Oversensing of P wave

N° 12
Manufacturer Abbott Device ICD Field Sensing

This 65-year-old woman suffering from a severe ischemic cardiomyopathy underwent implantation of an Epic dual chamber ICD with an integrated (the anode of the sensing circuit is the defibrillation electrode of the right ventricular lead) bipolar ventricular lead. The patient presented after receiving a shock during exercise in absence of prior ill feeling.

Graph and trace
  1. The atrial events on the EGM were sensed appropriately by the atrial channel, then inappropriately by the ventricular channel (VS) immediately thereafter. The QRS following the P wave was sensed as a second, very rapid, ventricular EGM and was classified in the VF zone (F). This second signal is sensed after ventricular blanking, an absolute refractory period triggered by VS. This oversensing was intermittent; the morphology of the atrial signal sensed in the ventricle was different from the reference (x), in contrast with the true ventricular signal (>78% similarity);
  2. Oversensing occurred over several consecutive cycles. An episode of VF was diagnosed after 16 F cycles; the return of sinus rhythm counter was never filled, because F cycles fell between the VS cycles. A shock, not visible on this tracing, was delivered at a later time.

Systematic oversensing of a supernumerary cardiac signal results in the sensing of 2 morphologically different signals per cardiac cycle. Oversensing of the P wave occurs preferentially when the defibrillating electrode of an integrated bipolar lead straddles the tricuspid annulus, and when the sensed PR interval is longer than the ventricular blanking. Several solutions can be considered: 1) program a lower ventricular sensitivity in hope of eliminating the supernumerary signal due to the sensing of the P wave. This programming change incurs the serious risk of underdetecting VF. The induction of VF should be repeated with this new setting of sensitivity to verify that VF is properly detected; 2) force atrial pacing by increasing the back-up rate or by implementing a specific algorithm. Increasing the back-up pacing rate might avoid reaching the maximum sensitivity, though the outcome is highly unpredictable; 3) in this patient, the ventricular lead had to be repositioned in order to eliminate oversensing entirely, advancing the entire defibrillation electrode (which, in this case, was the sensing anode) inside the right ventricle.

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