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 impla nt registration data and returned product analysis without censoring w hen follow-up data are missing. Methods. We studied 73 patients (mean [+/-SD] age 77 +/- 8 years) undergoing implantation of a ventricular-i nhibited (VVI) pacemaker who were subsequently found to be at increase d risk of experiencing premature pacemaker failure. Survival curves fo r 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 l oss 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 medi an pace maker survival time of 37 months. Twenty three patients died, all of causes unrelated to the pacemaker system; median patient surviv al 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 ha lf the pacemaker failure rate, ignoring censoring inflates the device survival estimate by greater than or equal to 10% from the median surv ival onward. Conclusions. When medical device survival curves are gene rated by implant registration data and returned product analysis, they should be adjusted for unreported loss of follow-up.