4.7 Pacemaker Rhythmias, Spike Analysis & Pacemaker Malfunctions

Pacemaker Rhythmias and Analysis

Artificial pacemakers are devices implanted to deliver electrical impulses to the heart when the intrinsic conduction system is too slow or fails (e.g., symptomatic bradycardia, Mobitz II, third-degree heart block). Evaluating pacemaker function on an ECG is a core competency, as identifying malfunctions rapidly can prevent severe patient decompensation.

The NBG Pacemaker Code

Pacemaker functions are described using a standardized 3- to 5-letter code developed by the North American Society of Pacing and Electrophysiology (NASPE) and the British Pacing and Electrophysiology Group (BPEG) (NBG code). The first three positions are the most critical for ECG interpretation:

Comparison Table: NBG 5-Letter Pacemaker Code

• Example: VVI - Paces the Ventricle, Senses the Ventricle, and Inhibits pacing if an intrinsic ventricular beat is sensed. The classic demand pacemaker.
• Example: DDD - Paces both Atrium and Ventricle, Senses both, and can Trigger or Inhibit based on intrinsic activity. Closely mimics physiologic pacing.

Normal Pacemaker Function on ECG

Pacemaker activity is visible on the ECG as thin, vertical, straight lines called pacer spikes. The location of the spike relative to the cardiac cycle indicates which chamber is being paced.

1. Atrial Pacing: A pacing spike appears just before the P wave. The resulting P wave may look slightly different from a native sinus P wave.
2. Ventricular Pacing: A pacing spike appears just before the QRS complex. Because the impulse originates in the ventricular muscle (usually the right ventricular apex) rather than the normal conduction system, the resulting QRS complex is wide (≥ 0.12s) and often resembles a Left Bundle Branch Block pattern.
3. Dual-Chamber Pacing (A-V Sequential): Two spikes are seen in one cardiac cycle. The first spike is followed by a P wave (atrial capture), and the second spike, occurring after a programmed PR interval, is followed by a wide QRS complex (ventricular capture).

Pacemaker Malfunctions

Identifying pacemaker failure is critical. There are three primary types of malfunction identifiable on an ECG. Each carries specific clinical implications and requires distinct troubleshooting approaches.

Comparison Table: Pacemaker Malfunctions

1. Failure to Pace (Failure to Fire)

• ECG Finding: Complete absence of pacing spikes where they should be expected based on the programmed rate.
• Result: The heart rate drops below the pacemaker's set rate, leading to bradycardia or asystole if no intrinsic escape rhythm emerges.
• Causes: Dead battery, broken lead wire, over-sensing (the pacemaker inappropriately senses noise and inhibits pacing).

2. Failure to Capture

• ECG Finding: A pacing spike is clearly visible, but it is not followed by the appropriate waveform (no P wave after an atrial spike, or no QRS complex after a ventricular spike).
• Result: The pacemaker fires, but the electrical energy is insufficient to depolarize the myocardium.
• Causes: Lead dislodgment, low voltage output, increased myocardial stimulation threshold (e.g., due to ischemia or electrolyte imbalance).

3. Failure to Sense (Under-sensing)

• ECG Finding: Pacing spikes occur inappropriately, firing regardless of the patient's intrinsic electrical activity. You will see pacing spikes falling on top of native P waves, QRS complexes, or dangerously, on T waves (R-on-T phenomenon).
• Result: The pacemaker fails to "see" the patient's own heartbeat and competes with the intrinsic rhythm. This is particularly dangerous if a ventricular pacing spike lands on a T wave.
• Causes: Lead dislodgment, sensitivity set too low, fibrous tissue around the lead tip.