Closure of foramen ovale occurs immediately after the birth due to the decrease in resistance of pulmonary circulation with the first breath of infant. This will increase blood flow to left atrium resulting in high left atrial pressure. This high pressure will close the foramen ovale.
PFO is a condition in which septum primum and septum secundum fail to fuse after birth. In the beginning, this will lead to a left to right flow of blood. It is an acyanotic condition at the time of birth because a small amount of oxygenated blood from the left atrium will flow back to the right atrium due to the pressure gradient. This oxygenated blood will re-enter pulmonary circulation.
Over the long period, volume and pressure overload in the pulmonary artery will cause hypertrophy and hyperplasia (pulmonary stenosis like situation) of the pulmonary artery. This will cause an increase in pressure on the right side of the heart resulting in a flow of deoxygenated blood from right atrium to left atrium via foramen ovale. This reversal of blood flow is called as eissenmenger syndrome. Thus, the main complication that can develop due to patent foramen ovale is an increased risk of paradoxical emboli.
Diagnosis: transesophageal echocardiography is an investigation of choice in adults with suspected paradoxical emboli.
Paradoxical emboli triad: raised right atrial pressure, the venous source of thrombosis, and the presence of PFO.
Management: most PFO are left untreated because they usually fuse off later in the life.
Indication for the closure of PFO: any PFO > 25mm is an indication for surgical closure. Another indication is when a patient develops paradoxical emboli. Give aspirin (in low-risk population) or aspirin + warfarin (in high-risk population) followed by closure of PFO (percutaneous closure or surgically).
Adapted from: Fundamentals of Cardiology: Concise Review for USMLE and General Medical Boards