Upward bias in estimates of pacemaker reliability: effect of unreported patient mortality

OBJECTIVES: This study attempted to determine the effect of unreported patient deaths on estimates of pacemaker reliability. BACKGROUND: The reliability of pacemakers is usually reported with reference to implant registration data and returned product analysis without censoring when follow-up data are missing. METHODS: We studied 73 patients (mean +/- SD] age 77 +/- 8 years) undergoing implantation of a ventricular-inhibited (VVI) pacemaker who were subsequently found to be at increased risk of experiencing premature pacemaker failure. Survival curves for patients and pacemakers were constructed by the Kaplan-Meier method with appropriate censoring at the time of unrelated death or elective explantation of a normal device. To examine the effect of unreported loss of follow-up data, patient mortality was then ignored, and follow-up for pacemakers without known failure was assumed to continue to the date of analysis. RESULTS: There were 13 device failures, with a median pacemaker survival time of 37 months. Twenty-three patients died, all of causes unrelated to the pacemaker system; median patient survival time was only 44 months. Ignoring this attrition inflated follow-up time from 122 to 188 patient-years and reduced the apparent pacemaker failures at 30 months by almost half, from 37% to only 20%. Modeling the process shows that when the patient mortality rate is more than half the pacemaker failure rate, ignoring censoring inflates the device survival estimate by > or = 10% from the median survival onward. CONCLUSIONS: When medical device survival curves are generated by implant registration data and returned product analysis, they should be adjusted for unreported loss of follow-up.