Pacemakers use a 5-letter code. The first 3 letters are the most important
1st Letter: Chamber Paced
A= Atrium
V= Ventricle
D= Dual (A+V)
2nd Letter: Chamber Sensed
A= Atrium
V= Ventricle
D= Dual (A+V)
O= None
3rd Letter: Response after Sensing (i.e. Response to sensed beat)
I = Pacing Inhibited
T= Pacing Triggered
D= Dual (I+T)
O= None
4th Letter: Programmability (Program features)
P = Rate & Output
M = Multiprogramable
C = Communicating
R = Rate adaptive
O = None
5th Letter: Arrhythmia Control
P = pacing
S= shock
D= Dual (P+S)
O = None
Examples:
VVI = Ventricle paced, ventricle sensed; pacing inhibited if beat sensed.
VVIR = Demand ventricular pacing with physiologic response to exercise.
DDD = Atrium & ventricle can both be paced; atrium & ventricle both sensed; pacing triggered in each chamber if beat not sensed
DDDR = AV concordance with physiologic response.
More Terminology:
Unipolar vs. Bipolar
Single vs. Dual leads
Asynchronous vs. Synchronous pacing: Asynchronous pacers are fixed-rate, and do not sense atrial or ventricular myopotentials.
Further Reading
https://www.unc.edu/~rvp/old/RP_Anesthesia/Basics/Pacers.html