MESURE D'IMPEDANCE POUR DISPOSITIF IMPLANTABLE

IMPEDANCE MEASUREMENT FOR AN IMPLANTABLE DEVICE

Abrégé français

Cette invention concerne un procédé et un appareil servant à déterminer l'impédance subie par un conducteur associé à un dispositif implantable. A cet effet, un signal de courant essentiellement constant est appliqué à une première et à une seconde borne du conducteur. On mesure alors la tension sur ces première et seconde bornes et on détermine l'impédance sur ces première et seconde bornes en fonction du signal de courant constant et de la tension mesurée.

Abrégé anglais

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.

Revendications

CLAIMS
WHAT IS CLAIMED:
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.
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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.
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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:
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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
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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.
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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.
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Description

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IMPEDANCE MEASUREMENT FOR AN
IMPLANTABLE DEVICE
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
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.
2. DESCRIPTION OF THE RELATED ART
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. Patent 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
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series of electrical pulses to the vagus (or other cranial) nerve
intermittently
throughout the day, or over another predetermined time interval.
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.
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
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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.
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.
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.
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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.
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
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mathematically estimated. However, this type of ineasureinent 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.
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
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.
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
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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.
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.
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
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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.
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
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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.
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.
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
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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.
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.
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 ternninal of the lead. A voltage across the first and second
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measured, and an impedance across the first and second terminals is determined
based
upon the constant current signal provided and the measured voltage.
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
furrther
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.
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.
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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.
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BRIEF DESCRIPTION OF THE DRAWINGS
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:
Figure lA is a stylized diagram of an implantable medical device suitable for
use in the present invention implanted into a patient's body;
Figure 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;
Figure 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;
Figure 1D shows a lead and electrodes suitable for use in the present
invention attached to a vagus nerve of a patient;
Figure 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;
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Figure 3 is a stylized diagram of an output current signal provided by the
implantable medical device of Figures 1 and 2, provided to illustrate certain
stimulation parameters in accordance with one illustrative embodiment of the
present
invention;
Figure 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;
Figure 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;
Figure 6 is a more detailed flowchart illustrating a method of performing the
charge depletion calculation indicated in Figure 4, in accordance with one
illustrative
embodiment of the present invention;
Figure 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 Figure 4, in accordance with one illustrative embodiment of
the present
invention;
Figure 8, is a block diagram of the stimulation unit shown in Figure 2, in
accordance with one illustrative embodiment of the present invention;
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Figure 9 is a block diagram of the impedance measurement unit shown in
Figure 2, in accordance with one illustrative embodiment of the present
invention;
Figure 10 is a flowchart of a method of performing an impedance
measurement, in accordance with one illustrative embodiment of the present
invention; and
Figure 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.
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
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
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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.
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.
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,
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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.
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.
Figures 1 A-1 D illustrate a generator 110 having main body 112 comprising a
case or shell 121 (Figure 1A) with a connector 116 (Figure 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
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comprises a bipolar stimulating electrode pair (Figure 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 (Figure 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.
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 (Figure 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 US 5,531,778, although other known coupling techniques may be
used.
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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.
In certain embodiments of the invention, eye movement sensing electrodes
133 (Figure 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.
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
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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.
The pulse generator 110 may be programmed with an external computer 150
using progranuning 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) comxnunication between the computer 150 (Figure lA) 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 bafteries, 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.
Figure 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
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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.
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
Figure 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
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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.
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.
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,
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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 conununication 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 Figure 2.
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 Figure 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.
Figure 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 Figure 2) to lead assembly 122 and electrode
assembly
125 in an embodiment of the present invention. This illustration is presented
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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: US 4,702,254, US 5,025,807, US 4,867,164,
and
US 6,622,088 (epilepsy); US 5,188,104 and US 5,263,480 (eating disorders); US
5,215,086 (migraine headaches); US 5,231,988 (endocrine disorders); 5,269,303
(dementia); US 5,299,569 and US 6,622,047 (neuropsychiatric disorders); US
5,330,513 and US 6,721,603 (pain); US 5,335,657 (sleep disorders); US
5,540,730
(motility disorders); US 5,571,150 (coma); US 5,707,400 (refractory
hypertension);
US 6,587,719 and US 6,609,025 (obesity); US 6,622,041 (congestive heart
failure).
Each of the foregoing patents is hereby incorporated by reference herein in
its
entirety.
In one embodiment of the invention, the IMD 200 determines EOS and ERI
values by using a known initial battery charge (Qo) and a predetermined EOS
battery
charge (QEOS) indicative of the end of useful battery service, together with
the charge
actually depleted (Qd) by the IMD (calculated from the current usage rates for
idle
and stimulation periods (ri and rs), and the length of the respective idle and
stimulation
periods), to calculate for a desired time point how much useful charge remains
on the
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battery (Q) 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 (Qd) is known, the current usage rates are then
applied to the
remaining useful charge Qr to determine the time remaining until EOS and/or
ERI.
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.
Consistent with the foregoing, Figure 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

CA 02574438 2007-01-18
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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.
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 (ri) and stimulation periods (rs) 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; and
rs may also be referred to as "current usage," "current rate," "current
consumption,"
"charge depletion," "depletion rate" or similar terms.
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
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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 Figure 5 and the accompanying
description below.
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 descri ption of the
step 420
of calculating the electrical charge depleted is provided in Figure 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
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step 430 of calculating the time to ERI and/or EOS is provided in Figure 7 and
the
accompanying description below.
Referring now to Figure 5, a flowchart diagram is provided depicting in
greater detail the step 410 (Figure 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.
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
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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.
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.
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Referring again to Figure 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 r3. 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.
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
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and/or initialization steps of Figure 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.
Turning now to Figure 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; 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
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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.
As noted, the current rate ri 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).
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.
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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 rs 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.
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 progra.mmed 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
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current usage rate during future stimulation cycles based upon the settings
implemented during the programming or maintenance operation.
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.
As illustrated in Figure 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
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the stimulation settings change, the device maintains a substantially accurate
reflection of the actual charge that has been depleted by the IMD 200, and
fiiture
depletion calculations are based on the depletion rate for the newly
programmed
settings.
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.
The calculations associated with steps 620, 640 and 650 may be expressed
mathematically. In particular, the total charge available from the battery
Qtot after it is
placed in the IMD 200 may be represented as the difference between an initial
battery
charge Qo and the EOS battery charge QEOS, as expressed in Equation 1.
Qcoc = Qo - QEOS Equation 1
The charge depleted by the IMD 200 during idle periods Q; (step 620) may be
expressed as the idle period current usage rate r; multiplied by the total
duration of all
idle periods At; according to equation 2.
Q; = r; x EAt; Equation 2
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;. Similarly,
the
charge depleted during stimulation periods Qs (step 640) may be expressed as
the
stimulation period current usage rate rs multiplied by the total duration of
all
stimulation periods Ats according to equation 3.

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QS = rs xE Ats Equation 3
Again, where multiple stimulation rates are used the equation will be solved
for each stimulation rate and the results summed. The total charge depleted Qd
is the
sum of Qi and Qs, as shown in equation 4.
Qd = Qi + Qs Equation 4.
Finally, the charge remaining until EOS (Q) at any arbitrary point in time is
the difference between the total energy or charge available Qtot and the
charge actually
depleted from the battery Qd at that same timepoint, as expressed in equation
5 (step
650).
Qr = Qtot - Qd Equation 5
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.
Turning now to Figure 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 Figure 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.
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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 rl may
also be
calculated and reflected in subsequent calculations of total charge depleted
(step 720).
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 (ri) and
the stimulation period current usage rate (rs) 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 (rt) of the
device
(step 725). Following updates to the stimulation and/or idle period current
usage rates
rs and r;, the updated rates are then used to calculate a new total charge
remaining Qr,
by a method substantially as shown in Figure 6 and previously described. Once
the
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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 rt and
the total
charge remaining Qr 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= Qr / rt Equation 6
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 (QERI) above the EOS charge, QEOS. In this embodiment, the time
period
remaining until the ERI signal could be calculated by dividing QEOs by the
total
depletion rate rt and subtracting the resulting time period from the time to
EOS as
calculated in equation 6.
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.
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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.
Turning now to Figure 8, a block diagram is provided depicting in further
detail an embodiment of the stimulation unit 250 of Figure 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
(Figure 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
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two lead wires 122 in this embodiment is provided with a separate socket and
connector 116, as shown in Figure 1C. In another embodiment, two leads 122 may
be
combined into a single coaxial cable (as shown in Figures 1A and 1D), with a
single
socket providing both coaxial connectors 116.
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.
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

CA 02574438 2007-01-18
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voltage across the leads 122 will provide an indication of the actual
impedance
experienced by the leads 122.
Turning now to Figure 9, a block diagram depiction of one embodiment of the
impedance measurement unit 265 from Figure 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.
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,
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in the stimulation controller 230, in the power source controller 220, or in
any other
portion of the IMD 200.
In an alternative embodiment, the calculation described as being performed by
the impedance calculation unit 930 may alternatively be perfonned 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.
Turning again to Figures lA-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 (Figure 1A) in which the
electronics described in Figure 2 are enclosed. Coupled to the main body 112
is a
header 114 (Figure 1A) designed with terminal connectors 116 (Figure 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
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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 (Figure 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.
The system illustrated in Figures lA-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.
Turning now to Figure 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
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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 (Figure 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.
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 (Figure 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 (Figures lA-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.
Figure 11 provides a flowchart depicting the steps for performing the
calibration of the A/D converter 920 (Figure 9). In a preferred embodiment,
the
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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 Figure 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).
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

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multiple measured voltages. Such a calibration over a range of impedances may
provide further improved accuracy.
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.
The particular embodiments displosed 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.
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