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Sleep related hypoxemia carries a broad differential diagnosis. Right-to-left shunting is a known cause of hypoxemia that is not correctable with supplemental oxygen. A patent foramen ovale (PFO) is an intra-cardiac shunt that results in hypoxemia and in rare instances can lead to platypnea-orthodeoxia syndrome (POS). POS is characterized by dyspnea and hypoxemia in the upright position, which improves when supine. We present a case of nocturnal hypoxemia and PFO with a unique clinical presentation consistent with reverse POS discovered on PSG. Report of Case 57-year-old male with Class I obesity and a remote diagnosis of mild sleep-disordered breathing not treated with CPAP was referred to Sleep Medicine for excessive daytime sleepiness, snoring, and witnessed apneas. Polysomnography revealed basal oxygen saturation of ~88% during sleep. There was no evidence of apnea or hypopnea. Hypoxemia was not correctable with supplemental oxygen, suggestive of shunt physiology. Oxygen saturation was normal during upright exercise. Pulmonary hypertension was ruled out. Transesophageal echocardiogram revealed PFO with right-to-left shunt. Overall presentation was consistent with reverse POS secondary to PFO, which will be treated with percutaneous trans-catheter closure. Conclusion This is a rare case of reverse POS secondary to PFO, with shunt physiology initially suspected during Sleep Medicine evaluation based on PSG. Reverse POS is characterized by dyspnea and hypoxemia while supine, as opposed to upright as in classic POS. The precise pathophysiology of reverse POS is unclear. This case emphasizes the need to consider reverse POS in patients with supine hypoxemia refractory to oxygen therapy. Such findings on PSG should prompt further workup for causes of right-to-left shunt. This diagnosis should not be overlooked, as the underlying abnormality is often correctable. In this patient, we expect hypoxemia to resolve with percutaneous trans-catheter closure of PFO.

1249 Reverse platypnoea-orthodeoxia syndrome: A rare cause of hypoxemia during in lab polysomnography