U.S. patent application number 17/498545 was filed with the patent office on 2022-02-24 for neurostimulation system and method for automatically adjusting stimulation and reducing energy requirements using evoked action potential.
The applicant listed for this patent is Boston Scientific Neuromodulation Corporation. Invention is credited to Stephen Carcieri.
Application Number | 20220054843 17/498545 |
Document ID | / |
Family ID | |
Filed Date | 2022-02-24 |
United States Patent
Application |
20220054843 |
Kind Code |
A1 |
Carcieri; Stephen |
February 24, 2022 |
NEUROSTIMULATION SYSTEM AND METHOD FOR AUTOMATICALLY ADJUSTING
STIMULATION AND REDUCING ENERGY REQUIREMENTS USING EVOKED ACTION
POTENTIAL
Abstract
A neurostimulation system comprising stimulation output
circuitry configured for delivering stimulation pulses to target
tissue in accordance with a set of stimulation parameters. The
neurostimulation system comprises monitoring circuitry configured
for continuously measuring action potentials evoked in the target
tissue in response to the delivery of the stimulation pulses to the
target tissue, memory configured for storing a characteristic of a
reference evoked action potential, and at least one processor
configured for initiating an automatic mode, in which a
characteristic of the measured evoked action potentials is compared
to the corresponding characteristic of the reference evoked action
potential, and one or more stimulation parameter values in the set
of stimulation parameters are adjusted to decrease or increase the
energy level of the stimulation pulses, thereby evoking action
potentials in the target tissue having substantially the same
corresponding characteristic as the reference evoked action
potential.
Inventors: |
Carcieri; Stephen; (Los
Angeles, CA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Boston Scientific Neuromodulation Corporation |
Valencia |
CA |
US |
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Appl. No.: |
17/498545 |
Filed: |
October 11, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15395684 |
Dec 30, 2016 |
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17498545 |
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14187043 |
Feb 21, 2014 |
9533148 |
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15395684 |
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61768295 |
Feb 22, 2013 |
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International
Class: |
A61N 1/36 20060101
A61N001/36; A61B 5/00 20060101 A61B005/00; A61B 5/377 20060101
A61B005/377; A61N 1/372 20060101 A61N001/372 |
Claims
1. (canceled)
2. A method performed using a Spinal Cord Stimulation (SCS) system
configured to deliver SCS for a pain therapy; the method
comprising: delivering at least two levels of neurostimulation,
including a first level and a second level, to target tissue in or
near a spine according to a programmed therapy schedule, wherein
the first and the second levels of neurostimulation correspond to a
first and a second templates, respectively, wherein the first and
second templates are stored in memory, the first and the second of
the at least two levels of neurostimulation correspond to first and
second sets of stimulation parameter values, respectively;
verifying, throughout a course of the pain therapy, whether the
neurostimulation delivered using the stimulation parameter values
for the first and second sets of stimulation parameters is
effective for the pain therapy by monitoring whether the
stimulation is causing evoked action potentials, wherein the
verifying includes: determining waveform characteristic values of
action potentials throughout the course of the therapy, including
determining a first set of waveform characteristic values
corresponding to times when the first level of neurostimulation is
delivered and determining a second set of waveform characteristic
values corresponding to times when the second level of
neurostimulation is delivered; and comparing the first set and the
second set of waveform characteristic values to waveform
characteristic values in the first and second templates,
respectively; and maintaining an efficacious therapeutic effect
throughout the course of therapy, including: continuing to use the
first set of stimulation parameter values to deliver the first
level of neurostimulation when the first set of waveform
characteristic values similarly correspond to the waveform
characteristic values in the first template, and adjusting the
first set of stimulation values when the first set of waveform
characteristic values do not similarly correspond to the waveform
characteristic values in the first template; and continuing to use
the second set of stimulation parameter values to deliver the
second level of neurostimulation when the second set of waveform
characteristic values similarly correspond to the waveform
characteristic values in the second template, and adjusting the
second set of stimulation values when the second set of waveform
characteristic values do not similarly correspond to the waveform
characteristic values in the second template.
3. The method of claim 2, wherein the adjusting the first set of
the stimulation values or the second set of stimulation values
includes adjusting at least one of a pulse amplitude, a pulse
width, or a pulse rate.
4. The method of claim 2, wherein the adjusting the first set of
the stimulation values or the second set of stimulation values
includes adjusting at least one of a duty cycle or a burst
rate.
5. The method of claim 2, wherein the adjusting the first set of
the stimulation values or the second set of stimulation values
includes adjusting an electrode combination.
6. The method of claim 2, wherein the waveform characteristic
values include values for at least one of peak delay, width,
amplitude, or waveform morphology.
7. The method of claim 2, wherein evoked action potentials include
an evoked compound action potential or an evoked compound muscle
action potential.
8. The method of claim 2, wherein evoked action potentials include
at least one of therapeutic evoked action potentials or side-effect
evoked action potentials.
9. The method of claim 2, wherein first or second sets of waveform
characteristic values do not similarly correspond to the waveform
characteristic values in the first or second templates,
respectively, when at least one of the first or second sets of
waveform characteristic values is different from the waveform
characteristic values in the first or second sets of waveform
characteristic values.
10. The method of claim 2, wherein first or second sets of waveform
characteristic values do not similarly correspond to the waveform
characteristic values in the first or second templates,
respectively, when at least one of the first or second sets of
waveform characteristic values is different from the waveform
characteristic values in the first or second sets of waveform
characteristic values by more than a tolerance threshold.
11. The method of claim 2, wherein first or second sets of waveform
characteristic values do not similarly correspond to the waveform
characteristic values in the first or second templates,
respectively, when at least one of the first or second sets of
waveform characteristic values is different from the waveform
characteristic values in the first or second sets of waveform
characteristic values for more than a threshold time period.
12. The method of claim 2, wherein first or second sets of waveform
characteristic values do not similarly correspond to the waveform
characteristic values in the first or second templates,
respectively, when at least one of the first or second sets of
waveform characteristic values is different from the waveform
characteristic values in the first or second sets of waveform
characteristic values for more than a threshold number of
measurements.
13. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on a patient's
movement.
14. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on a patient's
temperature.
15. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on a patient's blood
flow.
16. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on an
electrocortigram or, an electroencephalogram.
17. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on a tissue or
transcutaneous oxygen tension.
18. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on a chemical species
concentration.
19. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on a glucose
concentration.
20. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on an impedance
measurement
21. The method of claim 2, wherein the adjusting the first set or
the second set of stimulation values is based on whether the
patient is asleep or awake.
Description
RELATED APPLICATION DATA
[0001] The present application is a continuation of U.S.
application Ser. No. 15/395,684, filed Dec. 30, 2016, which is a
continuation of U.S. application Ser. No. 14/187,043, filed Feb.
21, 2014, now issued as U.S. Pat. No. 9,533,148, which claims the
benefit under 35 U.S.C. .sctn. 119 to U.S. provisional patent
application Ser. No. 61/768,295, filed Feb. 22, 2013. The foregoing
applications are hereby incorporated by reference into the present
application in their entirety.
FIELD OF THE INVENTION
[0002] The present inventions relate to tissue stimulation systems,
and more particularly, to systems and methods for adjusting the
stimulation provided to tissue to minimize the energy requirements
of the systems.
BACKGROUND OF THE INVENTION
[0003] Implantable neurostimulation systems have proven therapeutic
in a wide variety of diseases and disorders. Pacemakers and
Implantable Cardiac Defibrillators (ICDs) have proven highly
effective in the treatment of a number of cardiac conditions (e.g.,
arrhythmias). Spinal Cord Stimulation (SCS) systems have long been
accepted as a therapeutic modality for the treatment of chronic
pain syndromes, and the application of spinal stimulation has begun
to expand to additional applications, such as angina pectoris and
incontinence. Deep Brain Stimulation (DBS) has also been applied
therapeutically for well over a decade for the treatment of
refractory Parkinson's Disease, and DBS has also recently been
applied in additional areas, such as essential tremor and epilepsy.
Further, in recent investigations, Peripheral Nerve Stimulation
(PNS) systems have demonstrated efficacy in the treatment of
chronic pain syndromes and incontinence, and a number of additional
applications are currently under investigation. Furthermore,
Functional Electrical Stimulation (FES) systems such as the
Freehand system by NeuroControl (Cleveland, Ohio) have been applied
to restore some functionality to paralyzed extremities in spinal
cord injury patients.
[0004] Each of these implantable neurostimulation systems typically
includes one or more electrode carrying stimulation leads, which
are implanted at the desired stimulation site, and a
neurostimulation device implanted remotely from the stimulation
site, but coupled either directly to the stimulation lead(s) or
indirectly to the stimulation lead(s) via a lead extension. Thus,
electrical pulses can be delivered from the neurostimulation device
to the electrode(s) to activate a volume of tissue in accordance
with a set of stimulation parameters and provide the desired
efficacious therapy to the patient. In particular, electrical
energy conveyed between at least one cathodic electrode and at
least one anodic electrode creates an electrical field, which when
strong enough, depolarizes (or "stimulates") the neurons beyond a
threshold level, thereby evoking action potentials (APs) that
propagate along the neural fibers. A typical stimulation parameter
set may include the electrodes that are sourcing (anodes) or
returning (cathodes) the modulating current at any given time, as
well as the amplitude, duration, and rate of the stimulation
pulses.
[0005] The neurostimulation system may further comprise a handheld
patient programmer to remotely instruct the neurostimulation device
to generate electrical stimulation pulses in accordance with
selected stimulation parameters. The handheld programmer in the
form of a remote control (RC) may, itself, be programmed by a
clinician, for example, by using a clinician's programmer (CP),
which typically includes a general purpose computer, such as a
laptop, with a programming software package installed thereon.
[0006] Of course, neurostimulation devices are active devices
requiring energy for operation, and thus, the neurostimulation
system may oftentimes includes an external charger to recharge a
neurostimulation device, so that a surgical procedure to replace a
power depleted neurostimulation device can be avoided. To
wirelessly convey energy between the external charger and the
implanted neurostimulation device, the charger typically includes
an alternating current (AC) charging coil that supplies energy to a
similar charging coil located in or on the neurostimulation device.
The energy received by the charging coil located on the
neurostimulation device can then be used to directly power the
electronic componentry contained within the neurostimulation
device, or can be stored in a rechargeable battery within the
neurostimulation device, which can then be used to power the
electronic componentry on-demand.
[0007] Typically, the therapeutic effect for any given
neurostimulation application may be optimized by adjusting the
stimulation parameters. Although the threshold for evoking action
potentials may be a good indication of whether a desired
therapeutic result is achieved, it is usually not directly
observable when programming the neurostimulation device. For this
reason, the programmer of the neurostimulation system is often
required to identify the efficacy threshold and the side-effect
threshold based on the patient's perception. For instance, the
programmer of the neurostimulation system may identify the efficacy
threshold by asking the patient whether the pain is relieved or
perceived paresthesia, and record the set of stimulation parameters
of that stimulation level. Similarly, the side-effect threshold is
identified by adjusting the stimulation until the patient perceives
any undesired side-effects such as slurred speech or involuntary
muscle contraction, and records the set of stimulation parameters
of that stimulation level. Then, the neurostimulation system is
configured with a certain set of stimulation parameters to generate
stimulation at an arbitrary level within the therapeutic window so
that the stimulation is perceptible by the patient without causing
any undesirable side effects.
[0008] There are a few issues that need to be considered when using
this approach. Many neurostimulation therapies take time to develop
the clinical benefit. For example, the patient may need to be on a
certain level of stimulation for a few hours or even days before he
or she can actually feel the pain relief or regain muscles
mobility. Also, the side effect threshold is often not perfectly
correlated with the therapeutic effect. Therefore, relying on the
subjective clinical assessment (e.g., perception threshold) at the
acute setting and configuring the stimulation parameters may result
in an erroneous therapeutic window. Moreover, various changes,
including postural changes, leads movement and tissue maturation,
may occur in the patient during the course of therapy, and the
stimulation parameters may need to be re-calibrated using the same
unreliable subjective clinical assessment approach, thus the
therapeutic window is often chosen to be very broad. That is, the
gap between the efficacy threshold and the side-effect threshold is
set as far as possible. In order to prevent under-stimulation and
over-stimulation, a set of stimulation parameters are chosen to
generate a stimulation pulse at the mid-level of the wide
therapeutic window. The set of stimulation parameters for
generating such stimulation pulse is more energy-intensive than
necessary to achieve the therapy, which in turn causes decreased
battery life, more frequent recharge cycles, and/or in the case
where non-chargeable primary cell devices are used, more frequent
surgeries for replacing the battery.
[0009] There, thus, remains a need to decrease the energy
requirements for neurostimulation therapy.
SUMMARY OF THE INVENTION
[0010] In accordance with the present inventions, a
neurostimulation system is provided. The system comprises
stimulation output circuitry configured for delivering stimulation
pulses to target tissue in accordance with a set of stimulation
parameters (e.g., at least one of a pulse amplitude, a pulse width,
a pulse rate, a duty cycle, a burst rate, and an electrode
combination), monitoring circuitry configured for continuously
measuring action potentials evoked in the target tissue (e.g., one
of an evoked compound action potential and an evoked compound
muscle action potential) in response to the delivery of the
stimulation pulses to the target tissue, memory configured for
storing a characteristic of a reference evoked action potential
(e.g., at least one of peak delay, width, amplitude, and waveform
morphology), which may be a therapeutic evoked action potential or
a side-effect evoked action potential, and at least one processor
configured for initiating an automatic mode, in which a
characteristic of the measured evoked action potentials is compared
to the corresponding characteristic of the reference evoked action
potential, and one or more stimulation parameter values in the set
of stimulation parameters are adjusted to decrease or increase the
energy level of the stimulation pulses, thereby evoking action
potentials in the target tissue having substantially the same
corresponding characteristic as the reference evoked action
potential.
[0011] In one embodiment, the processor(s) is configured for
triggering the automatic mode based on one or more of the following
pre-defined conditions: (a) immediately upon measuring evoked
action potentials having a characteristic different from the
characteristic of the reference evoked action potential; (b) upon
measuring evoked action potentials having a characteristic
different from the characteristic of the reference action evoked
action potential by more than a predetermined tolerance threshold;
(c) upon measuring evoked action potentials having a characteristic
different from the characteristic of the reference evoked action
potential for more than a predetermined time period, and (d) upon
measuring evoked action potentials having a characteristic
different from the characteristic of the reference evoked action
potential for more than a predetermined number of measurements.
[0012] In another embodiment, the processor(s) is configured for
halting or resuming the automatic mode based on one or more
conditions comprising patient's movement, patient's temperature,
patient's blood flow, electrocortigram, electroencephalogram,
tissue or transcutaneous oxygen tension, glucose concentration,
impedance measurement, chemical species concentration, and whether
the patient is asleep or awake. The processor(s) may be configured
for selecting the stimulation parameter to be adjusted and the step
size for the adjustment. In an optional embodiment, the
processor(s) is configured for generating an alert upon initiating
the automatic stimulation adjustment mode, thereby allowing manual
adjustment of the one or more stimulation parameter values. The
processor(s) may be configured for alternately using two or more of
the reference evoked action potentials based on a predefined
therapeutic schedule.
[0013] In one embodiment, the automatic mode is an automatic
stimulation adjustment mode. In this case, the processor(s) may be
configured for using the comparison between the measured evoked
action potentials and the reference evoked action potential to
determine whether the stimulation pulses delivered to the target
tissue was an over-stimulation or an under-stimulation of the
target tissue, and the stimulation parameter value(s) may be
adjusted to gradually decrease or increase the energy level of the
stimulation pulses, respectively, until the measured evoked action
potentials have substantially the same characteristic as the
reference evoked action potential.
[0014] In another embodiment, the automatic mode is an automatic
power consumption optimization mode. In this case, the stimulation
parameter value(s) may be adjusted to decrease the energy level of
the stimulation pulses, thereby evoking action potentials in the
target tissue having substantially the same corresponding
characteristic as the reference evoked action potential.
Furthermore, the memory may be configured for storing a threshold
stimulation parameter value and a template identifying the
characteristic of the reference evoked action potential, and the
processor(s) may be configured for (a) adjusting at least one
stimulation parameter value in the threshold stimulation parameter
set by a step size; (b) measuring an action potential evoked in the
target tissue by actuating the stimulation output circuitry to
generate a stimulation pulse in accordance with the stimulation
parameter value(s); (c) comparing the measured evoked action
potential to the template; (d) replacing the threshold stimulation
parameter value in the threshold stimulation parameter set with the
adjusted stimulation parameter value(s) when the characteristic of
the measured evoked action potential matches the template, and (e)
repeating steps (a)-(d) to identify the most energy efficient set
of stimulation parameters capable of generating evoked action
potential from the target tissue having substantially the same
characteristic as the reference evoked action potential.
[0015] The stimulation output circuitry, the monitoring circuitry,
the processor(s), and the memory may be implemented in a single
device, such as an implantable electric pulse generator. In another
embodiment, the stimulation output circuitry, the monitoring
circuitry, the processor(s), and the memory may be implemented
within a plurality of devices.
[0016] Other and further aspects and features of the invention will
be evident from reading the following detailed description of the
preferred embodiments, which are intended to illustrate, not limit,
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present disclosure is further described in the detailed
description which follows, in reference to the noted plurality of
drawings, by way of non-limiting examples of preferred embodiments
of the present disclosure, in which like characters represent like
elements throughout the several views of the drawings.
[0018] FIG. 1 is a plan view of an exemplary neurostimulation
system according to an embodiment of the present disclosure.
[0019] FIG. 2 is a profile view of an implantable pulse generator
(IPG) used in the neurostimulation system of FIG. 1.
[0020] FIG. 3 is a plan view of the neurostimulation system of FIG.
1, illustrated in the context of Spinal Cord Stimulation (SCS) used
in a patient.
[0021] FIG. 4 is a schematic block diagram showing exemplary
internal components configurations of the IPG of FIG. 2.
[0022] FIG. 5 is a plan view of a hand-held remote control (RC)
that can be used in the neurostimulation system of FIG. 1.
[0023] FIG. 6 is illustrates an exemplary user interface displayed
by the RC of FIG. 5 to provide a means for the user to control the
operation of the IPG of FIG. 2.
[0024] FIG. 7 is a schematic block diagram of exemplary internal
components of the RC of FIG. 5.
[0025] FIG. 8 is a timing waveform diagram that depicts an
exemplary evoked action potential measurement.
[0026] FIG. 9 is a flow diagram illustrating an exemplary method
for automatically adjusting therapy according to an embodiment of
the present disclosure.
[0027] FIG. 10 is a flow diagram illustrating an exemplary method
for minimizing the energy consumption in the IPG of FIG. 2.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0028] The present disclosure relates to a system and method for
automatically minimizing the power consumption of neurostimulation
systems while maintaining the stimulation pulse at efficacious
level. The neurostimulation system of the present disclosure uses
evoked action potential as an indicator for determining the
effectiveness of therapeutic effect of electrical stimulation pulse
at the target neural tissue. Evoked action potential is electrical
signal generated by the nerve tissues in response to sensory or
external stimuli. Characteristics of an evoked action potential
that correlates to a certain therapeutic effect is stored as a
template (for example, reference evoked action potential) for
matching against other electrophysiological signals that are
recorded later. The comparison between the characteristics of the
recorded evoked action potential and the characteristics of the
targeted evoked action potential (i.e., the template) provides an
objective assessment as to the effectiveness of the stimulation.
This objective and quantitative measurement allows for the system
to automatically adjust the stimulation parameters to maintain the
efficacious therapeutic effect with the minimal power consumption
requirement.
[0029] In this disclosure, various technical features are described
in relation to a spinal column stimulation (SCS) system. The SCS
system is configured to apply at least one stimulus to targeted
neural tissue to provide one or more medical, psychiatric, and/or
neurological therapeutic effects. However, it should be appreciated
that the disclosure may not be so limited to an SCS system, but
rather the features disclosed herein may be used with any other
types of implantable electrical stimulation systems. For example,
the present disclosure may be used as part of a pacemaker, a
defibrillator, a cochlear modulator device, a retinal modulator
device, a modulator device configured to produce coordinated limb
movement, a cortical modulator device, a deep brain modulator
device, an occipital nerve modulator device, a peripheral nerve
modulator device, a micro-modulator device, or in any other tissue
modulator device configured to treat urinary incontinence, sleep
apnea, shoulder sublaxation, headache, and similar ailments.
[0030] Turning first to FIG. 1, an exemplary SCS system 10
(hereinafter referred to as "the system") generally includes one or
more implantable stimulation leads 12(1) and 12(2), an implantable
pulse generator (IPG) 14, an external remote controller RC 16, a
clinician's programmer (CP) 18, an External Trial Stimulation (ETS)
20, and an external charger 22.
[0031] The IPG 14 may be physically connected to the stimulation
leads 12 via one or more percutaneous lead extensions 24. Each of
the stimulation leads 12 may carry a plurality of electrodes 26
arranged in an array. In the illustrated embodiment, the
stimulation leads 12 are percutaneous leads, and to this end, the
electrodes 26 are arranged in-line along the stimulation leads 12.
In other embodiments, the electrodes 26 may be arranged in a
two-dimensional pattern on a single paddle lead or a cuff-shaped
lead. Also, it should be appreciated that the number of stimulation
leads and electrodes may vary depending on the type of
neurostimulation system and its application. As will be described
in further detail below, the IPG 14 includes a pulse generation
circuitry that delivers the electrical stimulation in the form of
an electrical pulse train to the electrode array 26 according to a
set of stimulation parameters.
[0032] The ETS 20 may also be physically connected via the
percutaneous lead extensions 28 and external cable 30 to the
stimulation leads 12. The ETS 20, which may include a similar pulse
generation circuitry as the IPG 14, can also deliver electrical
stimulation in the form of an electrical pulse train to the
electrode array 26. The main difference between the ETS 20 and the
IPG 14 is that the ETS 20 is a non-implanted device. Such a device
may be used when it is difficult to implant a neurostimulation
device due to the patient's condition. The ETS 20 can also be used
as a trial basis after the stimulation leads 12 have been implanted
and prior to implantation of the IPG 14, to test the responsiveness
of the stimulation that is to be provided. For purposes of brevity
and clarity, only the IPG 14 will be referred in this disclosure.
However, it should be understood that all functionalities of the
IPG 14 described herein can also be performed by the ETS 20 to the
extent that the functionality does not depend on implantation of
the ETS 20. Thus, any functions described herein with respect to
the IPG 14 can likewise be performed with respect to the ETS
20.
[0033] The RC 16 may be used to telemetrically control the ETS 20
via a bi-directional RF communications link 32. Once the IPG 14 and
stimulation leads 12 are implanted, the RC 16 may be used to
telemetrically control the IPG 14 via a bi-directional RF
communications link 34. Such control allows the IPG 14 to be turned
on or off and to be programmed with different stimulation parameter
sets. The IPG 14 may also be operated to modify the programmed
stimulation parameters to actively control the characteristics of
the electrical stimulation energy output by the IPG 14.
[0034] The CP 18 provides clinician detailed stimulation parameters
for programming the IPG 14 and ETS 20 in the operating room and in
follow-up sessions. The CP 18 may perform this function by
indirectly communicating with the IPG 14 or ETS 20, through the RC
16, via an IR communications link 36. Alternatively, the CP 18 may
directly communicate with the IPG 14 or ETS 20 via an RF
communications link (not shown). The clinician detailed stimulation
parameters provided by the CP 18 are also used to program the RC
16, so that the stimulation parameters can be subsequently modified
by operation of the RC 16 in a stand-alone mode (i.e., without the
assistance of the CP 18).
[0035] The RC 16 and CP 18 may provide a user interface for the
programmer to analyze various therapeutic feedbacks from the
patient, including evoked compound action potentials (eCAP) and/or
compound muscle action potentials (eMAP). The neurostimulation
system may include various sensors to obtain a variety of
additional therapeutic feedbacks from the patient such as patient's
body activities, temperature, blood flow, electrocortigram,
electroencephalogram, tissue or transcutaneous oxygen tension,
glucose concentration, electrode impedance, intra/extra cellular
potential or electrical current, as well as chemical species
concentration, which may be monitored and analyzed via the RC 16
and/or the CP 18.
[0036] As will be described in further detail below, the
therapeutic feedback may be utilized by the IPG 14 in automatically
adjusting the stimulation parameters. In some embodiments, the
therapeutic feedback may be analyzed by the RC 16 or the CP 18, and
these external programming devices may automatically generate a
suitable stimulation parameter set for the IPG 14 or make
adjustments to the stimulation parameters stored in the IPG 14. The
RC 16 and the CP 18 may perform this function by directly
communicating with the IPG 14. Indirect communication may also be
possible. For instance, the CP 18 can retrieve the therapeutic
feedback from the IPG 14 via the RC 16, and provide a new
stimulation parameter set or a control signal (e.g., signals for
adjusting the stimulation parameters, operation modes) to the IPG
14 via the RC 16.
[0037] The external charger 22 is a portable device used to
transcutaneously charge the IPG 14 via an inductive link 38. Once
the IPG 14 has been programmed, and its power source has been
charged by the external charger 22 or otherwise replenished, the
IPG 14 may function as programmed without the RC 16 or CP 18 being
present. For purposes of brevity, the details of the ETS 20 and
external charger 22 will not be described herein. Details of
exemplary embodiments of these devices are disclosed in U.S. Pat.
No. 6,895,280, which is expressly incorporated herein by
reference.
[0038] Referring now to FIG. 2, the external features of the
stimulation leads 12 and the IPG 14 will be briefly described. One
of the stimulation leads 12(1) has eight electrodes 26 (labeled
E1-E8), and the other stimulation lead 12(2) has eight electrodes
26 (labeled E9-E16). Of course, the actual number and shape of
leads and electrodes may vary based on the intended application of
the neurostimulation system. Further details describing the
construction and method of manufacturing percutaneous stimulation
leads are disclosed in U.S. Pat. No. 8,019,439, entitled "Lead
Assembly and Method of Making Same," and U.S. Pat. No. 7,650,184,
entitled "Cylindrical Multi-Contact Electrode Lead for Neural
Stimulation and Method of Making Same," which are expressly
incorporated herein by reference. In alternative embodiments,
surgical paddle leads can be utilized, the details of which are
disclosed in U.S. Patent Publication. No. 2007/0150036 A1, entitled
"Stimulator Leads and Methods for Lead Fabrication," which is
expressly incorporated herein by reference.
[0039] The IPG 14 comprises an outer case 40 for housing the
electronic and other components (described in further detail
below), and a connector 42 to which the proximal ends of the
stimulation leads 12 mate in a manner that electrically couples the
electrodes 26 to the electronics within the outer case 40. The
outer case 40 is composed of an electrically conductive,
biocompatible material, such as titanium, and forms a hermetically
sealed compartment wherein the internal electronics are protected
from the body tissue and fluids. In some cases, the outer case 40
may serve as an electrode.
[0040] The IPG 14 includes a pulse generation circuitry that
provides electrical stimulation energy to the electrodes 26 in
accordance with a set of stimulation parameters. Such parameters
may include electrode combinations, which define the electrodes
that are activated as anodes (positive), cathodes (negative), and
turned off (zero). The stimulation parameters may further include
pulse amplitude (measured in milliamps or volts depending on
whether the IPG 14 supplies constant current or constant voltage to
the electrodes), pulse width (measured in microseconds), pulse rate
(measured in pulses per second), duty cycle (pulse width divided by
cycle duration), burst rate (measured as the stimulation energy on
duration X and stimulation energy off duration Y), as well as pulse
shape.
[0041] With respect to the pulse patterns provided during operation
of the system 10, electrodes that are selected to transmit or
receive electrical energy are referred to herein as "activated,"
while electrodes that are not selected to transmit or receive
electrical energy are referred to herein as "non-activated."
Electrical energy delivery will occur between two (or more)
electrodes, one of which may be the IPG outer case 40. Electrical
energy may be transmitted to the tissue in a monopolar or
multipolar (for example, bipolar, tripolar and similar
configurations) fashion or by any other means available.
[0042] Monopolar delivery occurs when a selected one or more of the
lead electrodes 26 is activated along with the case 40 of the IPG
14, so that electrical energy is transmitted between the selected
electrode 26 and outer case 40. In this setting, the electrical
current has a path from the energy source contained within the IPG
outer case 40 to the tissue and a sink path from the tissue to the
energy source contained within the case. Monopolar delivery may
also occur when one or more of the lead electrodes 26 are activated
along with a large group of lead electrodes located remotely from
the one or more lead electrodes 26 so as to create a monopolar
effect; that is, electrical energy is conveyed from the one or more
lead electrodes 26 in a relatively isotropic manner. Bipolar
delivery occurs when two of the lead electrodes 26 are activated as
anode and cathode, so that electrical energy is transmitted between
the selected electrodes 26. Tripolar delivery occurs when three of
the lead electrodes 26 are activated, two as anodes and the
remaining one as a cathode, or two as cathodes and the remaining
one as an anode.
[0043] The electrical energy (i.e., stimulation pulse) may be
delivered between electrodes as monophasic electrical energy or
multiphasic electrical energy. Monophasic electrical energy
includes a series of pulses that are either all positive (anodic)
or all negative (cathodic). Multiphasic electrical energy includes
a series of pulses that alternate between positive and negative.
For example, multiphasic electrical energy may include a series of
biphasic pulses, with each biphasic pulse including a cathodic
(negative) stimulation pulse and an anodic (positive) recharge
pulse that is generated after the stimulation pulse to prevent
direct current charge transfer through the tissue, thereby avoiding
electrode degradation and cell trauma.
[0044] That is, a charge is conveyed through the electrode-tissue
interface via current at an electrode during a stimulation period
(the length of the stimulation pulse), and then pulled back off the
electrode-tissue interface via an oppositely polarized current at
the same electrode during a recharge period (the length of the
recharge pulse). The recharge pulse may be active, in which case,
the electrical current is actively conveyed through the electrode
via current or voltage sources, or the recharge pulse may be
passive, in which case, the electrical current may be passively
conveyed through the electrode via redistribution of the charge
flowing from coupling capacitances present in the circuit.
[0045] As shown in FIG. 3, the stimulation leads 12 are implanted
within the spinal column 46 of a patient 48. The preferred
placement of the stimulation leads 12 is adjacent, i.e., resting
near, or upon the dura, adjacent to the spinal cord area to be
stimulated. The stimulation leads 12 will be located in a vertebral
position that depends upon the location and distribution of the
chronic pain. For example, if the chronic pain is in the lower back
or legs, the stimulation leads 12 may be located in the mid- to
low-thoracic region (e.g., at the T9-12 vertebral levels). Due to
the lack of space near the location where the electrode leads 12
exit the spinal column 46, the IPG 14 is generally implanted in a
surgically-made pocket either in the abdomen or above the buttocks.
The IPG 14 may, of course, also be implanted in other locations of
the patient's body. The lead extensions 24 facilitate locating the
IPG 14 away from the exit point of the electrode leads 12. As there
shown, the CP 18 communicates with the IPG 14 via the RC 16.
[0046] Turning next to FIG. 4, one exemplary embodiment of the IPG
14 will now be described. The IPG 14 includes modulation output
circuitry 50 configured for generating electrical modulation energy
in accordance with an electrical pulse train having a specified
pulse amplitude, pulse rate, pulse width, duty cycle, burst rate,
and shape under control of control logic 52 over data bus 54. The
pulse rate and the duration of stimulation may be controlled by
analog circuitry, or digital timer logic circuitry 56 controlling
the analog circuitry, and which may have a suitable resolution,
e.g., 10 .mu.s. In alternative embodiments, a continuous modulating
waveform may be generated by the stimulation output circuitry 50 in
a manner described in U.S. Provisional Patent Application Ser. No.
61/646,773, entitled "System and Method for Shaped Phased Current
Delivery," which is expressly incorporated herein by reference. The
stimulation energy generated by the stimulation output circuitry 50
is output via capacitors C1-C16 to electrical terminals 58
corresponding to electrodes E1-E16.
[0047] The stimulation output circuitry 50 may either include
independently controlled current sources for providing stimulation
pulses of a specified and known amperage to or from the electrical
terminals 58, or independently controlled voltage sources for
providing stimulation pulses of a specified and known voltage at
the electrical terminals 58 or to multiplexed current or voltage
sources that are then connected to the electrical terminals 58. The
operation of this stimulation output circuitry 50, including
alternative embodiments of suitable output circuitry for performing
the same function of generating stimulation pulses of a prescribed
amplitude and width, is described more fully in U.S. Pat. Nos.
6,516,227 and 6,993,384, which are expressly incorporated herein by
reference.
[0048] The IPG 14 also includes monitoring circuitry 60 for
monitoring the status of various nodes or other points 62
throughout the IPG 14, e.g., power supply voltages, temperature,
battery voltage, and the like. To the extent that the previously
discussed therapeutic feedback is utilized by the IPG 14, the
monitoring circuitry 60 can monitor the therapeutic feedback using
one or more sensors. However, if the indicators are electrical
measurements, the sensors may be the electrodes 26. Because the
electrodes 26 already carried in the body may be used for
electrical measurements, the evoked action potential measurement
techniques described in the present disclosure may not require a
separate sensor. For other types of therapeutic feedbacks, however,
separate sensors (not shown) may be used to take the non-electrical
measurements. Specific implementations of other optional sensors
will depend on the nature of the therapeutic feedback to be
measured.
[0049] The IPG 14 further includes processing circuitry in the form
of a microcontroller (pc) 64 that controls the control logic 52
over data bus 66, and obtains status data from the monitoring
circuitry 60 via data bus 68. The IPG 14 additionally controls the
timer logic 56. The IPG 14 further includes memory 70 and
oscillator and clock circuit 72 coupled to the microcontroller 64.
The microcontroller 64, in combination with the memory 70 and
oscillator and clock circuit 72, thus include a microprocessor
system that carries out the automatic stimulation adjustment and
the power consumption optimization functions according to a series
of executable instructions (e.g., programs) stored in the memory
70. Alternatively, for some applications, the function provided by
the microprocessor system may be carried out by a suitable state
machine.
[0050] Thus, the microcontroller 64 generates the necessary control
and status signals, which allow the microcontroller 64 to control
the operation of the IPG 14 in accordance with a selected operating
program and stimulation parameters. In controlling the operation of
the IPG 14, the microcontroller 64 is able to individually generate
electrical energy at the electrodes 26 using the stimulation output
circuitry 50, in combination with the control logic 52 and timer
logic 56, thereby allowing each electrode 26 to be paired or
grouped with other electrodes 26, including the monopolar case
electrode, to control the polarity, pulse amplitude, pulse rate,
pulse width, and pulse duty cycle through which the electrical
energy is provided.
[0051] The IPG 14 further includes an alternating current (AC)
receiving coil 74 for receiving programming data (e.g., the
operating program and/or stimulation parameters) from the RC 16
and/or CP 18 in an appropriate modulated carrier signal, and
charging and forward telemetry circuitry 76 for demodulating the
carrier signal it receives through the AC receiving coil 74 to
recover the programming data, which programming data is then stored
within the memory 70, or within other memory elements (not shown)
distributed throughout the IPG 14.
[0052] The IPG 14 further includes back telemetry circuitry 78 and
an alternating current (AC) transmission coil 80 for sending
informational data sensed through the monitoring circuitry 60 to
the RC 16 and/or CP 18. The back telemetry features of the IPG 14
also allow its status to be checked. For example, when the RC 16
and/or CP 18 initiates a programming session with the IPG 14, the
capacity of the battery is telemetered, so that the RC 16 and/or CP
18 can calculate the estimated time to recharge. Any changes made
to the current stimulation parameters are confirmed through back
telemetry, thereby assuring that such changes have been correctly
received and implemented within the implant system. Moreover, upon
interrogation by the RC 16 and/or CP 18, all programmable settings
stored within the IPG 14 may be uploaded to the RC 16 and/or CP
18.
[0053] Notably, when the microcontroller 64 carries out the
automatic stimulation adjustment function and the power consumption
optimization functions, the evoked action potential measurements
may be obtained directly from the electrodes 26 or received from
the monitoring circuitry 60, which may process the measurements to
eliminate artifacts and noises. Further, a blanking circuit (not
shown) may be used to suppress or eliminate unwanted electrical
noises. Other types of therapeutic feedbacks may also be obtained
and pre-processed via the monitoring circuitry 60 in the similar
manner. The threshold (e.g., the template) to which the evoked
action potentials or other therapeutic feedback indicators are
compared may be stored and recalled from the memory 70. To the
extent that the therapeutic feedbacks are entered into the RC 16 or
the CP 18 (e.g., if the therapeutic feedback indicators are
conscious feedback parameters), these feedback indicators can be
received from the RC 16 or CP 18 via the coil 74 and forward
telemetry circuitry 76. In contrast, if the RC 16, or alternatively
the CP 18, is used to perform the automatic stimulation adjustment
and the power consumption optimization techniques described herein,
the therapeutic feedbacks, to the extent that they are objectively
measured by sensor(s) coupled to the IPG 14, can be transmitted
from the IPG 14 to the RC 16 or CP 18 via the back telemetry
circuitry 78 and coil 80. The RC 16 or the CP 18 may perform the
necessary routine to adjust the stimulation parameters and transmit
the adjusted set of stimulation parameters to the IPG 14 so that
the IPG 14 can generate a stimulation pulse according to the
adjusted set of stimulation parameters.
[0054] The IPG 14 further includes a rechargeable power source 82
and power circuits 84 for providing the operating power to the IPG
14. The rechargeable power source 82 may, e.g., include a
lithium-ion or lithium-ion polymer battery. The rechargeable
battery 82 provides an unregulated voltage to the power circuits
84. Alternatively, the power source may be non-rechargeable primary
cell battery. The power circuits 84, in turn, generate the various
voltages 86, some of which are regulated and some of which are not,
as needed by the various circuits located within the IPG 14. The
rechargeable power source 82 is recharged using rectified AC power
(or DC power converted from AC power through other means, e.g.,
efficient AC-to-DC converter circuits, also known as "inverter
circuits") received by the AC receiving coil 74. To recharge the
power source 82, an external charger (not shown), which generates
the AC magnetic field, is placed against, or otherwise adjacent, to
the patient's skin over the implanted IPG 14. The AC magnetic field
emitted by the external charger induces AC currents in the AC
receiving coil 74. The charging and forward telemetry circuitry 76
rectifies the AC current to produce DC current, which is used to
charge the power source 82. While the AC receiving coil 74 is
described as being used for both wirelessly receiving
communications (e.g., programming and control data) and charging
energy from the external device, it should be appreciated that the
AC receiving coil 74 can be arranged as a dedicated charging coil,
while another coil, such as coil 80, can be used for bi-directional
telemetry.
[0055] Additional details concerning the above-described and other
IPGs may be found in U.S. Pat. No. 6,516,227, U.S. Patent
Publication No. 2003/0139781, and U.S. Pat. No. 7,539,538, entitled
"Low Power Loss Current Digital-to-Analog Converter Used in an
Implantable Pulse Generator," which are expressly incorporated
herein by reference. It should be noted that rather than an IPG,
the system 10 may alternatively utilize an implantable
receiver-stimulator (not shown) connected to leads 12. In this
case, the power source, e.g., a battery, for powering the implanted
receiver, as well as control circuitry to command the
receiver-stimulator, will be contained in an external controller
inductively coupled to the receiver-stimulator via an
electromagnetic link. Data/power signals are transcutaneously
coupled from a cable-connected transmission coil placed over the
implanted receiver-stimulator. The implanted receiver-stimulator
receives the signal and generates the stimulation in accordance
with the control signals.
[0056] Referring now to FIG. 5, one exemplary embodiment of an RC
16 is described. As previously discussed, the RC 16 is capable of
communicating with the IPG 14, CP 18, or ETS 20. The RC 16
comprises a casing 100, which houses internal componentry
(including a printed circuit board (PCB)), and a lighted display
screen 102 and button pad 104 carried by the exterior of the casing
100. In the illustrated embodiment, the display screen 102 is a
lighted flat panel display screen, and the button pad 104 includes
a membrane switch with metal domes positioned over a flex circuit,
and a keypad connector connected directly to a PCB. In an optional
embodiment, the display screen 102 has touchscreen capabilities.
The button pad 104 includes a multitude of buttons 106, 108, 110,
and 112, which allow the IPG 14 to be turned ON and OFF, provide
for the adjustment or setting of stimulation parameters within the
IPG 14, and provide for selection between screens.
[0057] In the illustrated embodiment, the button 106 serves as an
ON/OFF button that can be actuated to turn the IPG 14 ON and OFF.
The button 108 serves as a select button that allows the RC 106 to
switch between screen displays and/or parameters. The buttons 110
and 112 serve as up/down buttons that can be actuated to increase
or decrease any of stimulation parameters of the pulse generated by
the IPG 14, including the pulse amplitude, pulse width, and pulse
rate. For example, the selection button 108 can be actuated to
place the RC 16 in a "Pulse Amplitude Adjustment Mode," during
which the pulse amplitude can be adjusted via the up/down buttons
110, 112, a "Pulse Width Adjustment Mode," during which the pulse
width can be adjusted via the up/down buttons 110, 112, and a
"Pulse Rate Adjustment Mode," during which the pulse rate can be
adjusted via the up/down buttons 110, 112. Alternatively, dedicated
up/down buttons can be provided for each stimulation parameter.
Rather than using up/down buttons, any other type of actuator, such
as a dial, slider bar, keypad, or touch screen can be used to
increment or decrement the stimulation parameters.
[0058] In the present disclosure, the selection button 108 can also
be actuated to place the RC 16 in a "Power Consumption
Optimization" mode that calibrates a selected stimulation parameter
that minimizes the power consumption of the IPG 14 when delivering
the efficacious stimulation pulse. For example, FIG. 6 illustrates
a programming screen 150 that includes a power consumption
optimization trigger box 152 that can be checked to initiate a
process for optimizing the power consumption of the IPG 14.
Alternatively, the IPG 14 by itself, or the RC 16 may periodically
initiate the power consumption optimization process without user
intervention. In this case, the programming screen 150 may include
an ON/OFF check box 154 that can be checked to turn this feature on
and unchecked to turn this feature off. When the feature is turned
on, the IPG 14 or RC 16 may periodically initiate the power
consumption optimization process. The IPG 14 or RC 16 may be
prevented from initiating the power consumption optimization
process by turning the feature off.
[0059] The programming screen 150 may provide a user interface that
has a list of stimulation parameters and associated check boxes 156
that can be actuated to select the stimulation parameter that is to
be adjusted to minimize the power consumption of the IPG 14. For
example, the pulse rate can be selected by checking box 156a, the
pulse amplitude can be selected by checking box 156b, the pulse
width can be selected by checking box 156c, and the pulse duty
cycle can be selected by checking box 156d. Alternatively, in some
embodiments, the selected stimulation parameters may be kept at the
current value during the power consumption optimization process. If
the pulse rate is selected, for instance, the power consumption
optimization process will be performed by adjusting all other
stimulation parameters except the pulse rate. The programming
screen 150 may further include a threshold entry box 158 for
manually entering the values of target evoked action potential
characteristics, which will be compared against the evoked action
potential measurements during the neurostimulation therapy. For
example, quantitative values of peak delay, amplitude, and width of
the evoked action potential measured following the stimulation
pulse at the efficacy threshold may be entered. Corresponding
thresholds for other types of objective therapeutic feedback
measurements may be entered into the box 158 in the similar manner.
If the therapeutic feedback is subjective conscious feedback from
the patient, the programming screen 150 further includes a patient
feedback box 160 in which the user may input the therapeutic effect
of the electrical stimulation as a percentage of pain alleviated.
It is appreciated that the illustrated embodiment of the user
interface is not intended to be limiting. The user interface may be
formatted to include any number of layouts, as readily
understood.
[0060] Referring to FIG. 7, the internal components of an exemplary
RC 16 is described. The RC 16 generally includes a processor 114
(e.g., a microcontroller), memory 116 that stores various data
(e.g., stimulation parameters) and series of instructions
executable by the processor 114. The RC 16 may also include an
input/output circuitry 120 for receiving stimulation control
signals from the button pad 104 and transmitting status information
to the display screen 102, and a telemetry circuitry 118 for
outputting stimulation parameters to and receiving status
information from the IPG 14. As mentioned above, the processor 114
may generate a set of stimulation parameters from the input
received from the programmer (e.g., patient or clinician) via the
buttons 104, which may be transmitted to the IPG 14. Further, the
processor 114 may analyze the therapeutic feedbacks (e.g., evoked
action potential measurement) obtained from the IPG 14 via the
telemetry circuitry 118, and generate a set of stimulation
parameters. The analysis and the stimulation parameter adjustment
routines, which will be described below, may be based on the
executable instructions stored in the memory 116. The set of
stimulation parameters generated by the processor 114 may be
transmitted to the IPG 14 via the telemetry circuitry 118, and used
by the IPG 14 to generate a corresponding stimulation pulse. The
telemetry circuitry 118 can also be used to receive stimulation
parameters from the CP 18. Further details of the functionality and
internal componentry of the RC 16 are disclosed in U.S. Pat. No.
6,895,280, which has previously been incorporated herein by
reference.
[0061] Although the foregoing programming functions have been
described as being at least partially implemented in the RC 16, it
should be noted that these techniques may be at least, in part, be
alternatively or additionally implemented in the CP 18. It is to be
emphasized that the schematic illustrations shown in FIGS. 4-7 are
intended to be functional, and not limiting. Those skilled in the
art will be able to fashion appropriate circuitry, whether embodied
in digital circuits, analog circuits, software and/or firmware, or
combinations thereof, in order to accomplish the desired
functions.
[0062] The neurostimulation system of the present disclosure uses
evoked action potential as an indicator of therapeutic effect of
electrical stimulation energy delivered by the IPG 14.
Characteristics of an evoked action potential that correlate to a
certain therapeutic effect may be stored as a template for matching
against other evoked action potentials that are measured following
the stimulation pulses during the neurostimulation therapy. The
comparison between the characteristics of the measured evoked
action potential and the characteristics of the targeted evoked
action potential (for example, the template) provides an objective
assessment as to the effectiveness of the stimulation. This
objective and quantitative measurement allows for the system to
automatically adjust the stimulation parameters to maintain the
efficacious therapeutic effect at the minimal power consumption
requirement.
[0063] The evoked action potential measurement technique may be
performed by generating an electrical field at one of the
electrodes 26, which is strong enough to depolarize the neurons
adjacent the stimulating electrode beyond a threshold level,
thereby inducing the firing of action potentials (APs) that
propagate along the neural fibers. Such stimulation is preferably
supra-threshold, but not uncomfortable to the patient or cause
undesirable side effects. A suitable stimulation pulse for this
purpose is, for example, 4 mA for 200 .mu.S. In operation, one or
more of the electrodes 26 is activated to generate the electrical
field, and the same electrodes or electrodes near the targeted
tissue is configured to record a measurable deviation in the
voltage caused by the evoked action potential due to the
stimulation pulse at the stimulating electrode. As such, the
technique may be implemented by using one or more of the electrodes
26 already carried on the lead. However, a dedicated electrodes or
sensors may be used for measuring the evoked action potentials if
needed. It should be noted that the evoked action potential
measurement technique described above is only representative of
various ways that may be used.
[0064] FIG. 8 is a timing waveform diagram 800 that depicts one way
in which the evoked action potential measurement is made. As shown
in FIG. 8, at time t1, a waveform caused by the stimulus is
generated. Such a waveform may cause inaccuracy when averaging
multiple evoked action potentials to obtain a compound evoked
action potential. Therefore, the noise or artifact caused by the
stimulation pulse may be suppressed or eliminated by a filter,
which may be implemented with software or hardware (e.g. blanking
circuit). The actual pulse representing the recorded evoked action
potential is generated. The characteristics of an evoked action
potential may include peak delay, width, amplitude, as well as the
waveform morphology. If the stimulation pulse is applied on a
recurring basis (e.g., at a set frequency), then there is also a
period T between pulses.
[0065] In most cases, the level of stimulation (for example, the
strength of the stimulation) generated by the IPG 14 is adjusted
throughout the course of therapy. For instance, the stimulation
pulse from the IPG 14 is adjusted in order to maintain the
paresthesia at a comfortable level. Even the optimal stimulation
setting at one point can be rendered to sub-optimal due to a
variety of factors such as patient's postural changes, lead array
movement, scar tissue maturation, as well as other temporal or
permanent changes that may occur in the patient. With incorrect
level of stimulation pulses from the IPG 14, the paresthesia can be
lost or cause undesired side-effects to the patient. In some cases,
the original level of the stimulation pulse can be converted into
painful over-stimulation. For this reason, the neurostimulation
system 10 of the present disclosure may be configured to
continuously monitor the evoked action potentials after stimulating
the target tissue, and analyze the characteristic of the evoked
action potentials to match against the template. The evoked action
potential comparison provides a measure of how effective the
applied stimulus is at stimulating the targeted tissue. So long as
the recorded evoked action potential matches with the template, the
stimulation is providing the intended therapeutic effect at the
targeted tissue.
[0066] It is the energy content of the stimulation that is adjusted
by the system 10. The energy content, which is also referred herein
as stimulation pulse or the level of stimulation pulse, can be
increased or decreased by adjusting one or more of the stimulation
parameters such as the pulse rate, pulse amplitude, pulse width,
and pulse duty cycle. The electrode combination (e.g., selection of
anode(s) and cathode(s)) is another stimulation parameter which may
be adjustable in controlling the stimulation level. In one
embodiment, the system 10 may be configured to notify the patient
or the clinician when it records evoked action potentials that do
not match the template. In response to this alert, the patient or
the clinician may adjust the stimulation parameters by using the RC
16 or the CP 18. The stimulation parameter adjustment can be
performed manually by the clinician or the patient, or it can be
performed is semi-automatic manner, in which the system provides
suggested stimulation parameter values. As will be described in
further detail below, the system 10 may include a reference
database which may contain a list of previous evoked action
potential measurements obtained at each stimulation parameter set.
The reference database may contain other evoked action
potential/stimulation correlation information which may be used in
assisting the stimulation parameter adjustment.
[0067] In the preferred embodiment, the system 10 is configured to
automatically adjust one or more of stimulation parameters to alter
the stimulation level (e.g., energy content of the stimulation)
until evoked action potentials having the same characteristics as
the template are recorded via the recording electrodes 26. As
mentioned above, the reference database may also be used in this
configuration for the system 10 to automatically store the evoked
action potential measurements for each set of stimulation
parameters tried by the system, and use the information as
needed.
[0068] The automatic stimulation adjustment process described above
may be triggered based on various pre-defined conditions. For
example, the stimulation adjustment process may be initiated
immediately upon detecting an evoked action potential having one or
more of its characteristics that differ from the template. In some
cases, however, mismatching evoked action potentials can be caused
by a temporary postural change, an acute lead movement, or
temporary impedance change at the target stimulation site by
various other factors. Constantly adjusting stimulation parameters
to obtain perfectly matching evoked action potential in such cases
may render the system 10 rather inefficient. Accordingly, in some
embodiments, the system 10 may be provided with a specific
tolerance rate for each characteristic of the evoked action
potential so that the stimulation parameters are adjusted when the
deviation goes beyond the tolerance rate. Similarly, the system 10
may be configured to adjust the stimulation parameter if the system
10 records mismatching evoked action potentials for more than a
predetermined time period. Various other types of verification
mechanisms may be employed by the system 10 to determine whether
the recording of the mismatching evoked action potential is
temporary or permanent. Further, in some embodiments the
neurostimulation system is configured to determine whether an
estimate of measured evoked action potentials having
characteristics different than characteristics of the reference
evoked action potential is temporary or permanent.
[0069] FIG. 9 is flow chart that illustrates an exemplary method
for automatically adjusting the stimulation parameters to generate
the stimulation pulse that results in the evoked action potential
corresponding to the desired therapeutic effect. In step 910, the
system 10 identifies either under-stimulation or over-stimulation
of the target tissue. In step 920, when no evoked action potential
is measured from the recording electrodes 26 or the characteristics
of the evoked action potential indicate under-stimulation, the
system 10 adjusts the stimulation parameters in a way that the
stimulation pulse generated by the IPG 14 is increased. In step
930, the IPG 14 generates stimulation pulse according to the set of
adjusted stimulation parameters. In step 940, an evoked action
potential is measured following the stimulation at the target
tissue. In step 950, the characteristics of the recorded evoked
action potential are compared to the characteristics of the target
evoked action potential stored in the template. If the
characteristics of the evoked action potential match the template,
the intended therapeutic effect from the stimulation pulse is
verified. Otherwise, the system 10 goes through additional
iterations of the process, and the stimulation level is gradually
increased until the revoked action potential having the same
characteristics as the template is determined in step 950.
[0070] When the characteristics of evoked action potential measured
from the recording electrodes 26 indicate over-stimulation in step
910, the system 10 adjusts the stimulation parameters in a way that
the stimulation pulse generated by the IPG 14 is decreased in step
920. The system 10 gradually decreases the stimulation level until
revoked action potentials having the same characteristics as the
template is measured in step 950. If the characteristics of the
measured evoked action potential match the template, the system 10
may verify if the power consumption optimization process should be
carried out in step 960. This can be verified from the rules stored
in the database and/or based on various measurements obtained from
the sensors. Based on the result, the system 10 may simply continue
the therapy using the same stimulation parameters, or continue to
adjust the stimulation parameters to optimize power consumption of
the system 10 in step 970.
[0071] During the automatic stimulation adjustment process, the
system 10 increases or decreases the stimulation level by adjusting
the values of stimulation parameters by a step size (e.g., unit
size). Each stimulation parameter may have different step size, and
the amount of value per step size may be determined based the
desired resolution of adjustment. For example, the step size for
the pulse rate may be 10 Hz, and thus one step size increase in the
pulse rate is equivalent to increase of 10 Hz. The step size may be
decreased to, for instance 5 Hz for increased resolution (e.g.,
finer adjustment) of adjustment at the cost of longer processing
time (e.g., increased iteration of adjustment) to identify the
stimulation level that evokes action potential matching the
template. The step size may be increased to, for instance 20 Hz, if
shorter processing time is desired over more precise adjustment.
Similar as the pulse rate, the step size for pulse amplitude, pulse
width and pulse duty cycle, may be, for example, 0.1 mA, 10 .mu.s,
10%, respectively, and they may be increased or decreased in the
similar manner. As for the electrode combination stimulation
parameter, the step size may be the electrode spacing of the
stimulation leads, e.g., 5 mm, which may be controlled by
electrodes selection.
[0072] It should be appreciated that an equivalent or substantially
same evoked action potential may result from various alternative
sets of stimulation parameters. For example, a lower amplitude
stimulation pulse closer to the target tissue and a higher
amplitude stimulation pulse from a distance may result in the same
evoked action potential. Likewise, an evoked action potential
measurement in response to a lower amplitude stimulation pulse at
higher pulse rate (e.g., higher frequency") may be have
substantially same characteristics as the characteristics of evoked
action potential in response to a higher amplitude stimulation
pulse at slower pulse rate.
[0073] Accordingly, the system 10 may be configured to perform the
power consumption optimization process to identify a more
energy-efficient set of stimulation parameters. That is, even when
the system 10 is measuring evoked action potentials with the
characteristics that match with the template, the system 10 may
continue to adjust the stimulation parameters in an effort to find
a stimulation setting that uses less power and yet provide the
intended therapeutic effects. This power consumption optimization
function allows the system 10 to minimize the power consumption by
keeping the level of stimulation pulse at or just above the
efficacy level necessary for the desired therapeutic effect.
[0074] Generally, the greater the values of the pulse rate, pulse
amplitude, pulse width, and pulse duty cycle, the greater the
energy consumption required by the IPG 14. The electrode
combination is another stimulation parameter which may be used to
decrease the power consumption. Notably, the spacing between the
cathode(s) and anode(s) used to deliver the electrical energy may
dictate the energy consumption required to generate the electrical
energy. For example, if the spacing between the cathode(s) and
anode(s) is relatively small, there may be substantial shunting of
electrical current between the cathode(s) and anode(s), thereby
requiring higher energy consumption in the IPG 14. In contrast, if
the spacing between the cathode(s) and anode(s) is relatively
great, there may be insubstantial shunting of electrical current
between the cathode(s) and anode(s), thereby requiring lower energy
consumption in the IPG 14. Thus, the greater the spacing between
the cathode(s) and anode(s), the lesser the energy consumption
required to generate the electrical stimulation pulses in
accordance with this stimulation parameter value.
[0075] Although the system 10 may instruct the IPG 14 to output the
electrical stimulation pulses to electrodes 26 based on a set of
stimulation parameters, not all stimulation parameters in the set
need to be adjusted. This is especially true when the purpose of
adjustment is to simply obtain the evoked action potential that
matches the template. For example, one step size increase or
decrease in the pulse amplitude may be enough to evoke the targeted
action potential. On the other hand, each and every combination of
stimulation parameters may need to be tried with different values
when optimizing the power consumption of the system. Of course,
multiple iterations of parameter adjustments, stimulation using the
adjusted parameters, followed by the comparison of evoked action
potential characteristics to the template may be required to
finalize the power consumption optimization process.
[0076] Accordingly, the selection and the order of stimulation
parameters to be adjusted during the stimulation adjustment process
and the power consumption optimization process may be determined
based on a number of pre-defined stimulation parameter adjustment
rules and the mode in which the system is operating in. Also, the
step size for adjusting stimulation parameters may be dynamically
adjusted depending on the system's operating mode. For instance,
the system 10 may be operating in the "quick adjustment" mode, in
which the step size resolution is decreased (i.e., larger step
size). Using the lower resolution step size, the system 10 can
increase or decrease the stimulation level faster, thereby
identifying the stimulation pulse that results in the evoked action
potential sufficiently similar to the template.
[0077] FIG. 10 illustrates an exemplary stimulation parameter
adjustment routine which may be performed by the system. The
routine shown in FIG. 10 is described in relation to the power
consumption optimization process. However, it should be appreciated
that the same routine may be used during the automatic stimulation
adjustment process for calibrating the level of stimulation pulse
to meet the intended therapeutic effect. In the example shown in
FIG. 10, it is assumed that the initial stimulation parameter set
corresponding to the efficacious therapeutic effect is already
known. The very first stimulation parameter, referred herein after
as the "threshold stimulation parameter set," may be based on the
perception threshold. The initial configuration of the threshold
stimulation parameter set may be entered by the patient or the
clinician via the RC 16 or CP 18. It is preferred that the
threshold stimulation parameter set to be configured in a way that
the stimulation level is as close to the minimum stimulation level
necessary to achieve the therapeutic effect (i.e., efficacy
threshold). In step 1100, the IPG 14 stimulates the target tissue
using the threshold stimulation parameter set, and the evoked
action potential is measured.
[0078] As mentioned above, the evoked action potentials can be
measured by either the same electrodes that were used for the
stimulation or other electrodes near the stimulated neural tissues.
The evoked action potential measurements from the group of
stimulated neural elements (e.g., neurons, muscle fibers) may be
processed in an appropriate manner so that a reliable determination
of evoked action potential can be made. For instance, evoked action
potential measurements may be averaged to obtain evoked compound
action potentials or compound muscle action potentials. Further, an
artifact or noise suppression process may be implemented by using
hardware (e.g., blanking circuit) or software to prevent noises
(e.g., stimulation artifact) from contaminating the compound evoked
action potential measurement.
[0079] In step 1200, the evoked action potential that was measured
in response to the threshold stimulation is analyzed, and its
characteristics are saved as a template for matching against the
evoked action potential measurements that follow. As previously
mentioned, the characteristics of the evoked action potential may
include peak delay, width, amplitude, as well as waveform
morphology.
[0080] In step 1300, one or more stimulation parameters are reduced
and/or increased by a step size, or other wise adjusted, and the
IPG 14 generates the stimulation pulse according to the adjusted
set of stimulation parameters. As way of an example, the system 10
may automatically decrease the amplitude, pulse width or pulse rate
to find more energy efficient stimulation setting. The system 10
may also stimulate on electrodes with lower impedance, or
electrodes that are nearer to the location where evoked action
potentials are detected in order to determine if the same
potentials can be evoked with more energy efficient settings. Also,
the step size may vary depending on the desired resolution of the
adjustment. Generally, it is preferred to use a small step size
value for more accurate stimulation parameter adjustment. However,
a larger step size value may be used during automatic stimulation
adjustment process for increased processing time.
[0081] In the example shown in FIG. 10, amplitude is selected for
reduction, and thus the target tissue is stimulated with the lower
amplitude stimulation pulse. Assuming all other stimulation
parameters were unchanged, the amount of power required by the IPG
14 for generating the stimulation pulse with lower amplitude will
be less than the amount of power it needed for generating the
previous stimulation.
[0082] In step 1400, the evoked action potential in response to the
stimulation is measured. In step 1500, once the evoked action
potential is measured, various characteristics of the recorded
evoked action potential, such as peak delay, width, amplitude, as
well as waveform morphology, are compared to the corresponding
characteristics of the evoked action potential saved in the
template. If the characteristics of the newly recorded evoked
action potential match with the template, the desired therapeutic
effect by the stimulation is verified. In such case, in step 1600,
the threshold stimulation parameter set is replaced by the
stimulation parameter set with the lower amplitude. By using this
objective comparison, subjective feedback from the patient (e.g.,
perception threshold) is no longer needed in determining whether
the system 10 is providing the indented therapeutic effect.
[0083] In step 1800, the power consumption optimization process may
continue with the next iteration, including stimulation parameter
adjustment, stimulation according to the adjusted stimulation
parameter set followed by the comparison of evoked action potential
against the template, in order to identify even more
energy-efficient stimulation parameter sets. As described before,
this can be verified from the rules stored in the database and/or
based on various measurements obtained from the sensors in step
1700. The system 10 will continue to iterate through alternative
stimulation parameter sets that use less power than the latest
threshold stimulation parameter set. Each time when more
energy-efficient set of stimulation parameters capable of evoking
the targeted evoked action potential is found, it will replace the
previous threshold stimulation parameter set. Even if the
characteristics of the newly recorded evoked action potential do
not match with the template in step 1400, the power consumption
optimization process may still continue by adjusting different
stimulation parameters.
[0084] As described earlier, the selection of stimulation
parameters for adjustment, the order in which they are adjusted
throughout the adjustment routines (e.g., stimulation parameter
adjustment mode, power consumption optimization mode) as well as
the step size may be specified by the parameter selection rules and
the operating mode of the system 10. For example, some parameter
selection rules may prioritize lower number of adjustment
iterations during the stimulation parameter adjustment routine. By
way of an example, the amplitude and pulse rate parameters may be
adjusted by two or more step sizes in the first iteration, and then
adjust the electrode combination parameter in next iteration to
fine tune the stimulation energy level. Such a rule may be
particularly useful when the system is operating in the "quick
adjustment" mode, simply to find a set of stimulation parameters
that would provide sufficient stimulation at the target tissue.
[0085] Based on another stimulation parameter selection rule, the
system 10 may select and adjust stimulation parameters in the way
which would increase or decrease the stimulation level as little as
possible per adjustment iteration. For instance, a stimulation
parameter would be selected and adjusted in the smallest step size
(e.g., one step size or one half of step size) in a way to minimize
the fluctuation of the stimulation energy level during the
stimulation parameter adjustment routine. This rule may be
particularly helpful when the system is operating in the "power
consumption optimization" mode because the minimal fluctuation in
the stimulation level would allow the power optimization process to
be carried out even when the patient is in sleep.
[0086] Moreover, some stimulation parameter adjustment rules may be
based on specific correlation between a stimulation parameter and
the therapeutic effect. For instance, a certain rule can lock or
otherwise limit the adjustment of specific stimulation parameters.
By way of an example, the system 10 may be configured to maintain
the pulse rate high and adjust other stimulation parameters,
because high frequency stimulation is known to minimize the
paresthesia. Of course, various other stimulation parameter
selection rules may be defined by using any correlation between the
stimulation parameters and the therapeutic effect.
[0087] In step 1600, the system may be configured to end the power
consumption optimization process when all available adjustment
options have been tried. Also, the system 10 may be configured to
halt the power optimization process based on certain conditions
such as limited time period, limited number of adjustment
iterations, or feedbacks from various sensors (e.g., temporary
impedance changes, patient's movement, temperature changes), and
resume the power consumption optimization process in later time. In
this setting, the progress of the power consumption optimization
process, including the stimulation parameter sets and corresponding
evoked action potential measurements, may be saved in the reference
database, which may be implemented in the memory of IPG 14, RC 16
and/or the CP 18.
[0088] Identifying the most energy efficient set of stimulation
parameters may require considerably more time and iterations of
adjustments than simply identifying a stimulation parameter set
that results in the targeted evoked action potential. There is also
possibility that the patient may experience discomfort or other
undesired side effects while performing this lengthy power
consumption optimization cycle. Accordingly, in some embodiments,
the power consumption optimization function may be performed
periodically at predefined time (e.g., every other Monday at 12 AM)
or at the command of the patient (e.g., by receiving a command via
RC 16).
[0089] As mentioned, the system 10 may utilize sensors to determine
the state or condition of the patient, and initiate/resume the
power consumption optimization process. Such sensors may be carried
by the IPG 14, the stimulation leads 12, or can be separate from
these devices. The sensors may be adapted for various measurements
such as body activity (measured using accelerometer or electrode
impedance variation), body temperature, blood flow (peripheral or
central), electrocortigram, electroencephalogram, tissue or
transcutaneous oxygen tension, glucose concentration, electrode
impedance, intra- or extra-cellular potential or electrical
current, and chemical species concentration (intrathecally,
epidurally, or subcutaneously). These measurements may be used in
conjunction with some of the parameter adjustment rules as
discussed above. For example, the system 10 may be configured to
initiate/resume the power consumption optimization process when the
patient is in non-moving or otherwise stable state (e.g., stable
impedance measurement at the target tissue), or while sleeping.
[0090] In some other embodiment, the evoked action potential could
be used to define the upper limit of stimulation (e.g., side-effect
threshold) rather than the lower limit of stimulation (e.g.
efficacy threshold), such that the system 10 finds a setting that
is sub-threshold for evoking a potential with certain
characteristics while still evoking a potential with different
characteristics and providing energy efficient therapy. Also, in
some embodiments, the system may be configured to store multiple
templates (i.e., characteristics of two or more evoked action
potentials) and provide different level of stimulations based on a
therapeutic schedule programmed by the clinician. For instance, one
template can be used for matching against evoked action potential
measurements during the first two weeks of therapy, and use the
second template thereafter.
[0091] Optionally, the system may be placed into a "learning mode"
to create a reference table that captures the correlation between
various sets of stimulation parameters and the resulting evoked
action potential from the targeted tissue. In accordance with the
method of the present disclosure, this reference database may be
stored in the memory and recalled by the IPG 14, RC 16 and/or CP 18
to make nearly instantaneous corrective adjustments to stimulation
parameters. Moreover, in some embodiments, the waveform of the
evoked action potential may be analyzed through methods such as
principal component analysis, and the system 10 may match signals
in that space instead of using simple waveform or threshold for
matching to the template.
[0092] Although particular embodiments of the present inventions
have been shown and described, it will be understood that it is not
intended to limit the present inventions to the preferred
embodiments, and it will be obvious to those skilled in the art
that various changes and modifications may be made without
departing from the spirit and scope of the present inventions.
Thus, the present inventions are intended to cover alternatives,
modifications, and equivalents, which may be included within the
spirit and scope of the present inventions as defined by the
claims.
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