Author: Jared Scott, M.D. Educational Pearls The foramen ovale (FO) connects the left and right atria to allow oxygenated blood to bypass the developing lungs, it usually closes at birth but for some it remains patent (PFO). A PFO allows clots to cross from the venous to arterial circulation, increasing the likelihood of stroke. PFO is present in 25% of general population, present in 50% of those with stroke of unknown cause, and very common those with stroke under 50 years old. Treat with anticoagulation or surgical correction. References: http://www.heart.org/HEARTORG/Conditions/More/CardiovascularConditionsofChildhood/Patent-Foramen-Ovale-PFO_UCM_469590_Article.jsp#.WarsZZN95E4