Fault-tolerant elective replacement indication for implantable medical

Abstract

A pacemaker having a fault-tolerant elective replacement indicator (ERI) triggering scheme in which transient excursions of parameters used as criteria for triggering ERI are rejected as triggering events. Periodic assessments of certain indicia of battery depletion are made, and subjected to a long-term low-pass filtering operation in order to reduce the effects of transient excursions of the indicia which result from non-ERI conditions. Over a long period of time (e.g., a day) predetermined threshold values of the indicia of interest must be exceeded a predetermined number of times in order for the device to issue an ERI. In one disclosed embodiment of the invention, the battery's loaded terminal voltage and internal impedance are used as indicators of the battery's depletion level. Periodically, these values are measured and converted to digital values. The digital values are subjected to a low-pass filtering operation to prevent temporary or transient excursions of the impedance and voltage curves from causing ERI to be triggered. In another disclosed embodiment of the invention, a long term fading average of periodically measured values is maintained. When the measured values are found to fulfill the ERI criteria, assessment of the parameters of interest is performed at an increased rate. While assessments are performed at the increased rate, the continuously updated fading average value must fulfill the ERI triggering criteria at least a predetermined number of times before ERI is triggered. If the fading average fails to fulfill the ERI triggering criteria the required number of times, assessments are resumed at the first, slower periodic rate.