Objectives: The purpose of this study was to delineate retrograde His-Purkinje system conduction and reentry (V3 phenomenon) during left ventricular extrastimulation and compare them with right ventricular extrastimulation.
Background: The V3 phenomenon has been well described in the past during right ventricular extrastimulation; however, it has not been studied systematically during left ventricular extrastimulation.
Methods: Left and right ventricular pacing were performed in 13 patients. Retrograde and anterograde routes of impulse propagation were determined on the basis of the sequence of His (H) and right bundle (RB) potentials, H-RB intervals, as well as the QRS configuration and axis of V3 beats.
Results: During right ventricular pacing, retrograde conduction of V2, when discernible, occurred exclusively through the left bundle at all coupling intervals equal to or shorter than the His-Purkinje relative refractory period, with the exception of two isolated beats. During left ventricular extrastimulation, His bundle activation was through the left bundle in nine patients and through the right or left bundle in three other patients. In one patient, the route could not be determined. The V3 phenomena occurred in eight patients during right ventricular pacing. Seven patients had a left bundle branch block pattern QRS configuration, and one had a right bundle branch block pattern configuration. V3 beats occurred in five patients during left ventricular apex pacing: left bundle branch block pattern configuration in one patient and right bundle branch block pattern configuration in four. In three of these four patients, the reentry was interfascicular and limited to the left bundle branch system.
Conclusions: The left-sided His-Purkinje system is the preferred retrograde route of impulse propagation during both left and right ventricular extrastimulation. Reentry within the His-Purkinje system elicited by right ventricular extrastimulation involves both bundle branches, whereas this reentry tends to occur within the left-sided His-Purkinje system during left ventricular pacing.