Figure 1. Illustration of Atrial and Atrioventricular Septal Defects.

In Figure 1A, a mature, properly septated heart is depicted. The septum primum (green) has fused to the septum secundum (red), resulting in closure of the ostium secundum while interaction of the DMP (yellow), septum primum and major endocardial cushions (blue) results in closure of the ostium primum (OP). In Figure 1B, a patent foramen ovale is illustrated. Here, although the septum primum has not fused to the septum secundum, under normal hemodynamic conditions the two remain in contact to functionally partition the atria. If right atrial pressure were to exceed that of the left atrium, however, right-to-left shunting would result. In Figure 1C, an ostium secundum defect (OSD) is depicted; the flap valve (green) of the ostium secundum is not of sufficient length to prevent atrial communication. Figure 1D depicts an incomplete AVSD, a malformation in which atrial shunting is freely permitted due to persistence of the ostium primum. Complete AVSDs (1E) are characterized by both atrial and ventricular shunting is permitted. RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle; DMP, dorsal mesenchymal protrusion; SP, septum primum; SS, septum secundum; OSD, ostium secundum defect; OPD, ostium primum defect; PFO, patent foramen ovale; VSD, ventricular septal defect