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Undersensing of the intrinsic signal in cardiac devices has four possible causes. First, the intracardiac signal is inappropriate for sensing due to a low amplitude and/or slew rate; second, the intracardiac signal occurs in the refractory period of the device; third, reversion to a fixed rate pacing in case of continuous interference; and fourth, the amplitude of the electrogram is reduced by lead failure. Intracardiac recordings are helpful in differentiating these causes. After ventricular sensing or pacing the device starts a postventricular atrial refractory period (PVARP), which is programmable with a nominal setting of 320 ms. However, when ventricular sensing is not preceded by a P wave this ventricular beat is considered by the device to be a ventricular ectopic beat and the device switches to a PVARP of 400 ms, in order to prevent the initiation of pacemaker-mediated tachycardia, which might be evoked by ventricular ectopic beats with retrograde conduction. In Fig. 1, ventricular sensing is indicated in the marker channel by VS. From the ventricular EGM it is obvious that ventricular sensing is caused by T‑wave oversensing, which is not preceded by a P wave and is interpreted by the device to be a ventricular ectopic beat, resulting in an extended PVARP of 400 ms. By extending the PVARP to 400 ms the succeeding P wave falls in the extended PVARP (indicated by AR in Fig. 1) and is not followed by ventricular stimulation, resulting in electrocardiographic undersensing (Fig. 1). The problem was corrected by reducing ventricular sensitivity, which prevented T‑wave oversensing.
B.M. van Gelder is a clinical advisor for LivaNova, France. J. Schroemges and F.A.L.E. Bracke declare that they have no competing interests.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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F. A. L. E. Bracke
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