An atrial septal defect (ASD) is an abnormality that occurs during the separation of the primitive atrium, leaving an unclosed interatrial foramen between the left and right atria.

A crescent-shaped thin wall called the septum primum is formed and divides the posterior and superior portions of the inner wall of the primitive atrium. It then grows downward toward the atrioventricular orifice and finally meets and merges with the central endocardial cushion. Until then, the single-lumen primitive atrium is initially separated into left and right atria. Before the septum primum completely merges with the endocardial cushion, the foramen separating them is called foramen primum or ostium primum. After the closure of the septum primum occurs, its roots are spontaneously absorbed into a perforation to maintain communication between the left and right atria; the perforation is called the foramen secundum or ostium secundum (Fig. 11.1). At the same time, the right side of the foramen secundum is covered by another septum, called the septum secundum, which grows from anterior to posterior. The septum secundum is crescent shaped with a depressed inferior border, which forms the edge of the FO. A certain interspace is kept between the septum primum and septum secundum and is called the foramen ovale (Fig. 11.2). During development, blood from the umbilical vein flows from the RA into the LA through the foramen ovale to provide blood supply for the fetus.

The septum primum is thin and comprises the floor of the FO, acting like a flap. The septum secundum is composed of thick muscular tissue and forms the anterior and superior border, also called the Vieussens ring, of the FO. If the septum primum fails to close, an ostium primum defect is formed (Fig. 11.3). In another case, if the foramen secundum is too large or left uncovered by the septum secundum, it is called ostium secundum defect (Fig. 11.3).

During the development of the atrial septum, the venous sinus and pulmonary veins are also in constant development and shifting. The venous sinus moves to the RA, dilates, and becomes part of it, forming the opening orifices of the SVC, the IVC, and the coronary sinus (Fig. 11.1) in the RA, respectively. The right border of the venous sinus forms the crista terminalis of the RA, the lower portion of which forms the Eustachian and the Thebesian valves. If shifting of the venous sinus is insufficient, then development of the atrial septum is affected and a sinus venosus ASD, also called a superior vena cava defect, is present and located at the posterosuperior portion of the atrial septum (Fig. 11.3).

The term simple ostium primum defect refers to a residual ASD anterior to the orifice of the vena coronaria without involvement of the atrioventricular valve or the fissure on the MV. The inferior border of the defect is the junction of the left and right atrioventricular rings, with its anterior close to the aortic wall and its posterior margin close to the atrioventricular node. A simple ostium primum defect is not commonly seen.

Simple left ventricle–right atrium shunt refers to a type of ASD located at the atrial part of the membranous ventricular septum, superior to the parietal septum of the triple valve, and inferior to the anterior of the MV. The defect is usually small in size, connected with the AV, and without abnormality of the atrioventricular valve. Because the atrioventricular valve is not involved but the LV and RA are, no consensus has been reached on the classification of this type of abnormality. In most cases of left ventricle-right atrium shunt ASD, direct suturing can be performed without a patch; however, care must be taken not to damage the AV.

First, the presence of an ASD should be clearly confirmed, which can be accomplished only when the MV can be touched by a finger reaching through the defect; otherwise, a pulmonary vein trunk that drains into the RA or an enlarged orifice of the coronary sinus can be mistaken for an ASD.

Both the size and type of an ASD should be confirmed. A defect that lies posteriorly and superiorly to the coronary sinus is an ostium secundum defect; an ostium primum defect lies anteriorly and inferiorly to the orifice of coronary sinus.

Whether there is anomalous pulmonary venous drainage should be confirmed, which is easily done when there is partial anomalous pulmonary venous drainage (PAPVD) but may be difficult when there is total anomalous pulmonary venous drainage (TAPVD). If the LA chamber is found to be too small by intracardiac exploration and no pulmonary vein orifice is identified, TAPVD should be considered.