There are three main possible explanations if there is a vessel to the left of the aortic arch. A vessel formed by either the left brachiocephalic vein alone or both brachiocephalic veins that drains inferiorly into the coronary sinus is a left superior vena cava. If the vessel connects the left brachiocephalic vein to the Hemi-azygos vein, the vessel is a left superior intercostal vein. If the vessel arises from the left upper lobe pulmonary veins and drains into the left brachiocephalic vein but has no connection to the Hemi-azygos vein, the patient has partial anomalous pulmonary venous return.