C43-48 Unraveling the Hidden Shunt of Platypnea-Orthodeoxia Syndrome

Abstract Introduction Platypnea Orthodeoxia Syndrome (POS) is uncommon and often underdiagnosed. This condition presents with posture-dependent dyspnea (platypnea) and oxygen desaturation (orthodeoxia) that occur in the upright position and improve when supine. A high index of suspicion is essential for accurate diagnosis. Echocardiographic imaging, particularly with positional studies, plays an important role in identifying an underlying intracardiac shunt. Case Presentation A 79-year-old man with obstructive sleep apnea, hypertension, hyperlipidemia, coronary artery disease, paroxysmal atrial fibrillation on Xarelto presented with acute on chronic groin pain. During evaluation, he was incidentally found to be hypoxemic with oxygen saturations in the 80s despite a normal chest radiograph. His initial arterial blood gas (ABG) on 15 L of oxygen revealed pH 7.43, pCO2 40 mmHg, and PaO2 56 mmHg. Infectious and pulmonary evaluations were unremarkable. He developed recurrent desaturation when upright that improved when supine. His initial transthoracic echocardiography (TTE) was technically limited without a bubble study and unrevealing. A nuclear lung perfusion scan showed no evidence of pulmonary embolism or shunt. Serial ABGs confirmed positional hypoxemia: when upright on room air, pH 7.42, PaO2 71 mmHg with oxygen saturation 93%. When supine, pH 7.42 and PaO2 110 mmHg with oxygen saturation 98%. Right heart catheterization (RHC) revealed normal filling pressures, pulmonary pressures, and Qp:Qs 1.0 without shunt in the supine position. Transesophageal echocardiography (TEE) demonstrated a patent foramen ovale (PFO) with a bidirectional shunt on color Doppler and early positive saline contrast, associated with an atrial septal aneurysm and septal bowing into the left atrium. These findings established the diagnosis of POS secondary to a PFO with positional right-to-left shunting. Discussion POS is an underrecognized cause of positional hypoxemia characterized by right-to-left shunting that occurs when upright and resolves when supine. It most often results from an anatomic defect that creates an interatrial communication such as a patent foramen ovale. Functional or acquired conditions such as aortic dilatation can also be factors that contribute to shunt development. While a RHC may aid in the diagnosis, it can also yield false negative results because when performed in the supine position it may normalize atrial pressure and fail to identify a shunt, as in our case. A TTE can also be nondiagnostic. When clinical suspicion for POS remains high, a TEE with positional and provocative maneuvers should be pursued. In the absence of intracardiac communication, intrapulmonary causes should be evaluated using additional imaging modalities. This abstract is funded by: None