Figure 2: A. Opened right atrium showing two mechanisms for ASD2. Most frequently the defect (a) is between the superior border of septum primum (SP) and septum secundum (SS). The defect (b) can also be due to a hole in septum primum. The divided orifice of the inferior vena cava (IVC) is seen below the fossa ovalis and the orifice of the superior vena cava (SVC) above and rightward. The AV canal septum (*) is intact and the muscular floor of the atrial septum (ο) is well - formed. B - The opened left atrium in the same heart showing both types of ASD2 as in A. CS coronary sinus; MV mitral valve; TV tricuspid valve.