Atrial Septal Defect (ASD) Lecture Notes

Overview

• The atrial septum separates the right and left atrium of the heart.
• Atrial Septal Defect (ASD): An opening in a baby's heart between the two upper chambers.

Development of the Septum

Septum Primum

• Early Development: Tissue called septum primum grows downward, creating two separate chambers.
• Closure of Ostium Primum: The septum primum fuses with the endocardial cushion, closing the first gap (ostium primum).

Development of Ostium Secundum

• A hole appears in the upper area called ostium secundum.

Septum Secundum

• Grows downward just to the right of septum primum.
• Covers the ostium secundum like a curtain, leaving a small opening called the foramen ovale.
• Creates a makeshift valve that allows blood flow from the right atrium to the left atrium, but not the other way.

Blood Flow in Developing Fetus

• Oxygenated Blood Source: From the placenta.
• Pathway: Umbilical vein → right atrium → foramen ovale → left atrium → left ventricle → body.
• Avoids right ventricle and lungs.

Birth Changes

• Closure at Birth: Septum secundum and septum primum close the foramen ovale, transitioning to lung-based oxygenation.

Types and Causes of ASD

• General Cause: Septum between right and left atrium doesn’t close completely post-birth.
• Prevalence: Accounts for 10-15% of all congenital heart defects; most common in adults.

Ostium Secundum (90% of ASD cases)

• Cause: Septum secundum doesn’t fuse with septum primum, leaving foramen ovale open.

Ostium Primum (10% of ASD cases)

• Cause: Ostium primum doesn’t make it all the way down, leaving an opening.
• Association: Found in 25% of patients with Down syndrome and often with fetal alcohol syndrome.

Hemodynamic Consequences

• Pressure Difference: Higher pressure on left side shunts blood to right atrium.
• Effect: More oxygenated blood enters usually deoxygenated right side.
• Classification: Acyanotic heart defect (not blue) due to extra travel of oxygenated blood.
• Oxygen Saturation: Increased in right atrium, right ventricle, and pulmonary artery.
• Auditory Indicators: Splitting of S2 sound and possible systolic murmur.

Complications

Embolism Risk

• Example: Deep vein thrombosis leading to blood clot (embolus).
• Typical Path: Right atrium → right ventricle → lungs.
• Paradoxical Embolism: Embolus crosses ASD, enters systemic circulation, may reach brain.

Management and Treatment

• Monitoring: Some ASDs may close on their own, especially smaller openings.
• Surgery: Necessary for larger openings to prevent future complications (involves plugging or patching).