We report a patient with severe hypoxemia from a large (41%) right to left shunt through a patent foramen ovale after right ventricular myocardial infarction, and review 18 previous descriptions of patients with right to left shunting through patent foramen ovale. These shunts occur when right atrial pressure is elevated above left atrial pressure, or when the anatomic relationship of the interatrial septum to the inferior vena cava is altered. Since 15-35% of the population have a potentially patent foramen ovale, interatrial right to left shunting may occur more frequently than had previously been recognized, and should be considered in a differential diagnosis of hypoxemia.