Cardiac Implantable Electronic Devices (Pacemakers, ICDs, and CRT)
Codes and modes
The NBG code describes chamber paced, chamber sensed, response (I/T/D), and rate modulation. VVI paces and senses the ventricle and inhibits on a sensed event; DDD tracks the atrium and paces the ventricle when needed. Pacemaker syndrome arises from loss of AV synchrony, often in VVI pacing with intact retrograde conduction. Hysteresis lets the intrinsic rate run below the programmed base rate before pacing resumes.
Testing and programming
At implant and follow-up, measure sensing amplitude and pacing threshold. Program sensitivity near half the measured signal amplitude so the device sees intrinsic events without oversensing; set output at about twice the threshold (with a safety margin) for capture. Avoid undersensing (dangerous bradycardia/asystole) and oversensing (inappropriate inhibition).
ICDs and CRT
ICD detection zones distinguish ventricular fibrillation from ventricular tachycardia; antitachycardia pacing (ATP) can terminate slow VT before a shock, and superseding/charging behavior matters during programming. CRT requires biventricular pacing; indications commonly include reduced EF with a wide QRS, especially LBBB with QRS ≥150 ms. Loss of biventricular capture, due to AF or PVCs, defeats CRT.
Magnets and MRI
A pacemaker magnet generally causes asynchronous pacing (DOO/VOO) and may alter rate; an ICD magnet suspends tachyarrhythmia detection and therapy without affecting bradycardia pacing. MRI-conditional status depends on the specific system, leads, and programming protocol.
Rule of thumb
If the ECG shows pacing spikes with no capture, check output, threshold, lead integrity, and whether the patient is dependent; if spikes are absent when you expect them, think oversensing or battery/lead failure before reprogramming.