Patent foramen ovale

PFO is idiopathic.

The middle segment of the atrial septum is called the fossa ovalis. Often, there is a slit-like opening between the two parts of the septum called a PFO. In one autopsy series, PFOs were reported of 0.2 to 0.5 cm in 29% of patients and 0.6 to 1.0 cm in 6% of patients.[6]Thompson T, Evans W. Paradoxical embolism. Q J Med. 1930 Jan 1;23(90):135-52. During normal fetal development, the septum primum membrane develops from the top of the atrium downward to form part of the early interatrial septum separating the right and left atria. In PFO, this membrane is incomplete and is called septum primum atrial septal defect (ASD). A septum secundum occurs when another membrane forms from the bottom of the atrium upward and to the right of the septum primum. The two septa overlap and form an aberrant atrial septum, dividing the atria in half.

A PFO is a flap-like communication between the two atria. Because the septum secundum is to the right of the septum primum, there is no, or very little, left-to-right shunting. Because the pressure in both atria is generally low, there is very little shunting and the flap may remain closed. However, when the right atrial pressure rises transiently (e.g., during straining, Valsalva maneuver or coughing) this flap separates and allows blood to travel from the right atrium to the left atrium. This type of shunting may become more marked in cases where the right atrial pressure is elevated due to pathology such as pneumonia or pulmonary embolism. Shunting across a PFO may exacerbate hypoxemia at high altitude.[10]Levine BD, Grayburn PA, Voyles WF, et al. Intracardiac shunting across a patent foramen ovale may exacerbate hypoxemia in high-altitude pulmonary edema. Ann Intern Med. 1991 Apr 1;114(7):569-70. http://www.ncbi.nlm.nih.gov/pubmed/2001089?tool=bestpractice.com The size of the PFO also determines the amount of shunting because large defects will increase the allowed flow.[11]Kerut EK, Norfleet WT, Plotnick GD, et al. Patent foramen ovale: a review of associated conditions and the impact of physiological size. J Am Coll Cardiol. 2001 Sep;38(3):613-23. http://www.ncbi.nlm.nih.gov/pubmed/11527606?tool=bestpractice.com

The degree of blood shunting is not enough to be hemodynamically significant except under rare circumstances. In patients on mechanical ventilators, such aberrant flow may cause persistent hypoxemia. If a thrombus such as a deep vein thrombosis was to form in the legs and flow to the right atrium, it may travel through the PFO and enter the systemic circulation, causing a stroke or a peripheral arterial embolism.

Small versus large PFO[1]Homma S, Sacco RL, Di Tullio MR, et al. Effect of medical treatment in stroke patients with patent foramen ovale: Patent Foramen Ovale in Cryptogenic Stroke Study. Circulation. 2002 Jun 4;105(22):2625-31. http://circ.ahajournals.org/content/105/22/2625.full http://www.ncbi.nlm.nih.gov/pubmed/12045168?tool=bestpractice.com

• Small PFO: <10 microbubbles crossing into the left atrium after intravenous injection.

• Large PFO: >10 microbubbles crossing into the left atrium after intravenous injection. Bubbles appear in the left atrium within three cardiac cycles of injection to exclude the possibility of recirculation through a pulmonary arteriovenous malformation.

Septum primum ASD versus septum secundum ASD

• Septum primum ASD: an incomplete membrane formed from the top of the atrium downward separating the right and left atria.

• Septum secundum ASD: forms from the bottom of the atrium upward and to the right of the septum primum. The septa overlap and form an aberrant atrial septum, dividing the atria in half.