Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-12-08
2002-08-06
Jastrzab, Jeffrey F. (Department: 3737)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06430440
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to implantable electrical devices and, more particularly, an implantable cardiac stimulation device that incorporates a magnetoresistive-based position sensor which is adapted to sense the body position of a patient in which the device is implanted.
BACKGROUND OF THE INVENTION
Implantable cardiac stimulation devices, such as pacemakers and implantable cardioverter defibrillators (ICDs), have become increasingly sophisticated over the past several years. These devices are now capable of sensing the performance of a patient's heart and responsively applying therapeutic electrical stimulation to the heart that is tailored to correct the heart's performance.
In fact, current generation pacemakers are capable of sensing the activity level of the patient and then tailoring the delivery of pacing pulses to the patient's heart to match the activity level of the patient. For example, if the pacemaker ascertains that the patient is more active, the pacemaker increases the pacing rate so that the patient's heart beats more rapidly to provide an increased flow of blood to the patient. Conversely, when the patient is at rest, the pacemaker decreases the pacing rate so as to maximize the battery life of the pacemaker and also so that the heart rate of the patient more closely mirrors the normal function of the heart when the patient is at rest.
Typically, pacemakers and other implantable cardiac stimulation devices that provide therapy based at least in part on the activity of the patient, incorporate some sort of an activity sensor. Typically, the activity sensor is comprised of an accelerometer that provides a signal that is indicative of the activity level of the patient. The accelerometer is generally positioned within the implantable cardiac stimulation device's casing and the accelerometer provides a signal which is indicative of the acceleration experienced by the casing. It is, of course, understood that the greater the activity level of the patient, the more the casing is accelerated. Hence, the accelerometer is capable of providing a signal which is indicative of the activity level of the patient.
Typically, accelerometers that are used in implantable cardiac stimulation devices incorporate some sort of piezo-electric sensor. One such accelerometer is described in U.S. Pat. No. 5,425,750 to Moberg. This sensor incorporates a cantilevered beam with a weight mounted on the cantilevered end of the beam. The surface of the beam is coated with a piezo-electric crystal polymer. Acceleration of the casing containing the accelerometer results in the cantilevered beam bending in response to the acceleration. The piezo-electric crystal is mechanically deformed by this bending and thereby produces an electrical signal that is proportionate to the mechanical deformation of the crystal. This electric signal can be amplified and used to provide an indication of the activity level of the patient.
One difficulty associated with using these piezo-electric sensors is that the piezo-electric material is often very expensive. This is particularly true for the very sensitive sensors-that have to be used in implantable cardiac stimulation devices. The accelerometer as a whole cannot be very large in size as it has to be positioned in the limited confines of an implantable cardiac stimulation device casing. Hence, the piezo-electric material must also be relatively small in size which requires the piezo-electric material to be very sensitive in order to be able to provide an electrical signal that is reflective of the acceleration of the cardiac stimulation device casing. These types of piezo-electric materials are very expensive and increase the overall cost of the implantable cardiac stimulation device.
Also, piezo-electric acceleration sensors are inherently AC coupled. This type of sensor, i.e., an AC accelerometer, only produces an activity signal output that is proportional to the beam's rate of change of bending and cannot be used to sense the position of the patient's body. To accommodate for this lack of body position information, systems have been developed which attempt to determine the position of the patient's body based upon changes in these AC activity signals. By processing this derived position information in coordination with activity information, orthostatic and circadian based compensation can be provided to an otherwise chronotropically incompetent patient. Illustrative of such systems are U.S. Pat. No. 5,476,483 to Bornzin et al. and commonly-assigned copending U.S. application Ser. No. 09/359,025 to Park et al., both of which are incorporated by reference in their entirety.
Another difficulty associated with the piezo-electric acceleration sensors is that, even though very sensitive piezo-electric materials can be used, the limited amount of space that is taken up by the piezo-electric sensor can still be quite-considerable. As the implantable cardiac stimulation device casing is implanted within the body, it is desirable to minimize the size of each of the components that are positioned within the casing so as to reduce the overall size of the implantable cardiac stimulation device. Unfortunately, the piezo-electric based acceleration sensor must have a certain minimum amount of surface area in order for the material to mechanically deform sufficiently so as to provide a usable signal indicative of the patient's activity level. Consequently, the minimum size of an accelerometer of this type is comparatively large and is not subject to a significant reduction in size.
Moreover, the piezoelectric crystals used in these types of activity sensors are under continuous repeated stress. This results in fatigue in the crystalline structure that can, ultimately, result in the activity sensor ceasing to work. It will, of course, be appreciated that replacement of inoperative activity sensors in implanted cardiac stimulation devices is impractical if not impossible due to the invasiveness of the procedure.
Hence, there is a need for a sensor which is capable of detecting the body position as well as the activity level of the patient and providing a signal indicative thereof which is both small in size and made of inexpensive components. To this end, there is a need for a sensor which does not require the use of a large amount of surface area, does not require the use of very sensitive piezo-electric materials, and is more resistant to fatigue related material failure.
SUMMARY OF THE INVENTION
The aforementioned needs are satisfied by the sensor of the present invention which is adapted for use with an implantable electrical device, e.g., a cardiac stimulation device. The sensor of the present invention incorporates a magnetoresistive sensor and a magnet that are mounted so as to be movable with respect to each other. Relative movement between the magnet and the magnetoresistive sensor produces a change in the resistance value of the magnetoresistive sensor. This change in the resistance value can be sensed by the application of a voltage to the sensor so that the resulting output signal is indicative of the body position of the patient.
In one aspect of the invention, the magnetoresistive sensor is mounted to a substrate and a permanent magnet is mounted on a bendable cantilevered beam that is attached to the substrate so as to position the permanent magnet in proximity to the magnetoresistive sensor. Movement of the substrate will result in the cantilevered beam bending and vibrating. The movement of the cantilevered beam results in the permanent magnet changing its relative position with respect to the magnetoresistive sensor thereby causing the resistance of the magnetoresistive sensor to change.
In one embodiment of the invention, the magnetoresistive sensor is comprised of a giant magnetoresistive (GMR) sensor that provides a differential output voltage which is indicative of the sensed magnetic field. The output signal varies in both amplitude and frequency and both of these variables ca
Gibson Scott
McNeil, II Kenneth R.
Shankar Balakrishnan
Vogel Alan B.
Jastrzab Jeffrey F.
Oropeza Frances P.
Pacesetter Inc.
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