Intraventricular conduction disturbances after correction of tetralogy of Fallot: can bifascicular and trifascicular block be diagnosed from the surface ECG?

To determine the origin of conduction disturbances commonly seen in the ECG after correction of Fallot's tetralogy, 36 children underwent electrophysiologic studies; these included endocardial recordings of right ventricular (RV) and left ventricular (LV) apical activation and His-bundle recordings. The surface ECG showed a pattern of complete right bundle branch block (RBBB) in 22, and RBBB with left-axis deviation (LAD)--so-called bifascicular block--in nine; the remaining five had RBBB, LAD, and a long PR interval, often considered as indicating trifascicular block. Eight of 22 patients with RBBB and normal axis had delayed RV apical activation (36%), indicating a proximal lesion. Of nine patients with RBBB and LAD, four only had delayed RV apical activation (44%); in the other five, therefore, the RBBB pattern was due to a peripheral lesion (ventriculotomy); they cannot be considered as having true bifascicular block. LV activation was not delayed in any case. In five cases, the surface ECG suggested trifascicular block (long PR in the presence of RBBB and LAD). His-bundle recordings showed the HV interval to be prolonged in only two cases, at the upper limit of normal in two, and short in one. The AH interval was prolonged in all and was mainly responsible for the long PR interval. Thus, the surface ECG is not a reliable tool for making a diagnosis of true bifascicular block and trifascicular damage after correction of Fallot's tetralogy. This may explain controversies existing about the prognosis of such conduction abnormalities.