A 2-year-old male with right isomerism was referred for supraventricular tachycardias. Atrial pacing study revealed that anterograde conduction was only through the posterior atrioventricular node. During the mapping of ventriculoatrial conduction, we identified a sharp potential resembling a His-bundle electrogram with a decremental property at the anterior wall of the common atrium. Catheter ablation for the potential eliminated the anterior ventriculoatrial conduction, thereby indicating retrograde activation of the possible anterior atrioventricular node.

A complex malformation of the heart was diagnosed by fetal echocardiography at 27 weeks of gestation. The study of the relationship of the abdominal great vessels, showing the inferior caval vein anterior and to the left of the aorta, made possible the diagnosis of right isomerism. This inferential guide to the arrangement of the thoraco abdominal organs proves to be as valid in the fetus as it is in the neonate and child. The examination of the abdominal great vessels as part of the fetal scan is of great value, enabling us to counsel mothers appropriately in the light of the poor prognosis of neonates with isomerism of atrial appendages and complex intracardiac lesions.