For the moment, there are two types of events: electrical shock and pacing. Electrical shock will depolarize all the cells simultaneously. Pacing will depolarize the cells in a given region of the myocardium--if they are not in refractory state.
Electrical shock is described by the moment of it's appearance.
Pacing is described by the region in which it is given (a paralelipiledic region), the moment of the first impulse and the rate of pacing.