Classification follows pacemaker code developed by the North American Society of Pacing and Electrophysiology (NASPE) and the British Pacing and Electrophysiology Group (BPEG). The code is expressed as a series of up to five letters:
Position I — The first position reflects the chamber(s) paced.
Position II — The second position refers to the chamber(s) sensed.
Position III — The third position refers to how the pacemaker responds to a sensed event.
Position IV — The fourth position reflects rate modulation, also referred to as rate responsive or rate adaptive pacing. (See 'Rate responsiveness' below.)
Position V — The fifth position is rarely ever utilized but specifies the location or absence of multisite pacing, defined as stimulation sites in both atria, both ventricles, more than one stimulation site in any single chamber, or a combination of these.
In practice it is common for only the first 3 letters to be used