Case Presentation: A 75 yo non-ambulatory woman with history of OSA, CVA, Atrial Fibrillation presented for urologic surgery, however was found to be dyspneic and hypoxic to spO2 84% on room air. These symptoms appeared to resolve when lying down, yet instantly returned when upright and often required non-rebreather (NRB) mask to stabilize. On presentation, vitals were BP 147/96, HR 98, RR 19, oxygen saturation 86% initially on room air, improved to 93% on 3L NC. Labs showed a Hgb 15.8, ABG pH 7.33, paCO2 41, paO2 106, Methemoglobin 0.4 on 3L NC with an A-a gradient of 70.9mmHg. CXR and high-resolution chest CT were negative for any signs of acute or chronic pulmonary disease.Initial echocardiography showed mild right ventricle dilation, left atrial dilation, and mobile interatrial septum. Repeat echo with agitated saline demonstrated an interatrial shunt, with moderate to severe early right to left shunting. Shunt fraction was measured to be 31.75%, substantially higher than normal 5%. Patient was observed to have multiple profound hypoxic episodes when sitting up, requiring non-rebreather (NRB) to recover. This hypoxia improved to >92% on room air air when the patient laid down. After extensive discussion between pulmonology and cardiology, patient underwent percutaneous closure of patent foramen ovale (PFO) for treatment of platypnea-orthodeoxia syndrome (POS). She was discharged on 1L NC, and one year post-procedure was documented to be 99% on room air.

Discussion: This patient demonstrated the classic clinical signs of POS with seated hypoxia that improved when the patient was supine. While PFO is a relatively common occurrence in adults (20-34%), POS is a rare complication of PFO, and few case reports exist surrounding the outcomes of patients where POS can be attributed to PFO. POS occurs when the venous return and right heart pressures are increased in a seated position, causing PFO to open and interatrial shunting to occur. In supine position, right heart pressures decrease, allowing the PFO to close and shunting to resolve. Rarely are PFOs closed to treat POS, and is only considered after an exhaustive investigation of pulmonary etiologies has been done. A shunt fraction measures the degree to which the shunt from known uncorrected PFO is impacting blood oxygen content. This is calculated by obtaining an ABG on room air, then administering 100% FiO2 via NRB for 20 minutes and then repeating an ABG. A shunt fraction greater than 5% indicates a high likelihood of an anatomical shunt. Furthermore, findings on echo should raise attention to a possible atrial septal defect, as mobile interatrial septum seen on echo predicts larger shunts across the PFO. It is likely that this patient’s undiagnosed OSA may have increased her likelihood of developing POS as OSA increases the comorbid occurrence of right to left shunts and is associated with increased risk of cryptogenic strokes, as seen in this patient.

Conclusions: In the work-up of hypoxia, POS should be considered in patients with seated hypoxia which improves with supine positioning. When a comprehensive work up of pulmonary etiologies is unrevealing, further workup to rule out cardiac etiologies is imperative. While POS prevalence is unknown, it is likely underdiagnosed and underrecognized. An interdisciplinary approach with both pulmonology and cardiology subspecialities is helpful to diagnose and treat POS. Closure of the PFO is rarely performed to solely treat POS however can improve quality of life and decrease morbidity.