Abstract Body (Do not enter title and authors here): Background: Acute hypoxemic respiratory is most often the result of common pathologies that are easily identifiable. However, in cases of refractory hypoxemia, further workup and management strategies must be deployed.

Case Description: An 82-year-old male with no significant cardiac history presented to an outside hospital with an acute coronary syndrome and a PCI was completed to an obstructed obtuse marginal artery. Angiography also noted chronic total occlusion (CTO) of the right coronary artery (RCA) with left to right sided collaterals. Within 24 hours of the procedure, the patient developed acute hypoxic respiratory failure. Right heart catheterization and echocardiography demonstrated new right ventricular (RV) failure. The patient required intubation as well as inotropic and pressor support for stabilization, and he was transferred to our institution for consideration of advanced therapies. A transthoracic echocardiogram with a bubby study was positive for right to left shunting, and a transesophageal echocardiogram confirmed presence of a patent foramen ovale (PFO). Management included decreasing positive end-expiratory pressure and administering an inhaled pulmonary vasodilator. This approach allowed for stabilization and the time needed for gradual recovery of his right sided function, eventually allowing for discontinuation of support.

Discussion: This case underscores the importance of developing a broad differential in rapidly deteriorating patients and quickly implementing appropriate diagnostic tests and treatment. This patient’s new right heart failure was likely secondary to disruption of collateral flow following his ischemic event. While septal defects are more commonly associated with left to right shunting, the patient’s new RV failure and elevated right sided filling pressures caused shunting of deoxygenated blood from right to left, driving his hypoxemia. Prompt evaluation and diagnosis of this patient’s PFO led to appropriate management. The use of a pulmonary vasodilator and minimization of positive airway pressures helped to reduce right sided filling pressures, stabilized hemodynamics, and improved oxygenation by decreasing shunting across the PFO, allowing the patient to recover from his ACS event. If refractory hypoxemia persisted and hemodynamics remained tenuous, more invasive measures including VA ECMO and oxy-RVAD would have been considered.