Atrial Septal Defect

Overview
  • 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.
Morphology
  • Overview
    • Defects of atrial septation can occur anywhere along the developing inter-atrial septum. True atrial septal defects affect either the septum primum or secundum are discussed below. A few other conditions result in pathophysiology similar to that of ASDs are discussed below although the atrial septum may be completely normal in such conditions.
  • True ASDs
    • Septum Primum Defect: Results from defective fusion of the Septum Primum with the Endocardial Cushions and is usually accompanied by defects in the atrioventricular valves
    • Septum Secundum Defect: Can result from a variety of defects in Septum Secundum development but generally affects the central part of the septum in the area of the foramen ovale.
  • ASD-like Conditions
    • Patent Foramen Ovale: (See page)
    • Sinus Venosus Defect: Results from an inappropriate communication between the pulmonary veins and the right atrium
Clinical Consequences
  • Because of higher left-than-right atrial pressures, ASDs result in left-right shunts of varying severity given the size of the defect. Such shunts are of little clinical consequence early in life as the heart possess significant functional reserve to accommodate the additional volume entering the right heart. However, through mechanisms that are still poorly understood, chronic elevations in pulmonary blood flow lead to remodeling of the pulmonary vasculature and development of pulmonary hypertension. This often results in compensatory right concentric ventricular hypertrophy which reduced right ventricular compliance, thus requiring higher right atrial pressures to maintain sufficient right ventricular diastolic filling. Right atrial pressures can elevate to exceed that of the left atrium, resulting in reversal of the shunt between the two chambers and development of right-left shunts during adult life together with consequent hypoxemia and cyanosis. Consequently, ASDs not detected incidentally by the presence of a heart murmur can often manifest in adult life with symptomology of a right-left shunt.
  • FYI: ASDs result in two characteristic changes in the normal heart sounds which stem from increased blood flow through the right heart. The most characteristic audible change is wide and fixed splitting of the S2, regardless of the respiratory cycle. Secondly, in some cases a systolic heart murmur may be auscultated, reflecting increased flow across the pulmonic valve.