Most common adult congenital lesion.

More common in women.

Classical Types

1. Ostium Primum – 15%; assoc with Mitral regurg, cleft anterior mitral leaflet. Large.
2. Ostium Secondum – 65%; assoc with Mitral regurg in elderly; isolated
3. Sinus venosus – 15%; superior (right PV and SVC mixing; common) or inferior (rare) type. assoc with p/tAPVC which may be Superior or Inferior (Scimitar syndrome)
4. Coronary sinus septal defect – rare; look for complex lesions
5. Vestibular defect

Mechanism and findings

• L-R shunt causes RA + RV dilatation –> SVT, fatigue, exercise intolerance, AF.
• L-R shunt worsens with age and LV compliance worsens –> HTN, MR, LV disease
• If RV HTN, then R-L shunt –> hypoxaemia, cyanosis, paradoxical embolus, reversible Pulm HTN or irreversible pulm obstruction
• Examination – S2 wide + fixed split; soft systolic pulmonary flow; precordial lift.
• ECG – RBBB; Left deviation in primum. Right deviation in sinus venosus and secundum.

Modern classification

If classification were to be based on embryological origin, then ASD is a defect in part of the atrial wall that directly separates the two atria. This means only those defects in the fossa ovalis+inferior rim are ASDs, and it has to be within the confines of its muscular rim. Hence only ‘Secondum’ ASDs are true ASDs. Other types of classical ASDs are based on their location in relation to the fossa ovalis, e.g. posterior to FO is SV-ASD.

In SV-ASD, the interatrial communication is due to the SVC or IVC entrances overriding both atria. The fossa within iteself may have a deficiency, but the rims must stay intact for it to be SV-ASD. In the superior  type, there is a defect in the wall as the SVC enteres the RA, with the right (superior) pulmonary veins that drain into the LA.

Septum secondum is the deep infolding of the parietal walls of the atria.