Behavior of the ventricular triggering period of DDD pacemakers

Fifty-seven patients with implanted DDD pulse generators (23 Intermedics Cosmos and 34 Medtronic Symbios) were studied to characterize the behavior of the so-called ventricular safety pace period or nonphysiologic AV delay (ventricular triggering period) in a variety of circumstances. The pulse generators were programmed to various combinations of upper rate, lower rate, and atrioventricular (AV) intervals. We then evaluated the response of the pulse generators to a variety of signals sensed by the ventricular channel during the ventricular triggering period (VTP). In the Cosmos DDD pulse generators, the VTP lengthened from its nominal value (100 ms) whenever the sum of the atrial escape (pacemaker VA) interval and the nominal value of the VTP was shorter than the upper rate limit interval. Extension of the VTP reflected the necessary delay required to maintain constancy of the (ventricular) upper rate limit interval. In contrast, the Medtronic Symbios DDD pulse generators exhibited constancy of the VTP even when the sum of the atrial escape (pacemaker VA) interval and the duration of the nonprogrammable VTP (110 ms) was shorter than the upper rate limit interval. This response was observed only with a programmed lower rate of 90 ppm, upper rate of 100 ppm, and AV interval of 250 ms. Under these circumstances, in the presence of crosstalk or myopotentials sensed in the VTP, the Symbios DDD pulse generator violated its ventricular upper rate limit interval, but remained faithful to its separate (but equal) atrial upper rate limit interval by omitting the succeeding atrial stimulus. The behavior of the VTP of DDD pulse generators, particularly its relation to the upper rate response, must be known for the proper interpretation of pacemaker function.