[Correlations between automaticity and conduction. Considerations in a case of left bundle branch block in phases 3 and 4 (author's transl)].

A case of phase-3 (tachycardia dependent) and phase-4 (bradycardia-dependent) left bundle branch block (LBBB) which illustrates some relationships between automaticity and conduction in the intraventricular conduction system is reported. The observations made were: a) a conspicuous spontaneous variability of the conductivity and block temporal zones in the left bundle branch (LBB), i.e. of the cycle duration determining either normal intraventricular conduction or block in the LBB; b) after isoproterenol, transient shortening of phase-3 block zone and disappearance of phase-4 block zone due to escape beats which rise from the LBB; c) transient change into not rate-dependent LBBB in consequence of catheter LBB injury and gradual successive reappearance of conductivity zone; d) short periods of 2:1 phase-4 LBBB. A coexisting phase-4 block in a LBB fascicle with electrocardiographic pattern not fulfilling the agreed diagnostic criteria for hemiblocks was also demonstrated. This intraventricular conduction defect was also characterized by conspicuous spontaneous variability of the block temporal zones. After isoproterenol, the duration of the cycle decreased where it occurred. After LBB traumatic injury, it lasted not rate-dependent longer than LBBB and no escape beats rising from the LBB injured fascicle appeared. During normal intraventricular conduction the ECG showed "ischemic" alterations in V1-V4 leads of variable degree related to LBB conductivity and block zone duration. Selective coronary angiograms were normal. These findings are discussed in an attempt to clarify the electrophysiological mechanism of rate-dependent intraventricular conduction defects.