Typical counterclockwise atrial flutter occurring despite absence of the inferior vena cava

A 74-year-old man with a structurally normal heart presented with typical atrial flutter, after treatment of atrial fibrillation with propafenone. Catheterization and computed tomographic imaging revealed absence of the inferior vena caval segment that normally traverses the liver to enter the right atrium. Abdominal venous return occurred via the hemi-azygous vein, draining into the superior vena cava. Hepatic veins inserted postero-inferiorly into the right atrium. Pacing atrial myocardium between the hepatic veins and the tricuspid valve resulted in concealed entrainment. Radiofrequency catheter ablation directed (via a superior approach from the right internal jugular vein) to this extraordinary "isthmus" abolished atrial flutter. The implications of this congenital abnormality on posterior barriers maintaining the atrial flutter circuit are discussed.