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Patient
Man implanted with a pacemaker operating in VVI mode for an atrial disease (alternating between sinus node dysfunction and episodes of atrial fibrillation); control electrocardiogram;
Trace
The electrocardiogram shows atrial fibrillation; the spontaneous QRS complexes are not detected and do not inhibit ventricular pacing which is effective; ventricular sensing defects;
Exergue
Atrial or ventricular undersensing is reflected on the electrocardiogram by the presence of fixed pacing rate artifacts despite the presence of spontaneous complexes which fail to inhibit pacing.
This tracing reveals ventricular undersensing, sometimes leading to pacing during a vulnerable period. In the absence of proper sensing, the pacemaker operates in asynchronous mode with no possibility of inhibition by intrinsic ventricles.