What ventricle is the pacemaker?

this is a particular setting on a pacemaker which essentially senses if the ventricle depolarises, if it does not the PM will activate it and ensure the heart carries on beating.



Also:

A demand pacemaker (there are several types) kicks in after your cardiac function varies outside a pre-set normal rate. It then brings your heartbeat into a more efficient rhythm. (Sinus-rhythm.)

A demand pacemaker does not take the place of an implanted defibrillator! If a genuinely life-threatening cardiac arrhythmia develops, it takes a defibrillator -- an implanted one or an external unit used by someone trained to respond -- to correct ventricular fibrillation. If it is not corrected, v-fib is fatal.