Total anomalous pulmonary venous return (TAPVR), Type I (supracardiac)

Total anomalous pulmonary venous return (TAPVR) occurs when the pulmonary veins fail to drain into the left atrium and instead form an aberrant connection with some other cardiovascular structure. Such abnormalities account for approximately 2% of cardiac malformations and are best differentiated according to the site at which the anomalous pulmonary veins terminate. Four types of TAPVR thus may be defined.

In type I, the most common (55% of cases), the anomalous pulmonary veins enter the systemic system at a supracardiac level. Typically, four anomalous pulmonary veins (two from each lung) converge directly behind the left atrium and form a common vein, known as the vertical vein, that passes anterior to the left pulmonary artery and the left main bronchus to join the innominate vein. On chest radiographs, the anomalous venous configuration resembles a snowman composed of two "balls": the dilated vertical vein on the left, the innominate vein on the top, and the superior vena cava on the right form the head of the snowman; the body of the snowman is formed by the enlarged right atrium.  Less commonly, anomalous supracardiac drainage to the left brachiocephalic vein, the right superior vena cava, or the azygos vein occurs. Venous obstruction in type I TAPVR is uncommon, but extrinsic obstruction may occur if the vertical vein courses between the left pulmonary artery anteriorly and the left main bronchus posteriorly.

Type II TAPVR (30% of cases) involves a pulmonary venous connection at the cardiac level. The pulmonary veins join either the coronary sinus or the right atrium.

Type III TAPVR (13% of cases) consists of a connection at the infracardiac or infradiaphragmatic level. The pulmonary veins join behind the left atrium to form a common vertical descending vein, which courses anterior to the esophagus and passes through the diaphragm at the esophageal hiatus. This vertical vein usually joins the portal venous system but occasionally connects directly to the ductus venosus, the hepatic veins, or the inferior vena cava. Type III TAPVR is virtually always accompanied by some degree of obstructed venous return. The obstruction of pulmonary venous flow causes cyanosis and, often, early and severe congestive heart failure. In addition, lymphangiectasia sometimes results from the obstruction of venous return through the vein that extends below the diaphragm. Heart size is usually normal, but there is severe interstitial pulmonary edema, thymic atrophy, and depression of the diaphragm.

Type IV TAPVR involves anomalous venous connections at two or more levels. In the most common pattern, the vertical vein drains into the left innominate vein, and the anomalous vein or veins from the right lung drain into either the right atrium or the coronary sinus.

The radiologic appearance of TAPVR varies according to the site of abnormal venous drainage and whether the flow is obstructed. The structure in which the anomalous vein terminates appears dilated. All the systemic venous and pulmonary venous blood enters the right heart, and the only path for its exit to the left heart is a communication in the atrial septum, usually a large atrial septal defect or patent foramen ovale. This right-to-left shunt is essential for survival. The right heart is prominent in TAPVR because of the increased flow volume, but the left atrium remains normal in size. In infants affected by TAPVR, cyanosis and congestive heart failure typically develop in the early neonatal period. Approximately one-third of patients with TAPVR also have other associated cardiac abnormalities; many have heterotaxy syndrome, with asplenia being most common.