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ECG Findings

  • Pacing spikes.

  • If a pacer spike is seen preceding a P wave, the patient is atrial paced. This can be intermittent.

  • If a pacer spike is seen preceding a QRS complex, the patient is ventricular paced. This can be intermittent. The QRS complex will typically be wide with a QRS duration > 120 ms. Right ventricle pacing leads will typically give an LBBB morphology, while left ventricle pacing leads will give an RBBB morphology.


  1. Pacemakers are designated by the “five letter” system. In this system, the letter “A” denotes atrium, “V” denotes ventricle, “D” denotes dual (both chambers), and “O” denotes neither. The first three letters are the most commonly used:

    1. First letter—designates chamber(s) paced

    2. Second letter—designates chamber(s) sensed

    3. Third letter—designates pacemaker response to sensed electrical activity: T: triggered—fires even when beat sensed, I: inhibitory—holds when beat sensed, D: dual—atrial triggered and ventricle inhibited

    4. Fourth letter—extra options: P: programmable, M: multiprogrammable, C: communicating, R: rate adaptation, O: none

    5. Fifth letter—cardioverting options: P: pacing, S: shocking, D: dual (P+S), O: none

  2. The two most common pacemaker malfunctions are failure to pace and failure to sense.

  3. The absence of paced complexes does not mean pacemaker malfunction if the pacemaker is set to inhibitory mode. When a normal native complex is detected within a preset time frame, the pacemaker will remain silent when it detects a native beat.

  4. Some ECG machines perceive small pacer spikes as artifact and do not reproduce them on the printed tracing.


Dual-Chamber Pacemaker, Paced Rhythm. (ECG contributor: James V. Ritchie, MD.)


Tiny pacer spikes (arrows) precede the P waves, and somewhat larger pacer spikes precede the QRS complexes (arrowheads). The QRS complexes are wide, with discordant T waves.

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