Platypnea-Orthodeoxia Syndrome Caused by a Patent Foramen Ovale and Interatrial Septal Aneurysm: A Case Report.

Platypnea-orthodeoxia syndrome (POS) is a rare condition characterized by positional hypoxemia, often due to right-to-left shunting across a patent foramen ovale (PFO) despite normal right-sided pressures. Early recognition is essential as symptoms may be subtle and imaging is often unrevealing. A 78-year-old female patient with a known PFO presented with one week of lower abdominal pain and mild exertional dyspnea without chest pain. She was profoundly hypoxic to 76% on room air, with improvement when supine and worsening when upright. Initial labs were unremarkable, and arterial blood gas revealed primary respiratory alkalosis with severe hypoxemia. Despite unremarkable chest radiography, CT angiography, and ECG, oxygen via high-flow nasal cannula at 100% FiO2 only minimally improved her SpO2, prompting consideration of right-to-left shunt physiology. Prior transthoracic echocardiograms had documented a small PFO without evidence of pulmonary hypertension or right-sided disease. Repeat echocardiogram demonstrated an atrial septal aneurysm and a positive bubble study. Transcatheter PFO closure was performed, and oxygen saturation improved immediately. She was weaned from the nasal cannula over three days. Post-procedural echocardiogram showed no residual shunt. This case highlights the importance of recognizing POS as a potential presentation of PFO-related right-to-left shunting, particularly when hypoxemia is refractory to supplemental oxygen and standard cardiopulmonary evaluation is unrevealing. Early closure can lead to dramatic clinical improvement.