We report on a 16-year-old boy after an event of cardiac arrest and initial treatment with a veno-arterial extracorporeal membrane oxygenator (ECMO). After a short stabilisation period a biventricular assist device (BVAD, Thoratec) was implanted. Although the BVAD was functioning well, the patient showed persisting hypoxemia. Transthoracic echocardiography revealed a patent foramen ovale with a high right-to-left shunt due to low aspiration pressures of the BVAD. The patient was successfully treated by interventional closure of the PFO with a 27-mm Amplatzer septal occluder and could easily be weaned from the respirator. Meanwhile the boy has successfully undergone heart transplantation. PFO has to be considered as a cause of arterial hypoxemia in patients supported with ventricular assist devices. The diagnosis of a PFO may be missed under ECMO-treatment. Interventional closure of a PFO can successfully be performed even if the patient is supported with a BVAD.