Pacemaker Three-Letter Code: Second Position
The second letter of the three-letter pacemaker code represents the chamber(s) sensed. 1
Understanding the NBG Pacemaker Code Structure
The standardized pacemaker nomenclature uses a systematic approach where each position has specific meaning 1:
Position Definitions
Position I (First Letter): Chamber(s) Paced 1
- O = none
- A = atrium
- V = ventricle
- D = dual (both atrium and ventricle)
Position II (Second Letter): Chamber(s) Sensed 1
- O = none
- A = atrium
- V = ventricle
- D = dual (both atrium and ventricle)
Position III (Third Letter): Response to Sensed Event 1
- O = none
- I = inhibited
- T = triggered
- D = dual (both triggered and inhibited)
Position IV (Fourth Letter): Rate Modulation 1
- O = none
- R = rate modulation present
Position V (Fifth Letter): Multisite Pacing 1
- O = none
- A = atrium
- V = ventricle
- D = dual chambers
Clinical Application Examples
Common Pacing Modes Explained
VVI Mode: The device paces the Ventricle, senses the Ventricle, and is Inhibited by sensed ventricular activity 1
DDD Mode: The device paces Dual chambers (atrium and ventricle), senses Dual chambers, and has a Dual response (inhibits pacing when intrinsic activity is sensed, triggers ventricular pacing after sensed atrial activity) 1
VDD Mode: The device paces only the Ventricle, but senses Dual chambers (both atrium and ventricle), allowing it to track atrial activity and maintain AV synchrony without atrial pacing capability 1
Key Clinical Pitfall
A common error is confusing sensing with pacing—the second letter specifically indicates which chamber(s) the device monitors for intrinsic electrical activity, not which chamber(s) it stimulates 1. This distinction is critical for understanding pacemaker function and troubleshooting device behavior in your elderly patient with near syncope 1.