Pacing threshold spikes months and years after implant.

To determine patterns of variation in chronic pacing thresholds, we made 4,942 threshold measurements in 257 patients with 312 leads, at times from implant to 295 months (median 17 months) including 1,053 determinations in 46 children less than 12 years old. Motivation was late sudden death in two single-ventricle pacemaker-dependent children with multiple possible death causes. At stimulus duration 0.5 +/- 0.04 msec, mean of the thresholds, measured 1 month or more after implant, was 1.3 +/- 0.66 volts (V) for endocardial electrodes and 2.8 +/- 1.39 V for epicardially applied electrodes. Highest mean thresholds were in the 6 to 12-year-old age group. In 34 leads studied at implant, again within a month and for at least three years thereafter, time of maximum threshold occurred after one month in 59%, independent of lead type or patient age. Of 107 leads with five or more measurements after 3 months use, gradual increase in threshold continued after 3 months in 24%. An additional 21% had at least one threshold that exceeded the post-three-months individual patient lead mean by three standard deviations. Most striking was the occurrence of transient several-volt increases and decreases in threshold as late as 8 years after lead implantation in at least three children. These temporary changes were detected initially transtelephonically by the vario method of threshold measurement. They occurred during minor illnesses such as summer colds, yet similar illnesses also occurred without threshold elevation. We suggest further study of pacing threshold variations in highly pacemaker-dependent children whose cardiac anatomy makes use of epicardial electrodes necessary.