A patent foramen ovale (PFO) is a type of atrial septal defect (ASD), or opening between the two upper chambers of the heart (left and right atria). "Patent" means "open", and "foramen ovale" means "oval window", named after the shape of the hole. 

All babies have a PFO before birth; in fact it is a very important part of the fetal blood flow.  The PFO usually closes within the first few days, weeks or months after birth, but in many people a very small PFO remains for years.  In about one-in-eight people it persists throughout their entire lives.  Unless a PFO us unusually large it is considered just part of the normal variation between individuals; just as some people have freckles and others do not, some people have a tiny PFO and others do not.  As long as it is not unusually large it is not considered a malformation or an abnormality.  A small PFO is very commonly noted as an incidental (normal) finding on an echocardiogram.

In some individuals the patent foramen ovale may be larger than usual, and this can result in some of the problems that accompany other types of atrial septal defects (see ASDs).  In occasional cases, these large PFOs may require closure with surgery or a special device placed during a cardiac catheterization.

Also, in rare situations even a small PFO can be a problem.  In a very small number of people, blood clots that form in the veins of the body can break loose in the circulation and pass through a PFO from the right to the left side of the heart to cause strokes, heart attacks or other problems related to blood clots.  This situation, termed a "paradoxical embolus", is extremely rare largely because the natural flow of blood through a PFO is from the left to the right side of the heart, making if very difficult for clots to pass "against the current" from the right to the left side.  This risk of this complication is very tiny in most people, and is not felt to outweigh the risk of surgical or catheter-directed closure of a patent foramen ovale in the vast majority of cases.  However, in a selected group of people who are felt to have had strokes or other problems related paradoxical emboli, or who are very prone to blood clots, it may be reasonable to consider closure of even a small PFO.  Even this recommendation remains controversial, however.

Patients with small PFOs of the usual type do not require any special precautions or limitations.  They do NOT require SBE prophylaxis (for example, antibiotics at the dentist).

For more information about PFOs, try some of the web links below. I cannot vouch for the contents, but with the Internet it's always a case of "browser beware".

American Heart Association 
A good site to see a brief description of the different types of heart disease and the treatments available

Boston Children's Hospital
Overview of atrial septal defects
Role of the PFO in the fetal circulation