The pattern found on this tracing corresponds to an extremely early extrasystole, originating from the distal right ventricular Purkinje network generating a ventricular fibrillation. The Purkinje network runs in the endocardium and divides into a thin right ventricular branch and two thicker left ventricular branches with numerous ramifications allowing rapid activation of the ventricular mass. There are currently a number of studies demonstrating the central role of the Purkinje network in the genesis and maintenance of ventricular fibrillations leading to sudden death. The arrhythmogenic nature of the cells in the Purkinje network resides in a greater sensitivity to early post-depolarization relative to surrounding myocardial cells. The pattern of the extrasystoles originating from the right ventricular Purkinje is relatively homogeneous whereas the pattern is more variable for the left ventricular Purkinje due to a richer and more complex arborization. During an electrophysiological study, a characteristic potential can be recorded at the level of the Purkinje network responsible for the arrhythmia; it is the preferred target for ablation procedures.