Four types of ASDs or interatrial communications exist: ostium primum, ostium secundum, sinus venosus, and coronary sinus defects (Fig. 65-5). (Ostium primum is discussed in the section on AV septal defect.) Ostium secundum defects occur from either excessive resorption of the septum primum or deficient growth of the septum secundum and are occasionally associated with anomalous pulmonary venous connection (<10%). Sinus venosus defects of the superior vena cava type occur at the cardiac junction of the superior vena cava, giving rise to a superior vena cava connected to both atria, and are almost always associated with anomalous pulmonary venous connection (right ≫ left). Sinus venosus–inferior vena cava defects are very uncommon and abut the junction of the inferior vena cava, inferior to the fossa ovalis. Coronary sinus septal defects are rare and arise from an opening of its wall with the left atrium, allowing …