A patent foramen ovale (PFO) is the failure of a fetal shunt to close normally after birth. During fetal circulation, blood takes an alternative path, called fetal shunts, in order to bypass the not yet functioning lungs to circulate oxygen rich blood throughout the body. These fetal shunts are referred to as foramen ovale,ductus venosus and ductus arteriosus. In utero, these shunts allow fetal oxygenation to occur through the placenta and at birth will normally close, allowing the aerated lungs to take over(McCance & Huether, 2019). The placenta sends oxygenated blood through the umbilical vein towards the baby’s liver where it is diverted, via the ductus venosus, towards the inferior vena cava (IVC) and to the baby’s right atrium. From there, the blood is shunted, due to the high pressure system of the IVC, from the right atrium to the left atrium through the foramen ovale. This shunting effectively bypasses the lungs and the oxygenated blood circulates to the aorta and out into the systemic circulation. Any remaining blood that did not shunt through the foramen ovale travels to the pulmonary artery where the ductus arteriosus redirects it back into the systemic system(McCance & Huether, 2019). Once the oxygenated blood circulates the body, the deoxygenated blood circulates back to the placenta through the umbilical arteries where the blood is aerated and the process begins again.
At birth the lungs expand with respiration and the umbilical cord is clamped. Respiration moves to its primary post-natal state where gas exchange occurs in the lungs and no longer in the placenta (McCance & Huether, 2019). Now the pressure gradient has shifted so the left side of the heart is greater than the right side. This reversal of pressure gradients causes the valve flaps of the foramen ovale to close by adhering to the flaps of the atrial septum. This process usually achieves anatomic closure within the first month of life (McCance & Huether, 2019). Until complete anatomic closure is achieved, any condition that stimulates an increase in right sided pressure or causes dilation of the right atrium can reopen the foramen ovale and create an acyanotic shunt(McCance & Huether, 2019). Such conditions can include RV failure, pulmonary hypertension and tricuspid atresia(McCance & Huether, 2019). A foramen ovale that fails to close or remains open is referred to as a patent foramen ovale.
For the majority of people living with PFO, there is not a problem, despite the leakage of deoxygenated blood. Most infants with a PFO will be asymptomatic as the disorder is acyanotic and therefore harder to detect(McCance & Huether, 2019). However, problems can arise when blood containing blood clots travel through the PFO and to the brain, causing transient ischemic attacks or strokes (American Heart Association, 2017). Increased intrathoracic pressure, as in a valsalva maneuver, can increase atrial pressures and increase window size in a patent foramen ovale (Lee et. al 2018). A PFO is diagnostically assessed through imaging such as a transesophageal or transthoracic echocardiography.
Since there is no way to diagnose a PFO except through the use of an echocardiogram, many PFOs are diagnosed when a doctor is conducting tests for a different problem, such as a previous stroke or for atrial fibrillation (Chen, X., et. al 2018).