Reversible Complete Heart Block following Cardiac Surgery

His bundle electrograms were recorded before and throughout the postoperative course in two children who developed temporary complete heart block after surgical correction for tetralogy of Fallot. In one the complete heart block was associated with prolonged duration of the QRS complex, and in the other there was no prolongation of the QRS complex. His bundle electrograms in the patient with widened QRS identified the site of block to be between the low atrium and the His bundle, and in the patient with a QRS of normal duration the block was distal to the His bundle. Complete heart block disappeared within 3 weeks in both patients. Subsequent His bundle studies showed normal A-V conduction in the first patient except for the presence of persistent complete right bundle-branch block. In the other, prolongation of the P-R interval was persistent, and atrial pacing at high rates produced a Wenckebach type of block distal to the His bundle. These studies indicate that electrophysiologically the site of complete heart block following definitive intracardiac surgery can occur at multiple sites. Furthermore, in complete heart block the site of the pacemaker may not be accurately predicted from routine electrocardiograms alone, and His bundle recordings may be necessary to clarify this site.