INTRODUCTION: A patent foramen ovale (PFO) is found in 25-30% of patients. The discovery is often made only on autopsy, as most PFOs are clinically silent and any inter-atrial blood exchange usually shunts from the left to right heart. Thus, when a patient presents with hypoxic respiratory failure, concern for presence of a PFO is rarely at the top of the differential. However, in the setting of elevated right heart pressures, PFOs can become of great hemodynamic importance and can lead to deadly complications, including right to left shunting and refractory hypoxic respiratory failure. We present an unusual case of constrictive pericarditis leading to significant shunting through a PFO.