Combined magnetic field detector and activity detector employing a capacitive sensor, for a medical implant

• US 5,709,225 A
• Filed: 09/20/1995
• Issued: 01/20/1998
• Est. Priority Date: 09/22/1994
• Status: Expired due to Fees

• Current Assignee: Pacesetter AB
• Original Assignee: Pacesetter AB
• Inventors: Budgifvars, Goran, Bergstrom, Inga
• Primary Examiner(s): Jastrzab, Jeffrey R.
• Application Number:
  US08/531,274
• Time in Patent Office: 853 Days
• Field of Search: 607/27-32, 607/17, 128/899, 128/903
• US Class Current: 128/899
• CPC Class Codes: A61N 1/36542 controlled by body motion, e.g. acceleration
• CPC Class Codes: G01R 33/02 Measuring direction or magnitude of magnetic fields or magnetic flux G01R33/20 takes precedence
• CPC Class Codes: G01V 3/081 the magnetic field is produced by the objects or geological structures characterised by the method of magnetic field measurement G01R33/00
• CPC Class Codes: Y10S 128/903 Radio telemetry

Abstract

A medical implant contains a magnetic field detector with a sensor whose capacitance changes in the presence of a magnetic field with a predefined strength, and a capacitance-controlled unit connected to the sensor. The capacitance-controlled unit sends a measurement signal to a detection unit. If a measurement signal parameter affected by capacitance exhibits a specific change over a predefined period of time, a detection signal is generated by the detection unit to indicate the presence of a magnetic field. The sensor'"'"'s capacitance also changes when the sensor moves, and this is utilized for measuring activity in an activity measurement circuit connected to the capacitance-controlled unit and for generating an activity signal on the basis of the measurement signal.

Claims

1. A medical implant comprising:

a capacitive accelerometer having a capacitance which changes upon motion of said capacitive accelerometer and at least partially coated with a material sensitive to magnetic fields which causes a change in the capacitance of said sensor in the presence of a magnetic field;

means connected to said capacitive sensor for generating a signal including a first component dependent on a change in said capacitance due to motion of said capacitive accelerometer and a second component dependent on a change in capacitance of said capacitive accelerometer in the presence of a magnetic field having a predetermined field strength;

means for generating an activity signal dependent on said first component;

means for generating a magnetic field detection signal dependent on said second component;

means for generating a medical therapy, said medical therapy being adjustable and having a plurality of therapy modes;

means connected to said means for generating a medical therapy adapted for delivering said medical therapy to a subject; and

control means, supplied with said activity signal and with said magnetic field detection signal, for controlling said means for generating a medical therapy for adjusting said medical therapy dependent on said activity signal and for changing a mode of said medical therapy dependent on said magnetic field detection signal.

2. A medical implant as claimed in claim 1 wherein said means for generating a signal comprises:

a capacitance-controlled unit connected to said capacitive accelerator which emits a measurement signal having at least one measurement signal parameter which changes dependent on the change in capacitance of said capacitive accelerometer; and

means connected to said capacitance-controlled unit and supplied with said measurement signal for generating said detection signal if said at least one measurement signal parameter changes to a predetermined value for a predetermined period of time.

3. A medical implant as claimed in claim 2 wherein said capacitance-controlled unit comprises a capacitance-controlled amplifier having a gain, and wherein said at least one measurement signal parameter comprises said gain.

4. A medical implant as claimed in claim 2 wherein said capacitance-controlled unit comprises a capacitance-controlled oscillator having an output frequency, and wherein said at least one measurement signal parameter comprises said output frequency.

5. A medical implant as claimed in claim 1 wherein said capacitive accelerometer comprises at least two electrode plates, moveable relative to each other, and said capacitance of said capacitive accelerometer changing dependent upon a distance between said at least two electrode plates.

6. A medical implant as claimed in claim 5 wherein at least one of said electrode plates is coated with said material sensitive to magnetic fields.