SUPERNORMAL CONDUCTION IN THE LEFT-BUNDLE-BRANCH

Supernormal Conduction. This report describes a patient with tachycard ia-dependent left bundle branch block (LBBB) and atrial extrasystoles, some of which were followed by an unexpectedly narrow QRS complex. Hi s-bundle recordings and premature atrial stimulation were performed to analyze the mechanism underlying the normalized intraventricular cond uction of some of the early atrial impulses. The results suggested the presence of supernormal conduction in the left bundle branch (LBB), b ecause: (1) the HV interval was identical in LBBB complexes and in ear ly narrow QRS complexes; (2) during single test stimulation using diff erent paced atrial cycle lengths, there was a well-defined range of H, H, intervals resulting in normalization of intraventricular conduction ; and (3) atrial pacing,vith a cycle length of 500 msec resulted in al ternation between wide and narrow QRS complexes. These findings rule o ut alternative mechanisms that could explain the unexpectedly normal i ntraventricular conduction of early impulses.