ASD, the commonest ASD overall, is usually isolated and is a defect of septum primum resorption higher up, whereas a primum defect sits at the cushions and therefore drags valve disease along with it. Both are strongly associated with Down syndrome, and a cleft mitral valve in a trisomy 21 infant is essentially diagnostic of the partial form.

How it's tested

A VSD in the membranous septum is the most common congenital cardiac anomaly, which is the single most likely fact to be asked from this page. For AVSD the association with Down syndrome is the hook, and the partial form is recognised by a cleft mitral valve causing regurgitation with no VSD present. On the atrial side, the exam's favourite point is that a patent foramen ovale is a failure of fusion, not a hole, and that it lets a venous clot reach the systemic circulation as a paradoxical embolus.

Go deeper

First Aid 2026 — CV/Embryology (p.284-286) · B&B — Atrial & ventricular septation · Mehlman — HY Cardio (PFO = ASD on USMLE, paradoxical emboli, Down syndrome AVSD)

High-yield images3

Atrial septation in five steps. (1) Septum primum grows down toward the dorsal endocardial cushion, narrowing the ostium primum. (2) Ostium secundum opens in the septum primum by cell death as the ostium primum closes. (3) Septum secundum develops to the right of septum primum. (4) Septum secundum covers most of the ostium secundum, and the residual opening is the foramen ovale. (5) The remaining septum primum degenerates into the one-way valve that closes the foramen at birth.

Atrial septation seen in the heart itself, four panels. Septum primum (green) descends toward the endocardial cushions leaving the ostium primum; the ostium secundum then perforates it; septum secundum (blue) grows down to its right and overlaps the opening, leaving the foramen ovale as the residual channel between the two septa.

Ventricular septation in three steps. (1) The muscular interventricular septum grows up from the apex, leaving the interventricular foramen open beside the atrioventricular canals. (2) The aorticopulmonary septum descends. (3) It fuses with the muscular septum to form the membranous interventricular septum, closing the foramen; failure here is the membranous VSD.

PreviousHeart Tube, Looping & Outflow Tract Formation NextHeart Embryology Derivatives

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