Atrioventricular block in situs ambiguus and left isomerism (polysplenia syndrome).

Three patients are described who had situs ambiguus and left isomerism (polysplenia syndrome) and advanced atrioventricular block. One presented with a complex bradyarrhythmia with Wenckebach block. The other two had congenital atrioventricular block with a narrow QRS at a ventricular rate of 80 per minute, an atrial rate of 150' per minute, and both had a P wave axis directed superiorly and to the right in one, and superiorly to the left in the other. This ECG pattern was not observed in more than 400 adult patients with complete A-V block treated in our service. It is our opinion that in infants and children with heart disease the presence of complete A-V block with narrow QRS and an unusual P waves axis directed superiorly is strongly suggestive of left isomerism. The incidence rate of complete A-V block in left isomerism is nearly twenty percent of the cases described.