Refractory Hypoxemia After Left Ventricular Assist Device Implantation: Underestimated Patent Foramen Ovale and Venovenous Extracorporeal Membrane Oxygenation-Associated Sump Effect on Bubble Study

Hypoxemia after left ventricular assist device (LVAD) implantation is a life-threatening complication with a wide differential diagnosis. One potentially catastrophic cause is right-to-left shunting through an unmasked or underestimated patent foramen ovale (PFO). We describe a 63-year-old man in whom an intraoperatively identified PFO with left-to-right flow was initially judged mild and unlikely to be clinically significant. However, on postoperative day 1 in the intensive care unit, profound refractory hypoxemia developed due to massive right-to-left shunting, requiring rescue venovenous (VV) extracorporeal membrane oxygenation (ECMO). Furthermore, a bedside agitated saline solution study (bubble study) performed by transesophageal echocardiography under full VV-ECMO support was confounded by the extracorporeal sump effect, wherein the high-velocity venous drainage cannula siphoned agitated saline solution and led to a significant underestimation of the intracardiac shunt. This case reinforces the argument for a low threshold for PFO closure at the time of LVAD implantation, given the inherent limitations of preoperative echocardiographic assessment in advanced heart failure: Even defects that appear mild may carry clinically significant risk after LVAD-mediated left atrial decompression. Additionally, clinicians should be aware that active ECMO venous drainage can mask or underestimate intracardiac shunting on bubble study and that transient reduction of extracorporeal flow is essential for accurate interpretation during concurrent ECMO support.