A Note About Pacemaker Nomenclature

Electronic pacemakers are typically identified with a three letter label, such as VVI or DDD.  What do these letters signify?
The first letter identifies the chamber (or chambers) of the heart that is stimulated:  A represents atria, V represents ventricles and D represents dual, or both.
The second letter identifies another function of the electronic pacemaker:  it has the capability of sensing native electrical activity.   In other words, if a beat occurs spontaneously, the pacer can detect that beat, and be programmed to respond to it.  The letters A, V and D are used to identify sensing, just as they were to identify stimulating.   O is used to signify no sensing.
The third letter describes responses to native beats.  There are two possible responses: to stimulate or to inhibit stimulation, and the letters I and D are used to signify these (a response of stimulation is only programmed to accompany inhibition, so that is Dual response).
Thus a pacer could be programmed to stimulate the atria at a given rate, and to ignore any native activity: this would be AOO pacing.   Likewise, it could be programmed to stimulate the ventricles and to sense native ventricular activity, responding by inhibiting its own pacing activity.  This would be VVI pacing, and has the effect of letting the pacer act as a backup for the native rhythm, stimulating a beat only when the native R-R interval exceeds a set limit.
Other examples of this nomenclature are provided on the following pages.