Atrial Septal Defect (ASD) USMLE Step 1 Pathophysiology

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Atrial Septal Defect (ASD) - USMLE Step 1 CVS Physiology , Dr G Bhanu Prakash
Atrial Septal Defect

Overview
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Atrial Septal Defects (ASDs) arise from defective closure of the inter-atrial septum and allow for an inappropriate right-left shunt of blood following birth. ASDs are the most common subtype of congenital heart disease.
Normal Development

Overview

The right and left atria emerge from a single chamber which is septated between the fourth and sixth weeks of development. Atrial septation develops in such a way as to allow significant shunting of blood from the emerging right atrium to the emerging left atrium, a physiological priority during fetal life. However, following birth this shunt must be rapidly ablated; consequently, the inter-atrial septum develops in such a way as to provide a "flap-like" system that can be shut quickly following delivery.
Septum Primum Development (First Wall)
The first septum to divide the common atrial chamber grows, ridge-like, from the superior aspect of the atrium downward and eventually fuses with the Endocardial Cushions which defines the inferior aspect of the chamber. The hole defined by the advancing edge of the Septum Primum and the Endocardial Cushions is known as the first hole (Ostium Primum). As the Septum Primum fuses to the Endocardial Cushions (Destroying the Ostium Primum), a second hole is formed in the middle of the Septum Primum, known as the Ostium Secundum (Second Hole), which continues to allow right-to-left shunting of blood.
Septum Secundum Development (Second Wall)
A second wall, known as the Septum Secundum, begins to develop directly to the right of the maturing Septum Primum. As the Septum Secundum develops, a hole is formed in its center known as the foramen ovale which continues to allow right-to-left shunting of blood. Because the superior aspect of the Ostium Primum eventually regresses, the Ostium Primum begins to act as a flap against the foramen ovale and in doing so allows only unidirectional movement of blood from the right to the left atrium.
Delivery
During fetal life the right atrial pressure is slightly higher than the left atrial pressure, allowing blood to shunt from the right to the left atrium and thus bypass the developing fetal lung. Following delivery, the atrial pressures switch, causing the flap-like Ostium Primum to shut the foramen ovale, limit shunting of blood between the two atria, and thus promoting blood to enter the now mature fetal lungs.

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