Rechargeable spinal cord stimulator system

• US 6,516,227 B1
• Filed: 07/26/2000
• Issued: 02/04/2003
• Est. Priority Date: 07/27/1999
• Status: Expired due to Term

• Current Assignee: Boston Scientific Neuromodulation Corporation (Boston Scientific Corporation)
• Original Assignee: Advanced Bionics Corporation (Sonova Holding AG)
• Inventors: Chen, Joey, Meadows, Paul, Peterson, David Karl-Lee, Mann, Carla M.
• Primary Examiner(s): LAYNO, CARL HERNANDZ
• Application Number:
  US09/626,010
• Time in Patent Office: 923 Days
• Field of Search: 607/43, 607/46, 607/5, 607/56, 607/117
• US Class Current: 607/46
• CPC Class Codes: A61N 1/0553 Paddle shaped electrodes, e.g. for laminotomy
• CPC Class Codes: A61N 1/36071 Pain
• CPC Class Codes: A61N 1/36185 Selection of the electrode configuration
• CPC Class Codes: A61N 1/37241 providing test stimulations
• CPC Class Codes: A61N 1/37247 User interfaces, e.g. input or presentation means
• CPC Class Codes: A61N 1/37252 Details of algorithms or data aspects of communication system, e.g. handshaking, transmitting specific data or segmenting data
• CPC Class Codes: A61N 1/3787 from an external energy source
• CPC Class Codes: H01H 85/201 for connecting a fuse in a lead and adapted to be supported by the lead alone
• CPC Class Codes: H01H 85/32 Indicating lamp structurally associated with the protective device

Abstract

A spinal cord stimulation (SCS) system provides multiple stimulation channels, each capable of producing up to 10 mA of current into a 1 KΩ load. The SCS system further includes a replenishable power source, e.g., a rechargeable battery, that requires only an occasional recharge, and offers a life of at least 10 years at typical settings. Each of the multiple stimulus channels of the system may be combined with other channels to deliver more than 10 mA of current. Additionally, the SCS system has the capability to stimulate simultaneously on all available channels. Each channel has at least two outputs (one positive and one negative) that can be mapped via a low impedance switching matrix to any electrode contact or the system case, thereby allowing a clinician to provide unique electrical stimulation fields for each current channel. Moreover, this feature, combined with multi-contact electrodes arranged in two or three dimensional arrays, allows “virtual electrodes” to be realized. When the system'"'"'s replenishable power source is fully charged, the user may operate the SCS system independent of external controllers or power sources. The replenishable power source may be replenished using non-invasive means. The SCS system monitors the state of charge of the internal power source and controls the charging process by monitoring the amount of energy used by the SCS system, and hence the state of charge of the power source. A suitable bidirectional telemetry link allows the SCS system to inform the patient or clinician regarding the status of the system, including the state of charge, and makes requests to initiate an external charge process. Processing circuitry within the implanted portions of the system automatically controls the applied stimulation pulses as a new burst of pulses begins in order to gradually ramp up the amplitude of the pulses to a desired level. Other processing circuitry allows electrode impedance measurements to be regularly made.

Claims

1. A spinal cord stimulation (SCS) system comprising implantable components, external components, and surgical components;

wherein the implantable components comprise a multichannel implantable pulse generator (IPG) having a rechargeable power source and an electrode array detachably connected to the IPG, the electrode array having a multiplicity of electrodes (En) thereon; and

wherein the surgical components comprise tools that assist a surgeon in positioning the IPG and electrode array; and

wherein the external components comprise a handheld programmer that may be selectively placed in telecommunicative contact with the IPG, a clinician programmer that is selectively coupled with the handheld programmer (HHP), and a portable charger that may be inductively coupled with the IPG in order to recharge the IPG power source, wherein the implantable components further comprise a lead extension that connects the electrode array to the IPG; and

wherein the surgical components further comprise an insertion needle and tunneling tools to aid in implanting the electrode array and lead extension.

2. A spinal cord stimulation (SCS) system comprising implantable components, external components, and surgical components;

wherein the implantable components comprise a multichannel implantable pulse generator (IPG) having a rechargeable power source and an electrode array detachably connected to the IPG, the electrode array having a multiplicity of electrodes (En) thereon; and

wherein the surgical components comprise tools that assist a surgeon in positioning the IPG and electrode array; and

wherein the external components comprise a handheld programmer that may be selectively placed in telecommunicative contact with the IPG, a clinician programmer that is selectively coupled with the handheld programmer (HHP), and a portable charger that may be inductively coupled with the IPG in order to recharge the IPG power source, wherein the external components further comprise;

a percutaneous extension for temporarily making an electrical connection with the implantable electrode array when first implanted,

an external trial stimulator electrically connected to the percutaneous extension, and

means for coupling the clinician programmer with the external trial stimulator.

3. A spinal cord stimulation (SCS) system comprising implantable components, external components, and surgical components;

wherein the implantable components comprise a multichannel implantable pulse generator (IPG) having a rechargeable power source and an electrode array detachably connected to the IPG, the electrode array having a multiplicity of electrodes (En) thereon; and

wherein the surgical components comprise tools that assist a surgeon in positioning the IPG and electrode array; and

wherein the external components comprise a handheld programmer that may be selectively placed in telecommunicative contact with the IPG, a clinician programmer that is selectively coupled with the handheld programmer (HHP), and a portable charger that may be inductively coupled with the IPG in order to recharge the IPG power source, wherein the IPG comprises;

an hermetically sealed case wherein the rechargeable power source and electronic circuitry are housed;

a processor IC, including memory circuits;

a digital IC coupled to the processor IC;

an analog IC controlled by the digital IC, the analog IC having a multiplicity of output current DACs, each output current DAC being connected through a coupling capacitor and header connector to a respective electrode on the electrode array, each output current DAC including circuitry that generates an output stimulus current having a selected amplitude and polarity;

an RF telemetry circuit within the sealed case that receives externally-generated programming signals that define current stimulation pulse parameters;

a rechargeable battery that provides operating power for the electronic circuitry housed within the hermetically sealed case;

a secondary coil;

a rectifier circuit; and

battery charger and protection circuitry that receives externally generated energy through the secondary coil and rectifier circuit, and uses the externally generated energy to charge the rechargeable battery.

4. The SCS system of claim 3 wherein the portable charger comprises:

a rechargeable battery;

a recharging base station that recharges the rechargeable battery from energy obtained from line ac power;

a primary coil;

a power amplifier for applying ac power derived from the rechargeable battery to the primary coil;

a back telemetry receiver for monitoring the magnitude of the ac power at the primary coil as applied by the power amplifier, thereby monitoring reflected impedance associated with energy magnetically coupled through the primary coil; and

an alarm generator that generates an audible alarm signal in response to changes sensed in the reflected impedance monitored by the back telemetry receiver.

5. The SCS system of claim 4 wherein the back telemetry receiver comprises:

alignment detection circuitry that detects when the primary coil is properly aligned with the secondary coil included within the IPG for maximum power transfer; and

charge complete detection circuitry that detects when the battery within the IPG is fully charged.

6. The SCS system of claim 5 wherein the alignment detection circuitry causes the alarm generator to broadcast a first audible tone when the primary coil is misaligned with the secondary coil, whereby the first audible tone stops being broadcast when the primary coil is properly aligned with the secondary coil.

7. The SCS system of claim 5 wherein the battery charger and protection circuitry within the IPG comprises:

monitoring circuitry that monitors the voltage of the battery and the charging current flowing to the battery; and

wherein the monitoring circuitry generates a flag signal when the battery voltage and battery charging current reach prescribed levels, which prescribed levels indicate the battery is fully charged; and

wherein the rectifier circuit is switchable between a full-wave rectifier circuit and a half-wave rectifier circuit, and

wherein the rectifier circuit is switched to operate as a full-wave rectifier circuit during charging of the battery, and wherein the flag signal causes the rectifier circuit to switch to a half-wave rectifier circuit when the battery is fully charged, whereby modulation of the rectifier circuit between a full-wave rectifier circuit and a half-wave rectifier circuit is used to indicate whether the battery is fully charged; and

wherein the charge complete detection circuitry within the external charger detects the switching of the rectifier circuit from a full-wave rectifier circuit to a half-wave rectifier circuit by the change in reflected impedance sensed at the primary coil.

8. The SCS system of claim 3 wherein the processing circuitry included within the IPG further comprises means for controlling the output current DACs so that the stimulation pulse magnitude is ramped up at the beginning of a stimulation burst.

9. The SCS system of claim 8 wherein the means for controlling the output current DACs includes means for increasing the current pulse amplitude while maintaining the pulse width at a constant value.

10. The SCS system of claim 3 wherein the processing circuitry included within the IPG further comprises means for controlling the output current DACs so that the stimulation pulse magnitude is ramped down at the ending of a stimulation burst.

11. The SCS system of claim 10 wherein the means for controlling the output current DACs includes means for decreasing the current pulse amplitude while maintaining the pulse width at a constant value.

12. The SCS system of claim 3 wherein the analog IC further includes a measurement circuit that measures the voltage at prescribed conditions on the circuit side of the coupling capacitor associated with any of the multiplicity of electrodes.

13. The SCS system of claim 12 wherein the processor IC further includes an analog-to-digital conversion circuit that converts the voltage measured by the measurement circuit to a digital value, which digital value is thereafter available to compute electrode impedance.

14. The SCS system of claim 12 wherein the analog IC further includes a sample and hold circuit for sampling and holding the voltage appearing across a selected pair of electrodes while a specified current pulse having a known current amplitude is applied thereto, and further wherein the IPG processing circuitry includes means for computing the impedance of the selected pair of electrodes based on the sampled voltage and known current amplitude.

15. The SCS system of claim 14 wherein the sample and hold circuit includes means for sampling the voltage across the selected pair of electrodes at a time that is approximately in the middle of the current pulse width applied to the selected pair of output nodes.

16. A spinal cord stimulation (SCS) system comprising implantable components, external components, and surgical components;

wherein the implantable components comprise a multichannel implantable pulse generator (IPG) having a rechargeable power source and an electrode array detachably connected to the IPG, the electrode array having a multiplicity of electrodes (En) thereon; and

wherein the surgical components comprise tools that assist a surgeon in positioning the IPG and electrode array; and

wherein the external components comprise a handheld programmer that may be selectively placed in telecommunicative contact with the IPG, a clinician programmer that is selectively coupled with the handheld programmer (HHP), and a portable charger that may be inductively coupled with the IPG in order to recharge the IPG power source, wherein the power source included within the IPG comprises a lithium-ion battery having at least a 720 mWHr capacity, said battery exhibiting a life of 500 cycles over 10 years with no more than 80% loss in capacity.

17. A spinal cord stimulation (SCS) system comprising:

implantable components;

external components; and

wherein the implantable components include a multichannel implantable pulse generator (IPG) having a replenishable power source and an electrode array having a plurality of electrodes, wherein the plurality of electrodes are detachably connected to the IPG; and

wherein the external components include a handheld programmer (HHP) that may be selectively placed in telecommunicative contact with the IPG, a clinician programmer that may be selectively placed in telecommunicative contact with the HHP, and a portable charger than may be inductively coupled with the IPG in order to replenish the IPG power source, and wherein the implantable components further include a lead extension for electrically connecting the electrode array to the IPG.

18. A spinal cord stimulation (SCS) system comprising:

implantable components;

external components; and

wherein the implantable components include a multichannel implantable pulse generator (IPG) having a replenishable power source and an electrode array having a plurality of electrodes, wherein the plurality of electrodes are detachably connected to the IPG; and

wherein the external components include a handheld programmer (HHP) that may be selectively placed in telecommunicative contact with the IPG, a clinician programmer that may be selectively placed in telecommunicative contact with the HHP, and a portable charger than may be inductively coupled with the IPG in order to replenish the IPG power source, and wherein the external components further include a percutaneous extension for temporarily making an electrical connection with the implantable electrode array when first implanted, an external trial stimulator electrically connected to the percutaneous extension, and means for coupling the clinician programmer with the trial stimulator.

19. A spinal cord stimulation (SCS) system comprising:

implantable components;

external components; and

wherein the implantable components include a multichannel implantable pulse generator (IPG) having a replenishable power source and an electrode array having a plurality of electrodes, wherein the plurality of electrodes are detachably connected to the IPG; and

wherein the external components include a handheld programmer (HHP) that may be selectively placed in telecommunicative contact with the IPG, a clinician programmer that may be selectively placed in telecommunicative contact with the HHP, and a portable charger than may be inductively coupled with the IPG in order to replenish the IPG power source, and wherein the multichannel IPG comprises;

an hermetically sealed case wherein the replenishable power source is housed;

a multiplicity of current sources within the sealed case;

a low impedance switching matrix within the sealed case having a multiplicity of input ports selectively connected to a multiplicity of output nodes;

a connector that provides detachable electrical connection with each of the multiplicity of output nodes of the switching matrix through respective feedthrough connectors, wherein the electrode array may connect with the multiplicity of output nodes of the switching matrix through the connector;

telemetry means within the sealed case for receiving externally-generated programming signals that define current stimulation pulse parameters; and

processing circuitry within the sealed case coupled to the current sources and switching matrix and responsive to stimulation pulse parameters that generates control signals that define current stimulation pulses.

20. The SCS system of claim 19 wherein the multiplicity of current sources are grouped in pairs, each pair including a positive current source and a negative current source connected to a common current source node, wherein each positive current source is capable of sourcing a current having a specified amplitude through the common current source node and low impedance switching matrix to an output (electrode) node, and wherein each negative current source is capable of sinking a current having a specified amplitude from the output (electrode) node through the low impedance switching matrix and the common current source node;

whereby different combinations of the multiplicity of current sources may be activated at different times to source and sink current through selected output (electrode) nodes in order to create stimulation pulses at the selected output (electrode) nodes having specified stimulation parameters.

21. The SCS system of claim 20 wherein the multiplicity of current sources comprises at least eight current sources.

22. The SCS system of claim 20 wherein the IPG processing circuitry comprises control logic, timer logic, a microcontroller circuit, and memory circuitry coupled to the microcontroller, and wherein the control logic, timer logic and microcontroller are responsive to programming signals stored in the memory circuitry so as to generate stimulation current pulses having a specified amplitude, width and repetition rate.

23. The SCS system of claim 22 wherein the IPG processing circuitry further includes means for ramping up the stimulation pulse magnitude at the beginning of a stimulation burst and ramping down the stimulation pulse magnitude at the ending of a stimulation burst.

24. A spinal cord stimulation (SCS) system comprising:

implantable components;

external components; and

wherein the implantable components include a multichannel implantable pulse generator (IPG) having a replenishable power source and an electrode array having a plurality of electrodes, wherein the plurality of electrodes are detachably connected to the IPG; and

wherein the external components include a handheld programmer (HHP) that may be selectively placed in telecommunicative contact with the IPG, a clinician programmer that may be selectively placed in telecommunicative contact with the HHP, and a portable charger than may be inductively coupled with the IPG in order to replenish the IPG power source, and wherein the multichannel IPG comprises;

an hermetically sealed case wherein the replenishable power source and electronic circuitry are housed;

a processor IC, including memory circuits;

a digital IC coupled to the processor IC;

an analog IC controlled by the digital IC, the analog IC having a multiplicity of independent bi-directional output current DACs, each output current DAC being connected through a coupling capacitor and header connector to a respective electrode on the electrode array, each output current DAC including circuitry that generates an output stimulus current having a selected amplitude and polarity;

an RF telemetry circuit within the sealed case that receives externally-generated programming signals that define current stimulation pulse parameters;

a secondary coil;

a rectifier circuit; and

power source replenisher and protection circuitry that receives externally generated energy through the secondary coil and rectifier circuit, and uses the externally generated energy to replenish the replenishable power source.

25. The SCS system of claim 24 wherein the portable charger comprises:

a rechargeable battery;

a recharging base station that recharges the rechargeable battery from energy obtained from line ac power;

a primary coil;

a power amplifier for applying ac power derived from the rechargeable battery to the primary coil;

a back telemetry receiver for monitoring the magnitude of the ac power at the primary coil as applied by the power amplifier, thereby monitoring reflected impedance associated with energy coupled through the primary coil; and

an alarm generator that generates an audible alarm signal in response to changes sensed in the reflected impedance monitored by the back telemetry receiver.

26. The SCS system of claim 25 wherein the back telemetry receiver comprises:

alignment detection circuitry that detects when the primary coil is properly aligned with the secondary coil included within the IPG for maximum power transfer; and

replenish complete detection circuitry that detects when the power source within the IPG is fully replenished.

27. The SCS system of claim 26 wherein the alignment detection circuitry causes the alarm generator to broadcast a first audible tone when the primary coil is misaligned with the secondary coil, whereby the first audible tone stops being broadcast when the primary coil is properly aligned with the secondary coil.

28. The SCS system of claim 26 wherein the power source replenisher and protection circuitry within the IPG comprises:

monitoring circuitry that monitors the voltage of the power source and the replenishing current flowing to the power source; and

wherein the monitoring circuitry generates a flag signal when the power source voltage and power source replenishing current reach prescribed levels, which prescribed levels indicate the power source is fully replenished; and

wherein the rectifier circuit is switchable between a full-wave rectifier circuit and a half-wave rectifier circuit, and

wherein the rectifier circuit is switched to operate as a full-wave rectifier circuit during replenishing of the power source, and wherein the flag signal causes the rectifier circuit to switch to a half-wave rectifier circuit when the power source is fully replenished, whereby modulation of the rectifier circuit between a full-wave rectifier circuit and a half-wave rectifier circuit is used to indicate whether the power source is fully replenished; and

wherein the replenish complete detection circuitry within the external charger detects the switching of the rectifier circuit from a full-wave rectifier circuit to a half-wave rectifier circuit by the change in reflected impedance sensed at the primary coil.

29. The SCS system of claim 28 wherein the power source included within the IPG comprises a lithium-ion battery having at least a 720 mWHr capacity.

30. An implantable pulse generator (IPG) system for use with a spinal cord stimulation system, the IPG system comprising an implantable pulse generator and an external portable charger, wherein the IPG comprises:

an hermetically sealed case;

electronically circuitry including memory circuits, housed within said hermetically sealed case, said electronic circuitry including a multiplicity of independent bi-directional output current sources, each output current source being connected to an electrode node;

a multiplicity of coupling capacitors, each coupling capacitor being connected to a respective one of said electrode nodes;

a header connecter attached to said sealed case, the header connecter having a multiplicity of feedthrough pins that pass therethrough, wherein each of said multiplicity of coupling capacitors is connected on the sealed side of said case to one of said feedthrough pins;

an electrode array having a multiplicity of electrodes thereon external to said sealed case, wherein each electrode of the multiplicity of electrodes is detachably electrically connected to one of said feedthrough pins on a nonsealed side of said sealed case;

wherein each output current source generates an output stimulus current having a selected amplitude and polarity that, when the output current source is enabled, is directed to the electrode connected thereto through its respective feedthrough pin and coupling capacitor;

a rechargeable battery that provides operating power for the electronic circuitry;

a secondary coil;

a rectifier circuit; and

battery charger and protection circuitry that receives externally generated energy through the secondary coil and rectifier circuit, and uses the externally generated energy to charge the rechargeable battery.

31. The IPG system of claim 30 wherein the portable charger comprises:

a second rechargeable battery;

a recharging base station that recharges the second rechargeable battery from energy obtained from line ac power;

a primary coil;

a power amplifier for applying ac power derived from the second rechargeable battery to the primary coil;

a back telemetry receiver for monitoring the magnitude of the ac power at the primary coil as applied by the power amplifier, thereby monitoring reflected impedance associated with energy magnetically coupled through the primary coil; and

an alarm generator that generates an audible alarm signal in response to changes sensed in the reflected impedance monitored by the back telemetry receiver.

32. The IPG system of claim 31 wherein the back telemetry receiver comprises:

alignment detection circuitry that detects when the primary coil is properly aligned with the secondary coil included within the IPG for maximum power transfer; and

charge complete detection circuitry that detects when the battery within the IPG is fully charged.

33. The IPG system of claim 32 wherein the alignment detection circuitry causes the alarm generator to broadcast a first audible tone when the primary coil is misaligned with the secondary coil, whereby the first audible tone stops being broadcast when the primary coil is properly aligned with the secondary coil.

34. The IPG system of claim 33 wherein the battery charger and protection circuitry within the IPG comprises:

monitoring circuitry that monitors the voltage of the rechargeable battery and the charging current flowing to the rechargeable battery; and

wherein the rectifier circuit is switchable between a full-wave rectifier circuit and a half-wave rectifier circuit, and

wherein the rectifier circuit is switched to operate as a full-wave rectifier circuit during charging of the rechargeable battery, and wherein the rectifier circuit switches to a half-wave rectifier circuit when the rechargeable battery voltage and rechargeable battery charging current reach prescribed levels, which prescribed levels indicate the rechargeable battery is fully charged, whereby modulation of the rectifier circuit between a full-wave rectifier circuit and a half-wave rectifier circuit is used to indicate whether the rechargeable battery is fully charged; and

wherein a change in reflected impedance at the primary coil indicates a switching of the rectifier circuit from a full-wave rectifier circuit to a half-wave rectifier circuit, and hence indicates whether the rechargeable battery is fully charged.

35. The IPG system of claim 30 wherein the rechargeable battery included within the IPG comprises a lithium-ion battery having at least a 720 mWHr capacity.

36. The IPG system of claim 30 wherein the electronic circuit included within the IPG further includes means for controlling the output current sources so that the stimulation pulse magnitude is ramped up at the beginning of a stimulation burst.

37. The IPG system of claim 36 wherein the means for controlling the output current sources includes means for increasing the current pulse amplitude while maintaining the pulse width at a constant value.

38. The IPG system of claim 30 wherein the electronic circuit included within the IPG further includes means for controlling the output current sources so that the stimulation pulse magnitude is ramped down at the ending of a stimulation burst.

39. The IPG system of claim 38 wherein the means for controlling the output current sources includes means for decreasing the current pulse amplitude while maintaining the pulse width at a constant value.

40. The IPG system of claim 30 wherein the electronic circuitry further includes a measurement circuit that measures the voltage at prescribed conditions on the circuit side of the coupling capacitor associated with any of the multiplicity of electrodes.

41. The IPG system of claim 40 wherein the electronic circuitry further includes an analog-to-digital conversion circuit that converts the voltage measured by the measurement circuit to a digital value, which digital value is thereafter available to compute electrode impedance.

42. The IPG system of claim 30 wherein the electronic circuitry further includes a sample and hold circuit for sampling and holding the voltage appearing across a selected pair of electrodes while a specified current pulse having a known current amplitude is applied thereto, and further wherein the electronic circuitry includes means for computing the impedance of the selected pair of electrodes based on the sampled voltage and known current amplitude.

43. The IPG system of claim 42 wherein the sample and hold circuit includes means for sampling the voltage across the selected pair of electrodes at a time that is approximately in the middle of the current pulse width applied to the selected pair of output nodes.

44. The IPG system of claim 43 wherein the specified current pulse comprises a current pulse having a pulse width of approximately 20 μ

s and a pulse amplitude of about 1 mA.

45. A spinal cord stimulation (SCS) system comprising implantable components and external components, wherein the implantable components comprise a multichannel implantable pulse generator (IPG) having a power source and an electrode array detachably connected to the IPG, the electrode array having n electrodes thereon, where n is an integer of at least eight;

and wherein the external components comprise a handheld programmer (HHP) that may be selectively placed in telecommunicative contact with the IPG in order to provide limited control of the IPG, and a clinician programmer that may be selectively coupled with the HHP in order to program operation of the IPG, and wherein the multichannel IPG includes an independent bi-directional current source for each of the n electrodes, and wherein each of the bi-directional current sources is electrically connected through a coupling capacitor to one of said n electrodes, and further wherein each of the current sources may be separately controlled to actively direct a current pulse of a selected amplitude and polarity to its respective electrode.

46. The SCS system of claim 45 wherein the n electrodes may be grouped into m channels, where m is an integer of at least two, and wherein a current pulse is generated for a first channel by actively directing a current pulse of a respective amplitude and polarity for a time period equal to a selected pulse width to a first group of anodic electrodes within the first channel, while actively directing a current pulse of a respective amplitude and opposite polarity for the selected pulse width to a second group of cathodic electrodes within the first channel, the sum of all the anodic amplitude currents in the first channel being equal to the sum of all the cathodic amplitude currents, whereby charge balance is maintained in the first channel;

and wherein a current pulse is generated for a second channel by actively directing a second current pulse of a respective amplitude and polarity for a time period equal to a selected second pulse width to a selected group of anodic electrodes within the second channel, while actively directing a second current pulse of a respective amplitude and opposite polarity for the selected second pulse width to a selected group of cathodic electrodes within the second channel, the sum of all the anodic amplitude currents in the second channel being equal to the sum of all the cathodic amplitude currents, whereby charge balance is maintained in the second channel; and

so on, for each of the m channels, whereby charge balance is maintained in each of the m channels.

47. The SCS system of claim 46 wherein the number of channels comprises four (m=4).

48. The SCS system of claim 46 wherein, for each channel, after the current pulse is actively directed to the selected group of cathodic and anodic electrodes for the selected pulse width, which selected pulse width comprises a first phase period, the selected electrodes are subjected to a second phase period during which passive discharge occurs.

49. The SCS system of claim 46 wherein, for each channel, after the current pulse is actively directed to the selected group of cathodic and anodic electrodes for the selected pulse width, which selected pulse width comprises a first phase period, the selected electrodes are subjected to an active second phase period during which a current pulse of the opposite polarity is directed to the selected group of cathodic and anodic electrodes with an amplitude and duration that maintains charge balance, thereby creating an active biphasic stimulation pulse.

50. The SCS system of claim 49 wherein the amplitude and duration of the current pulse directed to the selected group of cathodic and anodic electrodes during the second phase period is equal and opposite to that applied during the first phase period, thereby creating an active symmetrical biphasic stimulation pulse.

51. The SCS system of claim 46 further including arbitration means for selectively preventing overlap of current pulses amongst the m channels.

52. The SCS system of claim 51 wherein the arbitration means includes means for selectively defining a programmable hold-over period of time that, when enabled, begins following the first phase period, and during which the start of any pulses from any other non-overlapping channel is delayed until the end of the hold-over period.

53. The SCS system of claim 46 wherein the power source of the IPG comprises a rechargeable battery, and wherein the IPG includes means for non-invasively charging the rechargeable battery.

54. A method of charging a rechargeable battery carried within an implantable pulse generator (IPG), the IPG having a secondary coil antenna through which electromagnetic energy may be coupled to non-invasively transfer energy into the IPG;

the method comprising;

(a) generating external to the IPG an alternating electromagnetic energy field;

(b) broadcasting the electromagnetic energy field from a primary coil external to the IPG;

(c) receiving the broadcast electromagnetic energy through the secondary coil, whereby an alternating voltage is induced in the secondary coil;

(d) rectifying the induced alternating voltage to produce a dc voltage;

(e) recharging the battery with a charging current derived from the dc voltage;

(f) monitoring the charging current and a voltage across the battery as the battery is charged;

(g) changing the manner in which the rectifying is done in step (d) as soon as the battery voltage and charging current monitored in step (f) reach prescribed levels, which prescribed levels signify that the battery is fully charged;

(h) sensing the change in rectifying that occurs in step (g) from a location external to the IPG; and

(i) providing an alarm signal at a location external to the IPG upon sensing the change in rectifying in step (h).

55. The method of claim 54 wherein step (g) comprises switching from a full-wave rectifier circuit to a half-wave rectifier circuit.

56. The method of claim 55 wherein step (h) comprises monitoring reflected impedance at the primary coil, which reflected impedance changes when the half-wave rectifier circuit is switched for the full-wave rectifier circuit.

57. A method of transmitting information from an implant device to an external device, the implant device having a secondary coil for receiving electromagnetic energy from an external source, and a rectifier circuit for rectifying the received electromagnetic energy, the method comprising:

monitoring from a location within the implant device at least one parameter associated with the implant device indicative of information needed at a location external to the implant device;

modulating the rectifier circuit between a full-wave rectifier circuit and a half-wave rectifier circuit as a function of the monitored parameter; and

sensing from a location external the implant device the modulation of the rectifier circuit.

58. The method of claim 57 wherein sensing the modulation of the rectifier circuit from a location external the implant device comprises sensing changes in reflected impedance associated with electromagnetic energy being coupled into the implant device from a location external the implant device.