Pacemakers

A standardized five-letter code is used to describe the function of each pacemaker.

1. The first letter designates the chamber paced (O, none; A, atrium; V, ventricle; D, dual [both atrium and ventricle]).
2. The second letter describes the chamber sensed (O, none; A, atrium; V, ventricle; D, dual).
3. The third letter describes the pacemaker's response to sensed events (O, none; I, inhibition of pacemaker output; T, triggering of pacemaker output; D, dual response: spontaneous atrial and ventricular activity inhibit atrial and ventricular pacing and atrial activity triggers a ventricular response).
4. The fourth letter indicates the presence or absence of rate modulation (O, no rate modulation; R, rate modulation present).
5. The fifth letter specifies the presence and type of multisite pacing (O, none; A, more than one stimulation site in either atrium, stimulation sites in each atrium, or a combination of the two; V, more than one stimulation site in either ventricle, stimulation sites in each ventricle, or a combination of the two; D, any combination of A and V).
6. For example, a VVI pacemaker will sense and pace the ventricle yet will be inhibited and will not fire if an R wave is detected. A DDD pacemaker senses and paces both the atrium and ventricle. A VVIRV pacemaker has ventricular inhibitory pacing with rate modulation and multisite ventricular pacing. This mode is often used in patients with heart failure, chronic atrial fibrillation, or intraventricular conduction delay. A DDDRD pacemaker has dual-chamber pacing with rate modulation and multisite pacing in both the atrium (or atria) and ventricle(s).