Patent Foramen Ovale
- As described in our discussion of Atrial Septal Defect, the embryonic foramen ovale is shut by closure of the Septum Primum flap against the foramen following birth. In most individuals, closure of the flap is followed by fusion of the tissues within six months, thus permanently sealing the foramen ovale and resulting in its anatomic remnant known as the Fossa Ovalis.
- In nearly 20% of individuals this fusion does not occur, leaving the foramen ovale patent during adult life. This is of little clinical significance in normal individuals whose higher left than right atrial pressures will keep the flap shut. However, in situations where right atrial pressure is elevated, the flap can re-open, allowing for an inappropriate right-left shunt of blood. Not only does this affect oxygenation of blood but it can allow an anatomical hole through which venous thrombi can bypass the pulmonary circulation and directly thromboemoblize into the systemic circulation, termed "Paradoxical Thromboembolism".