U.S. patent application number 10/900573 was filed with the patent office on 2006-02-02 for impedance measurement for an implantable device.
Invention is credited to Randolph K. Armstrong, Scott A. Armstrong.
Application Number | 20060025828 10/900573 |
Document ID | / |
Family ID | 37102064 |
Filed Date | 2006-02-02 |
United States Patent
Application |
20060025828 |
Kind Code |
A1 |
Armstrong; Randolph K. ; et
al. |
February 2, 2006 |
Impedance measurement for an implantable device
Abstract
A method and an apparatus for determining an impedance
experienced by a lead associated with the implantable device. A
substantially constant current signal is provided through a first
terminal and a second terminal of the lead. A voltage across the
first and second terminals is measured. An impedance across the
first and second terminals is determined based upon the constant
current signal and the measured voltage.
Inventors: |
Armstrong; Randolph K.;
(Houston, TX) ; Armstrong; Scott A.; (Danbury,
TX) |
Correspondence
Address: |
CYBERONICS, INC.
LEGAL DEPARTMENT, 6TH FLOOR
100 CYBERONICS BOULEVARD
HOUSTON
TX
77058
US
|
Family ID: |
37102064 |
Appl. No.: |
10/900573 |
Filed: |
July 28, 2004 |
Current U.S.
Class: |
607/28 ; 607/27;
607/45; 607/48 |
Current CPC
Class: |
A61N 1/3708 20130101;
G01R 31/389 20190101 |
Class at
Publication: |
607/028 ;
607/027; 607/045; 607/048 |
International
Class: |
A61N 1/18 20060101
A61N001/18 |
Claims
1. A method for determining an impedance associated with a lead
assembly in an implantable medical device, said lead assembly
comprising a first terminal and a second terminal, said method
comprising: providing a controlled current signal through said
first terminal and said second terminal; measuring a voltage across
said first and second terminals; and determining the impedance
across said first and second terminals based upon said constant
current signal and said voltage.
2. The method of claim 1, wherein said controlled current is a
substantially constant current.
3. The method of claim 2, wherein providing a substantially
constant current signal comprises providing a current signal that
is independent of the impedance across said lead terminals.
4. The method of claim 2, wherein measuring said voltage across
said first and second terminals comprises measuring said voltage
that results from said constant current signal.
5. The method of claim 2, wherein determining an impedance across
said first and second terminals of said lead assembly further
comprises dividing said measured voltage by the magnitude of said
constant current signal.
6. The method of claim 2, wherein determining an impedance across
said first and second terminals of said lead assembly comprises
dividing said measured voltage by the magnitude of said constant
current signal, and then subtracting an impedance associated with
said first and second terminals.
7. The method of claim 2, wherein measuring said voltage across
said first and second terminals comprises converting said voltage
signal from analog to digital using an analog-to-digital
converter.
8. The method of claim 2, further comprising calibrating said
analog-to-digital converter using a known impedance and driving a
known current signal across said known impedance to calibrate said
analog-to-digital converter based upon a resultant theoretical
voltage.
9. An implantable medical device, comprising: an amplifier circuit
for providing a substantially constant current signal through a
first terminal and a second terminal of a lead assembly; a voltage
measurement unit to measure a voltage across said first and second
terminals; and an impedance measurement unit to determine an
impedance between said first and second terminals based upon said
constant current signal and said measured voltage.
10. The implantable medical device of claim 9, wherein said
implantable medical device is a vagus nerve stimulator device.
11. The implantable medical device of claim 9, further comprising
an analog-to-digital converter to convert said voltage from an
analog signal to a digital signal for measurement.
12. The implantable medical device of claim 9, wherein said voltage
measurement unit is adapted to measure the voltage resulting from
an application of said constant current signal across said first
and second terminals of said lead assembly.
13. The implantable medical device of claim 9, wherein said
impedance measurement unit is adapted to divide the magnitude of
said voltage by the magnitude of said constant current signal.
14. The implantable medical device of claim 9, wherein said
impedance measurement unit is adapted to divide the magnitude of
said voltage by the magnitude of said constant current signal and
subtract an impedance value associated with said first and second
terminals.
15. A system for measuring the impedance of a lead assembly in an
implantable medical device, said lead assembly comprising first and
second terminals, comprising: an external device for performing
remote communications with an implantable medical device; an
implantable medical device adapted to communicate with said
external device and to deliver a stimulation signal to a patient
via a lead assembly, said implantable device comprising: an
amplifier circuit for providing a substantially constant current
signal through said first terminal and said second terminal of said
lead assembly; a voltage measurement unit to measure a voltage
across said first and second terminals; an impedance measurement
unit to determine an impedance between said first and second
terminals based upon said constant current signal and said voltage;
and a communications unit for providing said impedance
determination to said external device.
16. The system of claim 15, wherein said implantable device is a
vagus nerve stimulator.
17. The system of claim 15, wherein said communications unit is
capable of providing wireless communications with said external
device.
18. The system of claim 17, wherein said external device comprises
a computer system capable of executing at least one of a
communication program and a data acquisition program.
19. The system of claim 18, wherein said external device is capable
of transmitting software code to said implantable device, and said
implantable medical device is capable of receiving and executing
said code.
20. The system of claim 15, wherein said implantable medical device
further comprises an analog-to-digital converter to convert said
voltage from an analog signal to a digital signal for
measurement.
21. The system of claim 15, wherein said voltage measurement unit
is adapted to measure the voltage that resulting from an
application of said constant current signal across said first and
second terminals of said lead assembly.
22. The system of claim 15, wherein said impedance measurement unit
is adapted to divide said voltage by the magnitude of said constant
current signal.
23. The system of claim 15, wherein said impedance measurement unit
is adapted to divide said voltage by the magnitude of said constant
current signal, and subtract an impedance value associated with
said first and second terminals.
24. An apparatus for determining an impedance associated with a
lead medical assembly of an implantable medical device, comprising:
means for providing a substantially constant current signal through
a first terminal and a second terminal of said lead assembly; and
means for measuring a voltage across said first and second
terminals; and means for determining an impedance across said first
and second terminals based upon said constant current signal and
said measured voltage.
25. A computer readable program storage device encoded with
instructions that, when executed by a computer, performs a method
for determining an impedance associated with a lead assembly having
first and second terminals and comprising a component of an
implantable medical device, comprising: providing a substantially
constant current signal through said first terminal and said second
terminal of said lead assembly; measuring a voltage across said
first and second terminals while said constant current is provided;
and determining an impedance across said first and second terminals
based upon said constant current signal and said measured
voltage.
26. The computer readable program storage device of claim 25,
wherein providing a constant current signal comprises providing a
current signal that is independent of the impedance across said
first and second terminals of said lead assembly.
27. The computer readable program storage device of claim 25,
wherein measuring said voltage across said first and second
terminals comprises measuring said voltage resulting from said
constant current signal.
28. The computer readable program storage device of claim 25,
wherein determining an impedance across said first and second
terminals comprises dividing said measured voltage by the magnitude
of said constant current signal.
29. The computer readable program storage device of claim 25,
wherein said lead assembly further comprises first and second lead
wires coupled to tissue of a patient, and wherein said step of
determining an impedance across said first and second terminals
comprises dividing the magnitude of said voltage by the magnitude
of said constant current signal, and subtracting an impedance
associated with said first and second lead wires.
30. The computer readable program storage device of claim 25,
wherein measuring said voltage across said first and second
terminals further comprises converting said voltage signal from
analog to digital using an analog-to-digital converter.
31. The computer readable program storage device of claim 25,
further comprising calibrating said analog-to-digital converter
using a known impedance and driving a known current signal across
said known impedance to calibrate said analog-to-digital converter
based upon a resultant theoretical voltage.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to implantable medical
devices, and more particularly to methods, apparatus, and systems
for monitoring power consumption and impedance characteristics
relating to implantable medical devices.
[0003] 2. Description of the Related Art
[0004] There have been many improvements over the last several
decades in medical treatments for disorders of the nervous system,
such as epilepsy and other motor disorders, and abnormal neural
discharge disorders. One of the more recently available treatments
involves the application of an electrical signal to reduce various
symptoms or effects caused by such neural disorders. For example,
electrical signals have been successfully applied at strategic
locations in the human body to provide various benefits, including
reducing occurrences of seizures and/or improving or ameliorating
other conditions. A particular example of such a treatment regimen
involves applying an electrical signal to the vagus nerve of the
human body to reduce or eliminate epileptic seizures, as described
in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby
incorporated by reference in its entirety in this specification.
Electrical stimulation of the vagus nerve may be provided by
implanting an electrical device underneath the skin of a patient
and performing a detection and electrical stimulation process.
Alternatively, the system may operate without a detection system if
the patient has been diagnosed with epilepsy, and may periodically
apply a series of electrical pulses to the vagus (or other cranial)
nerve intermittently throughout the day, or over another
predetermined time interval.
[0005] Many types of implantable medical devices, such as
pacemakers and drug infusion pumps, typically include custom
integrated circuits that are complex, expensive, and specific to
the intended use. These systems also typically employ proprietary
communications techniques to transfer information between the
implant and an external programmer. The custom circuitry is
developed because of the need to keep power consumption at a
minimum, to conform to the allowable size for implantable devices,
and to support the complexity of the detection and communication
techniques, while still supplying the particular intended
therapy.
[0006] Typically, implantable medical devices (IMDs) involving the
delivery of electrical pulses to body tissues, such as pacemakers
(heart tissue) and vagus nerve stimulators (nerve tissue), comprise
a pulse generator for generating the electrical pulses and a lead
assembly coupled at its proximal end to the pulse generator
terminals and at its distal end to one or more electrodes in
contact with the body tissue to be stimulated. One of the key
components of such IMDs is the power supply, ordinarily a battery,
which may or may not be rechargeable. In many cases surgery is
required to replace an exhausted battery. To provide adequate
warning of impending depletion of the battery and subsequent
degradation of the operation of the IMD, various signals may be
established and monitored. One such signal is an elective
replacement indicator (ERI) that may indicate that an electrical
device component, such as a battery, has reached a point where
replacement or recharging is recommended. Another indicator may be
an end of service (EOS) signal, which may provide an indication
that the operation of the implanted device is at, or near,
termination and delivery of the intended therapy can no longer be
guaranteed. ERI and EOS are commonly used indicators of the present
status of an IMD battery. ERI is intended to be a warning signal of
an impending EOS indication, providing sufficient time (e.g.,
several weeks or months) in typical applications to schedule and
perform the replacement or recharging.
[0007] Generally, battery-powered IMDs base the EOS and the ERI
signals on battery voltage and/or battery impedance measurements.
One problem associated with these methodologies is that, for many
battery chemistries, these measured battery characteristics do not
have monotonically-changing values with respect to remaining
charge. For example, lithium/carbon monofluoride (Li/CFx) cells
commonly used in neurostimulators and other IMDs have a relatively
flat voltage discharge curve for the majority of their charge life,
and present status of the battery cannot be accurately and
unambiguously determined from a measured battery
characteristic.
[0008] Another problem associated with this methodology is the
variability of current consumption for a specific device's
programmed therapy or circuitry. This variability, combined with
the uncertainty of the battery's present status prior to ERI or
EOS, hinders reliable estimation of the anticipated time until
reaching ERI or EOS. For scheduling purposes, it is desirable to
have a constantly available and reliable estimate over all
therapeutic parameter ranges and operation settings of the time
until the device will reach EOS, and provide an indication, similar
in purpose to ERI, when that time reaches a specific value or
range.
[0009] Impedance measurements are used to assess the integrity of
the electrical leads that deliver the stimulation provided by a
pulse generator. A change in the impedance across the leads that
deliver the electrical pulses may be indicative either of changes
in a patient's body or in the electrical leads themselves. For
example, damage in the lead, which may be induced by a break in one
or more filaments in a multifilament lead wire, or changes in the
body tissue where stimulation is delivered, may affect the efficacy
of the stimulation therapy. Therefore, it is desirable for changes
in the lead impedance, which may be indicative of various changes
or malfunctions, to be accurately detected.
[0010] For instance, the integrity of the leads that deliver
stimulation is of interest to insure that the proper therapy dosage
is delivered to the patient. Some IMDs, most notably pacemakers,
provide a voltage-controlled output that is delivered to one or
more body locations (such as the heart). Other IMDs, such as a
vagus nerve stimulator device developed by Cyberonics, Inc.,
provide a current-controlled output. Generally, however,
state-of-the-art measurements of lead impedance involve an analysis
of the delivery of a voltage signal from a capacitive (C) energy
storage component through the resistive (R) lead impedance and an
examination of the decay of that signal based upon a time-constant
proportional to the product of the resistance and capacitance (RC).
The total equivalent impedance present at the leads and the known
energy source total equivalent capacitance cause a time-constant
discharge curve. As the voltage on the capacitance is discharged
through the resistance, the exponential decay of this voltage may
be monitored to determine the decay time constant RC. From that
time constant and an estimate of the known equivalent capacitance
C, the equivalent resistance R presented by the leads may be
mathematically estimated. However, this type of measurement may
lead to inaccuracies for a number of reasons, including the fact
that the discharging of the voltage signal may be affected by other
resistances and capacitances in the system, the accuracy of the
capacitor, the time, voltage, and algorithmic accuracies of the
measurement system, and the like. It would be desirable to have a
more efficient and accurate method, apparatus, and/or system to
measure or assess the impedance present at the leads that deliver
an electrical stimulation or therapy.
[0011] The present invention is directed to overcoming, or at least
reducing, the effects of, one or more of the problems set forth
above.
SUMMARY OF THE INVENTION
[0012] In one aspect of the present invention, a method is provided
for projecting an end of service date and/or elective replacement
indication of a power supply in an implantable medical device, the
power supply having an initial electrical charge and a final
electrical charge. According to a preferred embodiment, the method
comprises determining an active charge depletion of an IMD,
determining an inactive charge depletion of the implantable device,
and determining a time period until an end of service (EOS) and/or
elective replacement indication (ERI) of a power supply associated
with the IMD based upon the active charge depletion, the inactive
charge depletion, and the initial and final (EOS) battery
charges.
[0013] In another embodiment, a method for projecting an end of
service and/or an elective replacement indication of an IMD having
a power supply with an initial electrical charge and a final
electrical charge comprises determining a current usage rate for at
least one future idle period, and determining charge depleted
during at least one previous idle period. The method also comprises
determining a current usage rate for at least one future
stimulation period, and determining charge depleted during at least
one previous stimulation period. A total charge depleted by the IMD
is determined based upon the charges depleted during the at least
one previous idle period and the at least one previous stimulation
period, respectively. A total future charge depletion is determined
based upon the current usage rate during the at least one future
stimulation period and the current usage rate during the at least
one future idle period. A time period until an end of service (EOS)
and/or ERI of a power supply (e.g., a battery) of the IMD is
determined based upon the total charge depleted and the total
future charge depletion, as well as the initial and final (EOS)
battery charges.
[0014] In a further embodiment of the present invention, a method
is provided for projecting an end of service date and/or elective
replacement indication of a power supply in an implantable medical
device, the power supply having an initial electrical charge and a
final electrical charge. According to a preferred embodiment, the
method comprises determining a charge depletion of an IMD and
determining a time period until an end of service (EOS) and/or
elective replacement indication (ERI) of a power supply associated
with the IMD based upon the charge depletion and the initial and
final (EOS) battery charges.
[0015] In a further embodiment of the present invention, a method
for projecting an end of service and and/or elective replacement
indication of an IMD having a power supply with an initial
electrical charge and a final electrical charge comprises
determining a previous active depleted charge of an IMD and
determining a future or potential active current usage rate of the
IMD. The method also comprises determining a previous inactive
depleted charge of the IMD and determining a future or potential
inactive current usage rate of the IMD. A time period until an EOS
and/or ERI of a power supply associated with the implantable device
is determined based upon the previous active depleted charge, the
potential active current usage rate, the previous inactive depleted
charge, the potential inactive current usage rate, and the initial
and final (EOS) battery charges.
[0016] In another aspect of the present invention, an implantable
medical device is provided for projecting an end of service and/or
an elective replacement indication of a power supply in the IMD.
The IMD comprises a battery with an initial electrical charge and a
final electrical charge to provide power for at least one operation
performed by the implantable device. The device further comprises a
stimulation unit operatively coupled to the battery, the
stimulation unit providing a stimulation signal to at least one
body location. The stimulation unit preferably comprises an
electrical pulse generator, but may alternatively comprise a drug
pump, a magnetic field generator, a mechanical vibrator element, or
other device for stimulating body tissue. The IMD also preferably
comprises a controller operatively coupled to the stimulation unit
and the battery. The controller is adapted to determine an active
current usage rate and an inactive current usage rate of the IMD,
as well as an active electrical charge depleted by the battery
during stimulation and an inactive electrical charge depleted by
the battery during inactive periods in which no electrical
stimulation is provided to the patient. The controller is further
adapted to determine a time period until an end of service of a
power supply associated with the IMD based upon the active current
usage rate and the inactive current usage rates, the active and
inactive electrical charges depleted, and the initial and final
electrical charges of the battery.
[0017] In still another aspect, the present invention comprises an
IMD for projecting an EOS and/or an ERI of a battery. The IMD
comprises a battery with an initial and a final (EOS) electrical
charge, a stimulation unit providing an electrical stimulation
signal, and a controller. The controller is adapted to determine
first and second active current usage rates for current usage in a
first stimulation therapy and a second stimulation therapy,
respectively. The controller is also adapted to determine first and
second inactive (i.e., non-stimulating) current usage rates in a
first inactive mode and a second inactive mode, respectively. In
addition, the controller is adapted to determine an active
electrical charge depleted by the battery during stimulation and an
inactive electrical charge depleted during inactive periods. The
controller also determines a time period until an EOS and/or an ERI
of the battery, based upon the first and second active current
usage rates, the first and second inactive current usage rates, the
active and inactive electrical charges depleted, and the initial
and final electrical battery charges.
[0018] In another aspect of the present invention, a system is
provided for projecting an EOS and/or an ERI of a power supply of
an IMD. The system comprises an external device (i.e., a device
outside the body of the patient) for performing remote
communications with the IMD, and the IMD is also capable of
communicating with the external device as well as delivering a
stimulation signal to the patient. The IMD comprises a battery to
provide power for delivering the stimulation signal, a
communications unit to provide communications between the external
device and the IMD, and a stimulation unit operatively coupled to
the battery for providing a stimulation signal. The system also
comprises a controller operatively coupled to the stimulation unit
and to the battery. The controller comprises a charge depletion
circuit for determining both an active charge depletion and an
inactive charge depletion of the IMD. The controller further
comprises an EOS/ERI circuit for determining a time period until an
end of service and/or an elective replacement indication of a power
supply associated with the implantable device, based upon the
active charge depletion, the inactive charge depletion, and the
original and EOS battery charges.
[0019] In yet another aspect of the present invention, a computer
readable program storage device encoded with instructions is
provided for projecting an end of service and/or an elective
replacement indication of a power supply in an IMD. The computer
readable program storage device is encoded with instructions that,
when executed by a computer, determine an active charge depletion
and an inactive charge depletion of the IMD, and also determines a
time period until an end of service and/or an elective replacement
indication of a power supply associated with the IMD based upon the
determined active charge depletion, the determined inactive charge
depletion, and the initial and final battery charges.
[0020] In another aspect of the present invention, a method is
provided for determining an impedance presented by a lead
associated with an IMD. In the method, a substantially constant
current signal is provided through a first terminal and a second
terminal of the lead. A voltage across the first and second
terminals is measured, and an impedance across the first and second
terminals is determined based upon the constant current signal
provided and the measured voltage.
[0021] In another aspect of the present invention, an IMD is
provided that comprises circuitry for determining an impedance
presented by a lead associated with the IMD. The IMD comprises an
amplifier circuit for providing a substantially constant current
signal through a first terminal and a second terminal of a lead.
The IMD further comprises a voltage measurement unit to measure a
voltage across the first and second terminals. The implantable
device additionally comprises an impedance determination unit to
determine an impedance between the first and second terminals based
upon the constant current signal and the voltage.
[0022] In another aspect of the present invention, a system is
provided for determining an impedance experienced by a lead
associated with an IMD. The system comprises an external device
communicating with the IMD, and the IMD is in turn adapted to
communicate with the external device and to deliver a stimulation
signal to a lead coupled to the IMD. The IMD comprises an amplifier
circuit for providing a substantially constant current signal
through a first terminal and a second terminal of the lead. The IMD
also includes a voltage measurement unit to measure a voltage
across the first and second terminals, and an impedance
determination unit to determine an impedance between the first and
second terminals based upon the constant current signal and the
measured voltage. The IMD may also include a communications unit
for communicating data relating to the impedance determination to
the external device.
[0023] In yet another aspect of the present invention, a computer
readable program storage device encoded with instructions is
provided for determining an impedance experienced by a lead coupled
to an IMD. The computer readable program storage device is encoded
with instructions that when executed by a computer, preferably
within the IMD, provides a substantially constant current signal
through a first terminal and a second terminal of the lead,
measures a voltage across the first and second terminals, and
determines an impedance across first and second terminals based
upon the constant current signal and the voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The invention may be understood by reference to the
following description taken in conjunction with the accompanying
drawings, in which like reference numerals identify like elements,
and in which:
[0025] FIG. 1A is a stylized diagram of an implantable medical
device suitable for use in the present invention implanted into a
patient's body;
[0026] FIG. 1B is a stylized diagram of another embodiment of an
implantable medical device suitable for use in the present
invention implanted into a patient's body;
[0027] FIG. 1C illustrates an implantable medical device suitable
for use in the present invention, showing the header and electrical
connectors for coupling the device to a lead/electrode
assembly;
[0028] FIG. 1D shows a lead and electrodes suitable for use in the
present invention attached to a vagus nerve of a patient;
[0029] FIG. 2 is a block diagram of an implantable medical device
and an external unit that communicates with the implantable medical
device, in accordance with one illustrative embodiment of the
present invention;
[0030] FIG. 3 is a stylized diagram of an output current signal
provided by the implantable medical device of FIGS. 1 and 2,
provided to illustrate certain stimulation parameters in accordance
with one illustrative embodiment of the present invention;
[0031] FIG. 4 is a flowchart representation of a method of
providing a warning signal relating to a power supply of the
implantable medical device, in accordance with one illustrative
embodiment of the present invention;
[0032] FIG. 5 is a flowchart representation of a method of
performing a calibration of a charge depletion tabulation, in
accordance with one illustrative embodiment of the present
invention;
[0033] FIG. 6 is a more detailed flowchart illustrating a method of
performing the charge depletion calculation indicated in FIG. 4, in
accordance with one illustrative embodiment of the present
invention;
[0034] FIG. 7 is a more detailed flowchart illustrating a method of
performing an end-of-service (EOS) and/or an elective replacement
indication (ERI) determination, as indicated in FIG. 4, in
accordance with one illustrative embodiment of the present
invention;
[0035] FIG. 8, is a block diagram of the stimulation unit shown in
FIG. 2, in accordance with one illustrative embodiment of the
present invention;
[0036] FIG. 9 is a block diagram of the impedance measurement unit
shown in FIG. 2, in accordance with one illustrative embodiment of
the present invention;
[0037] FIG. 10 is a flowchart of a method of performing an
impedance measurement, in accordance with one illustrative
embodiment of the present invention; and
[0038] FIG. 11 is a flowchart of a method of performing a
calibration of an A/D converter used for impedance measurement, in
accordance with one illustrative embodiment of the present
invention.
[0039] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof have been shown
by way of example in the drawings and are herein described in
detail. It should be understood, however, that the description
herein of specific embodiments is not intended to limit the
invention to the particular forms disclosed, but on the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0040] Illustrative embodiments of the invention are described
herein. In the interest of clarity, not all features of an actual
implementation are described in this specification. In the
development of any such actual embodiment, numerous
implementation-specific decisions must be made to achieve the
design-specific goals, which will vary from one implementation to
another. It will be appreciated that such a development effort,
while possibly complex and time-consuming, would nevertheless be a
routine undertaking for persons of ordinary skill in the art having
the benefit of this disclosure.
[0041] Embodiments of the present invention provide methods and
apparatus for monitoring and/or estimating the electrical charge
depletion of an implantable medical device (IMD). Estimating
battery life may be based upon estimated future charge depletion
and actual past charge depletion. Embodiments of the present
invention provide for an elective replacement indicator (ERI)
signal to provide a warning for performing an electrical diagnostic
operation upon the IMD. This electrical diagnostic operation may
include replacing an electrical component in the IMD, performing
additional evaluation(s) of the operation of the IMD, replacing or
recharging a power source of the IMD, and the like. A more detailed
description of an IMD suitable for use in the present invention is
provided in various figures and the accompanying description
below.
[0042] Generally, IMDs contain power storage devices or battery
units to provide power for the operations of the IMD. Embodiments
of the present invention determine an estimated usable life
remaining in the battery unit based upon determining initial and
final battery charges, charge depleted by operations of the IMD,
and a future depletion rate. Embodiments of the present invention
may be performed in a standalone manner within the IMD itself, or
in conjunction with an external device in communication with the
IMD. Utilizing embodiments of the present invention, an end of
service (EOS) signal or an ERI signal may be provided, indicating
that the IMD is at or near termination of operations and/or the
battery power has reached a level at which replacement should be
considered to avoid interruption or loss of therapy to the
patient.
[0043] Other embodiments of the present invention provide for
determining the lead impedance. This process involves determining
the voltage across a lead associated with the IMD, based upon the
delivery of a constant current signal. The impedance may be
measured on demand or at predetermined periodic intervals to detect
significant changes in impedance across the leads of the IMD.
Changes in the impedance may be logged and time-stamped, and saved
in a memory in the IMD for diagnostic considerations. Voltage and
current measurements associated with the IMD may be calibrated
using various impedance measurements in order to enhance the
accuracy of lead impedance measurements.
[0044] FIGS. 1A-1D illustrate a generator 110 having main body 112
comprising a case or shell 121 (FIG. 1A) with a connector 116 (FIG.
1C) for connecting to leads 122. The generator 110 is implanted in
the patient's chest in a pocket or cavity formed by the implanting
surgeon just below the skin (indicated by a dotted line 145),
similar to the implantation procedure for a pacemaker pulse
generator. A stimulating nerve electrode assembly 125, preferably
comprising an electrode pair, is conductively connected to the
distal end of an insulated electrically conductive lead assembly
122, which preferably comprises a pair of lead wires (one wire for
each electrode of an electrode pair). Lead assembly 122 is attached
at its proximal end to the connector 116 on case 121. The electrode
assembly is surgically coupled to a vagus nerve 127 in the
patient's neck. The electrode assembly 125 preferably comprises a
bipolar stimulating electrode pair (FIG. 1D), such as the electrode
pair described in U.S. Pat. No. 4,573,481 issued Mar. 4, 1986 to
Bullara. Persons of skill in the art will appreciate that many
electrode designs could be used in the present invention. The two
electrodes are preferably wrapped about the vagus nerve, and the
electrode assembly 125 is preferably secured to the nerve 127 by a
spiral anchoring tether 128 (FIG. 1D) such as that disclosed in
U.S. Pat. No. 4,979,511 issued Dec. 25, 1990 to Reese S. Terry, Jr.
and assigned to the same assignee as the instant application. Lead
assembly 122 is secured, while retaining the ability to flex with
movement of the chest and neck, by a suture connection 130 to
nearby tissue.
[0045] In one embodiment, the open helical design of the electrode
assembly 125 (described in detail in the above-cited Bullara
patent), which is self-sizing and flexible, minimizes mechanical
trauma to the nerve and allows body fluid interchange with the
nerve. The electrode assembly 125 preferably conforms to the shape
of the nerve, providing a low stimulation threshold by allowing a
large stimulation contact area with the nerve. Structurally, the
electrode assembly 125 comprises two electrode ribbons (not shown),
of a conductive material such as platinum, iridium,
platinum-iridium alloys, and/or oxides of the foregoing. The
electrode ribbons are individually bonded to an inside surface of
an elastomeric body portion of the two spiral electrodes 125-1 and
125-2 (FIG. 1D), which may comprise two spiral loops of a
three-loop helical assembly. The lead assembly 122 may comprise two
distinct lead wires or a coaxial cable whose two conductive
elements are respectively coupled to one of the conductive
electrode ribbons 125-1 and 125-2. One suitable method of coupling
the lead wires or cable to the electrodes comprises a spacer
assembly such as that disclosed in U.S. Pat. No. 5,531,778,
although other known coupling techniques may be used. The
elastomeric body portion of each loop is preferably composed of
silicone rubber, and the third loop 128 (which typically has no
electrode) acts as the anchoring tether 128 for the electrode
assembly 125.
[0046] In certain embodiments of the invention, eye movement
sensing electrodes 133 (FIG. 1B) may be implanted at or near an
outer periphery of each eye socket in a suitable location to sense
muscle movement or actual eye movement. The electrodes 133 may be
electrically connected to leads 134 implanted via a catheter or
other suitable means (not shown) and extending along the jawline
through the neck and chest tissue to the stimulus generator 110.
When included in systems of the present invention, the sensing
electrodes 133 may be utilized for detecting rapid eye movement
(REM) in a pattern indicative of a disorder to be treated, as
described in greater detail below.
[0047] Alternatively or additionally, EEG sensing electrodes 136
may optionally be implanted in spaced apart relation through the
skull, and connected to leads 137 implanted and extending along the
scalp and temple and then along the same path and in the same
manner as described above for the eye movement electrode leads.
Electrodes 133 and 137, or other types of sensors, may be used in
some embodiments of the invention to trigger administration of the
electrical stimulation therapy to the vagus nerve 127 via electrode
assembly 125. Use of such sensed body signals to trigger or
initiate stimulation therapy is hereinafter referred to as a
feedback loop mode of administration. Other embodiments of the
present invention utilize a continuous, periodic or intermittent
stimulus signal applied to the vagus nerve (each of which
constitutes a form of continual application of the signal)
according to a programmed on/off duty cycle without the use of
sensors to trigger therapy delivery. This type of delivery may be
referred to as a prophylactic therapy mode. Both prophylactic and
feedback loop administration may be combined or delivered by a
single IMD according to the present invention. Either or both modes
may be appropriate to treat the particular disorder diagnosed in
the case of a specific patient under observation.
[0048] The pulse generator 110 may be programmed with an external
computer 150 using programming software of the type copyrighted by
the assignee of the instant application with the Register of
Copyrights, Library of Congress, or other suitable software based
on the description herein, and a programming wand 155 to facilitate
radio frequency (RF) communication between the computer 150 (FIG.
1A) and the pulse generator 110. The wand 155 and software permit
noninvasive communication with the generator 110 after the latter
is implanted. The wand 155 is preferably powered by internal
batteries, and provided with a "power on" light to indicate
sufficient power for communication. Another indicator light may be
provided to show that data transmission is occurring between the
wand and the generator.
[0049] FIG. 2 illustrates one embodiment of an IMD 200 (which may
comprise pulse generator 110) for performing neurostimulation in
accordance with embodiments of the present invention. In one
embodiment, the implantable medical device 200 comprises a battery
unit 210, a power-source controller 220, a stimulation controller
230, a power regulation unit 240, a stimulation unit 250, an
impedance measurement unit 265, a memory unit 280 and a
communication unit 260. It will be recognized that one or more of
the blocks 210-280 (which may also be referred to as modules) may
comprise hardware, firmware, software, or any combination of the
three. The memory unit 280 may be used for storing various program
codes, starting data, and the like. The battery unit 210 comprises
a power-source battery that may be rechargeable or
non-rechargeable. The battery unit 210 provides power for the
operation of the IMD 200, including electronic operations and the
stimulation function. The battery unit 210, in one embodiment, may
be a lithium/thionyl chloride cell or, more preferably, a
lithium/carbon monofluoride (Li/CFx) cell. The terminals of the
battery unit 210 are preferably electrically connected to an input
side of the power-source controller 220 and the power regulation
unit 240.
[0050] The power-source controller 220 preferably comprises
circuitry for controlling and monitoring the flow of electrical
power to various electronic and stimulation-delivery portions of
the IMD 200 (such as the modules 230-265 and 280 illustrated in
FIG. 2). More particularly, the power-source controller 220 is
capable of monitoring the power consumption or charge depletion of
the implantable medical device 200 and is capable of generating the
ERI and the EOS signals. The power-source controller 220 comprises
an active charge-depletion unit 222, an inactive charge-depletion
unit 224, and an ERI/EOS calculation unit 226. The active
charge-depletion unit 222 is capable of calculating the charge
depletion rate of the implantable medical device 200 during active
states, and may comprise sub-units to calculate the charge
depletion rates of a plurality of active states comprising
different charge depletion rates. The active state of the
implantable medical device 200 may refer to a period of time during
which a stimulation is delivered by the implantable medical device
200 to body tissue of the patient according to a first set of
stimulation parameters. Other active states may include states in
which other activities are occurring, such as status checks and/or
updates, or stimulation periods according to a second set of
stimulation parameters different from the first set of stimulation
parameters. The inactive charge-depletion unit 224 is capable of
calculating the charge depletion rate of the implantable medical
device 200 during inactive states. Inactive states may also
comprises various states of inactivity, such as sleep mode, wait
modes, and the like. The ERI/EOS calculation unit 226 is capable of
performing calculations to generate an ERI signal and/or an EOS
signal. One or more of the active charge-depletion unit 222, the
inactive charge-depletion unit 224, and/or the ERI/EOS calculation
unit 226 may be hardware, software, firmware, and/or any
combination thereof.
[0051] The power regulation unit 240 is capable of regulating the
power delivered by the battery unit 210 to particular modules of
the IMD 200 according to their needs and functions. The power
regulation unit 240 may perform a voltage conversion to provide
appropriate voltages and/or currents for the operation of the
modules. The power regulation unit 240 may comprise hardware,
software, firmware, and/or any combination thereof.
[0052] The communication unit 260 is capable of providing
transmission and reception of electronic signals to and from an
external unit 270. The external unit 270 may be a device that is
capable of programming various modules and stimulation parameters
of the IMD 200. In one embodiment, the external unit 270 is a
computer system that is capable of executing a data-acquisition
program. The external unit 270 is preferably controlled by a
healthcare provider such as a physician, at a base station in, for
example, a doctor's office. The external unit 270 may be a
computer, preferably a handheld computer or PDA, but may
alternatively comprise any other device that is capable of
electronic communications and programming. The external unit 270
may be used to download various parameters and program software
into the IMD 200 for programming the operation of the implantable
device. The external unit 270 may also receive and upload various
status conditions and other data from the IMD 200. The
communication unit 260 may comprise hardware, software, firmware,
and/or any combination thereof. Communications between the external
unit 270 and the communication unit 260 may occur via a wireless or
other type of communication, illustrated generally by line 275 in
FIG. 2.
[0053] Stimulation controller 230 defines the stimulation pulses to
be delivered to the nerve tissue according to parameters and
waveforms that may be programmed into the IMD 200 using the
external unit 270. The stimulation controller 230 controls the
operation of the stimulation unit 250, which generates the
stimulation pulses according to the parameters defined by the
controller 230 and in one embodiment provides these pulses to the
connector 116 for delivery to the patient via lead assembly 122 and
electrode assembly 125 (see FIG. 1A). Various stimulation signals
provided by the implantable medical device 200 may vary widely
across a range of parameters. The Stimulation controller 230 may be
hardware, software, firmware, and/or any combination thereof.
[0054] FIG. 3 illustrates the general nature, in idealized
representation, of an output signal waveform delivered by the
output section of a pulse generator 110 (such as stimulation unit
250 shown in FIG. 2) to lead assembly 122 and electrode assembly
125 in an embodiment of the present invention. This illustration is
presented principally for the sake of clarifying terminology,
including the parameters of signal on-time, off-time, frequency,
pulse width, and current. In the treatment of a neuropsychiatric
disorder in an exemplary implementation, the stimulation unit 250
of the IMD 200 delivers pulses having a desired output signal
current and frequency, with each pulse having a desired output
signal pulse width. The pulses are delivered for the duration of
the output signal on-time (stimulation period), and are followed by
the output signal off-time during which no output signal is
delivered (idle period). This periodic stimulation reduces the
symptoms of the neuropsychiatric disorder. Stimulation parameters
suitable for treatment of a variety of medical conditions can be
found in the following patents: U.S. Pat. No. 4,702,254, U.S. Pat.
No. 5,025,807, U.S. Pat. No. 4,867,164, and U.S. Pat. No. 6,622,088
(epilepsy); U.S. Pat. No. 5,188,104 and U.S. Pat. No. 5,263,480
(eating disorders); U.S. Pat. No. 5,215,086 (migraine headaches);
U.S. Pat. No. 5,231,988 (endocrine disorders); U.S. Pat. No.
5,269,303 (dementia); U.S. Pat. No. 5,299,569 and U.S. Pat. No.
6,622,047 (neuropsychiatric disorders); U.S. Pat. No. 5,330,513 and
U.S. Pat. No. 6,721,603 (pain); U.S. Pat. No. 5,335,657 (sleep
disorders); U.S. Pat. No. 5,540,730 (motility disorders); U.S. Pat.
No. 5,571,150 (coma); U.S. Pat. No. 5,707,400 (refractory
hypertension); U.S. Pat. No. 6,587,719 and U.S. Pat. No. 6,609,025
(obesity); U.S. Pat. No. 6,622,041 (congestive heart failure). Each
of the foregoing patents is hereby incorporated by reference herein
in its entirety.
[0055] In one embodiment of the invention, the IMD 200 determines
EOS and ERI values by using a known initial battery charge
(Q.sub.0) and a predetermined EOS battery charge (Q.sub.EOS)
indicative of the end of useful battery service, together with the
charge actually depleted (Q.sub.d) by the IMD (calculated from the
current usage rates for idle and stimulation periods (r.sub.i and
r.sub.s), and the length of the respective idle and stimulation
periods), to calculate for a desired time point how much useful
charge remains on the battery (Q.sub.r) until the EOS charge is
reached, and how long at projected current usage rates the device
can operate until EOS or ERI. Once the charge actually depleted by
operation of the device (Q.sub.d) is known, the current usage rates
are then applied to the remaining useful charge Q.sub.r to
determine the time remaining until EOS and/or ERI.
[0056] The present invention allows EOS and ERI determinations to
be made without measurements or calculations of internal battery
impedance or other battery parameters. Instead, the device
maintains a precise record of the current used during idle and
stimulation periods, and subtracts the charge represented by the
current used from the total available battery charge to determine
the charge remaining on the battery. Because the relative duration
of stimulation and idle periods are determined by the stimulation
programming parameters of the IMD, a determination of EOS and ERI
can be calculated in a straightforward manner based upon the
current usage rates associated with the programming parameters.
[0057] Consistent with the foregoing, FIG. 4 provides a flowchart
depiction of a method for determining the remaining time to EOS
and/or ERI based on known or determined IMD characteristics such as
battery charge and current usage rates. In one embodiment, the
current usage of the IMD 200 is calibrated during manufacture (step
410). Current drawn by the IMD from the battery is defined as
electrical charge per unit time. The total charge depleted from the
battery as a result of the operations of the IMD may be determined
by multiplying each distinct current rate used by the IMD by its
respective time used. In one embodiment, as part of the
calibration, during manufacturing, a power supply capable of
generating known currents and voltages may be used to characterize
the power consumption or current depletion of the implantable
medical device 200 during its stimulation and idle modes The power
consumption data thus obtained is preferably stored in a memory of
the IMD.
[0058] Once the charge usage characteristics of the IMD are known,
the battery may be subsequently installed into the implantable
medical device 200 for operation and therafter a record of power
consumed by the implantable medical device 200 is maintained. In a
particular embodiment, the calibration step 410 involves
calibration of current usage for idle periods (r.sub.i) and
stimulation periods (r.sub.s) of the device. Current may thus be
used as a proxy value for electrical charge depletion, and the
calibration step allows a precise determination of the amount of
electrical charge used by the device after implantation. As used
herein, the terms "depletion rate," "consumption rate," and "usage
rate" may be used interchangeably and refer to the rate at which
electrical charge is depleted from the battery. However, as noted
above, current may be used as a proxy for electrical charge, and
where this is the case, current rates r.sub.i and r.sub.s may also
be referred to as "current usage," "current rate," "current
consumption," "charge depletion," "depletion rate" or similar
terms.
[0059] As previously noted, the IMD 200 has a number of settings
and parameters (e.g., current, pulse width, frequency, and
on-time/off-time) that can be changed to alter the stimulation
delivered to the patient. These changes result in different current
usage rates by the IMD 200. In addition, circuit variations from
device to device may also result in different current usage rates
for the same operation. Calculations and estimations are preferably
performed during the manufacturing process in order to calibrate
accurately and precisely the current usage rates of the IMD 200
under a variety of stimulation parameters and operating conditions.
A calibration of the current usage rates and a determination of the
charge present on the battery at the time of implant allow a more
accurate assessment of actual and predicted charge depletion after
the IMD 200 is implanted. The initial charge on the battery may
include a safety factor, i.e., the charge may be a "minimum charge"
that all batteries are certain to possess, even though many
individual batteries may have a significantly greater charge.
Nothing herein precludes a determination of a unique initial charge
for each individual battery. However, it will be recognized that
such individual determinations may not be economically feasible. A
more detailed illustration and description of the step (410) of
calibrating current usage and initializing the battery charge for
the implantable medical device 200 is provided in FIG. 5 and the
accompanying description below.
[0060] After calibrating the current usage characteristics of the
IMD 200, the IMD may be implanted and subsequently a charge
depletion calculation is performed (step 420). This calculation may
be performed by the IMD itself, the external unit 270, or by both,
and includes determining the actual electrical charge depleted from
the battery 210 and estimating future current usage (i.e.,
depletion rates), which are then used to calculate an elective
replacement indication (ERI) and/or an end of service (EOS) signal
(step 430). A more detailed illustration and description of the
step 420 of calculating the electrical charge depleted is provided
in FIG. 6 and the accompanying description below. In step 430 an
estimated time until an elective replacement indication will be
generated and/or the estimated time until the end of service are
calculated utilizing the initial battery charge, the actual charge
consumed and the estimated future charge depletion calculated in
light of the calibration performed during manufacture. A more
detailed description and illustration of the step 430 of
calculating the time to ERI and/or EOS is provided in FIG. 7 and
the accompanying description below.
[0061] Referring now to FIG. 5, a flowchart diagram is provided
depicting in greater detail the step 410 (FIG. 4) of calibrating
and initializing the IMD 200 during manufacturing. In one
embodiment, the current rates for the IMD 200 during stimulation
are calibrated (block 510). During manufacturing, several different
combinations of measurements may be calibrated. More specifically,
measurements of charge depletion relating to different types of
pulses (i.e., pulses having different stimulation parameters) are
calibrated to ensure that current usage measurements for the IMD
are accurate over a wide range of stimulation parameters. In other
words, various pulses having a range of current amplitudes, pulse
widths, frequencies, duty cycles and/or lead impedances into which
the pulses are delivered are used to calibrate the measurement of
current usage during stimulation to establish a baseline of the
measurement of charge depletion for various types of pulses. All
operational variables relating to or affecting the current usage
rates of the IMD may be considered.
[0062] More particularly, during manufacture of the IMD 200,
several combinations of data points relating to various current
rates resulting from various combinations of pulse parameters are
used in one embodiment to generate a linear equation that relates
various pulse parameters to current rate, which may then be used to
determine charge depletion. For example, for a first stimulation,
pulses of a certain frequency are provided and for a second
stimulation, the frequency of the pulses used may be doubled.
Therefore, the estimated current usage rate for the second
stimulation may be estimated to be approximately double that of the
power consumption or charge depleted due to the first stimulation.
As another example, a first stimulation may be of a first pulse
width and a second stimulation may be of a pulse width that is
double that of the width of the first pulse. Therefore, a
relationship between the pulse width to the current consumption of
the second pulse may be estimated to be approximately double that
of the current usage rate of the first pulse. In one embodiment, a
graph may be generated using the various types of stimulation
versus the current consumption associated with that
stimulation.
[0063] As yet another example, a first stimulation pulse may have a
first current amplitude and a second stimulation may have a current
amplitude that is double that of the first stimulation pulse.
Therefore, the current consumption of the second stimulation pulse
may be estimated to be approximately double that of the current
consumption of the first stimulation pulse. The power consumption
is directly proportional to the current consumption. Therefore, a
relationship of a pulse parameter to current usage rate may be
estimated or measured such that an interpolation may be performed
at a later time based upon the linear relationship developed during
the calibration of the power consumption during stimulation. It may
be appreciated that the relationships of some pulse parameters to
current usage rate may not be a simple linear relationship,
depending upon such pulse characteristics as the type of pulse
decay (i.e., square wave, exponential decay), for example.
Nevertheless, calibration of current usage rate for various pulse
parameters may be performed by routine calculation or experiment
for persons of skill in the art having the benefit of the present
disclosure.
[0064] Referring again to FIG. 5, current usage during an idle
(i.e., non-stimulating) period is calibrated in step 520. From the
idle current consumption and the stimulation current consumption
calibration, the overall current consumption may be modeled based
upon programmed settings. It should be noted that while the
invention as shown in the drawings describes a device having two
current usage patterns associated with an idle period and a
stimulation period, such a two-state embodiment is described solely
for clarity, and more complex embodiments are possible involving a
third state such as, by way of nonlimiting example, a current usage
rate associated with electrical sensing of the lead electrodes,
which may be defined by a third current rate r.sub.3. Four-state or
even higher state embodiments are possible, although where the
differences in current usage rates are small, or where a particular
current usage rate comprises only a tiny fraction of the overall
time of the device, the complexity required to implement and
monitor the time such current rates are actually used by the device
may render the device impractical. These multi-state embodiments
may be implemented if desired, however, and remain within the scope
and spirit of the present invention.
[0065] Using the calibration of current usage during stimulation
periods (step 510) and idle periods (step 520), a calculation may
optionally be made to initialize the charge depleted, if any,
during manufacturing operations, such as the charge depleted during
testing of the device after assembly (block 530). In a preferred
embodiment, all of the calibrations are performed with a calibrated
current source device, and not a battery, and in this case there is
no charge depletion during manufacturing operations. In another
embodiment, the amount of charge depleted during manufacturing may
small, in which case the initialization procedure may also be
omitted. The calibration and/or initialization steps of FIG. 5
allow the IMD 200, via power-source controller 220, to maintain a
running tally of how much charge has been depleted from the device.
When the battery unit 210 is first inserted into the implantable
medical device 200, the charge depleted is generally initialized to
zero so that a running tabulation may begin from zero for
maintaining a running tally of the charge depleted from the battery
over the life of the implantable medical device 200. In one
embodiment, the charge depleted tally is incremented throughout the
operating life of the device and at any point the running tally may
be subtracted from the known initial charge of the battery to
determine the remaining charge. In an alternative embodiment, the
charge depleted tally could be initialized to the value of the
battery initial charge and the tally decremented throughout the
device operation and directly used as the remaining charge. In
either implementation, information relating to the baseline charge
remaining on the battery at the end of manufacturing may be
retained to calculate the estimated time to EOS or ERI.
[0066] Turning now to FIG. 6, a flowchart depiction of the step 420
of calculating charge depleted by the device is provided in greater
detail. For simplicity, only the two-current state of a single idle
period and a single stimulation period is shown. Embodiments having
additional current usage rates are included in the present
invention. The IMD 200 may determine a current depletion rate
r.sub.i for idle periods (block 610). The rate is preferably stored
in memory. In one embodiment, the determination is made by the IMD
200 after implantation. In a preferred embodiment, the idle current
depletion rate may be a rate determined during manufacturing (i.e.,
a rate calibrated in step 520) and stored in the memory 280. An
idle period is defined as a time period when the implantable
medical device 200 is not performing active stimulation, i.e., is
not delivering a stimulation pulse to the electrodes. Various
electronic functions, such as tabulation and calculation of numbers
or execution of various software algorithms within the IMD 200 may
take place during the idle period.
[0067] As noted, the current rate r.sub.i during idle periods 610
may be predetermined during the manufacturing process (step 520)
and may include various considerations, such as the power
consumption of the operation of various electronics in the
implantable medical device 200, even though no active stimulation
may be taking place during that time period. However, since the
implantable medical device 200 may be occasionally reprogrammed
while still implanted inside a patient's body, the number and
duration of idle periods may vary according to the duty cycle and
frequency of the stimulation pulses. Therefore, the IMD 200 (e.g.,
via the power source controller 220 in the device) may maintain a
running tabulation of the idle periods, and for each idle period a
certain amount of charge depleted during the idle period (i.e., off
time) is tabulated and stored in memory 280 (step 620).
[0068] It will be appreciated that the depleted charge may be
obtained in a number of different ways, each within the scope of
the present invention. Specifically, the total time of all idle
periods since implantation, initialization, or since a previous
idle power depletion calculation, may be maintained as a running
total idle time in memory, or alternatively a running tally of
charge depleted during idle periods may be maintained. While these
values are different numerically, they are directly related by
simple equations as discussed more fully hereinafter. At an update
time, the total idle time may be periodically accessed and
multiplied by the idle period current usage rate to determine the
total power depleted during idle periods since implantation,
initialization, or the previous calculation.
[0069] The IMD 200 may also maintain in memory 280 a tabulation of
current usage rates (i.e., charge depletion) for a wide range of
stimulation settings (step 630). In another embodiment, theoretical
charge depletion calculations relating to particular types of
stimulation may be provided to the IMD 200. The stimulation
parameter settings may then be used by the device to maintain a
running tabulation of the charge depleted during stimulation
periods using a current usage rate r.sub.s calculated from the
pulse width, pulse amplitude, pulse frequency, and other parameters
which may impact the current usage rate. This method provides
specific current usage rates for a variety of stimulation parameter
settings and lead impedances without requiring the storage of
current usage rates for all possible stimulation parameter settings
and lead impedances.
[0070] In one embodiment, the charge depleted may be stored in
micro-amp seconds; however, various other measurement units may be
utilized. In one embodiment, the IMD 200 itself may be capable of
calculating the current usage rate for a particular combination of
programmed output settings based upon a known relationship between
current usage rates and different combinations of programmed
settings. The relationship may then be used to interpolate a
particular current usage rate for a particular combination of
programmed output settings. However, in order to reduce the
computation load on the device, some or all of these calculations,
including the interpolation, are preferably performed by an
external programmer 270. Therefore, upon programming or performing
routine maintenance of the implantable medical device 200, the
external unit 270 may perform the calculations to determine the
current usage rate during future stimulation cycles based upon the
settings implemented during the programming or maintenance
operation.
[0071] For example, if the stimulation for a particular patient is
set to a particular pulse width, the external device 270 may factor
in the calibration data and determine a current usage rate for a
particular set of stimulation settings. Therefore, for each
stimulation period, the charge that is depleted is tabulated for
the stimulation period ("on-time") by multiplying the stimulation
time by the current usage rate and a running tabulation is
maintained (block 640). For example, if the predetermined current
usage rate for each second of stimulation at a particular
combination of parameter settings is 100 microamps, and the
stimulation is 30 seconds long, a calculation is made by
multiplying the 30 second time period for the stimulation, by the
100 microamps to arrive at 3000 micro amp seconds of charge
consumed, which is then added to a running charge consumption
tally.
[0072] As illustrated in FIG. 6, the sum of the tabulations of the
charge depleted for the idle period (off-time or inactive period;
step 620) and the charge depleted for the stimulation period
(on-time or active period; step 640) are added to arrive at a total
charge depleted by the IMD 200 (block 650). It will be appreciated
that the sum of idle period and stimulation charge depletion may
occur at the conclusion of one or more cycles of idle period and
stimulation period, or continuously throughout idle periods and
stimulation periods. Occasionally during the operational life of
the IMD 200, various stimulation parameters may be changed to
provide different types of stimulation. However, utilizing the
steps described herein, a running tally (or a periodically updated
tally) of the charge depletion is maintained, such that even when
the stimulation settings change, the device maintains a
substantially accurate reflection of the actual charge that has
been depleted by the IMD 200, and future depletion calculations are
based on the depletion rate for the newly programmed settings.
[0073] The memory 280 may store the results of the charge
calculations (step 660). The data stored may include both the
current usage rates for idle and stimulation periods of the IMD
200, as well as the total charge depleted. This data may be
utilized by the IMD 200 and/or external unit 270 to determine
various aspects of the device, including the amount of remaining
battery life.
[0074] The calculations associated with steps 620, 640 and 650 may
be expressed mathematically. In particular, the total charge
available from the battery Q.sub.tot after it is placed in the IMD
200 may be represented as the difference between an initial battery
charge Q.sub.0 and the EOS battery charge Q.sub.EOS, as expressed
in Equation 1. Q.sub.tot=Q.sub.0-Q.sub.EOS Equation 1
[0075] The charge depleted by the IMD 200 during idle periods
Q.sub.i (step 620) may be expressed as the idle period current
usage rate r.sub.i multiplied by the total duration of all idle
periods .DELTA.t.sub.i according to equation 2.
Q.sub.i=r.sub.i.times..SIGMA..DELTA.t.sub.i Equation 2
[0076] Where multiple idle rates are present, the above equation
will be solved for each idle current usage rate and the results
summed to obtain Q.sub.i. Similarly, the charge depleted during
stimulation periods Q.sub.s (step 640) may be expressed as the
stimulation period current usage rate r.sub.s multiplied by the
total duration of all stimulation periods .DELTA.t.sub.s according
to equation 3. Q.sub.s=r.sub.s.times..SIGMA..DELTA.t.sub.s Equation
3
[0077] Again, where multiple stimulation rates are used the
equation will be solved for each stimulation rate and the results
summed. The total charge depleted Q.sub.d is the sum of Q.sub.i and
Q.sub.s, as shown in equation 4. Q.sub.d=Q.sub.i+Q.sub.s Equation
4.
[0078] Finally, the charge remaining until EOS (Q.sub.r) at any
arbitrary point in time is the difference between the total energy
or charge available Q.sub.tot and the charge actually depleted from
the battery Q.sub.d at that same timepoint, as expressed in
equation 5 (step 650). Q.sub.r=Q.sub.tot-Q.sub.d Equation 5
[0079] This may be accomplished by counters that record the amount
of time the device uses the idle current usage rate(s) and the
stimulation current usage rate(s), respectively, which are then
multiplied by the applicable current usage rate to obtain the total
consumed charge during the idle and stimulation periods.
Alternatively, separate registers may directly maintain a running
tally of the charge depleted during stimulation periods and idle
periods, respectively.
[0080] Turning now to FIG. 7, a more detailed flow chart depicting
the calculation of the time to the end of service (EOS) and/or
elective replacement indicator (ERI) signals, as indicated in step
430 of FIG. 4, is illustrated. The IMD 200 is programmed for
delivering to the patient electrical pulses having predetermined
parameters (step 710). Programming the stimulation settings may be
performed during manufacturing and/or by a healthcare provider when
the external unit 270 gains communication access to the IMD 200.
Occasionally, medical personnel may determine that an alteration of
one or more of the stimulation parameters is desirable.
Implementation of such changes may easily be accomplished to
optimize the therapy delivered by the IMD. Alternatively, as part
of a routine diagnostic process, a predetermined change to the
stimulation settings may be performed. Additionally, the IMD 200
may have multiple sets of stimulation parameters stored in memory
and may switch between the different stimulation modes represented
by those parameters at preset times or at the occurrence of certain
physiological events. When a change in one or more stimulation
parameter settings is implemented (whether by programming or
accessing data from memory), the IMD 200 and/or the external unit
270 may determine an updated stimulation period current usage rate
rs associated with the new parameter settings, and subsequent
updates to the total charge consumed will be based upon the new
stimulation period current usage rate (step 720). The rates may
either be stored in memory or calculated from an equation by
interpolation among known current rates for known parameter
settings, as previously described. It is also possible that changes
to the software or firmware of the device could change the idle
period depletion rate, in which event a new idle period current
usage rate r.sub.i may also be calculated and reflected in
subsequent calculations of total charge depleted (step 720).
[0081] Because the duty cycle (on-time to off-time ratio) is also a
programmed parameter, the present invention allows both the idle
period current usage rate (r.sub.i) and the stimulation period
current usage rate (r.sub.s) to be combined into a single rate for
purposes of projecting future energy or charge depletion and
calculating a time to EOS and/or ERI. This rate represents the
total current usage rate (r.sub.t) of the device (step 725).
Following updates to the stimulation and/or idle period current
usage rates r.sub.s and r.sub.i, the updated rates are then used to
calculate a new total charge remaining Q.sub.r, by a method
substantially as shown in FIG. 6 and previously described. Once the
total charge remaining is retrieved from memory, the remaining time
to an activation of an EOS is calculated (step 730) by using the
total depletion rate r.sub.t and the total charge remaining Q.sub.r
on the battery until EOS. More particularly, the time remaining is
calculated by dividing the remaining charge by the total depletion
rate as shown in Equation 6. t=Q.sub.r/r.sub.t Equation 6
[0082] At a predetermined time period before the end of service of
the battery unit 210 is reached, an ERI signal, which may prompt
the healthcare provider and/or the patient to schedule elective
replacement of an electronic device, may be asserted to provide a
warning. ERI is typically determined as simply a predetermined
time, for example from 1 week to 1 year, more typically 6 months,
earlier than EOS. In an alternative embodiment, the ERI signal may
be defined as a particular charge level remaining (Q.sub.ERI) above
the EOS charge, Q.sub.EOS. In this embodiment, the time period
remaining until the ERI signal could be calculated by dividing
Q.sub.EOS by the total depletion rate r.sub.t and subtracting the
resulting time period from the time to EOS as calculated in
equation 6.
[0083] The time to EOS provides a warning to the healthcare
provider and/or patient that the energy or charge supply will be
depleted very shortly. Therefore, the time to EOS is reported to
the implantable medical device 200 and/or to the external device
270 (block 740). The ERI is also reported to the implantable
medical device 200 and/or to the external device 270, which is then
brought to the attention of the patient and/or a medical
professional.
[0084] In addition to battery life, for diagnostic purposes the
impedance of the various leads that deliver stimulation provided by
the IMD 200 is also of interest. Lead impedance measurements and
known output current signal characteristics may be used to
calculate consumed stimulation charge. Sudden changes in lead
impedance may indicate any of a number of changes in the operation
of the implantable medical device 200. Changes in impedance may
indicate that the leads delivering the stimulation have moved or
have been damaged, or that the patient's body where the stimulation
was delivered may have changed in some way.
[0085] Turning now to FIG. 8, a block diagram is provided depicting
in further detail an embodiment of the stimulation unit 250 of FIG.
2. The stimulation unit 250 of the IMD 200 comprises an op amp unit
820, which may comprise one or more operational amplifiers that are
capable of delivering a controlled current signal for stimulation.
In one embodiment, the controlled current is a constant current or
a substantially constant current. The stimulation unit 250 may also
comprise an amplifier control circuitry unit 810 that may contain
circuitry and/or programmable logic to control the operation of the
op amps 820. Additionally, the stimulation unit 250 may be coupled
to leads 122, which may comprise a pair of signal wires capable of
delivering an electrical signal to an electrode pair 125-1 and
125-2 (FIG. 1D) each coupled to a distal end of one of the leads
122. The leads 122 (and the electrodes 125-1 and 125-2) are capable
of providing a complete circuit between the implantable medical
device 200 and the region of the body/tissue to which the
electrodes are attached, which may be approximated as an equivalent
impedance. Each lead 122 may comprise a single strand wire or, more
preferably, a multi-strand wire braided or otherwise coupled
together as a single functional wire. Each of the two lead wires
122 in this embodiment is provided with a separate socket and
connector 116, as shown in FIG. 1C. In another embodiment, two
leads 122 may be combined into a single coaxial cable (as shown in
FIGS. 1A and 1D), with a single socket providing both coaxial
connectors 116.
[0086] Embodiments of the present invention provide for utilizing
the delivery of a constant current signal for delivery of
stimulation, and measurement of the impedance experienced by the
leads 122. In a preferred embodiment, the controlled or constant
current signal provided by the stimulation unit 250 is independent
of the impedance experienced across the leads 122. For example,
even if the impedance experienced across the leads 122 changes, the
op amp 820, in conjunction with the amplifier control circuitry
810, adjusts to deliver a controlled or constant current despite
the change in the impedance experienced across the leads 122.
[0087] Since a controlled, constant current is delivered despite
variations in the impedance across the leads 122, the voltage
across the lead terminals provide an indication of the lead
impedance. For example, if the nerve tissue to which the leads 122
are connected has an impedance of 1000 ohms, a particular
stimulation may call for a one milliamp constant current signal. In
this case, even if a 5000 ohms impedance is experienced across the
leads 122, the stimulation unit 250 will still provide a one
milliamp current. Hence, the power may vary but the current remains
constant. In other words, the op amp 820 will stabilize itself
utilizing various circuitry, including the amplifier control
circuitry 810, to provide a constant current signal even if the
impedance experienced by the leads 122 varies during the period the
signal is provided. Therefore, using Ohm's Law, V=IR, a measurement
of the voltage across the leads 122 will provide an indication of
the actual impedance experienced by the leads 122.
[0088] Turning now to FIG. 9, a block diagram depiction of one
embodiment of the impedance measurement unit 265 from FIG. 2 is
provided. In one embodiment, the impedance measurement unit 265
comprises a voltage measurement unit 910, an A/D converter (analog
to digital converter) 920 and an impedance calculation unit 930.
The voltage measurement unit 910 is capable of measuring or
determining the voltage differential between the terminals of the
leads 122. The signal from the voltage measurement unit 910 is
generally an analog signal, which may be sent to the A/D converter
920. The A/D converter 920, which preferably has been calibrated
prior to the operation of the IMD 200, will convert the analog
voltage measurement signal to a digital signal. In alternative
embodiments of the present invention the impedance measurement unit
265 may be implemented without the use of the A/D converter 920 and
still remain within the scope of the present invention.
[0089] Although certain embodiments may be implemented without it,
the A/D converter 920 may be beneficial for enhancing the
resolution of the voltage signal, thereby providing for enhanced
analysis of the voltage across the leads 122. Based upon the
voltage across the leads 122, and the constant current signal
provided by the stimulation unit 250, the impedance calculation
unit 930 calculates the impedance by dividing the voltage across
the lead terminals 122 by the current delivered by the stimulation
unit 250. The impedance calculation unit 930 may be a hardware
unit, a software unit, a firmware unit, or any combination thereof,
which may be located in various portions of the IMD 200, including
in the impedance measurement unit 265, in the stimulation
controller 230, in the power source controller 220, or in any other
portion of the IMD 200.
[0090] In an alternative embodiment, the calculation described as
being performed by the impedance calculation unit 930 may
alternatively be performed by the external unit 270, which may
receive the signal relating to the constant current stimulation
signal and the measured voltage signal. One of the advantages of
utilizing the embodiments provided by the present invention is that
substantially any size of a constant or controlled current stimulus
signal may be used to perform the impedance measurement, thereby
conserving battery power of the implantable medical device 200.
Accordingly, the smallest stimulation signal that may reliably be
provided by the stimulation unit 250 may be used to perform the
impedance measurement. Thus, the impedance measurement may be made
without imposing a significant charge depletion burden on the
battery. Additionally, the impedance of the leads 122 themselves is
also accounted for when analyzing the impedance. Furthermore, the
A/D converter 920 may be calibrated prior to the operation of the
implantable medical device 200, for example, during the
manufacturing process.
[0091] Turning again to FIGS. 1A-1D, the leads 122 are shown
connected to tissue (e.g., nerve tissue 127) in a patient's body
and to the IMD 200. The implantable medical device 200 may comprise
a main body 112 (FIG. 1A) in which the electronics described in
FIG. 2 are enclosed. Coupled to the main body 112 is a header 114
(FIG. 1A) designed with terminal connectors 116 (FIG. 1C) for
connecting to leads 122. The main body 112 may comprise a titanium
case 121 and the header 114 may comprise a biocompatible polymer
such as polyurethane or acrylic. The leads 122 projecting from the
header 114 may be attached to the tissue utilizing a variety of
methods for attaching the leads 122 to tissue. A first end of the
leads 122 is coupled to connector(s) 116 on the header 114, and a
distal end is coupled to the tissue by electrodes 125-1 and 125-2,
which together provide a cathode and an anode (FIG. 1D). Therefore,
the current flow may take place from one electrode 125-1 to a
second electrode 125-2 via the tissue, thereby delivering the
stimulation.
[0092] The system illustrated in FIGS. 1A-1D may be viewed as an
electrical circuit that includes a current or voltage source (i.e.,
the battery 210 of the IMD 200) being connected to an impedance
(i.e., the equivalent impedance of the tissue) via a pair of wires
(i.e., the leads 122). The total impedance connected to the IMD 200
includes the impedance of the lead wires 122 as well as the
impedance across the terminals 116 of the leads 122 to the tissue.
One of the biggest components of the impedance experienced by
terminals 116 on the header 114, to which the leads 122 are
connected, is the impedance of the tissue. Therefore, if a break in
any one portion of the lead wires 122 occurs (such as a break in
one or more strands of a multistrand wire), the impedance may rise
significantly, which may provide an indication that a break in the
lead wire 122 has occurred.
[0093] Turning now to FIG. 10, a flowchart depicting steps for
determining the impedance experienced by the leads 122 of the IMD
200 is provided. As shown in step 1010, stimulation is delivered by
the IMD 200 to the tissue of the patient by one of a number of
available stimulation delivery modes, such as a constant current
signal pulse (step 1010). To conserve battery power, impedance may
be determined using a small magnitude and/or short duration pulse.
The resultant voltage induced across the leads 122 is measured
(block 1020) upon delivery of the stimulation signal. Voltage
measurement may be performed by a voltage measurement unit 910
(FIG. 9) during delivery of the stimulation current signal. The IMD
200 adjusts the time at which the voltage is measured such that it
occurs while the stimulation current signal is being delivered.
[0094] An analog-to-digital (A/D) conversion is preferably
performed on the voltage signal (block 1030). Although, embodiments
of the present invention may be performed without utilizing an A/D
converter 920, in a preferred embodiment an A/D converter 920 (FIG.
9) is used to provide precise resolution of the voltage signal. The
A/D converter 920 is preferably calibrated prior to the conversion
of the voltage signal from analog to digital. Finally, the
impedance is calculated utilizing the amplitude of the current
delivered for stimulation and the corresponding voltage
measurement, as shown in step 1040. The voltage resulting from the
current signal delivered as stimulation is divided by the value of
the current to arrive at the total impedance across the terminals
116 of the header 114 (FIGS. 1A-1D). In one embodiment, the
predetermined impedance of the lead 122 itself may be subtracted to
arrive at the impedance across the lead terminals 116, which
corresponds to the impedance of the tissue 1030. Various
operational adjustments to the operation of the IMD 200 may be made
based upon the determination of the impedance across the terminals
116.
[0095] FIG. 11 provides a flowchart depicting the steps for
performing the calibration of the A/D converter 920 (FIG. 9). In a
preferred embodiment, the calibration of the A/D converter 920 is
performed prior to implanting the IMD 200 in the body of the
patient, more preferably during the manufacturing process of the
IMD 200. Referring to FIG. 11, a predetermined, known impedance is
provided for the calibration of the A/D converter 920, as depicted
in step 1110. The known impedance is electrically connected across
the two distal ends of leads 122 (which may or may not include
electrode assembly 125), and the other ends of the lead wires 122
are connected to the terminals 116 of header 114 (step 1220). With
the leads 122 connected between the IMD 200 and the known
impedance, a constant current test signal is driven through the
lead 122, through the known impedance, and back to the IMD 200
(step 1130).
[0096] The constant current test signal may comprise a series of
individual constant current signals that may vary in duration of
current amplitude from one signal to another in the series of test
signals, provided that each individual test pulse comprises a
constant current. During the delivery of each constant current test
pulse to the known impedance, a corresponding voltage resulting
from the driving of the constant current is measured across the
terminals 116 of the IMD 200 (step 1240). This measurement of
voltage at the terminals 116 allows a comparison to a theoretical
indication of what the measurement should be by calculation from
the known current being driven, and the known impedance across the
leads 122. This theoretical voltage calculation value is then used
with the actual voltage measured across the terminals 116 to
calibrate the A/D converter 920 (block 1150). Calibration of the
A/D converter 920 should provide improved accuracy for measurements
subsequently processed by the A/D converter 920. In another
embodiment, the calibration process may be performed using multiple
known impedances and corresponding resulting multiple measured
voltages. Such a calibration over a range of impedances may provide
further improved accuracy.
[0097] Utilizing embodiments of the present invention, a more
accurate assessment of the status of the battery and the impedance
experienced by the leads 122 may be assessed, thereby providing
better warnings to the user and/or to a healthcare provider
assessing the operations of the IMD 200. Various end of service
signals (EOS) and/or elective replacement indication (ERI) signals
may be provided to indicate the status of the operation of the IMD
200. Additionally, the impedance experienced by the leads 122 of
the IMD 200 may be analyzed to assess the integrity of the leads
122 or any drastic changes in the tissue to which the stimulation
signal is provided.
[0098] The particular embodiments disclosed above are illustrative
only, as the invention may be modified and practiced in different
but equivalent manners apparent to those skilled in the art having
the benefit of the teachings herein. Furthermore, no limitations
are intended to the details of construction or design herein shown,
other than as described in the claims below. The particular
embodiments disclosed above may be altered or modified and all such
variations are considered within the scope and spirit of the
invention. Accordingly, the protection sought herein is as set
forth in the claims below.
* * * * *