Platypnea-Orthodeoxia Syndrome Presenting With Secondary Erythrocytosis.

Platypnea-orthodeoxia syndrome (POS) is a rare condition characterized by positional dyspnea and hypoxemia that may cause secondary erythrocytosis through chronic hypoxic stimulation. We describe a 71-year-old woman with an 80-pack-year smoking history who presented with persistent erythrocytosis and progressive positional dyspnea. Workup revealed an atrial septal defect with bidirectional shunting and normal erythropoietin, consistent with secondary erythrocytosis from suspected POS rather than polycythemia vera. In patients with unexplained erythrocytosis and positional dyspnea, evaluation for intracardiac shunting should be considered. Percutaneous closure is an effective treatment when the etiology is cardiac.