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Hypoxia caused by persistent left superior vena cava connecting to the left atrium a rare clinical entity.

We relate the case of a 40-year-old man with a history of premature birth and dextroposition of the heart who presented for an evaluation of persistent hypoxia. An unrevealing pulmonary evaluation and agitated-saline echocardiogram led to cardiac magnetic resonance imaging. This revealed a very unusual finding: a persistent left superior vena cava with insertion into the left atrium and a small connecting vein between the right and left superior venae cavae. The implications, embryology, and pathogenesis of this rare condition are discussed.