Control of externally induced current in implantable medical devices

• US 6,209,764 B1
• Filed: 04/30/1999
• Issued: 04/03/2001
• Est. Priority Date: 04/30/1997
• Status: Expired due to Term

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Stein, Marc T., Hartlaub, Jerome T.
• Primary Examiner(s): Getzow, Scott M.
• Application Number:
  US09/303,002
• Time in Patent Office: 704 Days
• Field of Search: 607/63, 607/5, 607/9
• US Class Current: 223/94
• CPC Class Codes: A61N 1/32 alternating or intermittent currents applying electric fields by inductive or capacitive coupling A61N1/40; microwave apparatus A61N5/02

Abstract

A current limiting apparatus that is adapted to be operatively connected as part of a conductive loop formed by a medical device implanted within a living organism having electrically excitable tissue. This apparatus limits unwanted current in the conductive loop that may be induced by a significant level of an external signal such as a time-alternating electromagnetic field. This apparatus includes a switch for introducing a high impedance into the conductive loop when the switch is turned off. The apparatus also includes a control circuit that controls the switch to be turned on when the conductive loop should be closed to stimulate the electrically excitable tissue for a therapeutic effect. The control circuit turns the switch off whenever the living organism enters an area having a significant level of external signal that may induce unwanted current in the conductive loop to limit the unwanted current. The living organism can manually initiate turning off the switch when the living organism is about to enter an area having such an external signal. Alternatively, such an external signal is sensed to turn the switch off automatically when such an external signal is sensed.

Claims

1. A method for limiting unwanted current, 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 method including the steps of:

A. closing said conductive loop when said medical device is stimulating said electrically excitable tissue for achieving a therapeutic effect if the level of said external signal is less than said external signal threshold level; and

B. opening said conductive loop by generating and maintaining a high impedance within said conductive loop whenever the level of said external signal is greater than said external signal threshold level.

2. The method for limiting unwanted current as recited in claim 1, further including the step of:

C. sensing said external signal to automatically perform step B.

3. The method for limiting unwanted current as recited in claim 2, wherein said step C includes the step of:

D. sensing the level of said unwanted current induced in said conductive loop, wherein a significant level of said unwanted current in said conductive loop indicates that the level of said external signal is greater than said external signal threshold level.

4. The method for limiting unwanted current as recited in claim 1, wherein said medical device includes stimulation electrodes, implanted near said electrically excitable tissue, and an implantable pulse generator operatively connected to said stimulation electrodes via a lead wire, said implantable pulse generator having a case that acts as a reference electrode with respect to said stimulation electrodes.

5. A current limiting apparatus that is adapted to be operatively connected as part of a conductive loop formed by a medical device implanted within a living organism having electrically excitable tissue, said current limiting apparatus comprising:

a switch, adapted to be operatively connected within said conductive loop, that turns on to close said conductive loop and that turns off to open said conductive loop; and

means for controlling said switch to turn said switch on when said medical device is stimulating said electrically excitable tissue to achieve a therapeutic effect if the level of an external signal is less than an external signal threshold level, and to turn said switch off whenever the level of said external signal is greater than said external signal threshold level, said external signal being an induced current caused by said conductive loop.

6. The current limiting apparatus of claim 5, wherein said switch introduces and maintains a high impedance within said conductive loop when the level of said external signal is greater than said external signal threshold level.

7. The current limiting apparatus of claim 5, further comprising:

an external signal sensor for sensing said external signal, wherein said means for controlling turns said switch off when said sensor senses that the level of said external signal is greater than said external signal threshold level.

8. The current limiting apparatus of claim 7, wherein said external signal sensor senses the level of said unwanted current induced in said conductive loop, wherein a significant level of said unwanted current in said conductive loop indicates that the level of said external signal is greater than said external signal threshold level.

9. The current limiting apparatus of claim 5, wherein said medical device includes stimulation electrodes, implanted near said electrically excitable tissue, and an implantable pulse generator operatively connected to said stimulation electrodes via a lead wire, said implantable pulse generator having a case that acts as a reference electrode with respect to said stimulation electrodes.

10. A current limiting apparatus that is adapted to be operatively connected as part of a conductive loop formed by a medical device implanted within a living organism having electrically excitable tissue, said current limiting apparatus comprising:

means for switching said conductive loop between being closed and being open; and

a control circuit, operatively connected to said means for switching, that controls said means for switching to close said conductive loop when said medical device is stimulating said electrically excitable tissue to achieve a therapeutic effect if the level of an external signal is less than an external signal threshold level, and that controls said means for switching to open said conductive loop whenever the level of said external signal is greater than said external signal threshold level, said external signal being an induced current caused by said conductive loop.

11. The current limiting apparatus of claim 10, wherein said means for switching introduces and maintains a high impedance within said conductive loop when the level of said external signal is greater than said external signal threshold level.

12. The current limiting apparatus of claim 10, further comprising:

means for sensing said external signal, wherein said control circuit controls said means for switching to open said conductive loop when said means for sensing senses that the level of said external signal is greater than said external signal threshold level.

13. The current limiting apparatus of claim 10, wherein said medical device includes stimulation electrodes, implanted near said electrically excitable tissue, and an implantable pulse generator operatively connected to said stimulation electrodes via a lead wire, said implantable pulse generator having a case that acts as a reference electrode with respect to said stimulation electrodes.

14. A method for limiting unwanted current, 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 method including the steps of:

A. closing said conductive loop when said medical device is stimulating said electrically excitable tissue for achieving a therapeutic effect if the level of said external signal is less than said external signal threshold level;

B. opening said conductive loop by generating and maintaining a high impedance within said conductive loop whenever the level of said external signal is greater than said external signal threshold level and generates a voltage build-up to a positive polarity at a node within said conductive loop; and

C. opening said conductive loop by generating and maintaining a high impedance within said conductive loop whenever the level of said external signal is greater than said external signal threshold level and generates a voltage build-up to a negative polarity at said node within said conductive loop.

15. A current limiting apparatus that is adapted to be operatively connected as part of a conductive loop formed by a medical device implanted within a living organism having electrically excitable tissue, said current limiting apparatus comprising:

a sensor for detecting a voltage build up in said conductive loop to either one of a positive polarity and a negative polarity when the level of an external signal is greater than a threshold level;

a switch, adapted to be operatively connected within said conductive loop, that turns on to close said conductive loop and that turns off to open said conductive loop; and

a control circuit, operatively coupled to the sensor and the switch, for controlling said switch to turn said switch on when said medical device is stimulating said electrically excitable tissue to achieve a therapeutic effect if the level of said external signal is less than an external signal threshold level, and to turn said switch off whenever the sensor detects the voltage build up in the conductive loop to either one of the positive polarity and the negative polarity when the level of said external signal is greater than said external signal threshold level.

16. A method for limiting unwanted current, 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 method including the steps of:

A. closing said conductive loop when said medical device is stimulating said electrically excitable tissue for achieving a therapeutic effect if the level of said external signal is less than said external signal threshold level; and

B. opening said conductive loop by generating and maintaining a high impedance within said conductive loop whenever the level of said external signal is greater than said external signal threshold level, said step B farther including the steps of;

opening a first switch device coupled within said conductive loop whenever the level of said external signal is greater than said external signal threshold level; and

opening a second switch device coupled within said conductive loop whenever the level of said external signal is greater than said external signal threshold level,

wherein, the first switch device and the second switch device are operatively coupled in series within the conductive loop such that the first switch device prevents parasitic current flow through said second switch device when the external signal causes voltage build-up to a positive polarity within the conductive loop, and such that said second switch device prevents parasitic current flow through said first switch device when the external signal causes voltage build-up to a negative polarity within the conductive loop.