Foramen ovale (changes after birth)

The foramen ovale closes after birth. Initially, this is due to a functional equalization of pressures within the atria that apposes the septum primum and the septum secundum. Equalization of pressure occurs because of:

• lung inflation leading to reduced pulmonary vascular resistance
  • foramen ovale is an interatrial communication that permits blood from the inferior vena cava to freely enter the left atrium in utero (right-to-left shunt)
  • at birth, the left atrial pressure exceeds the right atrial pressure (thus reversing the right-to-left shunt of the fetal circulation)
    • this forces the septum primum against the septum secundum, functionally closing the foramen ovale
  • the duct closes by a prostaglandin dependent mechanism, the signal for closure being a rise in circulating oxygen levels
  • eventally the septa eventually fuse and leave a remnant of the foramen ovale, the fossa ovalis

Functional closure can be reversed during the immediate neonatal period; this occurs during bouts of crying when cyanosis may be seen and there is a re-emergent right-to-left shunt.

Anatomical fusion of the septum primum and secundum occurs by one year of age in 80% of the population. Fibrosis results in the fossa ovalis of the interatrial septum. The remaining 20% have a foramen which is only potentially patent. The normal pressures within the atria ensure that it remains closed; the septum primum acts as a flap-valve against the septum secundum due to the greater pressures within the left atrium compared to the right atrium.