Pacemakers originated more than thirty years ago. The pacemaker was a box the size of a large doughnut which sent impulses down the wire to stimulate the ventricle at a fixed rate. The heart rate was factory pre-set and the pacer paced regardless of the patient's own rhythm. The device weighed a little less than a pound.

The capability to change the pacing rate came as a big improvement a few years thereafter. The drawback was that the patient had to have another surgery each time the rate was to be changed; the surgeon was required to physically access the pacemaker and change the rate with a screwdriver. In addition, the pacers were powered by five or six nickel-cadmium battery packs that required imminent replacement of the box every two years.

The development of the lithium iodine battery and the advent of microchips revolutionized the pacer design and led to what we have today - a multifunctional, programmable device that has a multitude of diagnostic features. It has downsized from the bulky "deck of cards" to a device that is as small as two half-dollar coins stacked up.

Because of the expanded functions, a coding system had to be developed which would decribe accurately at a glance the type of mode the pacemaker was in. Fig. 1 shows a table of codes developed by NASPE (North American Society for Pacing and Electrophysiology). This type of coding is used in North America and most developed countries.

For practical purposes, the most frequently used modes are:

AAI -	Atrial; paced, sensed and inhibited by the atrium;
VVI -	Ventricular; paced, sensed and inhibited by the ventricle;
DDD -	Dual; both atrium and ventricle are paced and sensed, and the pacemaker is inhibited by either chamber;
DDI -	Dual; both atrium and ventricle are paced and sensed, and the pacemaker is inhibited by either chamber. In addition, when atrial activity exceeds the programmed upper rate, pacer reverts to VVI pacing only.
DVI -	Dual; paces both chambers but does not sense the atrium; it is inhibited by the ventricle only.