Pacemaker terminology

Pacemaker terminology

Rheobase: Minimum voltage which will capture the myocardium at any pulse width.
Chronaxie: Pulse width needed for capture at twice the rheobase voltage
Electrogram (EGM; egram): Intracardiac signal picked up by the intracardiac electrode
Slew rate: Slope of the electrogram
Current of injury: ST elevation occurring in the ECG monitored from the electrode tip as the lead makes contact with the myocardium

Pacemaker blanking period: To prevent damage to the sensing circuit by the pacing spike, the sensing circuit is turned off for a short period immediately after the spike is delivered.

Refractory period is the period after the blanking period during which a sensed event does not reset the lower rate interval. But an event sensed in this period will extend the ventricular refractory period. A signal sensed during the refractory period is designated Vr and a signal sensed beyond the refractory period is designated Vs.

Noise mode response: Sensing multiple noise signals causes reversion to interference mode and pacing will occur in asynchronous mode. This prevents asystole due to oversensing.
T wave oversensing need not be associated with tall T wave on surface ECG. But the T wave of the EGM will be sufficient for resetting the lower rate interval – both QRS and T wave will be designated as Vs in the report.

Sensed AV interval is always kept lower than the paced AV interval because the P wave is sensed only later due to the low slew rate of the P wave.

Atrial based versus ventricular based DDD pacemakers: In atrial based pacemakers, AA interval is constant while VA interval is constant for ventricular based pacemakers.