Interpretation of cardiac implantable electronic device (CIED) electrocardiograms requires understanding standard pacing modes, timing cycles, and distinguishing benign pseudomalfunctions from true device failure.
Figure 11.1: 12-lead ECG of a Ventricular Paced Rhythm (VVI mode) showing pacing spikes preceding wide, LBBB-like QRS complexes with discordant ST-T changes. Rendered with solid white background.
1. NBG Generic Pacemaker Code
POSITION I POSITION II POSITION III POSITION IV POSITION V Paced Chamber Sensed Chamber Response to Sense Programmability Multisite Pacing A (Atrium) A (Atrium) T (Triggered) R (Rate Modulated) A (Atrium) V (Ventricle) V (Ventricle) I (Inhibited) O (None) V (Ventricle) D (Dual: A+V) D (Dual: A+V) D (Dual: T+I) D (Dual: A+V) O (None) O (None) O (None)
2. True Pacemaker Malfunctions vs Benign Pseudomalfunctions
| Malfunction Category | Electrocardiographic Presentation | Underlying Mechanism | Clinical Action |
|---|---|---|---|
| Failure to Pace | Absence of pacing spikes despite heart rate dropping below lower rate limit. | Lead fracture, battery depletion, over-sensing of myopotentials/EMI. | Emergency device interrogation; lead replacement. |
| Failure to Capture | Pacing spike present, but not followed by a captured P wave or QRS complex. | Lead dislodgement, exit block (hyperkalemia, fibrosis), output programmed too low. | Increase pacing output voltage/pulse width; correct hyperkalemia; reposition lead. |
| Undersensing | Pacing spikes fire randomly within intrinsic complexes during refractory periods. | Lead sensing threshold set too high; low intrinsic signal amplitude. | Reprogram sensitivity to lower numerical value ($ ext{mV}$); risk of R-on-T and VF. |
| Oversensing | Unwarranted pause in pacing due to inappropriate sensing of T waves, myopotentials, or crosstalk. | Sensitivity set too low; electrical interference. | Increase sensitivity threshold; program blanking periods. |
| Pacemaker-Mediated Tachycardia (PMT) | Rapid ventricular pacing at Maximum Tracking Rate (MTR) in DDD mode. | Retrograde P wave sensed by atrial lead triggering ventricular output (Endless Loop). | Apply clinical magnet over pulse generator to revert to asynchronous DOO mode; lengthen PVARP. |
| Hysteresis (Pseudomalfunction) | Heart rate drops below programmed lower rate limit before pacing initiates. | Normal feature designed to encourage intrinsic rhythm. | No intervention required. |
| Fusion / Pseudofusion Beat | Pacing spike falls on top of an intrinsic QRS complex with hybrid morphology. | Normal simultaneous depolarization from intrinsic and paced wavefronts. | No intervention required. |
3. Cardiac Resynchronization Therapy (CRT)
- Biventricular Pacing: Right ventricular apical lead + Coronary sinus left ventricular epicardial lead.
- ECG Signature of Effective CRT: $R$ wave in lead $V_1$ (positive anterior vector from LV posterior pacing) and negative $QS$ in leads I and aVL.