The superior vena cava (SVC) is formed by the confluence of the right and left innominate veins and enters the right atrium at its upper pole. The SVC lies along the right sternal border. The right innominate vein lies more vertical and is shorter then the left innominate vein. The SVC is about 7 cm long. This vein is valveless and is joined by the azygous vein entering the SVC posteriorly.1 Early in development, two paired anterior cardinal veins form the primitive venous return as the left anterior vein involutes and contributes to the left brachiocephalic vein. The junction of the primitive left anterior cardinal vein (LACV) and the right anterior cardinal vein thus forms the SVC. The primitive cardinal veins coalesce to the atrium via the sinus venosus. The sinus venosus then becomes the coronary sinus (Figure 19-1).
Schematic diagram illustrating the embryologic development of the central veins, posterior view. (A) shows the paired venous drainage into the primitive heart. (B) and (C) illustrate the fate of the embryonic veins with vitelline veins fusing to form the inferior vena cava, the right anterior cardinal vein becoming the superior vena cava, and the left horn of the sinus venosus becoming the coronary sinus.
Failure of the left anterior vein to involute is present about 0.3% in the general population but approaches 5% in those with congenital heart disease.2 The persistent left SVC empties to the coronary sinus in 90% and to the left atrium 10%. This seemingly tedious embryologic sequence has profound clinical impact in consideration of the persistent left SVC or duplicated SVC. In the case of persistent left SVC connecting to the left atrium, this imposes the risk of right-to-left shunt of air or emboli causing potentially stroke or other systemic embolization. Analogous to the supracardiac subset of total anomalous pulmonary venous connection, a persistent LACV that communicated with the left atrium provides a connection between systemic and pulmonary venous systems. When the persistent left SVC connects to the coronary sinus, any catheter-based venous access may inadvertently the coronary sinus, resulting in cardiac tamponade. Isolated LACV can be treated with ligation. In addition to contrast-enhanced cross-sectional imaging and venography, diagnosis may also be confirmed echocardiographically by coronary sinus dilatation and passage of contrast into it from a left arm vein1 (Figure 19-2). The coronary sinus is a frequent conduit for placement of a pacemaker lead in a coronary vein. The incidence of coronary sinus dissection with pacemaker lead insertion is about 6%, but perforation or cardiac tamponade is rare.3 A single left-sided SVC is quite rare and may be confused with partial anomalous pulmonary venous return. Another venous anomaly that can be confused for left-sided SVC is a hemiazygous vein draining into the left brachiocephalic vein.