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Pacemaker Malfunction

David L. Hayes, MD; and Ronald E. Vlietstra, MB, ChB
[+] Article, Author, and Disclosure Information

From the Mayo Clinic and Mayo Foundation, Rochester, Minnesota; the Watson Clinic, Lakeland, Florida. Requests for Reprints: David L. Hayes, Mayo Clinic, 200 First Street SW, Rochester, MN 55905.

Copyright 2004 by the American College of Physicians

Ann Intern Med. 1993;119(8):828-835. doi:10.7326/0003-4819-119-8-199310150-00009
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The field of cardiac pacing has expanded rapidly in recent years. Engineering improvements and microprocessor technology have resulted in a vast increase in pacemaker technology. The high-tech edge of cardiac pacing often discourages all but the pacemaker specialist from approaching a malfunctioning pacemaker. Electrocardiographic signs of pacemaker malfunction can be grouped into four categories: failure to output, failure to capture, undersensing, and inappropriate pacemaker rate. For each of these categories, there may be true malfunctions and pseudomalfunctions. In addition, environmental sources of electromagnetic interference, both within and outside the hospital environment, can result in pacemaker malfunction. Approaching pacemaker malfunction with these categories in mind should help minimize confusion.


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Figure 1.
Example of oversensing.

An electrocardiographic recording from a patient with a dual-chamber bipolar pacing system shows five atrial pacing artifacts followed by paced ventricular activity after the first, second, fourth, and fifth. Paced ventricular activity is absent after the third paced atrial beat because of oversensing; that is, activity other than intrinsic ventricular activity was sensed by the ventricular sensing channel and resulted in failure to output. Many signals could be responsible for the oversensing. In this patient, electrical noise arose within a ventricular pacing lead.

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Figure 2.
Intermittent ventricular failure to capture in a patient with a dual-chamber pacemaker.

The first two and last two ventricular pacing artifacts result in ventricular depolarization. The five ventricular pacing artifacts in between fail to result in ventricular capture, so that the patient remains asystolic during that period. Effective atrial depolarization can be seen throughout the tracing.

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Figure 3.
Example of undersensing.

The electrocardiographic recording was obtained from a patient with a ventricular pacemaker programmed to 50 beats per minute. The first pacing artifact occurs simultaneously with an intrinsic QRS complex and represents normal function (pseudofusion). However, the second and third pacing artifacts occur inappropriately close to the preceding QRS complex (that is, at less than 1200 ms the programmed rate of the pacemaker) and represent failure to sense or undersensing. The fourth pacemaker artifact is appropriate because it occurs 1200 ms after the preceding paced QRS complex. The second pacing artifact does not result in ventricular depolarization. This is not failure to capture, because the ventricular pacing artifact is occurring only 260 ms after the intrinsic QRS complex and the ventricle is refractory.

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Figure 4.
Example of functional undersensing.

The electrocardiographic tracing was obtained from a patient with a ventricular pacemaker programmed to 70 beats/min. Of the five pacing artifacts shown, the first, fourth, and fifth result in effective ventricular depolarization. There is failure to capture with the second and third pacing artifacts. The third pacing stimulus occurs approximately 850 ms after the preceding intrinsic QRS complex. This timing represents normal sensing, that is, pacing at a rate of 70 beats/min. However, the fourth pacing stimulus occurs approximately 640 ms after the preceding intrinsic QRS complex, indicating that the preceding QRS complex was not sensed because it occurred during the pacemaker's ventricular refractory period. This is an example of functional undersensing; that is, undersensing was a function of the ventricular refractory period.

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Figure 5.
The atrioventricular interval as a single interval with two subportions.

The entire atrioventricular interval corresponds to the programmed value. The initial portion of the interval is the blanking period (crosshatched portion). If an intrinsic ventricular event occurs during this period, it will not be sensed; that is, functional undersensing occurs. This interval is followed by the cross-talk sensing window (gray portion). If an intrinsic ventricular event occurs during this period or if any other event is sensed on the ventricular sensing circuit during this period, ventricular safety pacing will occur. (Reprinted with permission of W.B. Saunders Company; From: Hayes DL. Timing cycles of permanent pacemakers. Cardiol Clin. 1992; 10:593-608.).

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Figure 6.
Pacemaker re-entrant tachycardia shown by an electrocardiographic tracing from a patient with a dual-chamber pacemaker.

Atrial and ventricular pacing stimuli precede the first three paced complexes at a rate of 80 beats/min, the programmed lower rate limit for the pacemaker. A premature ventricular complex follows the third paced ventricular complex. The premature ventricular complex is conducted in a retrograde fashion through the atrioventricular node and results in atrial activation. The retrograde atrial activation is in turn sensed by the pacemaker and initiates ventricular pacing. The pacing rate is limited to the programmed upper rate limit of 110 beats/min.

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