Case Presentation: A 65 year old lady with recent 70 pound weight gain presented with severe hypoxemia not corrected with supplemental oxygen. She complained of severe shortness of breath on sitting upright which resolved completely on lying flat. Pulse oximetry corroborated her complaints, with O2 saturation 80% in supine position and 97% when supine. Blood pressure was 110/70 in both supine and upright positions. Other clinical examination including cardiovascular examination was within normal limits. CT was negative for PE or lung parenchymal disease. Transthoracic echocardiogram was unrevealing due to extremely poor windows. Liver ultrasound showed no cirrhosis. Right heart catheterization showed normal right atrial and pulmonary pressures and no evidence of intracardiac shunting while supine. Supine transesophageal echocardiogram showed large atrial septal aneurysm and secundum atrial septal defect with bidirectional shunt. Sitting up, she desaturated to 80% and the ASA bulged continuously to the left, resulting in marked right to left shunt with saline contrast. She underwent successful transcatheter atrial septal defect closure with Amplatzer septal occluder device with complete resolution of symptoms and hypoxemia.

Discussion: Our patient demonstrated hypoxemia and dyspnea while standing and sitting up, both relieved by recumbency, pathognomonic for platypnea orthodeoxia syndrome (POS). Hypoxemia in the hospital setting and prompts an expeditious workup. Clues like presence of platypnea and orthodeoxia helped narrow the differential diagnosis and guided appropriate testing and intervention in our patient, with complete resolution of symptoms and hypoxemia. Differential diagnosis for POS includes either interatrial shunting either from a patent foramen ovale or atrial septal defect or intrapulmonary shunting as seen in hepatopulmonary syndrome associated with liver cirrhosis, pneumonectomy, pulmonary embolism or emphysema. Obesity or diaphragmatic paralysis are known precipitants of POS in presence of an interatrial communication.

Conclusions: POS describes a rare condition of dyspnea and hypoxemia induced by upright posture that resolves when supine. Two conditions must coexist: an interatrial communication and a functional component that streams shunt flow in the upright position. POS is a rare cause of dyspnea requiring a high index of suspicion, POS should be considered in patients with positional dyspnea and refractory hypoxemia. Due to the positional nature of POS, it is imperative to evaluate shunting in both supine and seated positions.