Implantable stimulator with means for determining an impedance and for selecting a stimulation based on the impedance

FIELD OF THE INVENTION

0001The present invention relates to implantable stimulator devices, such as spinal cord stimulators.

BACKGROUND

0002Implantable stimulation devices are devices that generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. Examples of implantable stimulator devices are disclosed in, e.g., <patcit id="pcit0001" dnum="US2011060387A1"><text>US 2011/060387 A1</text></patcit>, <patcit id="pcit0002" dnum="US2009024189A1"><text>US 2009/024189 A1</text></patcit>, <patcit id="pcit0003" dnum="US2011301665A1"><text>US 2011/301665 A1</text></patcit>, <patcit id="pcit0004" dnum="US2011009927A1"><text>US 2011/009927 A1</text></patcit> and <patcit id="pcit0005" dnum="EP2497531A1"><text>EP 2 497 531 A1</text></patcit>. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system to treat lower back pain, such as that disclosed in <patcit id="pcit0006" dnum="US6516227B"><text>U.S. Patent 6,516,227</text></patcit>. However, the present invention may find applicability in any implantable stimulator device

0003As shown in Figures 1A-1C, a SCS system typically includes an Implantable Pulse Generator (IPG) 10, which includes a biocompatible device case 12 formed of a conductive material such as titanium for example. The case 12 typically holds the circuitry and battery 14 (<figref idref="f0002">Fig. 2B</figref>) necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 10 is coupled to electrodes 16 via one or more electrode leads 18, such that the electrodes 16 form an electrode array. The electrodes 16 are carried on a flexible body 20, which also houses the individual signal wires 22 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead 18, labeled E₁-E₈,_. However, the number of electrodes on a lead, as well as the number of leads, are application specific and therefore can vary. The lead 18 couples to the IPG 10 using a lead connector 24, which is fixed in a non-conductive header material 26, which can comprise an epoxy for example.

0004As shown in the cross-section of <figref idref="f0002">Figure 2B</figref>, the IPG 10 typically includes an electronic substrate assembly including a printed circuit board (PCB) 30 containing various electronic components 32. Two coils (more generally, antennas) are generally present in the IPG 10: a telemetry coil 34 for transmitting/receiving data to/from an external controller 50; and a charging coil 36 for charging or recharging the IPG's battery 14 using an external charger (not shown). (<figref idref="f0001">Fig. 1B</figref> shows the IPG 10 with the case 12 removed to ease the viewing of the two coils 34 and 36).

0005<figref idref="f0002">Figure 2A</figref> shows a plan view of the external controller 50, and <figref idref="f0002">Figure 2B</figref> shows the external controller 50 in relation to the IPG 100 with which it communicates. The external controller 50 is shown as a traditional hand-held patient controller, although it could also comprise a clinician programmer of the type typically used in a clinician's office. (A clinician external controller would look differently, as one skilled in the art understands, and typically comprises computer). The external controller 50 is used to send data to and receive data from the IPG 10. For example, the external controller 50 can send programming data such as therapy settings to the IPG 10 to dictate the therapy the IPG 10 will provide to the patient. Also, the external controller 50 can act as a receiver of data from the IPG 10, such as various data reporting on the IPG's status.

0006As shown in <figref idref="f0002">Figure 2B</figref>, the external controller 50, like the IPG 100, also contains a PCB 52 on which electronic components 54 are placed to control operation of the external controller 50. The external controller 50 is powered by a battery 56, but could also be powered by plugging it into a wall outlet for example.

0007The external controller 50 typically comprises a user interface 60 similar to that used for a portable computer, cell phone, or other hand held electronic device. The user interface 60 typically comprises touchable buttons 62 and a display 64, which allows the patient or clinician to send therapy programs to the IPG 10, and to review any relevant status information reported from the IPG 10.

0008Wireless data transfer between the IPG 10 and the external controller 50 preferably takes place via inductive coupling. This typically occurs using a well-known Frequency Shift Keying (FSK) protocol, in which logic '0' bits are modulated at a first frequency (e.g., 121 kHz), and logic '1' bits are modulated at a second frequency (e.g., 129 kHz). To implement such communications, both the IPG 10 and the external controller 50 have communication coils 34 and 58 respectively. Either coil can act as the transmitter or the receiver, thus allowing for two-way communication between the two devices. This means of communicating by inductive coupling is transcutaneous, meaning it can occur through the patient's tissue 70.

0009The lead 18 in an SCS application is typically inserted into the epidural space 80 proximate to the dura 82 within the patient's spinal cord, as illustrated in cross section in <figref idref="f0003">Figure 3</figref>. The proximate portion of the lead 18 is tunneled through the patient where it is attached to the lead connector 24 of the IPG 10, which is implanted a somewhat distant location from the lead, such as in the upper portion of the patient's buttocks. Typically in an SCS application, there are two leads implanted in the left and right sides of the spinal column, so that stimulation therapy can be delivered by the IPG 10 to left- and right-branching nerves from the dura 82. However, only one such lead is shown in <figref idref="f0003">Figure 3</figref> for simplicity.

0010Once implanted, the patient is typically put though a fitting procedure to determine effective therapy to treat the patient's symptoms. This typically occurs in a clinician's office, and may be somewhat experimental in nature. (Some aspects of fitting may also occur prior to full implantation of the IPG 10 during an external trial stimulation phase, which is discussed in <patcit id="pcit0007" dnum="US20100228324A"><text>U.S. Patent Publication 2010/0228324</text></patcit> for example).

0011Generally speaking, during the fitting procedure, various stimulation parameters are applied by the IPG 10 to determine what feels best for the patient, and then such stimulation parameters can then be stored in the IPG 10 as a stimulation program. Stimulation parameters can include which electrodes 16 on the lead 18 are active, the polarity of the active electrodes (i.e., whether they act as anodes (current sources) or cathodes (current sinks)), the magnitude of the current pulses applied at the active electrodes (which may comprise either a voltage or current magnitude), the duration of the pulses, the frequency of the pulses, and other parameters. These stimulation parameters can be varied by the external controller 50-either a clinician programmer or a hand-held patient controller-which wirelessly communicates with the telemetry coil 34 in the IPG 10 to change and store the parameters in the IPG 10. After a stimulation program has been set by the clinician, and the patient has left the clinician's office, the patient can modify the stimulation parameters of that program using his hand-held patient controller.

0012The art has recognized that patients may benefit from the use of different stimulation at different times, and in particular depending on the patient's posture or activity. This is because the position of the lead 18 may move in the epidural space 50 as the patient changes moves, e.g., from supine (on one's back), to standing, to prone (on one's stomach). This is shown by the arrows in <figref idref="f0003">Figure 3</figref>. As seen, as the patient moves, the distance between the electrodes 16 and the dura 82 can change. This change in distance can warrant changes in therapy.

0013In the prior art, such changes in patient posture or activity were sensed by an accelerometer in the IPG 10. As is well known, an accelerometer can detect its position in three-dimensional (3D) space by assessing gravitational forces, and so can detect the 3D position of a patient in which it is implanted. According to this technique, a patient is somewhat relieved of the obligation to manually adjust his stimulation parameters when changing postures, because the IPG can recognize a change in body posture and remember the level of stimulation needed. Thus, in the prior art, an IPG 100 could sense when the patient changes posture; learn from previous experience and remember the patient's last comfortable setting for that posture; and respond by automatically adjusting stimulation to the patient's chosen setting for that posture.

SUMMARY OF THE INVENTION

0014The present invention relates to an implantable stimulator device according to claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

0015<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figures 1A-1B</figref> show different views of an implantable medical device, specifically an Implantable Pulse Generator (IPG).</li><li><figref idref="f0002">Figures 2A-2B</figref> show an external controller for communicating with the IPG.</li><li><figref idref="f0003">Figures 3, 4A, and 4B</figref> show an electrode leads positioned in an epidural space within a patient's spinal cord, and shows how the positioning of the lead can change based on a patient's posture.</li><li><figref idref="f0004">Figure 5</figref> shows an illustration of impedance monitoring circuitry in accordance with an embodiment of the invention for measuring an impedance between two electrodes of an IPG, including an impedance algorithm.</li><li><figref idref="f0005">Figure 6</figref> shows how the impedance algorithm can compile the impedance measurements into an impedance fingerprint indicative of the tissue impedance between various electrode combinations.</li><li><figref idref="f0006">Figure 7</figref> shows an example of how the impedance measurements in <figref idref="f0005">Figure 6</figref> for a given electrode combination can be taken at more than one frequency.</li><li><figref idref="f0007">Figure 8</figref> shows how the impedance algorithm interfaces with a fingerprint database comprising known impedance fingerprints and associated stimulation programs indicative of a patient posture, and interfaces with a fingerprint log comprising measured fingerprints.</li><li><figref idref="f0008">Figures 9A and 9B</figref> show a display of an external controller in communication with an IPG comprising the impedance monitoring circuitry, and show how a user can review and rename postures identified by the IPG, and modify the stimulation program associated with an identified posture.</li><li><figref idref="f0009">Figures 10A and 10B</figref> show how and when the impedance measurements comprising an impedance fingerprint can be taken during operation of a stimulation program.</li></ul>

DETAILED DESCRIPTION

0016The inventors see shortcomings from the prior-art-based method of using an accelerometer in an IPG to sense patient posture and thus to determine an appropriate stimulation program for the patient. For one, the prior art does not consider that forces other than accelerative (e.g., gravitational) forces might be impingent upon the patient in a manner indicative of posture and thus affecting therapy. This can lead to misleading assumptions about the patient's posture. For example, an accelerometer in an IPG might conclude that a patient lying prone and a patient standing but bending forward are in the same posture, when reality is otherwise. Likewise, an accelerometer might conclude that a patient standing up and a patient sitting down are in the same posture, again when this is not true.

0017The inventors see such distinctions as significant, particularly upon realizing that the positioning of electrodes on the lead 18 can vary relative to the dura 82 in unpredictable and non-uniform ways. Consider <figref idref="f0003">Figures 4A and 4B</figref>. In <figref idref="f0003">Figure 4A</figref>, it is assumed that the patient is lying prone, with the result that the lead 18 has generally moved away from the dura 82 by a constant distance, d, along its entire length. By contrast, in <figref idref="f0003">Figure 4B</figref>, it is assumed the patient is standing but bending forward. When the patient bends in this manner, the epidural space 80 and dura 82 will also bend while the lead 18 remains relatively straight, with the result that the distance between the electrodes and the dura 80 is not constant, with some electrodes closer and some farther away. Additionally, when the patient bends, the lead 18 can also shift laterally in the epidural space 80, as shown by the arrow in <figref idref="f0003">Figure 4B</figref>. This too changes the positioning of the electrodes relative to the dura 50. For example, consider a nerve branch 84 that causes pain for a patient and requires neurostimulation. In <figref idref="f0003">Figure 4A</figref>, activation of electrode E2 would be most likely to recruit and treat nerve branch 84. However lateral shifting of the lead in <figref idref="f0003">Figure 4B</figref> now brings electrode E1 closer to nerve branch 84, suggesting that activation of that electrode, rather than E2, would provide the best therapy for the patient.

0018Despite these postural differences, an accelerometer based approach might determine that both of the patient postures in <figref idref="f0003">Figures 4A and 4B</figref> are the same, and therefore might conclude that a single stimulation program is appropriate for both postures, when in reality, different stimulation programs for each posture may be beneficial.

0019In recognition of this fact, the inventors approach to determining patient posture, and hence what stimulation program is effective, relies on measurements taken at the electrodes. Specifically, and as will be discussed in further detail below, impedance measurements of the patient's tissue are taken at the electrodes, which measurements can comprise complex impedance measurements (magnitude and phase) taken at different frequencies. Such impedance measurements, which can be taken interleaved with stimulation therapy, are used to determine an "impedance fingerprint." This fingerprint can be compared to other known fingerprints stored in the IPG, which known fingerprints are associated with particular stimulation programs. When a measured fingerprint matches one stored in the IPG, the stimulation program associated with the stored fingerprint is automatically selected for patient therapy. If the stimulation program is changed for a given posture, such changes will be stored with its associated fingerprint to remember these changes for the next time the patient is in the relevant posture. Additionally, as new fingerprints are measured which do not match those stored, the IPG can learn and store a new stimulation program for such new fingerprint by remembering stimulation parameters selected by the patient when such fingerprint is encountered. As such, the IPG can learn new postures and new stimulation programs "on the fly" as simulation is occurring, and as the patient goes about his day.

0020Before discussing particulars of the technique, an example of impedance monitoring circuitry 100 useable in an IPG 10 in accordance with the technique is disclosed in <figref idref="f0004">Figure 5</figref>. For simplicity, the circuitry 100 is shown for only a single electrode E, but such circuitry would in an actual implementation be duplicated or shared so that the impedance at all electrodes can be monitored. The depicted impedance monitoring circuitry is but one example of circuitry capable of performing impedance measurements, and should not be taken in a limiting sense.

0021Electrode E is shown relative to another reference electrode, Ref, which (as will be discussed subsequently) could comprise another electrode on the lead 18 or the IPG's conductive case 12 (<figref idref="f0001">Fig. 1A</figref>). A patient's tissue intervening between the electrode and the reference is modeled 102 as an R-C network, with a capacitor C and resistor R1 in parallel, and that parallel combination coupled to a series resistor R2. Generally speaking, tissue fluids are modeled by R2, while cell membranes in the tissue are modeled by the parallel connection of C and R1. Tissue model 102 could be further refined, with different resistances and capacitances to better fit the reality of the environment of the tissue, but the simple three component model 102 serves for purpose of illustration. By virtue of the capacitance, the impedance Z of the tissue will be complex, as shown by the equations in <figref idref="f0004">Figure 5</figref>. As such, the AC voltage V produced across the tissue in response to a constant AC current through the tissue will be phased-shifted (θ) relative to the current, as shown in the waveforms.

0022In one example, the measurement signal for the impedance measurement is provided by a AC constant current source 105. The magnitude, |I|, and frequency, f, of the produced current is controlled by an impedance algorithm 120 operating in a microcontroller 110, which algorithm will be discussed in further detail below. Microcontroller 110 can comprise any known central processor or controller typically present in an IPG 100, or any logic circuitry more generally. The voltage drop across the current source can be monitored by a compliance voltage regulator 115, which is used to generate a compliance voltage V+ sufficient to provide the desired current I without loading, and in an efficient manner considerate of power draw from the IPG's battery 14. Suitable compliance voltage regulation circuitry 115 can be found in <patcit id="pcit0008" dnum="US61654606" dnum-type="L"><text>U.S. Patent Application Serial No. 61/654,606, filed June 1, 2012</text></patcit> with which the reader is assumed familiar. Typically, the magnitude of the constant current |I| used for the impedance measurement will be much lower than typical magnitudes used for therapeutic currents, and hence won't be noticed by the patient. For example, |I| can range from 1 microamp to 100 microamps. Current source 105, depending on its complexity, can also be used to generate the therapeutic stimulation pulses to the patient (see <figref idref="f0009">Fig. 10</figref>), or may be separate from the sources used to generate the stimulation pulses.

0023Monitoring of tissue impedance occurs under the control of the impedance algorithm 120 operating in the microcontroller 110, which algorithm, as will be discussed further later, can operate while the IPG 10 is providing stimulation to the patient. At an appropriate time for a measurement, the impedance algorithm 120 sends the magnitude, |I|, and frequency, f, of the measuring signal I to the current source 105. The resulting AC voltage V at electrode E resulting in response to the measurement signal I is then digitized at an Analog-to-Digital (A/D) converter 125, which samples the voltage at an appropriate rate (such as every ten degrees) and an appropriate number of times (such as over 180 degrees) to allow the impedance algorithm 120 to discern the waveform's magnitude, |V|, and phase, θ, relative to the current. (It is assumed here that the impedance algorithm 120 understands the relative phase θ between the voltage and the current by virtue of its control of the current source 105. However, if necessary, the phase of the current I can also be monitored, and compared with the voltage at a phase detector, to provide the relative phase θ to the impedance algorithm).

0024Once the voltage magnitude, |V|, and phase, θ, have been determined by the impedance algorithm 75, the complex impedance, Z, of the tissue can be calculated based upon the impedance algorithm 75's a priori knowledge of the current magnitude, |I|. Specifically, and as shown in the equations in <figref idref="f0004">Figure 5</figref>: <maths id="math0001"><math display="block"><mi mathvariant="normal">Z</mi><mo>=</mo><mi mathvariant="normal">V</mi><mo>/</mo><mi mathvariant="normal">I</mi><mo>=</mo><mfenced open="|" close="|"><mi mathvariant="normal">V</mi></mfenced><mo>/</mo><mfenced open="|" close="|"><mi mathvariant="normal">I</mi></mfenced><mo>∗</mo><msup><mi mathvariant="normal">e</mi><mrow><mo>−</mo><mi>jθ</mi></mrow></msup><mo>=</mo><mfenced open="|" close="|"><mi mathvariant="normal">Z</mi></mfenced><mo>∗</mo><msup><mi mathvariant="normal">e</mi><mrow><mo>−</mo><mi>jθ</mi></mrow></msup></math><img file="EP2928366B1_D0001.tif" /></maths> where the magnitude of the impedance, |Z|, comprises the ratio between the magnitudes of the voltage and the current (i.e., |Z|=|V|/|I|). The magnitude, |Z|, and phase, θ, of this impedance 130 are stored by the impedance algorithm 120 a given frequency. Such storage may be within memory in the microcontroller 110, or a separate memory associated with and controlled by the microcontroller. In a preferred embodiment, the impedance measurement 130 can be repeated at electrode E at other frequencies, thus allowing new magnitudes, |Z|, and phases, θ, to be determined and stored, as discussed further below.

0025<figref idref="f0005">Figure 6</figref> illustrates how such impedance measurements 130 can be compiled and processed by the impedance algorithm 120 to determine an impedance fingerprint 150 of the tissue. Different electrodes may act as the reference electrode (Ref; <figref idref="f0004">Fig. 5</figref>) for any particular electrode whose impedance is being measured. For example, in electrode combinations 140a, the conductive case 12 acts as the reference electrode (e.g., ground) relative to each of the measured electrodes. This is desirable to understand each electrode's impedance relative to something akin to a system ground, which a large case electrode essentially provides in the relatively conductive environment of a patient's tissue.

0026In electrode combinations 140b, the impedance at each electrode is measured relative to its neighbor: E2 to E1, E3 to E2, etc., and so such neighbor acts as the reference. This is desirable because the electrodes can have different distances to relevant structures. For example, and referring again to <figref idref="f0003">Figure 4B</figref> in which the tissue is bent relative to the lead 18, if the impedances of the epidural space 80 and the dura 82 differ, then it would be expected that the impedance between E4 and E3 (which are relatively close to the dura 82) and electrodes E8 and E7 (which are relatively far from the dura), would be significantly different. In short, the combined information provided by electrode combinations 140a and 140b should provide a reliable assessment of the tissue impedance relative to the electrodes. However, other electrode combinations are possible, and greater numbers of electrode combination impedance measurements will increase the resolution of the resulting fingerprint, and thus increase the likelihood of eventually automatically choosing an appropriate stimulation program based on postural changes. Moreover, an electrode combination 140 may also include more than simply a pair of two electrodes, and can instead comprise an impedance measurement taken between a more-complex tissue network defined by three or more electrodes.

0027Taking measurements at different frequencies allows the impedance algorithm 120 to compute values for the various components between the two electrodes in the tissue model 102-R1, R2, and C-to be determined. Because tissue model 102 contains three unknown values, measurements are taken at at least three frequencies, thus rendering a system of equations from which the impedance algorithm 120 can deduce R1, R2, and C between a given electrode combination 140.

0028<figref idref="f0006">Figure 7</figref> shows a simple example of three such measurements, taken at low intermediate, and high frequencies (f_low, f_int, and f_high), and assuming that no decoupling capacitors 103 (<figref idref="f0004">Fig. 5</figref>) are included in the measurement. (Decoupling capacitors 103 are discussed further below). The simulation assumes that R2=100 ohms, R1=900 ohms, and C=1µF, although experimentation may be required to determine values that are more indicative of a patient's actual tissue. If the frequency of f_low is small enough (e.g., tens of Hertz), the capacitor C can be assumed to be an open circuit in the tissue model 102, meaning the measured impedance magnitude (|Z|_low) is dominated by the sum of R1 and R2. Likewise, if the frequency of f_high is high enough (e.g., tens of kilohertz), the capacitor C can be assumed to be a short circuit, meaning the measured impedance magnitude (|Z|_high) is dominated by R2 alone. In other words, the values for R1 and R2 can be determined, or at least estimated, solely using the impedance magnitudes at these two frequencies. A third impedance measurement, f_int, is taken at some intermediate frequency, which may be one which experimentation teaches results in an impedance magnitude that is about half of the maximum magnitude expected (e.g., ½(R1+R2)). This third measurement renders an impedance of |Z|_int and θ_int, which can then be used to determine the value of the capacitor C. Specifically, and using the equations set forth in <figref idref="f0004">Figure 5</figref>: <maths id="math0002"><math display="block"><msub><mfenced open="|" close="|"><mi mathvariant="normal">Z</mi></mfenced><mi>int</mi></msub><mo>∗</mo><msup><mi mathvariant="normal">e</mi><mrow><mo>−</mo><mi>jθ</mi><mi>int</mi></mrow></msup><mo>=</mo><mi mathvariant="normal">R</mi><mn>2</mn><mo>+</mo><mfenced><mi mathvariant="normal">R</mi><mn>1</mn><mo>/</mo><mfenced><mn>1</mn><mo>+</mo><mi mathvariant="normal">R</mi><mn>1</mn><mo>∗</mo><mi>Cj</mi><mn>2</mn><msub><mi>πf</mi><mi>int</mi></msub></mfenced></mfenced></math><img file="EP2928366B1_D0002.tif" /></maths> which can be solved for C. However, this simple method, whereby R1 and R2 are determined by making simple assumptions about the capacitance, is not strictly necessary. Measurements at any three frequencies can be used, rendering a system of equations from which R1, R2, and C can be solved from (|Z|1, θ1, f1), (|Z|2, θ2, f2) and (|Z|3, θ3, f3).

0029While the particular tissue model 102 here assumes three unknown values, other more or less complex tissue models 102 could exist, warranting different numbers of minimal frequency measurements to allow for the values in such models to be solved. Inclusion of measurements at more than a minimal number of frequencies allows the unknown tissue model values to be determined with increased reliability. In one example, the impedance measurements can be taken over frequencies ranging from about 20 Hz to 50 kHz.

0030It is known in the art of implantable neurostimulators to connect decoupling capacitors 103 (<figref idref="f0004">Fig. 5</figref>) to stimulating electrodes to prevent the direct injection of DC current into a patient tissue, which can be a safety concern. If present, such decoupling capacitors 103 may be included in the impedance measurement, yielding a total measured impedance of <maths id="math0003"><math display="block"><mfenced open="|" close="|"><mi mathvariant="normal">Z</mi></mfenced><mo>∗</mo><msup><mi mathvariant="normal">e</mi><mrow><mo>−</mo><mi>jθ</mi></mrow></msup><mo>=</mo><mfenced><mn>2</mn><mo>/</mo><msub><mi mathvariant="normal">C</mi><mi>de</mi></msub><mi mathvariant="normal">j</mi><mn>2</mn><mi>πf</mi></mfenced><mo>+</mo><mi mathvariant="normal">R</mi><mn>2</mn><mo>+</mo><mfenced><mi mathvariant="normal">R</mi><mn>1</mn><mo>/</mo><mfenced><mn>1</mn><mo>+</mo><mi mathvariant="normal">R</mi><mn>1</mn><mo>∗</mo><mi>Cj</mi><mn>2</mn><mi>πf</mi></mfenced></mfenced></math><img file="EP2928366B1_D0003.tif" /></maths> where (2/C_dej2πf) comprises the series impedance of the two decoupling capacitors 103. Because the capacitances of the decoupling capacitors, Cde, are known, they will not affect the ability to discern the unknown tissue values of R1, R2, and C by measuring at three different frequencies. (Note however that the inclusion of the decoupling capacitors 103 in the impedance measurement does not allow for the simple approximation of R1 and R2 at high and low frequencies, as shown in <figref idref="f0006">Figure 7</figref>).

0031Referring again to <figref idref="f0005">Figure 6</figref>, once the impedance algorithm 120 has computed the tissue model values (R1, R2, and C) for each of the electrode combinations 140, these values taken together comprise an impedance fingerprint (FP) 150. While it is informative that the fingerprint 150 comprises the solved-for values of electrical components in the tissue model 102, this is not strictly necessary. Instead, the fingerprint 150 can be comprised of any other suitable metric or metrics indicative of the complex impedance of the various electrode combinations 140. For example, the portion of the impedance fingerprint 150 for any given electrode combination 140 can comprise the raw impedance magnitudes and phases (|Z|, θ) measured at the various frequencies, or some other processing of this raw information short of solving tissue model values. Indeed, because it is not strictly necessary to solve tissue model values, it is not strictly necessary that the fingerprint 150 comprise information taken at any particular number of frequencies. Experience may teach that complex impedances measured at a single frequency may be sufficient to reliably discern patient posture. Additionally, the impedance fingerprint 150 may involve processing the impedance measurements or tissue model values for each of the electrode combinations together into a new metric.

0032The impedance fingerprint 150 is stored by the impedance algorithm 120 in a fingerprint database 200, as shown in <figref idref="f0007">Figure 8</figref>. The fingerprint database 200, like the impedance measurements 130, can be stored in any suitable memory in the IPG 10. The fingerprints 150 in the database 200 are each associated with a stimulation program (SP) 160, which may be determined during a fitting procedure with a clinician, or which may be new fingerprints learned on the fly, as discussed below.

0033Stimulation programs SPa-SPf are assumed in the illustrated example of <figref idref="f0007">Figure 8</figref> to be determined during a fitting procedure. For example, the clinician will ask the patient to stand, and then instruct the IPG 100 using an external controller 50 to take and store an impedance fingerprint 150 (FPa). While still standing, the clinician will adjust the stimulation parameters using the external controller 50-which again may comprise one of more of which electrodes are active, the polarity of the active electrodes, the magnitude, duration, and frequency of the current pulses applied at the active electrodes, or other parameters-to determine a stimulation program 160 that provides desirable therapy for the patient in the standing position (SPa). The fingerprint (FPa) and stimulation program (SPa) are then associated in the fingerprint database 200. Thereafter, the clinician will have the patient take new postures, and repeat the above procedure to determine various fingerprints (FPx) and stimulation programs (SPx) and store and associate them in the fingerprint database 200.

0034It is not strictly necessary that the fingerprint database 200 be pre-populated with posture related fingerprints 150 and associated stimulation programs 160 during a fitting procedure, as the system can learn and store new fingerprints and stimulation programs on the fly, as discussed further below. Both learned fingerprints and fingerprints determined during a fitting session are said to be pre-determined once stored in the database 200.

0035It should be noted that the fingerprints 150 stored in fingerprint database 200 may from time to time require updating. Such updating is useful when it is recognized that the physiology around the IPG 10 and lead 18 can change over the course of its useful life. For example, scar tissue may develop around the electrodes 16 on lead 18 or the case 12, or the lead 18 may over time settle into a particular position in the epidural space 80, either of which may require altering of the fingerprints 150 in the database 200. As such, it should be understood that the fingerprints 150 in database 200 will from time to time be updated, either in a clinician's office as described above, or under the control of the patient. For example, the microcontroller 110 in the IPG 10 can be controlled to communicate to the external controller 50 when it is reasonable to update the various fingerprints stored in the database 200, perhaps every six months or so. When the patient is so notified by the external controller 50, the user interface 60 of the external programmer 50 can be programmed to walk the patient through a fitting procedure similar to that described above, allowing new impedance measurements to be taken in the different postures understood by the database 200, and to update the stored fingerprints 150 associated with these postures. However, and again, such periodic re-fitting and pre-population of the fingerprint database 200 is not strictly necessary if fingerprints are learned and stored in the database 200 on the fly, as discussed further below.

0036The database 200 may optionally store a textual description 170 of the posture, which again may be entered into the external controller 50 by the clinician during the fitting procedure. This can be useful to allow the external controller 50 to query the various postures stored in the fingerprint database 200. For example, by communicating the textual description 170 of the currently-running stimulation program 160 to the external controller 50, the patient can verify that that program matches the patient's actual posture. Displaying a textual description 170 of the posture at the external controller 50 is also useful to allow the patient to modify that stimulation program, or to select a different stimulation program, as shown in example display 64 of the external controller 50 in <figref idref="f0008">Figure 9A</figref>. In a preferred embodiment, if the patient manipulates user interface 60 of the external controller 50 to modify any of the simulation parameters for a stimulation program 160 currently operating in the IPG 10, as identified by user selection 66, the stimulation program 160 in the fingerprint database 200 will be automatically updated with the new parameters so that these new parameters will be applied when the IPG 10 senses the fingerprint 150 associated with that stimulation program 160. This allows the IPG 10 to learn new parameters for the stimulation programs based on patient feedback.

0037Once the fingerprint database 200 has been populated or updated in this manner, the impedance algorithm 120 need merely take fingerprint measurements from time to time, match them to those stored in the database 200 if possible, and choose the associated stimulation program 160 from the database 200 to provide patient therapy. In this regard, and as shown in <figref idref="f0007">Figure 8</figref>, measured fingerprints 180 can be stored in a fingerprint log 210 associated with a timestamp, and (optionally) the stimulation program 190 running at that time. Log 210 as before can be stored in any suitable memory in the IPG 10. Although only strictly necessary to the disclosed technique to store the currently-measured fingerprint 180, storing a time-stamped history of fingerprints in the log 210 will allow the impedance algorithm 120 to perhaps identify new postures currently unknown to the database 200, as explained further below. The log 210 may be limited in its capacity to store only a reasonable number of fingerprint measurements 180, for example, those occurring over the course of a day.

0038At a given time (e.g., t3), the impedance algorithm 120 determines the currently-measured fingerprint 180 in the log 210 (e.g., FPc). If the impedance algorithm 120 can match this measured fingerprint 180 to a fingerprint 150 stored in the database 200 (e.g., FPc), the impedance algorithm 120 will cause stimulation program SPc associated with FPc in the database 200 to be applied to the patient. If the impedance algorithm 120 cannot match the measured fingerprint 180 to a fingerprint 150 in the database 200, the impedance algorithm 120 may take no action with regard to modifying the therapy currently being applied to the patient. Or, the unrecognized measured fingerprint 180, once learned on the fly, can be stored in the database 200 and associated with an appropriate stimulation program, as described in the next paragraph. One skilled in the art will understand that matching the measured fingerprints 180 in log 210 with the stored fingerprints 150 in database 200 may require a comparison that is acceptable within some margin of error, as it cannot be expected that any given measured fingerprint 180 will exactly match one stored in the database 200. For example, it may be sufficient that all values (R1, R2, C) in the two fingerprints 180 and 150 match within 20%, or that 85% of them match within 10%, or such other statistical metrics that may be suitable based upon routine experimentation and empirical data.

0039Other postures in the fingerprint database 200, such as "posture 1," may presently be unknown, but may be determined based on new impedance fingerprints that the IPG 100 learns on the fly. The fingerprint log 210 previously discussed is useful in this regard. The impedance algorithm 120 can from time to time check the fingerprint measurements 180 in the log 210 to see if a particular fingerprint is occurring with relevant frequency. For example, suppose a particular patient likes to sleep on his right side-a posture that was not addressed during the fitting procedure. The impedance algorithm 120 may notice a measured fingerprint 180 (e.g., FPg) associated with this posture in log 210 that occurs frequently, but is unknown to the impedance algorithm 120 because it doesn't match with any of the fingerprints 150 stored in the database 200. When assessing measured fingerprints 180 to determine the frequency of their occurrence, it cannot be assumed that any two (or more) measured fingerprints 180 in log 210 will match exactly, and again the impedance algorithm 120 may need to employ statistical metrics to make this determination.

0040When a predictable but unknown fingerprint (e.g., FPg) is present in log 210, the impedance algorithm 120 may determine a new posture, and can add this new fingerprint (perhaps as averaged where it variously occurs in log 180) to the database 200. If this measured fingerprint 180 is associated with stimulation program(s) 190 in the log 210, which stimulation programs may have resulted from patient changes, some indication of the associated stimulation program(s) 190 can be stored in the database 200 as well.

0041For example, if the stimulation programs associated with FPg are not uniform in the log 210 (e.g., SPw-SPz), the impedance algorithm 120 can average them in some manner to determine and store a singular stimulation program (SPg) with FPg in the database 200. Or, the impedance algorithm 120 may simply store the last stimulation program in the log 210 associated with FPg in the database 200 (e.g., SPz, which may simply default to the preceding stimulation program SPd). If review of the log 210 results in no important information relevant to stimulation associated with the new fingerprint-for example, if SPw-z do not indicate some change by the patient beyond the last stimulation program that was applied when the new fingerprint was encountered-then SPg can simply be associated with default or currently-pending stimulation parameters in the database 200. Once entered in the database 200, the stimulation program SPg associated with new fingerprint FPg can be updated based on the patient's modification of the stimulation parameters while in such posture, i.e., while FPg is currently being applied by the IPG 10, as explained earlier.

0042Thereafter, once new fingerprint FPg is recognized and learned by the impedance algorithm 120 in the log 210 and stored in database 200, the IPG 10 can automatically switch therapy to its associated program (SPg) in the database 200 when this now pre-determined fingerprint is encountered, even though the IPG 10 has no a priori understanding of the patient's posture based upon fitting. If textual descriptions 170 of posture are provided in the database 200, the IPG 10 may assign a generic textual description to such new and unidentified posture (i.e., "posture 1").

0043At some point in future, the IPG 10 can attempt to communicate with the external controller 50 to inform the patient that a new posture has been identified, and allow the patient to name this new posture. This is shown in <figref idref="f0008">Figure 9B</figref>. As shown, the display 64 of the external controller 50 identifies that the current posture of the patient is generically described as "posture 1," and allows the patient to rename the description of this posture to something meaningful to the patient per user selection 67, for example "right side." If this textual description is updated by the patient, the external controller 50 would telemeter this new textual description 170 to database 200 to overwrite the "posture 1" placeholder provisionally assigned in the database 200 for the relevant stored fingerprint 150 (FPg). If the patient does not choose to update the textual description per selection 67, there is no consequence to patient therapy: the impedance algorithm 120 can still change the stimulation program based on its detection of fingerprint FPg, including any patient modifications captured within stimulation program 160 with which it is associated.

0044The above example by which an unknown fingerprint FPg is determined and ultimately associated with a stimulation program SPg in the fingerprint database 200 illustrates that the fitting procedure described earlier is not strictly necessary-that is, it is not necessary to pre-populate the database 200 with fingerprint taken at known postures and associated stimulation programs taken at known patient postures. Just as sleeping on the patient's right side was initially unknown to the IPG 10, all of the postures represented in <figref idref="f0007">Figure 8</figref> may be unknown but eventually discovered. For example, the impedance algorithm may notice that a fingerprint associated with a standing patient occurs frequently (FPa), and over time-and based on patient adjustment of the stimulation parameters while in that posture-can eventually learn and store the stimulation program (SPa) to associate with that fingerprint in the database 200. Thus, the impedance algorithm 120 can entirely self-populate the fingerprint database 200 thus allowing the IPG 10 to learn which stimulation programs best suit the patient for a given measured impedance fingerprint.

0045Even if physiological changes (scarring) eventually cause changes to the learned fingerprints, such changes can be tracked and updated automatically. For example, an initial fingerprint FPa associated with a standing position may over time change, in which case the impedance algorithm 120 will recognize the change, and may eventually determine a new fingerprint (e.g., FPh) for this position, which is stored in the fingerprint database 200 and associated with its stimulation program (e.g., SPh). The earlier fingerprint FPa for this position may thus over time become moot, in which case it will never match the currently-measured fingerprints, and hence its associated stimulation program SPa will never be chosen by the impedance algorithm 120 for patient therapy. In this regard, the impedance algorithm 120 can monitor how often fingerprints in the database 200 are matched with those currently being measured, and if particular fingerprints have not been matched for some time, the impedance algorithm may eventually delete such now-moot fingerprints and their associated stimulation programs from the database 200.

0046The disclosed technique is further beneficial because its reliance on electrical measurements means that therapy can be adjusted as a result of any factor affecting electrode impedance, whether based upon the patient's posture or not. In this regard, "posture" should be understood as including anything that can affect the positioning or impedance of electrodes in the patient, including static postures, active postures (such as walking or running), gravity, accelerative forces, etc.

0047The tissue impedance measurements 130 (<figref idref="f0005">Fig. 6</figref>) upon which the disclosed technique are based are preferably taken during the provision of a stimulation program to the patient, and <figref idref="f0009">Figures 10A and 10B</figref> shows various ways in which this can occur. Shown are the current pulses being applied to a particular electrode, which comprises an active electrode in the stimulation program currently operating. One skilled will realize that some other electrode will act as the return path for these current pulses, but this is not shown for simplicity. Also, more than two electrodes may be active to provide a more complex therapy to the patient, but again this is not shown.

0048The therapeutic stimulation pulses in this example are bi-phasic, meaning that they have portions of opposite polarities. This is common in the neurostimulation art to assist in charge recovery from the tissue. To further assist in such recovery, the pulses are followed by a passive recovery period. Biphasic pulses and recovery periods are explained in further detail in <patcit id="pcit0009" dnum="US61654603" dnum-type="L"><text>U.S Provisional Patent Applicant Serial No. 61/654,603</text></patcit>, which with the reader is assumed familiar.

0049The recovery period is followed by a quiet period where no electrode is providing therapy to the patient. These quiet periods are logical times to take the impedance measurements 130 described earlier, which can occur in different ways. Whether impedance measurements can be taken during the quiet periods without interrupting or delaying the pulses depends on the duration of such quiet periods, which in turn depends on the frequency, f_pulse, of the pulses, and the duration of the pulses and the recovery periods. Whether impedance measurements can be taken during the quiet periods without interrupting or delaying the pulses also depends on the frequency f at which the impedance measurement is taken. Because a given impedance measurement may requiring sampling the resulting voltage V acorns the tissue for some significant portion of its cycle (e.g., 180 degree as noted earlier), the measurement can be taken without interruption if this 180-degree timeframe can fit within the duration of the quiet period-i.e., if 1/(2f) ≤ t_quiet-a condition that is more easily met when the frequency of the impedance measurement is higher. If this condition is met, an impedance measurement 130x for at least one electrode combination 140 can be taken during each quiet period, as shown in <figref idref="f0009">Figure 10A</figref>.

0050In the example discussed earlier in an IPG 10 with eight electrodes, 45 impedance measurements 130 would need to be taken to comprise a full fingerprint 150 for each of the fifteen electrode combinations 140 (eight from each electrode to the case, and seven between nearest neighbors) taken at three frequencies. Such measurements will generally not be difficult to take in a reasonable amount of time for updating the fingerprints 180 in the log 210, which need occur only every minute or longer. For example, for pulses occurring at f_pulse = 200 Hz, the 45 impedance measurements can be taken in less than half a second. Of course, greater number of electrodes, electrode combinations 140, or measurement frequencies would increase this time. Although the fingerprints 180 are measured and stored in the log 210 periodically, such measurements need not occur at consistent intervals of time.

0051If it is not possible to fit a given impedance measurement into a given quiet period-for example, when measuring impedance at lower frequencies-then some of the pulses may require delay or deletion, as shown in <figref idref="f0009">Figure 10B</figref>. As shown, the quiet period has been delayed to accommodate the time needed to take an impedance measurement, which depending on the measurement frequency and other timing consideration of the pulse train, may require simply delaying the issuance of a next pulse slightly, or which may require deleting one or more pulses in the train. Because delaying or deleting pulses impacts patient therapy, impedance measurements may only be taken once during a measurement period 250, which should be long enough to allow many uninterrupted therapy pulses 260 to occur at their specified frequency, f_pulse. For typical timings, it is not expected that the delay periods in <figref idref="f0009">Figure 10B</figref> would be significant enough that the patient would notice any change in therapy. Moreover, for typical timings the measurement period 250 will typically be short enough to make the required numbers of measurements (45) in a reasonable time to update the fingerprint 180 in the log 210 (again, at least every minute or so).

0052Various changes can be made to the particular implementation details describe herein. For example, a constant AC voltage source could be used in lieu of constant AC current source 105, in which case the resulting current through the tissue would be monitored to determine impedance.

0053While it was assumed that multiple measurement signals at different frequencies be applied between any two electrodes to determine the component values in the tissue model, this is not strictly necessary. For example, a single measurement signal I(t) = |I₁| * e^j2πf1t + |I₂| * e^j2πf2t + |I₃| * e^j2πf3t could be used that comprises three frequency components, f1, f2, and f3. This would allow the impedance algorithm to determine the complex impedance between any electrode combination, and thus the values of the components in the tissue model between them, using a single measurement signal. In this regard, it is not strictly necessary that the measurement signal be comprised of only the frequencies necessary to determine the complex impedance: any periodic signal from a Fourier transform standpoint providing a suitable number of frequencies will suffice, even if such measurement signal contains other significant frequencies, which may be filtered out if they are unnecessary. Provision of a suitable singular measurement signal comprising the requisite number of frequencies would require a more sophisticated current source 105, but such is within the skill of one in the art.

0054While, while the disclosed technique is particularly applicable to IPGs used in SCS applications, the technique is not so limited, and can be used with any multi-electrode implantable neurostimulation used in other portions of a patient's body for any purpose. In one additional example, the disclosed technique can be used with well-known Deep Brain Stimulators (DBS) as well.

0055Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the scope of the present invention as defined by the claims.