Persistence of the left superior vena cava is not uncommon in children with congenital heart disease and is occasionally found in adults at thoracotomy. The anomalous vena cava usually carries venous blood from the upper part of the body to the right atrium via the coronary sinus and has little functional significance. In the four patients we describe, however, the left superior vena cava drained arterial blood from some or all of the pulmonary veins into the left innominate vein, and thence into the right atrium. In three of them it was the sole channel by which oxygenated blood could reach the systemic circulation.We record these cases because we believe that the anomaly is more common than has been thought, and gives a clinical picture sufficiently distinctive to be recognized in life, and because it is one for which surgical relief may prove feasible. We have found only two similar cases of which an account of the physical signs has been given and in neither was the diagnosis made in life (Taussig, 1947).Before describing the clinical picture it is necessary to consider briefly the development of the superior vena cava. EMBRYOLOGY Marshall (1850), a century ago, appears to have been the first to give an embryological explanation of the persistence of a left superior vena cava.A modern account of the anomaly (Hamilton et al., 1945; Patten, 1946) is as follows. The venous system arises from the same network as the arterial system and the capillaries. Very soon three pairs of venous channels are recognizable, namely (i) the two vitelline veins, which pass from the yolk sac along the roof ofthe primative gut to meet (ii) the umbilical veins at the sinus venosus and (iii) the cardinal veins which also enter the sinus venosus. Each cardinal vein consists of two parts, the anterior cardinal or precardinal vein and the posterior cardinal or postcardinal vein. The precardinal and p6stcardinal veins unite on each side of the primative heart tube to form a short vessel, the common cardinal duct or duct of Cuvier which enters the horn of the sinus venosus (Fig. 1).The lung bud arises as an outgrowth from the primitive foregut. Both these structures are covered by a common venous network, the splanchnic plexus. The contiguous primitive pulmonary venous plexus and primitive splanchnic venous plexus drain into the paired precardinal and postcardinal veins and into the vitello-umbilical veins (Fig. 3A)
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