A 42-year-old white man with good exercise tolerance comes in for a follow-up appointment. He had a transthoracic echocardiogram (TTE) to evaluate an apical systolic murmur heard on routine physical examination. TTE shows mild to moderate mitral regurgitation with normal function. A slightly enlarged right atrial and ventricular cavity size prompted a micro-cavitation study which is positive for an intracardiac shunt.