A pacemaking system consists of a pulse generator (the pacemaker) and
one or more electrode leads. The pulse generator is about the size of a
match-box and weight 50-100g (approximately 2-4 ounces). Modern
pacemakers are encased entirely in metal and sealed to prevent body fluids
seeping into the unit. The generator consists of two parts: the power supply
or batteries, and the electronic circuitry (Fig. 1). 
The majority of pulse generators are powered by lithium type batteries,
These last longer than the mercury type batteries which were used previously. A
battery life over ten years is possible but the use of smaller batteries or more
complex pacemakers may reduce this predicted length of life. Nuclear-powered
pacemakers and rechargeable types of battery are no longer used because they
have proved inconvenient in practice. The generators contain electronic circuits
which draw energy from the batteries and transform this into a series of
electrical impulses. These are conducted down the electrode lead to the heart.
Each electrical impulse discharged by the pacemaker stimulates the heart to
contract and produce a heart-beat (Fig. 2). 
Peacemakers can be insensitive or sensitive to the natural heart rhythm.
Fixed rate pacemakers are insensitive and discharge electrical impulses to
stimulate the heart at a preset rate, ignoring the natural heart-beats. They may be
used when the heart rate is permanently slow (Fig. 3). 
Demand pacemakers contain circuits which are sensitive to the natural
heart-beat. They are suppressed by each natural heart-beat and discharge electrical impulses to stimulate the heart only when the heart misses a beat,
Most pacemakers are of the demand variety (Pig. 4), 
Programmable pacemakers can be “programmed” to work as either
fixed rate or demand. Even after implantation into a patient these
pacemakers can be adjusted by electromagnetic signals generated by an
external “programmer”. Not only is their mode (fixed rate or demand)
adjustable but the rate at which the pacemaker produces impulses (discharge
rate) and other characteristics can be changed to cope with the needs of the
patient (Fig. 5). 
Some pacemakers can automatically change the discharge rate without
external programming. These physiological pacemakers require more
complex electronic circuitry and often need more than one electrode lead,
particularly if they are being used to restore the normal sequence of
contraction of the chambers of the heart (Fig. 6). Increasingly special 
pacemakers are being used to interrupt or prevent fast heart rates and to
analyse and store information about the natural heart rhythm.