Supernormal and alternating conduction in intermittent bundle branch block and intermittent or concealed ventricular preexcitation. Electrophysiological study, mechanisms and clinical considerations

Abstract

Supernormal and alternating conduction are not rare in clinical arrhythmology, detectable at various levels of the conduction system in different pathophysiological conditions, and often associated, so as to justify the search for a possible link between them. In order to define a possible relationship between supernormal and alternating conduction, the electrophysiological data of two patient groups were analyzed. Group 1 included 9 patients with intermittent bundle branch block in the presence of supernormal conduction through the pathological branch. Group 2 included 16 patients with ventricular preexcitation, intermittent or concealed, in the presence of supernormal conduction through the atrioventricular accessory pathway. In group 1, 8 patients had a phase 3 and 1 patient a phase 3 and phase 4 bundle branch block. In all 9 patients, the area of phase 3 block was interrupted by a window of unexpected conductivity detectable by atrial premature stimulation. In 7/9 cases, by modulating the atrial rate, alternating conduction through the branch was observed, and the cardiac cycle of occurrence of the phenomenon was in the range of supernormal conduction in 6/7 cases. In group 2, 11 patients showed conduction through the accessory pathway only for a narrow range of cardiac cycles, during atrial premature stimulation. One patient had a prolonged accessory pathway refractory period (phase 3 block), but a window of unexpected supernormal conduction within the refractory zone was observed at atrial premature stimulation. Another patient presented a tachycardia and bradycardia-dependent block along the accessory pathway (phase 3 and phase 4 block) and the phase 3 block area was interrupted by a window of supernormal conduction. In the other three cases, supernormal conduction was manifested as unexpected resumption of conductivity through the accessory pathway for atrial cycles more precocious than the actual duration of its effective refractoriness. By modulating the atrial rate, alternating conduction through the accessory pathway was observed in 13/16 cases, and the cardiac cycle at which this phenomenon appeared falling in the range of supernormal conduction in 11/13 cases. It can be hypothesized that supernormal conduction is in relation with the presence of a phase of supernormal excitability experimentally demonstrated in the late phase of repolarization of cardiac cells, and that supernormal and alternating conduction are related phenomena, so that the evidence on ECG of alternating conduction through a pathological branch or an atrioventricular accessory pathway can be considered as a marker of the presence of supernormal conduction through the structure.