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Unexpected rhythms with normally functioning dual-chamber pacing systems

David L Hayes, MD
Section Editor
Mark S Link, MD
Deputy Editor
Brian C Downey, MD, FACC


A variety of electrocardiographic findings and rhythms may be encountered in which the pacing system is functioning normally. These include crosstalk, pacemaker-mediated or endless loop tachycardias, and repetitive non-reentrant ventriculo-atrial synchronous rhythms. These might be considered functional system malfunctions since the resulting arrhythmia may be deleterious to the patient, although the device is functioning normally and in accord with its programmed specifications.

A comprehensive discussion of the normal function and programming of permanent pacemakers is presented separately. (See "Permanent cardiac pacing: Overview of devices and indications" and "Modes of cardiac pacing: Nomenclature and selection".)


In a DDD pacemaker, a paced or sensed event in one channel initiates one or more timing circuits in the opposite channel. As a result, the release of an atrial output pulse will initiate an atrioventricular (AV) delay, which will permit a ventricular output pulse at the end of this interval if an R wave is not sensed. A ventricular event that is sensed during the AV interval, considered to be an R wave, will cancel the AV interval, inhibit the ventricular output pulse, and cause the pacemaker to initiate an atrial escape interval.

Current pacing systems, however, are incapable of recognizing morphologic characteristics of a complex. Any intrinsic complex, output decay, or artifact of electromagnetic interference of sufficient amplitude and frequency response that it is not filtered out by the sense amplifier is treated as a true signal. Although bipolar sensing configuration is believed to minimize far-field sensing, crosstalk can be seen with any sensing configuration. If any signal is interpreted as an R wave, and inhibits the ventricular output and resets the escape interval, the phenomenon is termed crosstalk-mediated ventricular output inhibition.

When AV nodal conduction is otherwise intact, crosstalk is asymptomatic. If there is first degree AV block, the effective paced rate may be slower than the programmed base rate, because the conducted R wave will occur after completion of the ventricular refractory period initiated by the oversensing of the atrial stimulus. The conducted R wave will be sensed, thereby resetting the atrial escape interval yet again. A clinical catastrophe may occur in the presence of complete AV block combined with crosstalk (in the absence of safety-pacing), as the system will effectively pace the atrium, but the ventricular output may be repeatedly inhibited, leaving the patient asystolic.

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Literature review current through: Nov 2017. | This topic last updated: Sep 14, 2017.
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