D104-07 Redirected Venous Flow Leading to Severe Hypoxemia: The Flow Phenomenon After Lung Transplantation

Abstract Introduction A patent foramen ovale (PFO) is present in 20-30% of adults and classically becomes symptomatic when right atrial (RA) pressure exceeds left atrial (LA) pressure. However, right-to-left shunting (RLS) may occur when venous inflow is redirected toward the interatrial septum, which is a pressure-independent mechanism termed the “flow phenomenon.” This has been described after thoracic procedures that alter cardiac or diaphragmatic anatomy, such as pneumonectomy or hemidiaphragm paralysis. We report a case of severe hypoxemia in a lung transplant recipient due to PFO-mediated shunting from right hemidiaphragm paralysis, despite normal right-sided pressures. Case A 64-year-old man with COPD who underwent right single-lung transplantation eight months earlier presented for a scheduled surveillance bronchoscopy to assess for rejection. In the preoperative area, he was found to be profoundly hypoxemic, so the procedure was deferred. He was noted to have platypnea and orthodeoxia. On arrival to the ICU, he required high-flow nasal cannula at 60 L/min and 100% FiO2. Imaging ruled out pulmonary embolism, infection, and signs of graft rejection (biopsy was deferred due to oxygen requirements). Transthoracic echocardiography revealed an interatrial shunt, and transesophageal echocardiography performed during right heart catheterization confirmed a right-to-left shunt through a PFO with venous flow from the superior vena cava (SVC) and inferior vena cava (IVC) directed toward the septum. Cardiac pressures were normal on TTE and catheterization. Diaphragm ultrasound demonstrated right hemidiaphragm paralysis, likely contributing to altered venous inflow geometry. The patient underwent percutaneous PFO closure, resulting in improvement in oxygenation and rapid weaning from high-flow oxygen to nasal cannula within 48 hours. Discussion This case illustrates a rare cause of severe hypoxemia due to a PFO-mediated RLS in the setting of altered venous flow rather than elevated RA pressure in a lung transplant patient. Right hemidiaphragm paralysis changed the orientation of the IVC/SVC, directing flow toward the PFO and enabling paradoxical shunting. Similar physiology has been described after pneumonectomy or other thoracic distortions. Recognition of this mechanism is crucial in post-transplant patients presenting with unexplained hypoxemia or platypnea-orthodeoxia, even with normal cardiac pressures. Demonstrating shunting via contrast echocardiography and confirming resolution after closure establishes the diagnosis. Percutaneous PFO closure remains the definitive and highly effective treatment. This abstract is funded by: None