A 35-year-old woman presented with three episodes of syncope. Clinical examination was unremarkable; in particular, she was acyanotic and had no cardiac murmurs. As part of her investigation, transthoracic echocardiography was performed. This showed a persistent left superior vena cava (SVC) and a dilated coronary sinus. Injection of agitated saline through an intravenous cannula in the left arm showed echo contrast in the left atrium and, to a lesser …