Schematic illustration of the possible explanation for the recording of local potentials. (To the left) Sinus beat. An ectopic pacemaker (EPM) is located in the high interventricular septum, surrounded by an inexcitable barrier of conduction impairment (grey shaded area). The electrical impulse of the sinus beat giving rise to the LVP (blue circle) is recorded at the end of the QRS complex, 60 ms after the onset of the local V potential (red circle), conducts slowly into the zone of depressed conductivity, and is blocked, thereby protecting the ectopic pacemaker by entrance block. (To the right) Ventricular premature beats. When the ectopic pacemaker fires, the impulse is conducted in the opposite direction through the zone of impaired conduction. This leads to the inversion of the polarity of the LVP _vpb compared with the LVP of the sinus beat and the LVP _vpb now precedes the local V _vpb with 45 ms.