1. heart septal defects, atrial Developmental abnormalities in any portion of the ATRIAL SEPTUM resulting in abnormal communications between the two upperchambers of the heart. Classification of atrial septal defects is based on location of the communication and types of incomplete fusion of atrial septa with the ENDOCARDIAL CUSHIONS in the fetal heart. They include ostium primum, ostium secundum, sinus venosus, and coronary sinus defects.