Case Presentation: An 82 year-old presented with acute onset hypoxia after a laparoscopic subtotal gastrectomy for stage I gastric adenocarcinoma. Upon extubation, she had positional episodes of desaturation to 79% when sitting up, with other vitals signs normal and no dyspnea. Her oxygen saturation was 91% when supine. Her lungs were clear to auscultation bilaterally. PaO2 was 45 mmHg on room air and increased to only 74 mmHg with 100% O2. Initial transthoracic echocardiogram with bubble study demonstrated a patent foramen ovale (PFO) with normal right atrial and pulmonary artery pressures and function. She had a stable 4.1 cm ascending thoracic aortic aneurysm and multiple, stable hepatic cysts. She was diagnosed with platypnea-orthodeoxia syndrome (POS) due to PFO and underwent successful percutaneous transfemoral closure of the defect. She was discharged saturating 97% on room air, regardless of position.

Discussion: Orthodeoxia, or the arterial deoxygenation that accompanies the positional change from supine to erect, requires two conditions to coexist: an anatomical component (interatrial communication such as a patent foramen ovale [PFO] or atrial septal defect [ASD]) and a functional component that redirects shunted blood flow through the atrial septum. These functional defects can either preferentially direct blood flow through the anatomical defect or cause a transient increase in right atrial pressure, reversing the left-right gradient. The former can be caused by conditions that direct the jet of deoxygenated blood through the interatrial communication by repositioning the atrial septum, such as an ascending aortic aneurysm, intracardiac lipoma, hepatic cyst distorting the right atrium, or aortic valve replacement.  Conditions which transiently increase right sided pressures include pulmonary embolism, pulmonary hypertension, pericardial effusion, pneumonectomy, chronic obstructive pulmonary disease and constrictive pericarditis.  Measured right sided pressures are typically normal in POS. The pathophysiology in our patient is likely due to her thoracic aortic aneurysm, elongating in the erect position and stretching the interatrial septum and PFO.  While a possibility, it is unlikely her hepatic cysts contributed to her presentation as they were small and there has only been one case report of a large liver cyst causing this presentation.

Conclusions: While PFOs can be seen in up to 29% of the general population, orthodeoxia is much more rare, with less than 200 cases described in the literature. With the elderly population increasing, the incidence of aortic aneurysms and other cardiovascular conditions distorting previously silent PFOs may increase as well, so it is important to maintain a high index of suspicion. Diagnosis is best made with a transesophageal echocardiogram with intravenous agitated bubble study demonstrating bubbles in the left atrium.