Interatrial Septum — Fossa Ovalis and Triangle of Koch
The interatrial septum forms the medial wall of the right atrium. It is derived developmentally from the septum primum and septum secundum.
Features of the Interatrial Septum (as seen from the right atrium)
1. Fossa Ovalis
- A shallow, oval/saucer-shaped depression in the lower part of the interatrial septum
- Represents the position of the septum primum (the original foramen ovale flap)
- The floor of the fossa ovalis is the thinnest part of the interatrial septum
2. Annulus Ovalis (Limbus Fossa Ovalis)
- The prominent, raised margin surrounding the fossa ovalis — distinct above and on the sides, deficient inferiorly
- Represents the free edge of the septum secundum
- Its anterior edge is continuous with the left end of the valve of the IVC (Eustachian valve)
- Also called limbus fossa ovalis
3. Patent Foramen Ovale (PFO)
- Normally, the foramen ovale closes at birth (when left atrial pressure exceeds right atrial pressure, pressing the septum primum against secundum)
- In ~25% of adults, a slit-like valvular opening persists between the upper part of the fossa ovalis and the limbus — the patent foramen ovale
- Usually functionally closed (pressure-sealed) but may open under raised right atrial pressure
- Clinical significance: PFO is a route for paradoxical embolism (venous thrombus crossing to the systemic circulation → stroke) and is associated with cryptogenic stroke
4. Triangle of Koch
A triangular area on the interatrial septum, immediately anterior to the coronary sinus opening. The AV node lies within this triangle.
Boundaries:
| Boundary | Structure |
|---|---|
| Anterior (base) | Septal leaflet (cusp) of the tricuspid valve |
| Posterior | Anterior margin of the opening of the coronary sinus |
| Superior (apex) | Tendon of Todaro — a subendocardial fibrous ridge running from the valve of the IVC to the central fibrous body |
The AV node lies at the apex of the Triangle of Koch, just anterior to the coronary sinus opening and above the tricuspid annulus.
Clinical significance: The Triangle of Koch is the key anatomical landmark for:
- Radiofrequency ablation of the AV node (for arrhythmias)
- Catheter-based procedures targeting the AV conduction system
- Avoiding inadvertent AV block during tricuspid valve surgery
5. Torus Aorticus
- An elevation in the anterosuperior part of the interatrial septum
- Produced by the bulging of the right posterior (non-coronary) sinus of the ascending aorta into the septal wall of the right atrium
Comparison: Right Atrium vs Left Atrium
| Feature | Right Atrium | Left Atrium |
|---|---|---|
| Receives blood from | SVC, IVC, coronary sinus (whole body) | 4 pulmonary veins (from lungs) |
| Pumps blood to | Right ventricle through tricuspid valve | Left ventricle through mitral valve |
| Cardiac contributions | Right border, part sternocostal surface, small part of base | Major part of base; part of upper border |
| Enlarged in | Tricuspid stenosis | Mitral stenosis |
| Septal landmark | Fossa ovalis (concave) | Fossa lunata (corresponding area, convex) |