Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1997-04-30
2001-03-06
Getzow, Scott M. (Department: 3737)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06198972
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to implantable medical devices, and more particularly to a method and apparatus for limiting unwanted current flow through electrically excitable tissue resulting from application of an external signal on an implanted medical device.
2. Description of the Related Art
The use of implantable medical devices for electrical stimulation of electrically excitable tissue is well known in the medical arts. Electrical stimulation of brain tissue has been used for tremor suppression. Moreover, electrical stimulation of peripheral nerve tissue has been used to promote blood circulation in patients having peripheral vascular disease. Additionally, electrical stimulation of the brain and nerve tissue of the spinal cord has been used for pain management. In such devices, electrodes deliver the stimulation signal to the electrically excitable tissue. The electrodes are operatively connected to an implantable pulse generator which is packaged in a case that is adapted to be implantable. Those electrodes are coupled to that pulse generator by a conductive lead wire.
A user having such an implanted medical device during normal life activities may be forced to go through a time-alternating electromagnetic field. Prevalent examples of sources of electromagnetic field are Electronic Article Surveillance (EAS) systems. Such systems detect theft and are found in the exit doorways of many stores and libraries. EAS systems typically emit an AC electromagnetic field for detecting theft of articles that have an attached electromagnetic tag.
When the user having an implanted medical device walks through an EAS system that emits a significant level of AC electromagnetic field, current may be induced in the electrically excitable tissue by the Faraday effect. According to the Faraday effect, when a conductive loop is disposed within a time-alternating electromagnetic field, a current is induced in that conductive loop. With the implanted medical device within the user, the conductive loop consists of the stimulation electrodes, the lead wire, the implantable pulse generator having a case, and the conductive medium of the body including the electrically excitable tissue between the stimulation electrodes and the case. The case in some implantable medical devices acts as a reference return electrode with respect to the stimulation electrodes and is composed of electrically conductive material.
The user may experience unwanted physiological effects from the current that is induced in the electrically excitable tissue of that conductive loop. This current may cause uncontrolled excitation of electrically excitable tissue which can lead to pain sensations and unwanted motor responses for the user. Thus, means for controlling that induced current is desired.
Objects of the Invention
Accordingly, a primary object of the present invention is to limit the current that may be induced within the conductive loop formed by an implanted medical device when a significant level of external signal such as an electromagnetic field is present. The induced current is limited automatically upon sensing a significant level of an external signal or manually with the user controlling the opening of the conductive loop when the user is about to enter an area having a significant level of external signal.
SUMMARY OF THE INVENTION
In a principal aspect, the present invention takes the form of an apparatus and method for limiting current flow, induced when the level of an external signal is greater than an external signal threshold level, in a conductive loop formed by a medical device implanted within a living organism having electrically excitable tissue. The present invention includes a switch that is adapted to be operatively connected within the conductive loop which includes the implanted medical device and the electrically excitable tissue. Additionally, a control circuit controls the switch to turn the switch on when the medical device is stimulating the electrically excitable tissue to achieve a therapeutic effect if the level of the external signal is less than the external signal threshold level. Moreover, the control circuit controls the switch to turn off whenever the level of the external signal is greater than the external signal threshold level, to thereby introduce a high impedance into the conductive loop.
The present invention can be applied to particular advantage when used with a sensor for sensing when the level of the external signal is greater than the external signal threshold level. The control circuit would automatically turn the switch off when the sensor senses that the level of the external signal is greater than the external signal threshold level. Alternatively, the living organism, having the implanted medical device, manually initiates the opening of the conductive loop. In this manner, the conductive loop can be opened manually to limit unwanted current that can result from application of a significant level of external signal on a closed conductive loop.
These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings.
REFERENCES:
patent: 4745923 (1988-05-01), Winstrom
patent: 5584870 (1996-12-01), Single et al.
patent: 5591218 (1997-01-01), Jacobson
patent: 5649965 (1997-07-01), Pons et al.
Hartlaub Jerome T.
Stein Marc T.
Banner & Witcoff , Ltd.
Getzow Scott M.
Medtronic Inc.
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