A 69-year-old patient presented with episodic, acute hypoxia and an increasing oxygen requirement. His hemoglobin oxygenation reached its nadir in the 80% to 85% range as measured by pulse oximetry while he was sitting upright. Oxygenation would improve in this patient to percentages in the upper 90s when he was in the supine position. He was found to have a large secundum atrial septal defect with bidirectional intracardiac shunting, left hemidiaphragmatic dysfunction, a dilated ascending aorta and a prominent Eustachian valve. The patient was stabilized with oxygen therapy, and the cardiology service provided definitive treatment via percutaneous shunt closure with a septal occluder.