What is the significance of a PFO? Cardiology Basics

What is the significance of a PFO? Cardiology Basics

PFO is short form for patent foramen ovale. Usually foramen ovale closes soon after birth so that there is no communication between the two atria in most persons. But occasionally, a small opening may persist and then it is called PFO. PFO is a valvular opening through the interatrial septum.

When the pressure in the left atrium rises after birth as the lungs become functional and increases pulmonary venous return, it presses on the left side of the foramen ovale and closes it. Even if there is a residual opening, most of the time there is no shunting of blood across the PFO because left atrial pressure is higher than that in the right atrium.

But occasionally as in straining, the pressure in the right atrium transiently rises above that in the left atrium so that blood can pass transiently from the right atrium to the left atrium across the PFO.

Another situation is a stretched open PFO. When the pressure in the right or left atrium rises significantly due to another disease, the atrium enlarges and stretches the interatrial septum along with it. When the PFO is stretched open, blood can flow either way, depending on which side has the higher pressure. If it is the left atrium which is enlarged, the stretched open PFO will shunt blood in a left to right direction and vice versa. If too much of right atrial blood with lower oxygen saturation reaches the left atrium, systemic arterial oxygen saturation falls, causing cyanosis.

Small PFO cannot produce much problem due to the shunting of blood across it unless it is stretched open by other diseases. But small blood clots from lower limb veins or veins of the abdomen can occasionally pass across it to the left atrium. This is a risky situation. The clot can move from the left atrium to the left ventricle and then to the aorta. From the aorta the clot can move to any part of the blood circulation. This is known as paradoxical systemic embolism, originating from the venous side.

If the embolus gets lodged in a cerebral artery and blocks it, a stroke may occur. Most important problem with paradoxical systemic embolism is stroke, though it can also get lodged in any other artery of the body and cause damage to that region. Some advocate closure of the PFO with a device, especially after a stroke, to prevent recurrence. This is considered only if the right to left shunting across the PFO is demonstrated.

PFO can be documented by an echocardiogram. It will also show right to left shunting during certain types of strain, indicating the risk of paradoxical systemic embolism. The colour Doppler echocardiogram shown here shows a left to right shunt across the PFO, most likely due to higher pressure in the left atrium, as discussed earlier.

Another test is transcranial Doppler, an ultrasound study of the head which looks for tiny air bubbles in the cerebral arteries. For detecting a right to left shunt across the PFO, agitated saline containing tiny air bubbles is injected into a right forearm vein. If the air bubbles are detected by the transcranial Doppler machine, it is presumed that tiny blood clots can also pass across the PFO to produce a stroke. Right to left flow of tiny air bubbles in the agitated saline will be seen in the left atrium on echocardiogram. This technique is known as saline contrast echocardiography. Air bubbles appear as white dots while the blood filled regions are black on echocardiogram.