Cardiac rhythm management system with lead impedance measurement

Abstract

An implanted cardiac rhythm management device, such as a pacemaker (pacer) or defibrillator provides a lead impedance measurement of the effective resistance of the leadwire connecting the device to electrodes in the heart. The lead impedance measurement is based on the amount of a voltage droop of a capacitively coupled pacing output voltage pulse over a fixed period of time that is shorter than the duration of the pacing pulse. The lead impedance measurement avoids the need for performing a natural logarithm function by using a look-up table of precalculated scaled resistance values corresponding to particular amounts of voltage droop. The lead impedance measurement is enhanced by performing interpolation and correcting for manufacturing tolerances of the particular cardiac rhythm management device.

What is claimed is: 
     
       1. An implantable cardiac rhythm management system comprising: a cardiac rhythm management device adapted for being implanted in a living organism;   a leadwire, coupled to the device, and providing at least one electrode;   a therapy circuit, coupled to the leadwire for providing electrical energy to the living organism;   a lead impedance measurement circuit, coupled to the leadwire and the therapy circuit, and providing an indication of a resistance of the leadwire based on a measured change in voltage resulting from the electrical energy provided to the living organism, wherein the lead impedance measurement is based on a predetermined set of resistances corresponding to a predetermined set of changes in voltage to which the measured change in voltage is compared;   an interpolation circuit providing a lead impedance measurement interpolated from values in the predetermined set of resistances and the predetermined set of changes in voltage; and   a correction factor circuit that is capable of adjusting the lead impedance measurement based on component variations from a nominal value in the device.