patent foramen ovale. Failure of anatomic fusion of the valve of the foramen ovale with the limbus of the fossa ovalis that normally occurs when left atrial pressure exceeds right atrial pressure after birth. There is no structural deficiency of tissue of the atrial septum. The foramen is functionally closed as long as left atrial pressure exceeds right atrial pressure, but can reopen if right atrial pressure rises. PFO is found in up to 35% of the adult population in pathological studies. The lower and variable prevalence reported in clinical series depends on the techniques used to find it. syn: probe-patent foramen ovale, PFO.