what is a PFO
1 A patent foramen ovale (PFO) is an opening covered by a flap of tissue in the wall of the heart that separates the two upper chambers or atria. 2  PFOs are very common, more than 20% of all humans have one.3  Estimates range from 20-30% depending on the decade of life. A patent foramen ovale (PFO) is an opening covered by a flap of tissue in the wall of the heart that separates the two upper chambers or atria. 2  PFOs are very common, more than 20% of all humans have one. 3  Estimates range from 20-30% depending on the decade of life.