Normal pacemaker function
The EGM displays pacemaker upper rate behaviour in the form of a Wenckebach response which is normal pacemaker function. The Wenckebach response is observed in a 5:4 pattern (5 atrial events to 4 ventricular events). It is recognised by finding group beating, progressive lengthening of the AV interval and intermittent pauses on the trace.
This response occurs when the atrial rate exceeds the programmed upper tracking rate (maximal tracking rate). The upper tracking rate being the fastest interval that the pacemaker is allowed to deliver ventricular pacing. The atrial rate in this example is approximately 140 bpm which is greater than the programmed upper tracking rate of 130 bpm. Atrial sensed events are followed by ventricular paced beats with progressive lengthening of the AV interval until an atrial event falls into the post ventricular atrial refractory period (PVARP) marked as an atrial refractory event (AR). The AR event does not trigger an AV interval (is not tracked) resulting in a short pause before the sequence repeats.
The progressive lengthening of the AV interval (Wenckebach response) is observed due to the interaction of two timing intervals, the AV interval (programmed at 80 ms) and the upper tracking rate interval. When an atrial sensed event occurs an AV interval is initiated. After 80 ms a ventricular paced beat is delivered. The next sensed atrial event initiates an AV interval but at 80 ms when the ventricular pace stimulus should be delivered it is delayed because the UTR interval has not yet been complete. (The ventricular pace stimulus can only be delivered when this interval is complete). This interaction results in progressive lengthen of the AV interval for subsequent sensed atrial events until an atrial event falls in the PVARP and is not tracked.
In this case the atrial activity was related to a paroxysmal atrial tachycardia. The mode was reprogrammed to DDIR and the patient started on a new antiarrhythmic medication. A plan was made to review the patient in the device clinic in 6 weeks to assess arrhythmia burden, symptoms and device settings and depending on findings to considered referral for catheter ablation.