Implatable electrical device incorporating a magnetoresistive-based motion sensor

• US 6,411,849 B1
• Filed: 12/08/1999
• Issued: 06/25/2002
• Est. Priority Date: 12/08/1999
• Status: Expired due to Fees

• Current Assignee: Pacesetter Incorporated (Abbott Laboratories Incorporated)
• Original Assignee: Pacesetter Incorporated (Abbott Laboratories Incorporated)
• Inventors: Gibson, Scott, McNeil, Kenneth R. II, Vogel, Alan B., Shankar, Balakrishnan
• Primary Examiner(s): Jastrzab, Jeffrey R.
• Assistant Examiner(s): Oropeza, Frances P.
• Application Number:
  US09/457,279
• Time in Patent Office: 930 Days
• Field of Search: 600/595, 607/4, 607/5, 607/7, 607/9, 607/11, 607/17, 607/19, 735/143.1
• US Class Current: 607/19
• CPC Class Codes: A61N 1/36542 controlled by body motion, e.g. acceleration

Abstract

An implantable cardiac device including an activity sensor that incorporates a magnetoresistive sensor and a magnet that are positioned so as to move relative to each other in response to activity of the patient when the device is implanted in the body of the patient. In one embodiment, the sensor includes a magnetoresistive sensor that is made from giant magnetoresistive (GMR) materials. In another embodiment, a magnet is positioned on a flexible cantilevered beam so as to be positioned adjacent the magnetoresistive sensor. Movement of the patient results in relative movement of the magnet with respect to the magnetoresistive sensor. The signal can be used by the implantable cardiac device'"'"'s processor for adjusting the delivery of therapeutic electrical stimulation such as by adjusting the pacing rate of pacing pulses being delivered to the patient'"'"'s heart.

Claims

1. An implantable cardiac device contained within a casing and adapted to be implanted in the body of a patient, the device comprising:

delivery means for delivering therapeutic electrical stimulation to the heart of the patient;

control means for controlling the delivery of therapeutic electrical stimulation to the heart of the patient; and

an activity sensor incorporating a first magnet and a magnetoresistive sensor, the activity sensor being adapted to be implanted in the body of the patient so that movement of the patient results in relative movement between the first magnet and the magnetoresistive sensor so that the magnetoresistive sensor provides a variable activity signal that is indicative of the activity of the patient; and

wherein

the control means controls the delivery of therapeutic electrical simulation to the heart of the patient responsive, at least in part, to the variable activity signal; and

means for generating a magnetic field to produce a substantially zero magnetic field at the magnetoresistive sensor when the first magnet is at rest.

2. The device of claim 1, wherein the delivery means comprises at least one lead that is adapted to be positioned adjacent to the wall of the heart and is further adapted to deliver pacing pulses to the heart.

3. The device of claim 2, wherein the control means comprises a processor that is positioned within the casing, wherein the processor is adapted to receive the variable activity signal and to use the variable activity signal to adjust the frequency of the pacing pulses that are to be delivered to the heart via the at least one lead.

4. The device of claim 3, wherein the control means is adapted to increase the pacing rate of pacing pulses that are delivered to the heart when the variable activity signal indicates that the patient is more active.

5. The device of claim 1, wherein the activity sensor additionally comprises:

a substrate having a first and a second surface that is positioned within the casing of the implantable cardiac device wherein the magnetoresistive sensor is mounted on the substrate;

and wherein the first magnet is movably mounted proximate to the substrate so that when the casing is implanted in the body of a patient, activity of the patient results in the magnet moving with respect to the magnetoresistive sensor so that the magnetoresistive sensor produces the variable activity signal that is proportional to the movement of the magnet with respect to the magnetoresistive sensor.

6. The device of claim 5, wherein the activity sensor further comprises:

a mounting block that is positioned on the first surface of the substrate; and

a flexible cantilevered beam that is attached to the mounting block so that the flexible cantilevered beam extends outward from the mounting block; and

wherein

the first magnet is mounted on the outer end of the flexible cantilevered beam so as to be positioned adjacent the magnetoresistive sensor.

7. The device of claim 6, wherein the means for generating a magnetic field comprises a second magnet that is positioned on the second surface of the substrate, wherein the second magnet is selected so that when the first magnet is positioned adjacent the magnetoresistive sensor and is at rest, the combined magnetic field of both the first magnet and the second magnet is substantially zero at the magnetoresistive sensor.

8. The device of claim 7, wherein the magnetoresistive sensor is formed out of giant magnetoresistive (GMR) materials.

9. The device of claim 1, wherein the magnetoresistive sensor is comprised of a bridge circuit having at least one resistor formed out of giant magnetoresistive (GMR) materials.

10. The device of claim 9, further comprising an activity circuit that receives an output voltage signal from the bridge circuit and provides an input signal to the control means that is indicative of the relative movement between the first magnet and the magnetoresistive sensor.

11. The device of claim 10, wherein the input signal comprises the variable activity signal and the variable activity signal received by the control means is indicative of both the frequency and magnitude of the relative motion of the first magnet and the magnetoresistive sensor.

12. The device of claim 10, wherein the activity circuit comprises:

a voltage controlled oscillator that receives the output signal from the bridge circuit and oscillates at a frequency that is proportional to the magnitude of the voltage provided by the bridge circuit; and

a counter which receives an output signal of the voltage controlled oscillator and provides a digital signal indicative of the oscillations of the voltage controlled oscillator; and

wherein

the control means periodically samples the counter value so that the counter value comprises the variable activity signal.

13. An implantable cardiac device adapted to be implanted in the body of a patient, the device comprising:

a casing adapted to be implanted within the body of a patient;

a controller for controlling the delivery of therapeutic electrical stimulation to the heart of the patient;

a first magnet positioned within the casing;

a magnetoresistive sensor positioned within the casing, wherein the first magnet and the magnetoresistive sensor are mounted so as to be moveable with respect to each other;

wherein

the magnetoresistive sensor produces a first signal that is indicative of the magnetic field sensed by the magnetoresistive sensor such that the first signal is indicative of the relative motion between the first magnet and the magnetoresistive sensor; and

an activity circuit that receives the first signal and provides a corresponding activity signal to the controller so as to allow the controller to adjust the delivery of therapeutic electrical stimulation to the heart of the patient responsive, atileast in part, to the activity signal; and

a second magnet that is positioned on the second surface of the substrate, wherein the strength of the second magnet is selected so that when the first magnet is positioned adjacent the magnetoresistive sensor and is at rest, the combined magnetic field of both the first magnet and the second magnet is substantially zero at the magnetoresistive sensor.

14. The device of claim 13, wherein the activity signal received by the controller is indicative of both the frequency and magnitude of the relative motion of the first magnet and the magnetoresistive sensor.

15. The device of claim 14, wherein the activity circuit comprises:

a voltage controlled oscillator that receives the output signal from the bridge circuit and oscillates at a frequency that is proportional to the magnitude of the voltage provided by the bridge circuit; and

a counter which receives the output signal of the voltage controlled oscillator and provides a digital signal indicative of the oscillations of the voltage controlled oscillator and wherein the controller periodically samples the counter value so that the counter value comprises the variable activity signal.

16. The device of claim 13, further comprising a substrate positioned within the casing the substrate having a first and a second surface wherein the magnetoresistive sensor is mounted on the substrate and wherein the first magnet is movably mounted to the substrate so that when the casing is implanted in the body of a patient, activity of the patient results in the first magnet moving with respect to the magnetoresistive sensor so that the magnetoresistive sensor produces the first signal that is indicative of the movement of the first magnet with respect to the magnetoresistive sensor.

17. The device of claim 15, further comprising:

a mounting block that is positioned on the first surface of the substrate; and

a flexible cantilevered beam that is attached to the mounting block so that the flexible cantilevered beam extends outward from the mounting block wherein the first magnet is mounted on the outer end of the flexible cantilevered beam so as to be positioned adjacent to the magnetoresistive sensor.

18. The device of claim 13, wherein the magnetoresistive sensor is comprised of a magnetoresistive sensor formed out of giant magnetoresistive (GMR) materials.

19. The device of claim 13, wherein the magnetoresistive sensor is comprised of a bridge circuit having at least one resistor formed out of giant magnetoresistive (GMR) materials.

20. The device of claim 13, wherein the controller is adapted to increase the pacing rate of pacing pulses that are delivered to the heart when the activity signal indicates that the patient is more active.