In this report we describe the case of a 37-year-old white woman with Ebstein's anomaly, who developed a rare syndrome called platypnea-orthodeoxia, characterized by massive right-to-left interatrial shunting with transient profound hypoxia and cyanosis. This shunt of blood via a patent foramen ovale occurred in the presence of a normal pulmonary artery pressure, and was probably precipitated by a propafenone overdose. This drug caused biventricular dysfunction, due to its negative inotropic effect, and hypotension, due to its peripheral vasodilatory effect. These effects gave rise to an increase in the right atrial pressure and a decrease in the left one with a consequent stretching of the foramen ovale and the creation of massive right-to-left shunting. In our case this interatrial shunt was very accurately detected at bubble contrast echocardiography.