A coding system for pacemaker functions can provide a simple description of pacemaker capabilities. One commonly used coding system employs three letters to describe functions. The first letter refers to the chamber paced by the pacemaker. The second refers to the chamber sensed by the pacemaker. The third refers to the pacemaker’s response to the sensed event.

In the example shown here, both chambers can pace if necessary (represented in the code by the first D, for dual). Both chambers are sensed (represented in the code by the second D, for dual). Because the third letter is D, the pacing will be either inhibited or triggered. If intrinsic electrical activity is sensed in either the atrium or ventricle, the pacemaker will inhibit (or withhold) pacing in whichever chamber that the electricity is sensed. Sensed electricity in one chamber (e.g., the atrium) can also trigger pacing in the other chamber (e.g., the ventricle).