216-36: A Case with Pacing Failure due to the Pitfall of Automatic Pacing Threshold Adjustment

Abstract

Although the automatic pacing output adjustment algorithm is smart and has been widely implemented in clinical pacemakers, we experienced a case with serious pacing failure through the pitfall of this system. A 63 year-old female underwent pacemaker implantation (MedtronicR Advisa DR) due to complete atrioventricular block in 2010. There were no specific episodes during the initial 4 years, but she suddenly suffered from syncope in 2014. The ECG showed irregular spontaneous heart beat with 30/min and uncaptured pacing spikes in rate of 60/min. The ventricular lead dislodgement was ruled out in the chest X-ray. The pacemaker interrogation revealed unchanged lead impedance, but the ventricular pacing out-put was set at 3.0 V with 0.6 ms through the automatic threshold adjustment, because the most recent ventricular pacing threshold was 0.625 V with 0.4 ms. We performed manual threshold testing with various AV intervals of 80, 120, 220 ms and various atrial rate of 100 and 120 bpm. Interestingly, the ventricular pacing threshold was influenced by the AV interval, i.e., the shorter the AV interval, the lower the pacing threshold (e.g., 1.00V, 1.25V, 1.75V at atrial rate of 100bpm, 1.00V, 1.00V, 1.50V at atrial rate of 120bpm, respectively). Because the routine automatic pacing threshold algorism uses a short AV interval (110 ms) to avoid fusion from native conduction, this algorism resulted in improper recommendation to use inappropriately lower pacing out-put in a programmed AV interval (160 ms). In addition, this case showed large day-to-day variation in pacing threshold ranging from 0.625V to 1.625V even in the same rate and AV interval. After reprogramming the ventricular output to provide an adequate safety margin (3.5V), consistent ventricular capture was achieved and no pacing failure was found in further observation.