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Background: There has been concern over declining bipolar (BP) impedance (Z) in aging polyurethane (PU) cardiac pacing leads. Subsequently, a prospective study was conducted comparing BP Z, threshold (Th), and R-wave sensing amplitude of 55D PU-insulated (Model 4024, Medtronic, Inc., Minneapolis, MN, USA) and silicone-insulated (Model 5024) leads. Methods: This study was initiated by The Iowa Heart Center. Patients with Model 4024 (N = 162) or 5024 (N = 120) pacing leads with at least 6 years implant time were enrolled and followed for an additional 5 years. Results: There was a significant drop in the mean BP Z for the Model 4024 population, between enrollment (6 years) and the final endpoint (11 years), which was in contrast to the Model 5024 which did not see a significant drop in its mean BP Z for this same period. The trend difference seen in the means between the two models was statistically significant (P < 0.0001). In addition, a statistically significant relationship was found between dropping BP Z and rising Th (P < 0.0001). The analysis showed that if BP Z dropped below 200 ohms, the probability of having a >3X increase over baseline, in Th at 2.5 V, increases from approximately 3-7% to as high as 30%. Conclusions: A significant drop in BP Z observed in the PU-insulated Model 4024 lead was not present in the silicone-insulated Model 5024 lead. The statistically significant relationship between dropping BP Z and rising Th helps to understand how to better manage patients with aging leads. (PACE 2012; 35:51-57) [ABSTRACT FROM AUTHOR]