Patent foramen ovale (PFO) is a prevalent congenital septal atrial defect usually without pathological significance. In certain pathogenic situations, PFO can trigger episodes of recurrent hypoxemia, a specific condition known as platipnea-orthodeoxia syndrome (POS). We report a 73 years old female presenting with dyspnea and low arterial oxygen saturation. On admission the patient had a hemoglobin saturation of 81% and an arterial oxygen partial pressure of 50 mmHg. After breathing 100% a 17% arterial-venous shunt was found (normal: less than 12%). A transesophageal echocardiogram and a cardiac catheterization showed the presence of a PFO. A percutaneous closure was performed.