Persistent left superior vena cava: a blessing in disguise.

Persistent left superior vena cava (SVC) is an uncommon condition, usually encountered during cannulation of the left subclavian vein. We describe a patient who required a cardiac device upgrade to cardiac resynchronization implantable cardioverter defibrillator for biventricular failure. The presence of a persistent left SVC proved to be a blessing in disguise in this patient as he had since developed total occlusion of the left subclavian-innominate system.