SEPTAL DEFECTS

Understanding the anatomy of septal defects is greatly facilitated if the heart is thought of as having three distinct septal structures: the atrial septum, the atrioventricular septum, and the ventricular septum (Figure 7.1). The normal atrial septum is relatively small. It is made up, for the most part, by the floor of the oval fossa. When viewed from the right atrial aspect, the fossa has a floor, surrounded by rims. The floor is derived from the primary atrial septum, or septum primum. Although often considered to represent a secondary septum, or septum secundum, the larger parts of the rims, specifically the superior, anterosuperior, and posterior components, are formed by infoldings of the adjacent right and left atrial walls. Inferoanteriorly, in contrast, the rim of the fossa is a true muscular septum (Figure 7.2). This part of the rim is contiguous with the atrioventricular septum, which is the superior component of the fibrous membranous septum. In the normal heart, this fibrous septum is also contiguous with the atrial wall of the triangle of Koch (Figure 7.3). In the past, we considered this component of the atrial wall, which overlaps the upper part of the ventricular musculature between the attachments of the leaflets of the tricuspid and mitral valves, as the muscular atrioventricular septum. As we discussed in Chapter 2, we now know that it is better viewed as a sandwich. This is because, throughout the floor of the triangle of Koch, the fibroadipose tissue of the inferior atrioventricular groove separates the layers of atrial and ventricular myocardium (Figure 7.4). From the stance of understanding septal defects, nonetheless, it is helpful to consider the entire area comprising the fibrous septum and the muscular sandwich as an atrioventricular separating structure, as it is absent in the hearts we describe as having atrioventricular septal defects.