An unusual form of "pacemaker mediated tachycardia".

Lower rate limit 60 ppm Maximum tracking rate 120 ppm Maximum sensor rate 120 ppm Paced AV delay 200–350 ms Sensed AV delay 200–350 ms A-refractory (PVARP) 250 ms V-refractory (VRP) 230–250 ms Case summary A 60-year-old woman underwent dual-chamber pacemaker implantation for sinus node dysfunction. The device was programmed to DDDR at a base rate of 60 bpm and a maximum sensor rate of 120 bpm. The initial device settings are listed in Table 1. On the second postprocedure day, the patient complained of palpitations and was noted to be in tachycardia. The EKG showed atrial pacing with 1:1 AV conduction at 120 bpm (Figure 1). Interrogation of the pacemaker revealed atrial pacing at 120 bpm with intrinsic conduction to the ventricle. During ventricular threshold testing, atrial pacing terminated spontaneously. We considered the possibility that the rapid atrial pacing could be secondary to inappropriate functioning of the rate response feature and hence turned it off. However, later that night, the same rhythm recurred with the atrium being paced at 120 bpm.What is the reason for rapid atrial pacing in this case?