Limitations to Activation of the Antitachycardia Burst Pacing Function of the Symbios 7008 Pacemaker in the Universal (DDD) Mode

The Symbios 7008 antitachycardia pacemaker was implanted in five patients for control of supraventricular tachycardia. Shortly after implantation in the first two patients, it was noted that the burst pacing sequence was not automatically activated by tachycardia when the pacemaker was in the DDD mode. Data from these two and the subsequent three patients were evaluated to explain this observation. The problem was primarily related to the operation of the device during the postventricular atrial refractory period. In all patients, the atrial electrogram encroached upon the programmed postventricular atrial refractory period because VA conduction during SVT was less than the lowest programmable interval (155 ms). Atrial events occurring during this interval will not trigger the tachycardia termination sequence. In all five patients, the size of the atrial electrogram decreased substantially (48 +/- 10%; mean +/- SD) during supraventricular tachycardia compared to sinus rhythm. In at least two of the five patients, decreased atrial size during supraventricular tachycardia may also have resulted in intermittent failure of atrial sensing during tachycardia, even at the most sensitive setting (0.6 mV). The latter may remain a problem even if the technical fault in SVT detection in the DDD mode were corrected. Two related problems were noted in the DDD mode: ventricular events during rapid SVT do not reset the low rate interval, resulting in random low rate pacing; and, automatic prolongation of atrial refractory period by two successive ventricular events without an intervening atrial sensed event compounds problems of atrial sensing. All of these problems were easily circumvented in all patients by noninvasive reprogramming to the DVI mode in which supraventricular tachycardia detection is based on ventricular sensing. These findings have implications for the future design of such devices.