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A 68-year-old female with a history of paroxysmal atrial fibrillation and complete atrioventricular (AV) block was referred for palpitations and suspicion of pacemaker dysfunction (model Philos II DR-T, Biotronik, Berlin, Germany). Baseline electrocardiogram (ECG) showed a dual-chamber pacemaker with an inappropriate behavior (Fig. 1, Panel A). Pacemaker programmed parameters were as follows: pacing mode DDD-R; basic rate 60 beats per minute (bpm); upper rate 130 bpm; dynamic paced AV delay 180/140 ms; dynamic sensed AV delay 135/95 ms; AV safety window 100 ms; atrial sensitivity 0.5 mV (bipolar); ventricular sensitivity 2.5 mV (bipolar); atrial and ventricular pulse amplitude and width 3.6 V and 0.40 ms, respectively (both unipolar). Pacemaker testing showed the following results: battery impedance 0.3 k ; sensed P wave 3 mV, sensed R wave 25 mV; atrial and ventricular pacing thresholds 0.6 and 0.4 V, respectively; atrial and ventricular lead impedances 442 and 560 , respectively (stable). Testing of the atrial pacing threshold in AAI mode at 70 bpm allowed the diagnosis to be made (Fig. 1, Panel B). What is the mechanism of the pacemaker “dysfunction”? How can it be solved?