Impedance measurement for an implantable device

Abstract

A method and an apparatus for determining an impedance experienced by a lead associated with the implantable device. A substantially constant current signal is provided through a first terminal and a second terminal of the lead. A voltage across the first and second terminals is measured. An impedance across the first and second terminals is determined based upon the constant current signal and the measured voltage.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates generally to implantable medical devices, and more particularly to methods, apparatus, and systems for monitoring power consumption and impedance characteristics relating to implantable medical devices. [0003] 2. Description of the Related Art [0004] There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and / or improving or ameliorating other conditions. A particular example of such a treatment regimen invol...

AI Technical Summary

Problems solved by technology

Many types of implantable medical devices, such as pacemakers and drug infusion pumps, typically include custom integrated circuits that are complex, expensive, and specific to the intended use.

One problem associated with these methodologies is that, for many battery chemistries, these measured battery characteristics do not have monotonically-changing values with respect to remaining charge.

For example, lithium / carbon monofluoride (Li / CFx) cells commonly used in neurostimulators and other IMDs have a relatively flat voltage discharge curve for the majority of their charge life, and present status of the battery cannot be accurately and unambiguously determined from a measured battery characteristic.

Another problem associated with this methodology is the variability of current consumption for a specific device's programmed therapy or circuitry.

This variability, combined with the uncertainty of the battery's present status prior to ERI or EOS, hinders reliable estimation of the anticipated time until reaching ERI or EOS.

For example, damage in the lead, which may be induced by a break in one or more filaments in a multifilament lead wire, or changes in the body tissue where stimulation is delivered, may affect the efficacy of the stimulation therapy.

However, this type of measurement may lead to inaccuracies for a number of reasons, including the fact that the discharging of the voltage signal may be affected by other resistances and capacitances in the system, the accuracy of the capacitor, the time, voltage, and algorithmic accuracies of the measurement system, and the like.

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