Table 4.13 - Types of Atrial Septal Defects.

Secundum type (or patent foramen ovale) ASD - results from abnormal resorption of the septum primum during formation of the foramen secundum; and/or large foramen ovale.
Endocardial cushion defect with primum type ASD - septum primum fails to fuse with the endocardial cushions, leaving patent foramen primum.
Sinus venosus type ASD - due to incomplete absorption of the sinus venosus into the RA; located high in the interatrial septum.
Common atrium - interatrial septum completely absent due to failure of septum primum and septum secundum to develop.