Registered Cardiovascular Invasive Specialist (RCIS) Practice Test

What type of atrial septal defect (ASD) is located in the middle third of the atrial septum?

1. Ostium Primum

2. Ostium Secundum

3. Sinus Venosus

4. Patent Foramen Ovale

The correct answer is: Ostium Secundum

The type of atrial septal defect located in the middle third of the atrial septum is known as ostium secundum. This defect is characterized by an opening in the septum that occurs in the fossa ovalis area, which is centrally located in the atrial septum. The ostium secundum defect is often due to inadequate development of the septum primum and septum secundum during fetal development, leading to a hole that allows for the left and right atrial communication. Understanding the anatomical positioning of different types of atrial septal defects helps identify their clinical significance and potential impact on heart function. In contrast, ostium primum defects are found at the lower part of the septum, usually associated with atrioventricular septal defects, while sinus venosus defects are located near the entrance of the superior vena cava at the upper aspect of the septum. The patent foramen ovale, although it also involves the fossa ovalis, is a normal embryological structure that typically closes after birth and does not represent a defect in the same way the ostium secundum does when it remains open.