Atrial Septal Defects and The Fetal Heart
Identifying the Five Types of ASDs
There are five primary types of atrial septal defects (ASDs):
Ostium Primum ASD – a defect located near the lower part of the atrial septum, often associated with abnormalities in the atrioventricular valves.
Ostium Secundum ASD – the most common type, occurring in the middle portion of the atrial septum.
Sinus Venosus ASD – an uncommon defect located near the superior vena cava or inferior vena cava, often linked with abnormal pulmonary vein connections.
Unroofed Coronary Sinus – a rare defect where part of the coronary sinus, which drains venous blood from the heart muscle, is missing, leading to a direct connection between the left atrium and the venous system.
Patent Foramen Ovale (PFO) – a small, flap-like opening between the atria that typically closes after birth but remains open in some individuals.
During fetal heart development, the heart evolves from a simple tubular structure into a complex organ with distinct chambers. A crucial part of this transformation is the formation of the atrial septum, which separates the right and left atria. Incomplete or abnormal development of this septum can lead to ASDs, the third most common congenital heart defect (Baruteau and Qureshi). They occur in approximately 1.6 per 1000 live births and account for about 10-15% of all congenital heart defects (Satou). The result is abnormal shunting of blood between the atria, causing oxygen-rich blood from the left atrium to mix with oxygen-poor blood in the right atrium.
While an unroofed coronary sinus and PFO may not be true ASDs, for the purpose of this lesson they have been included as abnormal communications between the right and left atria. Upgrade below to view this lesson and our entire library of other educational articles.