The authors report the case of a 63 year old woman who developed refractory arterial hypoxemia due to a right-to-left interatrial shunt through a patent foramen ovale during the acute phase of right ventricular infarction. The precarious haemodynamic condition of the patient contra-indicated surgical intervention and so the effects of the shunt were reduced by obstructing the atrial septal defect with the balloon of a Swan-Ganz catheter. The management of this type of shunt is discussed based on the degree of hypoxemia and the patient's haemodynamic status with reference to this particular case and a review of the literature.