Threshold behavior of electrodes in long-term ventricular pacing

In a study of the electrical stimulation threshold behavior of electrodes in long-term ventricular pacing, 120 patients were followed up for a minimum of 5 years after initial pacemaker implantation (range 5 to 11.3 years). In each patient, at least two postimplantation threshold measurements in milliamperes at a pulse duration of 1 msec were obtained at the time of pulse generator replacement. A plot of 380 long-term thresholds showed stability in 97 (81 %) of the 120 patients. Four subgroups, each exhibiting a different threshold pattern, were analyzed separately. An unexplained progressive increase in threshold occurred in 23 patients (19 percent). An additional 297 patients with low output or programmable pacemakers were assessed noninvasively for threshold variation for 1 month to 4 years after pacemaker implantation. None showed threshold increases of more than 2.5 milliamperes. Evaluation of 22 patients with Medtronic 5816 bipolar electrodes, attached to Medtronic 5870 capacitor discharge pulse generators, revealed high thresholds in 14 (64 percent), electrode removal for failure to pace was required in 9 (41 percent). Direct current leakage related to specific pacemaker circuits in these pulse generators (capacitor discharge) may be one cause of high threshold in these electrodes. The data indicate that there is overall threshold stability up to 11 years after pacemaker implantation and support the use of the newer low output pulse generators with increased longevity.