Editing Pacemaker (section)

=== Threshold ===
The heart is composed of an excitable tissue and an electrical pulse of a certain voltage is needed to "capture" the myocardium to make the heart function. The minimum voltage to achieve this is called the threshold. As this is a probabilistic property, the actual voltage used is higher than the threshold (usually by a 50–100% margin). Additionally, the electrical pulse has a time component and proper description of a threshold is a voltage and pulse width.

All electrical circuits require a complete connection between the cathode and the anode. For some pacing leads, both connections to the heart are provided in a single lead ("bipolar") and some only provide a single connection ("unipolar"). In unipolar, the second connection is internally from the heart to the generator through the body (blood, tissue, etc).

Also important is the [[electrical impedance|impedance]]. The lower the impedance, the more current is needed to achieve the threshold voltage and lowers the battery life. The impedance is affected by the integrity of the pacing lead and the electrode-tissue interface of the lead with the heart.