PATENT DOCUMENT

Abstract:
Apparatus and method for independently delivering a plurality of therapy programs in an implantable medical device. A therapy controller configures the device to generate independent pulse trains associated with a plurality of therapy programs and dynamically configures the electrodes to deliver the independent pulse trains to the patient. Once configured, the implantable medical device delivers the plurality of therapy programs to the patient wherein the therapy programs may overlap in time.

Full Description:
RELATED APPLICATIONS 
   This disclosure is related to the following co-pending applications:
         a. “Voltage/Current Regulator Improvements for an Implantable Medical Device” by inventor Goblish, et al., having U.S. patent application Ser. No. 10/133,702, and filed on Apr. 26, 2002;   b. “Detection of Possible Failure of Capacitive Elements in an Implantable Medical Device” by inventors Heathershaw, et al., having U.S. patent application Ser. No. 10/133,925, and filed on Apr. 26, 2001;   c. “Recharge Delay for an Implantable Medical Device” by inventors Goblish, et al., having U.S. patent application Ser. No. 10/133,703, and filed on Apr. 26, 2002;   d. “Wave Shaping for an Implantable Medical Device” by inventors Jensen et al., having U.S. patent application Ser. No. 10/133,573, and filed on Apr. 26, 2002;   e. “Automatic Waveform Output Adjustment for an Implantable Medical Device” by inventors Acosta et al., having U.S. patent application Ser. No. 10/133,961, and filed on Apr. 26, 2002; and   f. “Programmable Waveform Pulses For An Implantable Medical Device” by inventors Goblish et al., having U.S. patent application Ser. No. 10/133,906, and filed on Apr. 26, 2002.
 
which are not admitted as prior art with respect to the present disclosure by their mention in this section.
       

   FIELD OF THE INVENTION 
   This invention relates generally to implantable medical devices, and more particularly to techniques for providing a multiple independent stimulation channels in an implantable medical device. 
   BACKGROUND OF THE INVENTION 
   The medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions. Depending upon medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. Clinicians use medical devices alone or in combination with drug therapies and surgery to treat patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life. One type of medical device that can be used is an Implantable Neuro Stimulator (INS). 
   An INS generates an electrical stimulation signal that is used to influence the human nervous system or organs. Electrical contacts carried on the distal end of a lead are placed at the desired stimulation site such as the spine and the proximal end of the lead is connected to the INS. The INS is then surgically implanted into an individual such as into a subcutaneous pocket in the abdomen. The INS can be powered by an internal source such as a battery or by an external source such as a radio frequency transmitter. A clinician programs the INS with a therapy using a programmer. The therapy configures parameters of the stimulation signal for the specific patient&#39;s therapy. An INS can be used to treat conditions such as pain, incontinence, movement disorders such as epilepsy and Parkinson&#39;s disease, and sleep apnea. Additional therapies appear promising to treat a variety of physiological, psychological, and emotional conditions. As the number of INS therapies has expanded, greater demands have been placed on the INS. Examples of some INSs and related components are shown and described in a brochure titled Implantable Neurostimulation Systems available from Medtronic, Inc., Minneapolis, Minn. 
   The effectiveness of the therapy as provided by the INS is dependent upon its capability of adjusting the electrical characteristics of the stimulation signal. For example, stimulation waveforms can be designed for selective electrical stimulation of the nervous system. Two types of selectivity may be considered. First, fiber diameter selectivity refers to the ability to activate one group of nerve fibers having a common diameter without activating nerve fibers having different diameters. Second, spatial selectivity refers to the ability to activate nerve fibers in a localized region without activating nerve fibers in neighboring regions. 
   The basic unit of therapy is a “therapy program” in which amplitude characteristics, pulse width, and electrode configuration are associated with a pulse train for treatment of a specific neurological conduction in a specific portion of the body. The pulse train may comprise a plurality of pulses (voltage or current amplitude) that are delivered essentially simultaneously to the electrode configuration. Typically, a pulse train is delivered to the patient using one or more electrode. The INS may be able to adjust the therapy program, for example, by steering the pulse train so that it affects desired portions of the neurological tissue to be affected. Alternatively, the INS may be able to adjust various parameters of the pulse train including, for example, the pulse width, frequency and pulse amplitude. 
   It is often desirable, however, for the INS to simultaneously provide multiple therapy programs to the patient. For example, it may be desirable to provide multiple therapy programs to treat the neurological condition being treated in various parts of the body. Alternatively, the patient may have more than one condition or symptom that needs to be treated. Moreover, multiple therapy programs could serve to provide sub-threshold measurements, patient notification, and measurement functions. 
   It is therefore desirable to provide an INS that is capable of delivering multiple independent therapy programs to the patient. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention discloses techniques for delivering multiple independent therapy programs to the patient by an implantable medical device system. In accordance with an embodiment of the invention, the implantable medical device has a generator for generating necessary voltage signals for the therapy programs, one or more regulators configuring pulse trains from the voltage signals associated with the therapy programs, and a switching unit for dynamically selecting and configuring the electrodes that are to deliver each therapy program to the patient. A controller is also provided that configures the generator, the regulators, and the switching unit in accordance with the therapy programs. Once configured, the implantable medical device delivers the independent pulse trains associated with the therapy programs to a patient. The therapy programs may be simultaneous or overlapping in time. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an environment of an exemplary Implantable Neuro Stimulator (INS); 
       FIG. 2  shows an INS block diagram; 
       FIG. 3  shows an INS basic operation flowchart; 
       FIG. 4  shows a telemetry module block diagram; 
       FIG. 5  shows a telemetry operation flowchart; 
       FIG. 6  shows a recharge module block diagram; 
       FIG. 7  shows a recharge module operation flowchart; 
       FIG. 8  shows a power module block diagram; 
       FIG. 9  shows power module operation flowchart; 
       FIG. 10  shows a therapy module block diagram; 
       FIG. 11  shows a therapy module operation flowchart; 
       FIG. 12  shows a therapy measurement module block diagram; 
       FIG. 13  shows a therapy measurement module operation flowchart; 
       FIG. 14  shows a stimulation engine system according to an embodiment of the present invention; 
       FIG. 15A  shows a logic flow diagram for detecting an out-of-regulator condition according to an embodiment of the present invention; 
       FIG. 15B  shows an electrical configuration corresponding to a regulator according to an embodiment of the present invention; 
       FIG. 16  shows a logic flow diagram for detecting a faulty coupling capacitor according to an embodiment of the present invention; 
       FIG. 17  shows a first configuration for a set of regulators according to an embodiment of the present invention; 
       FIG. 18  shows a second configuration for a set of regulators according to an embodiment of the present invention; 
       FIG. 19  shows a stimulation waveform according to an embodiment of the present invention; 
       FIG. 20  shows a state diagram for a finite state machine to form the stimulation waveform as shown in  FIG. 19  according to an embodiment of the present invention; 
       FIG. 21  shows wave shaping of a stimulation pulse shown in  FIG. 19  according to an embodiment of the present invention; 
       FIG. 22  shows a first apparatus that supports wave shaping as shown in  FIG. 21  according to an embodiment of the present invention; 
       FIG. 23  shows a second apparatus that supports wave shaping as shown in  FIG. 21  according to an embodiment of the present invention; 
       FIG. 24  shows a logic flow diagram representing a method for supporting wave shaping according to an embodiment of the present invention; 
       FIG. 25  shows a stimulation arrangement according to prior art; and 
       FIG. 26  shows a stimulation arrangement according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Overall Implantable Medical Device System. 
     FIG. 1  shows the general environment of an Implantable Neuro Stimulator (INS) medical device  14  in accordance with a preferred embodiment of the present invention. The neurostimulation system generally includes an INS  14 , a lead  12 , a lead extension  20 , an External Neuro Stimulator (ENS)  25 , a physician programmer  30 , and a patient programmer  35 . The INS  14  preferably is a implantable pulse generator that will be available from Medtronic, Inc. with provisions for multiple pulses occurring either simultaneously or with one pulse shifted in time with respect to the other, and having independently varying amplitudes and pulse widths. The INS  14  contains a power source and electronics to send precise, electrical pulses to the spinal cord, brain, or neural tissue to provide the desired treatment therapy. In the embodiment, INS  14  provides electrical stimulation by way of pulses although alternative embodiments may use other forms of stimulation such as continuous electrical stimulation. 
   The lead  12  is a small medical wire with special insulation. The lead  12  includes one or more insulated electrical conductors with a connector on the proximal end and electrical contacts on the distal end. Some leads are designed to be inserted into a patient percutaneously, such as the Model 3487A Pisces-Quad® lead available from Medtronic, Inc. of Minneapolis Minn., and some leads are designed to be surgically implanted, such as the Model 3998 Specify® lead also available from Medtronic. The lead  12  may also be a paddle having a plurality of electrodes including, for example, a Medtronic paddle having model number 3587A. Those skilled in the art will appreciate that any variety of leads may be used to practice the present invention. 
   The lead  12  is implanted and positioned to stimulate a specific site in the spinal cord or the brain. Alternatively, the lead  12  may be positioned along a peripheral nerve or adjacent neural tissue ganglia like the sympathetic chain or it may be positioned to stimulate muscle tissue. The lead  12  contains one or more electrodes (small electrical contacts) through which electrical stimulation is delivered from the INS  14  to the targeted neural tissue. If the spinal cord is to be stimulated, the lead  12  may have electrodes that are epidural, intrathecal or placed into the spinal cord itself. Effective spinal cord stimulation may be achieved by any of these lead placements. 
   Although the lead connector can be connected directly to the INS  14 , typically the lead connector is connected to a lead extension  20  which can be either temporary for use with an ENS  25  or permanent for use with an INS  14 . An example of the lead extension  20  is Model 7495 available from Medtronic. 
   The ENS  25  functions similarly to the INS  14  but is not designed for implantation. The ENS  25  is used to test the efficacy of stimulation therapy for the patient before the INS  14  is surgically implanted. An example of an ENS  25  is a Model 3625 Screener available from Medtronic. 
   The physician programmer  30 , also known as a console programmer, uses telemetry to communicate with the implanted INS  14 , so a physician can program and manage a patient&#39;s therapy stored in the INS  14  and troubleshoot the patient&#39;s INS system. An example of a physician programmer  30  is a Model 7432 Console Programmer available from Medtronic. The patient programmer  35  also uses telemetry to communicate with the INS  14 , so the patient can manage some aspects of her therapy as defined by the physician. An example of a patient programmer  35  is a Model 7434 Itrel® 3 EZ Patient Programmer available from Medtronic. 
   Those skilled in the art will appreciate that any number of external programmers, leads, lead extensions, and INSs may be used to practice the present invention. 
   Implantation of an Implantable Neuro Stimulator (INS) typically begins with implantation of at least one stimulation lead  12  usually while the patient is under a local anesthetic. The lead  12  can either be percutaneously or surgically implanted. Once the lead  12  has been implanted and positioned, the lead&#39;s distal end is typically anchored into position to minimize movement of the lead  12  after implantation. The lead&#39;s proximal end can be configured to connect to a lead extension  20 . If a trial screening period is desired, the temporary lead extension  20  can be connected to a percutaneous extension with a proximal end that is external to the body and configured to connect to an External Neuro Stimulator (ENS)  25 . During the screening period the ENS  25  is programmed with a therapy and the therapy is often modified to optimize the therapy for the patient. Once screening has been completed and efficacy has been established or if screening is not desired, the lead&#39;s proximal end or the lead extension proximal end is connected to the INS  14 . The INS  14  is programmed with a therapy and then implanted in the body typically in a subcutaneous pocket at a site selected after considering physician and patient preferences. The INS  14  is implanted subcutaneously in a human body and is typically implanted near the abdomen of the patient. 
   System Components and Component Operation. 
     FIG. 2  shows a block diagram of an exemplary INS  200 . INS  200  generates a programmable electrical stimulation signal. INS  200  comprises a processor  201  with an oscillator  203 , a calendar clock  205 , a memory  207 , a system reset module  209 , a telemetry module  211 , a recharge module  213 , a power source  215 , a power management module  217 , a therapy module  219 , and a therapy measurement module  221 . In non-rechargeable versions of INS  200 , recharge module  213  can be omitted. Other versions of INS  200  can include additional modules such as a diagnostics module. All components can be configured on one or more Application Specific Integrated Circuits (ASICs) except the power source. Also, all components are connected to bi-directional data bus that is non-multiplexed with separate address and data lines except oscillator  203 , calendar clock  205 , and power source  215 . Other embodiments may multiplex the address and data lines. Processor  201  is synchronous and operates on low power such as a Motorola 68HC11 synthesized core operating with a compatible instruction set. Oscillator  203  operates at a frequency compatible with processor  201 , associated components, and energy constraints such as in the range from 100 KHz to 1.0 MHz. Calendar clock  205  counts the number of seconds since a fixed date for date/time stamping of events and for therapy control such as circadian rhythm linked therapies. Memory  207  includes memory sufficient for operation of the INS such as volatile Random Access Memory (RAM) for example Static RAM, nonvolatile Read Only Memory (ROM), Electrically Eraseable Programmable Read Only Memory (EEPROM) for example Flash EEPROM, and register arrays configured on ASICs. Direct Memory Access (DMA) is available to selected modules such as telemetry module  211 , so telemetry module  211  can request control of the data bus and write data directly to memory bypassing processor  201 . System reset module  209  controls operation of ASICs and modules during power-up of INS  200 , so ASICs and modules registers can be loaded and brought on-line in a stable condition. INS  200  can be configured in a variety of versions by removing modules not necessary for the particular configuration and by adding additional components or modules. Primary cell, non-rechargeable versions of INS  200  will not include some or all of the components in the recharge module. All components of INS  200  are contained within or carried on a housing that is hermetically sealed and manufactured from a biocompatible material such as titanium. Feedthroughs provide electrical connectivity through the housing while maintaining a hermetic seal, and the feedthroughs can be filtered to reduce incoming noise from sources such as cell phones. 
     FIG. 3  illustrates an example of a basic INS operation flowchart  300 . Operation begins with when processor  201  receives data from either telemetry  301  or from an internal source  303  in INS  200 . At receiving data step  305 , received date is then stored in a memory location  307 . The data  307  is processed by processor  201  in step  309  to identify the type of data and can include further processing such as validating the integrity of the data. After data  307  is processed, a decision is made whether to take an action in step  311 . If no action is required, INS  201  stands by to receive data. If an action is required, the action will involve one or more of the following modules or components: calendar clock  205 , memory  207 , telemetry  211 , recharge  213 , power management  217 , therapy  219 , and therapy measurement  221 . An example of an action would be to modify a programmed therapy. After the action is taken, a decision is made whether to prepare the action to be communicated in step  313 , known as uplinked, to patient programmer  35  or console programmer  30  through telemetry module  211 . If the action is uplinked, the action is recorded in patient programmer  35  or console programmer  30 . If the action is not uplinked, the action is recorded internally within INS  200 . 
     FIG. 4  shows a block diagram of various components that may be found within telemetry module  211 . Telemetry module  211  provides bi-directional communications between INS  200  and the programmers. Telemetry module  211  comprises a telemetry coil  401 , a receiver  403 , a transmitter  405 , and a telemetry processor  407 . Telemetry is conduced at a frequency in the range from about 150 KHz to 200 KHz using a medical device protocol such as described in U.S. Pat. No. 5,752,977 entitled “Efficient High Data Rate Telemetry Format For Implanted Medical Device” issued on May 19, 1998 and having named inventors Grevious et al.. Telemetry coil  401  can be located inside the housing or attached to the outside of the housing, and telemetry coil  401  can also function as the recharge coil if operation of the coil is shared or multiplexed. Receiver  403  processes a digital pulse representing the Radio Frequency (RF) modulated signal, knows as a downlink, from a programmer. Transmitter  405  generates an RF modulated uplink signal from the digital signal generated by telemetry processor  407 . Telemetry processor  407  may be a state machine configured on an ASIC with the logic necessary to decode telemetry signal during reception, store data into RAM, and notify processor  201  that data was received. Telemetry processor  407  also provides the logic necessary during transmission to request processor  201  to read data from RAM, encode the data for transmission, and notify the process that the data was transmitted. Telemetry processor  407  reduces some demands on processor  201  in order to save energy and enable processor  201  to be available for other functions. 
     FIG. 5  illustrates an example of a telemetry operation flowchart  500 . To begin telemetry, either the patient or the clinician uses patient programmer  35  or console programmer  30  and places the telemetry head containing telemetry coil  401  near INS  200  or the ENS. In step  501 , the RF telemetry signal is received through telemetry coil  401  and includes a wake-up burst that signals telemetry processor  407  to prepare telemetry processor  407  to receive incoming telemetry signals. Telemetry processor  407  is configured to receive a particular telemetry protocol that includes the type of telemetry modulation and the transmission rate of the incoming telemetry signal in step  503 . Telemetry receiver  403  demodulates the time base signal into digital pulses in step  505 . Telemetry processor  407  converts the digital pulses into binary data that is stored into memory. In step  509 , processor  201  will then take whatever action is directed by the received telemetry such as adjusting the therapy. Telemetry signal transmission is initiated by processor  201  requesting telemetry processor  407  to transmit data in step  551 . Telemetry processor  407  is configured for the desired telemetry protocol that includes the type of modulation and the speed for transmission in step  553 . Telemetry processor  407  converts the binary data into a time based digital pulses in step  555 . Transmitter  405  modulates the digital signal into an RF signal that is then transmitted through telemetry coil  401  to programmer  30  or  35  in step  559 . 
     FIG. 6  shows a block diagram of various components that may be found within recharge module  213 . Recharge module  213  provides controlled power to the battery (contained in power source  215 ) for recharging the battery and provides information to INS  200  about recharging status. Recharge module  213  regulates the charging rate of power source  215  according to power source parameters and keeps the temperature rise of INS  200  within acceptable limits so that the temperature rise does not create an unsafe condition for the patient. INS  200  communicates charging status to the patient&#39;s charger ( 213 ), so the patient charges at a level that prevents INS  200  from overheating while charges power source  215  rapidly. Recharge module  213  comprises a recharge coil, an Alternating Current (AC) over-voltage protection unit  601 , an AC to DC converter  603 , a recharge regulator  605 , a recharge measurement unit  607 , and a recharge regulator control  609 . Recharge module  213  charges the battery by receiving a power transfer signal with a frequency of about 5.0 KHz to 10.0 KHz and converting the power transfer signal into a regulated DC power that is used to charge the battery. The recharge coil can be the same coil as telemetry coil  401  if shared or multiplexed or the recharge coil can be a separate coil. AC over-voltage protection unit  601  can be a Zener diode that shunts high voltage to ground. AC to DC converter  603  can be a standard rectifier circuit. 
   Recharge regulator  605  regulates the voltage received from AC to DC converter  603  to a level appropriate for charging the battery. The recharge regulator control adjusts recharge regulator  605  in response to recharge measurements and a recharge program. The recharge program can vary based upon the type of device, type of battery, and condition of the battery. The recharge measurement block  607  measures current and voltage at regulator  605 . Based upon the recharge measurement, the regulation control can increase or decrease the power reaching power source  215 . 
     FIG. 7  illustrates an example of recharge module operation flowchart corresponding to recharge module  213 . Recharging INS  200  begins in the same manner as telemetry with either the patient or the clinician using patient programmer  35  or console programmer  30  and placing the telemetry head containing the recharge coil near INS  200  or the ENS. After the recharge signal is received in step  701 , it is converted to from AC to DC in step  703 . The DC signal is regulated in step  707 . Regulator output power is measured in step  707  and then fed back in step  705  in order to assist in controlling the regulator output power to an appropriate power level. Power source  215  is charged in step  709 , and the power source charge level is measured in step  711 . The measured power source charge level also is fed back in step  705 , so regulator  605  can control the regulator output to a level that is appropriate for power source  215 . Once recharge module  213  fully charges power source  215 , recharge module  213  can be configured to function as a power source for INS  200  while power is still received. 
     FIG. 8  shows a block diagram of various components that may be found within power management module  217 , and  FIG. 9  illustrates an example of a flowchart of power management module  217 . Power management module  217  provides a stable DC power source to INS  200  with voltages sufficient to operate INS  200  such as between about 1.5 VDC and 2.0 VDC. Power management module  217  includes a first DC to DC converter  801 , a second DC to DC converter  803 , and power source measurement component  805 . One or more additional DC to DC converters can be added to the power management module to provide additional voltage values for INS  200 . First DC to DC converter  801  and second DC to DC converter  803  can be operational amplifiers configured for a gain necessary for the desired output voltage. Power source measurement component  805  measures the power source and reports this measurement to processor  201 , so processor  201  can determine information about power source  215 . If processor  201  determines that power source  215  is inadequate for normal operation, processor  201  can instruct power management module  217  to initiate a controlled shutdown of INS  200 . 
   INS power source  215  typically provides a voltage sufficient for power management module  217  to supply power to INS  200  such as above 2.0 VDC at a current in the range from about 5.0 mA to 30.0 mA for a time period adequate for the intended therapy. INS power source  215  can be a physical storage source such as a capacitor or super capacitor, or power source  215  can be a chemical storage source such as a battery. The INS battery can be a hermetically sealed rechargeable battery such as a lithium ion (Li+) battery or a non-rechargeable battery such as a lithium thionyl chloride battery. The ENS battery can be a non-hermetically sealed rechargeable battery such as nickel cadmium or a non-rechargeable battery such as an alkaline. 
     FIG. 10  shows a block diagram of various components that may be found within therapy module  219 . Therapy module  219  generates a programmable stimulation signal that is transmitted through one or more leads to electrical contacts implanted in the patient. Therapy module  219  comprises a therapy controller (waveform controller)  1001 , a generator  1003 , a regulator module  1005 , and an electrical contact switches unit  1007 . Therapy controller  1001  can be a state machine having registers and a timer. Other embodiments of the invention may utilize other types of processors such as an ASIC, a microprocessor, a gate array, and discrete circuitry. Therapy controller  1001  controls generator  1003  and regulator module  1005  to create a stimulation signal. (A waveform generator that forms the stimulation signal may comprise generator  1003  and regulator module  1005 .) Generator  1003  assembles capacitors that have been charged by power source  215  to generate a wide variety of voltages or currents. Regulator module  1005  includes current/voltage regulators that receive a therapy current or voltage from generator  1003  and shape the stimulation signal according to therapy controller  1001 . Regulator module  1005  may include any number of devices or software components (active or passive) that maintains an output within a range of predetermined parameters such as current, voltage, etc. Electrical contact switches unit comprises solid state switches with low impedance such as Field Effect Transistor (FET) switches. The electrical contacts are carried on the distal end of a lead and deliver the stimulation signal to the body through an electrode. Additional switches can be added to provide a stimulation signal to additional electrical contacts. In the embodiment, therapy module  219  can deliver individual output pulses in the range from 0.0 Volts to 15.0 Volts into a range from about 1.0 Ohm to 10.0 K Ohms impedance throughout its operating parameter range to any combination of anodes and cathodes of up to eighteen electrical contacts for any given stimulation signal. Other embodiments can support a different voltage range, a different impedance range, or a different electrode arrangement. 
     FIG. 11  illustrates and example of operation with a flowchart of therapy module  219 . The therapy begins with the therapy controller  1001  configuring the generator  1003  according to the therapy program to provide appropriate voltage to regulator module  1005  in step  1101 . Therapy controller  1001  also configures regulator module  1005  to produce the stimulation signal according to the therapy program in step  1103 . Therapy controller  1001  also configures electrical contacts unit  1007  to so the stimulation signal is delivered to the electrical contacts specified by the therapy program in step  1105 . The stimulation signal is delivered to the patient through electrodes in step  1107 . After the stimulation signal is delivered to the patient, most therapies include a time delay in step  1109  before the next stimulation signal is delivered. 
     FIG. 12  shows a block diagram of various components that may be found within therapy measurement module  221 . Therapy measurement module  221  measures one or more therapy parameters at therapy module  219  to determine whether the therapy is appropriate. Therapy measurement module  221  includes a therapy voltage measurement component  1201 , a therapy current measurement component  1203 , and a therapy output measurement component  1205 . The therapy voltage measurements and therapy current measurements are taken periodically to perform therapy calculations. The therapy output measurement is a measurement of the delivered therapy that is used for safety and other purposes. 
     FIG. 13  illustrates an example of an operation flowchart of therapy measurement module  221 . In step  1301 , the therapy measurement operation begins by processor  201  setting up parameters of the therapy measurement to be taken (e.g. the specific stimulation signal to measure) and at which electrical contacts to perform the measurement. Before a therapy measurement is taken, a threshold determination is made whether a therapy measurement is needed in step  1303 . For some therapies, a therapy measurement may not be taken. When a therapy measurement is not taken, often a patient physiological measurement will be performed and reported to processor  201  for action or storage in memory in step  1305 . When a therapy measurement is desired, the therapy is delivered in step  1307  and then the therapy measurement is performed in step  1309 . The therapy measurement is reported to processor  201  for action or storage in memory in step  1311 . Examples of some actions that might be taken when the therapy measurement is reported include an adjustment to the therapy and a diary entry in memory that can be evaluated by the clinician at a later time. 
   Those skilled in the art will appreciate that the above discussion relating to the operation and components of the INS  14  serve as an example and that other embodiments may be utilized and still be considered to be within the scope of the present invention. For example, an ENS  25  may be utilized with the present invention. 
   Stimulation Engine. 
     FIG. 14  shows a stimulation engine system  1400  according to an embodiment of the present invention. Stimulation engine  1400  comprises therapy module  219  and therapy measurement block  221 . Therapy module  219  comprises generator control module  1003 , waveform controller (therapy controller)  1001 , regulators  1401 ,  1403 ,  1405 , and  1407 , and electrode controller (electrical contact switches unit)  1007 . Regulators  1401 - 1407  receive an input voltage from a capacitor bank comprising capacitors  1451 - 1465 . In the embodiment, capacitors  1451 - 1465  are associated as capacitor pairs such as described in U.S. Pat. No. 5,948,004 entitled “Implantable Stimulation Having An Efficient Output Generator” issued on Sep. 7, 1999 having named inventors Weijand et al. Capacitors  1451 - 1465  are charged by a battery  1467  during a recharging interval (during which a capacitor arrangement forms a charge configuration). If a capacitor pair is charged across battery  1467  in parallel and subsequently discharged across a load in series, the corresponding voltage (as provided to a regulator) is double of the voltage of battery  1467 . If a capacitor pair is charged across battery  1467  in series and subsequently discharged across the load in parallel, the corresponding voltage is one half the voltage of battery  1467 . The embodiment may utilize capacitor pairs both with a parallel configuration and with a series configuration in order to obtain a desired voltage level to a regulator. Moreover, other embodiments of the invention can utilize other types of capacitor configurations (e.g. capacitor triplets to obtain one third of the battery voltage and capacitor octets to obtain one eighth of the battery voltage) in order to achieve a desired level of voltage granularity to a regulator. Thus, any fraction of the battery voltage can be obtained by a corresponding capacitor configuration 
   In the embodiment, waveform controller  1001  (as instructed by processor  201 ) configures the capacitor bank through generator control  1003  in order to provide the required voltage inputs (corresponding to  1417 - 1423 ) to regulators  1401 - 1407 , respectively (during which the capacitor arrangement forms a stack configuration). Regulators  1401 - 1407  are instructed to generate stimulation pulses (as illustrated in  FIG. 19 ) at time instances by waveform controller  1001  through control leads  1409 - 1415 , respectively. In the embodiment, a voltage drop across a regulator (e.g.  1401 - 1407 ) is determined by a digital to analog converter (DAC) that is associated with the regulator and that is controlled by waveform controller  1001 . In the embodiment, waveform controller  1001  can independently control as many as four regulators ( 1401 - 1407 ) in order to form four independent simulation channels, although other embodiments may support a different number of regulators. Each stimulation channel is coupled to electrode controller  1007  through a coupling capacitor ( 1471 - 1477 ). Each stimulation channel can be coupled to at least one of sixteen electrodes (E 0 -E 15 ). Once again, variations of the embodiment may support different numbers of electrodes. An electrode may be either an anode or a cathode. 
   Therapy measurement block  221  monitors various components of the stimulation engine system  1400  for performance and diagnostic checks. To assist with its monitoring function, therapy measurement block  221  has associated holding capacitors  1491  and  1493 . Once again, variations of the embodiment may support different number of holding capacitors. At least one of the holding capacitors may be redundant in case the first capacitor has failed. As one example, therapy measurement block  221  monitors the voltage across a regulator in order to detect whether there is sufficient “headroom” (which is the voltage difference between the regulator&#39;s voltage input and voltage output). Some factors that may alter the “headroom” include a change of the voltage of battery  1467  and changing electrical characteristics of surrounding tissues (for example, caused by a movement in the placement of a lead). If a regulator does not have sufficient headroom, the regulator may not be able to regulate a stimulation pulse that has a constant amplitude over the duration of the pulse. Rather, the amplitude of the stimulation pulse may “droop.” In the embodiment, therapy measurement block  221  monitors input  1481  and input  1485  to determine the input voltage and the output voltage of regulator  1401 . (In the embodiment, regulators  1403 ,  1405 , and  1407  can be similarly monitored.) Typically the voltage drop across regulator should be 0.3 volts or greater in order to achieve adequate regulation. For example, if therapy measurement block  221  determines that the voltage drop across regulator  1401  is less than a minimum value, then therapy measurement block  221  may notify processor  201  about regulator  1401  experiencing an out-of-regulator condition. In such a case, processor  201  may instruct generator  1003  to associate another capacitor pair to the voltage input of regulator  1401  in order to increase the input voltage. (It is assumed that redundant capacitor pairs are available.) Also, processor  201  may store the occurrence of the out-of-regulator and report the occurrence over a telemetry channel through telemetry module  211 . The clinician may wish to recharge battery  1467  in such a case. 
   If battery  1467  has been recharged after additional capacitor pairs have been configured to compensate for a previous out-of-regulator condition of regulator  1401 , the voltage drop across a regulator may be greater than what is necessary to maintain adequate regulation. In such a case, therapy measurement block  221  may remove a capacitor pair that is associated with the voltage input of regulator  1401 . 
   In another embodiment of the invention, therapy measurement block  221  monitors the voltage of battery  1467 . If the voltage of battery  1467  is below a threshold value, therapy measurement block  221  reports the low battery condition to processor  201 . Consequently, processor  201  may instruct generator  1003  to configure capacitor pairs for the active regulators (e.g. regulators  1401 ,  1403 ,  1405 , and  1407 ). (It is assumed that there are a sufficient number of capacitor pairs.). As discussed below, in yet another embodiment of the invention, therapy measurement block  221  monitors various capacitive elements of the stimulation engine system  1400  for possible failure (e.g., holding capacitors  1491  and  1493  and coupling capacitors  1471 - 1477 ). 
   Automatic Waveform Output Adjustment. 
     FIG. 15A  shows a logic flow diagram  1500  for detecting an out-of-regulator condition. In step  1501 , therapy measurement block  221  measures the voltage drop across a regulator (e.g. regulator  1401 ). In step  1503 , therapy measurement block  221  determines whether the voltage drop is less that a threshold value. If not, therapy measurement block monitors another regulator (e.g. regulator  1403 ) in step  1505 . If so, then therapy measurement block  221  informs INS processor  201  about the out-of-regulator condition in step  1507 . In step  1509 , it is determined if a capacitor pair is available so that the capacitor pair may be added to the associated capacitor configuration. If so, a capacitor pair is added and another regulator is monitored. 
   Variations of the embodiment may detect a faulty capacitor of a capacitor pair. For example, if capacitor  1451  (C 1 ) is shorted, the associated voltage across capacitor  1451  is essentially zero. Consequently, the associated input voltage to a regulator is reduced, causing the voltage drop across the regulator to be reduced. With the logic shown in  FIG. 15A , another capacitor pair is configured in order to compensate for capacitor  1451  shorting. Moreover, additional logic steps can be included to detect a faulty capacitor and removing the faulty capacitor from service. In a variation of the embodiment, a capacitor pair is removed from the capacitor arrangement and another capacitor pair is added. If the voltage drop across the regulator is consequently within limits, the capacitor pair that was removed from the configuration is assumed to have a faulty capacitor. If a spare capacitor pair is not available, processor  201  may be notified so that programmer  30  or  35  can be alerted over the telemetry channel. In another embodiment, processor  201  may instruct the INS to shutdown in order to deactivate the generation of a stimulation waveform that is not with an acceptable range. 
   The embodiment may be used to detect other failure mechanisms. For example, rather than reconfiguring the capacitor configuration, an original regulator can be replaced with a spare regulator. If a voltage drop across the spare regulator is within an acceptable range, then the original regulator is determined to be faulty. However, if it is determined that the original regulator is not faulty, the capacitor arrangement (comprising C 1451 - 1465 ) can be tested. In one embodiment, the capacitors of the capacitor arrangement can be charged to a known voltage, such as the measured battery voltage, and the voltages across the capacitors can be measured by therapy measurement block  221 . If a voltage is low across a capacitor, the capacitor may be determined to be faulty. In such a case the capacitor may be replaced with a redundant capacitor. 
     FIG. 15B  shows an electrical configuration corresponding to regulators  1401 ,  1403 ,  1405 , and  1407 . The electrical configuration comprises an amplifier  1553  in which an output  1557  feeds into a negative input and a programmed input voltage  1555  feeds into a positive input of amplifier  1553 . Thus, amplifier  1553  is configured as a voltage follower amplifier (i.e. output  1557  should approximately equal programmed input voltage  1555  if the circuitry is operating properly). Amplifier  1553  receives a power supply voltage from a capacitor arrangement  1551  through a reg top  1559  and a reg bottom  1561 . 
   The embodiment corresponding to  FIG. 15A  measures a voltage drop across a regulator (e.g.  1401 ,  1403 ,  1405 , or  1407 ). In  FIG. 15B , the voltage drop across the regulator corresponds to a voltage difference between reg top  1559  and output  1557 . Moreover, other embodiments of the invention may utilize other electrical measurements in order to determine an out-of-regulator condition. In one embodiment, if output  1557  does not approximately equal programmed input voltage  1555 , therapy measurement block  221  may determine the occurrence of an out-of-regulator condition. In another embodiment, output  1557  (as measured by therapy measurement block  221 ) is compared with an expected output voltage. In the embodiment, processor  201  is cognizant of the configuration of capacitor arrangement  1551  and the battery voltage. Processor  201  may use electrical formulae that correspond to the known configuration in order to determine the expected output voltage. A sufficiently large difference between output  1557  and the expected output voltage is indicative of an out-of-regulator condition. In another embodiment, an out-of-regulator condition is detected when the voltage difference between reg top  1559  and reg bottom  1561  (corresponding to an input signal to regulator  1401 ,  1403 ,  1405 , or  1407 ) is less than programmed input voltage  1555 . 
   Detection and Correction of Possible Failure of Coupling Capacitor. 
   In the embodiment, a coupling capacitor (e.g.  1471 ,  1473 ,  1475 , and  1477 ) is used to transfer charge to an electrode. The accumulated voltage across the coupling capacitor is a measure of the charge that is transferred to the electrode. Moreover, the value of the coupling capacitor determines the maximum charge that can be transferred to the electrode for a given stimulation voltage. However, the coupling capacitor may fail in which the coupling capacitor becomes shorted. In such a case, the coupling capacitor becomes unable to limit excess charge. In order to detect a shorted condition, therapy measurement block  221  monitors the voltage drop across the coupling capacitor (e.g. capacitor  1471  which corresponds to regulator  1401 ). Inputs  1481  and  1483  enable therapy measurement block  221  to monitor the voltage drop across coupling capacitor  1471 . Similar inputs are provided for each other coupling capacitor ( 1473 ,  1475 , and  1477 ) in circuit. A voltage drop greater than or less than a prescribed range may be indicative of a possible failure in the coupling capacitor  1471 . 
   Once the system detects a failed coupling capacitor, it may take any number of corrective actions including, but not limited to, perform a corrective recharge to compensate for the failure, replacing the failed capacitor with another capacitor, notifying the implantable medical device or the physician programmer, and/or shutting down the implantable medical device.  FIG. 16  shows a logic flow diagram  1600  of one embodiment for detecting a faulty coupling capacitor and taking corrective action. In step  1601 , therapy measurement block  221  measures the voltage across the coupling capacitor (e.g. coupling capacitor  1471 ). Although a voltage drop measurement across the coupling capacitor is made, any measurement providing charge information would suffice to determine whether the capacitor has failed including, but not limited to, energy information going in and out of the capacitive element, and current information going in and out of the capacitive element. In step  1603 , if it is determined that the voltage drop is less than a predefined threshold, it is assumed that the coupling capacitor has malfunctioned and corrective action should be taken. Otherwise, step  1605  is executed and another coupling capacitor is monitored by therapy measurement block  221 . 
   In step  1607 , corrective action is taken by removing from service the coupling capacitor (e.g. coupling capacitor  1471 ) and its associated regulator (e.g.  1401 ) and notifying the INS processor  201 . In step  1609 , logic  1600  determines if a spare capacitor/regulator pair can be configured in order to assume the functionality of the faulty capacitor. In either case, the INS processor  201  may be notified. The INS processor  201  may then notify the clinician (i.e., the physician programmer  30 ) about the condition through the telemetry channel. If a spare regulator is available, the spare regulator is configured in step  1613  to assume the functionality of the regulator that was removed. INS processor  201  is informed in step  1615 . Step  1617  is executed, and another coupling capacitor is monitored. In other embodiments, other forms of corrective action may be taken. For example, the system can provide a charge balance pulse in an amount to compensate for the capacitive element being outside the predefined threshold. The charge balance pulse can be calculated by determining charge going in and going out of the coupling capacitor. For example, if the stimulation pulse is at a constant current, the system can determine the current amount and duration. The charge balance pulse can then be in an amount that zeros out the difference in the charges going in and going out of the coupling capacitor. In another example, the system can just notify the INS processor  201  and physician programmer  30  or it can just simply shut itself down from operation. 
   Other embodiments of the invention may monitor the coupling capacitor (e.g. coupling capacitor  1471 ) in order to detect whether the coupling capacitor becomes open. In such a case, the voltage drop across the coupling capacitor may exceed a predefined threshold. In this case, even the associated regulator/capacitor pair may become ineffective in the treatment of the patient. Therapy measurement block  221  may therefore remove the regulator/capacitor pair and configure a spare regulator. 
   In yet other embodiments, therapy measurement block  221  may measure other elements other than capacitive elements including, but not limited to, holding capacitors  1491  and  1493 . In one exemplary embodiment, therapy measurement block  221  measures the voltage of the battery using one of the holding capacitors  1491  or  1493 . After a certain time period (e.g., several seconds or several minutes), therapy measurement block  221  re-measures the voltage of the battery using the same holding capacitor  1491  or  1493 . Under proper operation of the holding capacitor  1491 , the two voltage measurements should be roughly the same. If the two voltage measurements vary by more than a predetermined threshold, however, there is likely a failure in the holding capacitor. Alternatively, if the original voltage measurement of battery is outside a predefined range, it may be indicative of a failed capacitor. For example, if the original voltage measurement of battery is be less than 2V, then it is likely that the holding capacitor has failed. This is the case since if the battery voltage had reached 2V, the circuitry would have already been shut down for purposes of conserving battery resources. In another alternative, if the holding capacitor is open circuited, the therapy measurement block  221  would have been unable to take the initial battery voltage measurement. Once the system determines a possible failure of the holding capacitor, it may then take appropriate action as discussed above (e.g., replacing holding capacitor with redundant capacitor, notifying the implantable medical device or physician programmer of capacitor failure, etc.). 
   Regulator Improvements. 
     FIG. 17  shows a first configuration for a set of regulators comprising regulators  1401 ,  1403 ,  1405 , and  1407  according to an embodiment of the present invention. The configuration shown in  FIG. 17  may be used to generate a Pulse Width “A” pulse (pwa)  1923  that is shown in FIG.  19 . Other embodiments may support a different number of regulators in order to generate a different numbers of corresponding waveforms. Capacitors  1451 ,  1453 ,  1455 ,  1457 ,  1459 ,  1461 ,  1463 , and  1465  have been charged by battery  1467  so that capacitors  1459  and  1461  have a 1.5 volt potential and capacitors  1451 ,  1453 ,  1455 ,  1457 ,  1463 , and  1465  have a 3.0 volt potential. In order to provide a 3.0 volt input to regulator  1403 , a 4.5 volt input to regulator  1407 , a 7.5 volt input to regulator  1405 , and a 13.5 volt input to regulator  1401 , a voltage reference  1711  is configured with respect to BPLUS of battery  1467 . Waveform controller  1101  configures the capacitors  1451 - 1465  and the voltage reference through generator control  1003 . The output of regulator  1403  is connected to anode  1703 ; the output of regulator  1407  is connected to anode  1707 ; the output of regulator  1405  is connected to anode  1705 ; the output of regulator  1401  is connected to anode  1701 ; and voltage reference  1711  is connected to cathode  1709 . 
     FIG. 18  shows a second configuration for a set of regulators comprising regulators  1401 ,  1403 ,  1405 , and  1407  according to an embodiment of the present invention. The configuration shown in  FIG. 18  may be used to generate a pulse width “B” pulse (pwb)  1915  that is shown in FIG.  19 . Capacitors  1451 - 1465  have the same voltage potential as shown in FIG.  17 . However, waveform controller  1001  configures a voltage reference  1811  to be the negative side of capacitor  1451  so that the input voltage to each regulator ( 1407 ,  1403 ,  1405 , and  1401 ) has a negative polarity rather than a positive polarity. As in the configuration shown in  FIG. 17 , cathode  1709  is connected to the voltage reference. Consequently, the voltage outputs of regulators  1407 ,  1403 ,  1405 , and  1401  have a negative polarity. Waveform controller  1001  also configures capacitors  1451 - 1465  so that capacitors  1451  and  1453  are between voltage reference  1811  and the input of regulators  1407  and  1403 , capacitors  1451 ,  1453 ,  1455 ,  1457  are between voltage reference  1811  and the input of regulator  1405 , and capacitors  1451 ,  1453 ,  1455 ,  1457 ,  1459 ,  1461 ,  1463 , and  1465  are between voltage reference  1811  and the input of regulator  1401 . 
   Table 1 compares the voltage outputs of regulators  1401 ,  1403 ,  1405 , and  1407  in  FIGS. 17 and 18 . 
   
     
       
             
           
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Comparison of Regulator Output Voltages for pwa and pwb 
             
             
               Configurations 
             
           
        
         
             
                 
               Pulse Width A 
               Pulse Width B 
             
             
                 
               Configuration 
               Configuration 
             
             
                 
                 
             
           
        
         
             
                 
               Anode 1701 
               12 volts 
                −11 volts 
             
             
                 
               Anode 1703 
                2 volts 
                 −5 volts 
             
             
                 
               Anode 1705 
                6 volts 
                 −6 volts 
             
             
                 
               Anode 1707 
                3 volts 
               −1.5 volts 
             
             
                 
                 
             
           
        
       
     
   
   With regulators  1401 ,  1403 ,  1405 , and  1407  having a capability of generating negative voltage, the risk of a charge accumulation that may damage surrounding tissue around stimulated electrodes is reduced. The required amplitude of a stimulation pulse pwa  1923  (as shown in  FIG. 19 ) varies with the type of therapy. 
   With a therapy pulse (e.g. pwa  1923 ) that is delivered to the tissue, it may be necessary to retract an equal amount of charge from the same tissue after the therapy pulse is completed. This retraction of charge is typically done in the form of a secondary pulse, or recharge pulse, which causes an equal amount of charge to flow in the opposite direction of the original therapy pulse. If the amount of charge in the secondary pulse does not equal the amount of charge in the therapy pulse, charge will accumulate on the electrode surface, and the chemical reactions at the electrode-tissue interface will not remain balanced, which can cause tissue and electrode damage. For example, the accumulated charge may be accompanied by electrolysis, thus causing hydrogen, oxygen and hydroxyl ions to form. As a result, the pH level of the immediate layer of fluid in the proximity of the electrode may deviate from its norm. PH variations may oscillate between pH  4  and pH  10  within a few microns of the electrode. Also, charge accumulation may cause dissolution of the electrode (e.g. platinum), resulting in lead corrosion and possible damage to tissue that encounters the resulting chemical migration. Thus, the reduction of the net charge that accumulates in the region of the treatment reduces the possibility of accompanying tissue damage and electrode damage. 
   As will be discussed in the context of  FIG. 19 , pwb pulse  1925  may have a negative polarity (as supported by the regulator configuration in FIG.  18 ). The negative charge that accumulates in the surrounding tissue during pwb pulse  1925  counterpoises the positive charge that accumulates during pwa pulse  1923 . 
   If the electrical characteristics between a stimulated electrode pair can be modeled as an equivalent circuit having a capacitor, the charge accumulated during pwa interval  1909  may be counterpoised by the charge accumulated during pwb interval  1915  if the product (amplitude of pwa  1923 )*(interval of pwa  1909 ) approximately equals the product (amplitude of pwb  1925 )*(interval of pwb  1915 ) when the polarities of pwa pulse  1923  and pwb pulse  1925  are opposite of each other. 
   Other embodiments of the invention may generate positive and negative current waveforms by converting a voltage pulse to a current pulse, in which the output from the regulator is driven through a resistance in the regulator. 
   Recharge Delay and Second Pulse Generation. 
     FIG. 19  shows stimulation waveform  1901  according to an embodiment of the present invention.  FIG. 19  shows waveform  1901  spanning a rate period interval  1902 . Waveform  1901  may repeat or may change waveform characteristics (corresponding to changing a waveform parameter) during a next rate period interval. Stimulation waveform  1901  may be programmed in order to customize a therapeutical treatment to the needs of the patient. An initial delay (delay_ 1 ) interval  1905  commences with a rate trigger event. The rate trigger event occurs at the beginning of each rate period interval. During a pulse width A (pwa) setup interval  1907 , capacitors  1451 - 1465  are moved from a charge configuration to a stack configuration. A pulse width pwa interval  1909  commences upon the completion of interval  1907 . During interval  1909 , regulators  1401 - 1407  apply voltage or current outputs to a set of electrodes (e.g. anodes) while corresponding electrodes (e.g. cathodes) are connected to a stimulation voltage reference. In the embodiment, pwa interval  1909  is programmable from 0 to 655 msec with increments of 10 microseconds, in which an associated timer is a 16-bit timer. 
   A second delay (delay_ 2 ) interval  1911  may begin upon the completion of pwa interval  1909 . During interval  1911 , all electrode connections remain open. In the embodiment, second delay interval  1911  is programmable from 0 to 655 msec with increments of 10 microseconds. 
   A pwb setup interval  1913  may begin upon the completion of second delay interval  1911 . During interval  1913 , capacitors  1451 - 1465  are moved from a charge configuration to a stack configuration. A pwb interval  1915  follows interval  1913 . During pwb interval  1915 , regulators  1401 - 1407  apply voltage or current outputs to the set of electrodes (e.g. anodes) while corresponding electrodes (e.g. cathodes) are connected to a stimulation voltage reference. In the embodiment, pwb interval  1915  is programmable from 0 to 655 with increments of 10 microseconds. 
   While the embodiment configures the stimulation pulse during pwa interval  1909  with a positive polarity and the stimulation pulse during pwb interval  1915  with a negative polarity, other embodiments may reverse the polarities. Moreover, other embodiments may configure both pulses during intervals  1909  and  1915  to have the same polarity. 
   A third delay (delay_ 3 ) interval  1917  begins upon completion of pwb interval  1915 . During interval  1917 , all electrodes connections remain open. In the embodiment, the third delay interval  1917  is programmable from 0 to 655 msec with increments of 10 microseconds. 
   A passive recharge interval  1919  is triggered by the completion third delay interval  1917 . During interval  1919 , electrodes may be connected to a system ground. In the embodiment, waveform controller  1001  (through passive recharge control  1491 ) passively recharges the connected electrodes in order to provide a charge balance in tissues that are adjacent to the connected electrodes. Passive recharging during interval  1919  may function to complete the recharging process that may be associated with pwb interval  1915 . In the embodiment, passive recharge interval  1919  is programmable from 0 to 655 msec with increments of 10 microseconds. A wait interval  1921  follows interval  1919  in order to complete rate period interval  1902 . In the embodiment, the rate period interval is programmable from 0 to 655 msec. In the embodiment, if the sum of the component intervals ( 1905 ,  1907 ,  1909 ,  1911 ,  1913 ,  1915 ,  1917 ,  1919 , and  1921 ) exceed the rate period interval, the rate period interval takes precedence over all components intervals in the event of a conflict. For example, all waveform timers are reloaded and a new waveform may commence with the occurrence of rate trigger event. 
   Pulses generated during pwa pulse interval  1909  and pwb interval  1915  may be used to stimulate surrounding tissues or may be used to assist in charge balancing. The effects of charge balancing during a pulse may be combined with charge balancing during passive recharge interval  1919  in order to obtain a desired charge balancing. (Recharging may provide charge balancing with active components or with passive components or both.) 
   Other embodiments of the invention may initiate rate period interval  1902  with a different interval than delay_ 1  interval  1905 . For example, other embodiments may define the beginning of rate period interval  1902  with passive recharge interval  1919 . Moreover, with the embodiment or with other embodiments, any of the delay intervals (delay_ 1  interval  1905 , delay_ 2  interval  1911 , delay_ 3  interval  1917 , wait interval  1921 ), pulse intervals (pwa interval  1909 , pwb interval  1915 ), setup intervals (pwa setup interval  1907 , pwb setup interval  1913 ), or passive recharge interval  1919  may be effectively deleted by setting the corresponding value to approximately zero. Also, other embodiments may utilize different time increments other than 10 microseconds. 
     FIG. 19  also shows a second waveform  1903  that is formed during the formation of  1901 . (In the embodiment, regulators  1401  and  1407  may be utilized to form four waveforms.) Waveform  1903  is phased with waveform  1901  (with each waveform having the same rate period interval). A pwa pulse  1927  (that is associated with waveform  1903 ) occurs after the completion of pwa pulse  1923  (that is associated with waveform  1901 ). The clinician may stimulate a set of electrodes with waveform  1901 . The subsequent stimulation of the set of electrodes by waveform  1903  may cause the firing of the neurons that may not be possible only with waveform  1901  or  1903  alone. In the embodiment, waveforms  1901  (corresponding to regulator  1401 ) and  1903  (corresponding to regulator  1403 ) may be applied to the same electrode or to two electrodes in close proximity. In the embodiment, if regulators  1401  and  1405  are configured to the same electrode, regulators  1401  and  1405  are configured in series for voltage amplitude waveforms and in parallel for current amplitude waveforms. 
   In the embodiment, the rate period interval of waveforms  1901  and  1903  are the same. However, other embodiments of the invention may utilize different rates periods for different waveforms. 
     FIG. 20  shows a state diagram that a finite state machine  2000  utilizes to form the waveforms as shown in  FIG. 19  according to an embodiment of the present invention. A finite state machine may be associated with each waveform that is generated by INS  200 . In the embodiment, state machine  2000  is implemented with waveform controller  1001 . Waveform controller  1001 , in accordance with state machine  2000 , controls generator  1003 , regulators  1401 - 1407 , passive recharge control  1491 , and electrode control  1007  in order to generate stimulation pulses in accordance with state machine  2000 . Moreover, waveform controller  1001  may obtain waveform parameters from processor  201 . The clinician may alter a waveform parameter (e.g. pwa pulse duration  1909 ) by sending an instruction over the telemetry channel through telemetry unit  211  to processor  201  in order to modify the waveform parameter. In the discussion of  FIG. 19 , it is assumed that wave shaping (as will be discussed in the context of  FIG. 21 ) is not activated. In  FIG. 20 , a state delay_ 1   2001  corresponds to first delay interval  1905 . A transition  2051  initiates a state pwa setup  2003  upon the expiration of interval  1905 . State  2003  corresponds to pwa setup interval  1913 . If wave shaping is activated, states ws_ 1   2005 , ws_ 2   2007 , and ws_ 3   2009  may be executed. (However, discussion of states  2005 ,  2007 , and  2009  are deferred until the discussion of  FIG. 21. ) A delay_ 2  state  2013  may be accessed directly from state delay_ 1   2001  through transition  2050  if pwa pulse is not generated during pwa interval  1909 . 
   Assuming that wave shaping is not activated, a state pwa  2011  is executed upon the completion of pwa setup interval  1907  through a transition  2053 . State pwa  2011  corresponds to interval pwa  1909  during which pwa pulse  1923  is generated. Upon the completion of interval  1909 , state delay_ 2   2013  is entered through a transition  2073 . State  2013  corresponds to delay_ 2  interval  1911 . If pwb pulse is generated, a pwb setup state  2015  is entered through transition  2077  upon the completion of delay_ 2  interval  1911 . If pwb pulse  1925  is not generated, a delay_ 3  state  2019  is entered through transition  2075  upon the completion of delay_ 2  interval  1911 . State pwb setup  2015  corresponds to pwb setup interval  1913  and state delay_ 3  state  2019  corresponds to delay_ 3  interval  1917 . 
   With the completion of pwb setup interval  1913 , if pwb pulse  1925  is to be generated, a pwb state  2017  is entered through transition  2079 . The pwb state  2017  corresponds to pwb interval  1915  during which the pwb pulse  1925  is generated. Upon the completion of pwb interval  1915 , delay_ 3  state  2019  is entered through transition  2081 . Upon the completion of delay_ 3  interval  1917 , finite state machine enters a passive recharge (pr) state  2021  through transition  2085  or a wait state  2023  through transition  2083 . The pr state  2021  may be circumvented if recharging during pwb  2017  state adequately eliminates a charge accumulation that occurs during pwa state  2003 . The pr state  2001  corresponds to passive charge interval  1919 . Upon the completion of passive recharge interval  1919 , state machine  2000  enters wait state  2023 , and remains in state  2023  until the completion of the rate period interval. State machine  2000  consequently repeats the execution of states  2001 - 2023 . 
   Other embodiments of the invention may support a different number of stimulation pulses (e.g. three, four, and so forth) during rate period interval  1902 . 
   Wave Shaping. 
     FIG. 21  shows a waveform  2101  in which stimulation pulse pwa  1923  is generated by wave shaping according to an embodiment of the present invention. Waveform  2101 , as shown in  FIG. 21 , spans a rate period interval  2102 . Wave shaping of pwa  1923  corresponds to a state ws_ 1   2005 , a state ws_ 2   2007 , and a state ws_ 3   2009  (as shown in finite state machine  2000  in  FIG. 20 ) and corresponds to a ws_ 1  duration  2109 , a ws_ 2  duration  2111 , and a ws_ 3  duration  2113  in FIG.  21 . Durations  2109 ,  2111 , and  2113  correspond to phases  1 ,  2 , and  3  of pwa pulse  1923 . In the embodiment, pwa pulse  1923  is synthesized in order to adjust the therapeutical effectiveness of pwa pulse  1923 . In the embodiment, without wave shaping, pwa pulse  1923  is essentially a rectangular pulse (flat-topped) as illustrated in FIG.  19 . 
   In the embodiment, pwa interval  1909  is subdivided into three phase intervals  2109 ,  2111 , and  2113 . During phase intervals  2109 ,  2111 , or  2113 , at least one parameter is associated with the stimulation waveform. In the embodiment, a parameter may correspond to characteristics of the stimulation waveform (e.g. a desired amount of rise during the phase) or may correspond to an electrode configuration in which the stimulation waveform is applied. In the embodiment, all other time intervals remain the same and all time intervals maintain the same order of succession (e.g. pwb  1925  follows pwa  1923 ) as in the case without wave shaping. During each of the three phases (ws_ 1   2150 , ws_ 2   2160 , and ws_ 3   2170 ) of pwa pulse  1923 , the output amplitude may be rising, falling, or constant across a phase. (Other embodiments may utilize a different number of phases. Typically, with a greater number of phases, one can achieve a better approximation of a desired waveform. The desired waveform may correspond to any mathematical function, including a ramp, a sinusoidal wave, and so forth.) Each of the three phases is defined by a register containing an initial output amplitude, a register containing a final output amplitude, and a register containing a number of clock periods in which the amplitude output remains constant across an incremental step. In the embodiment, a phase duration (e.g.  2109 ,  2111 , and  2113 ) is determined by:
 
(|final amplitude count−initial amplitude|+1)*(number of clock periods per step)
 
   The output amplitude changes by one amplitude step after remaining at the previous amplitude for a clock count equal to the value of the clock periods per step as contained in a register. The output amplitude range setting in a register determines a size of an amplitude step. (In the embodiment, the step size may equal 10, 50, or 200 millivolts.) 
   An example of wave shaping illustrates the embodiment as shown in FIG.  21 . The step size is 500 millivolts for phases  2150  and  2160  and 1 volt for phase  2170 . The master waveform generator clock is 10 microseconds. Durations  2109 ,  2111 , and  2113  are each 400 microseconds. During duration  2109 , the initial amplitude register contains 70 ( 46   16 ) and the final amplitude register contains 40 ( 28   16 ). The clock periods per step is 10 or 100 microseconds (10*10 microseconds). During duration  2109 , waveform  2103  starts at 3.5 volts and descends 0.5 volts every 100 microseconds until the amplitude value is 2.0 volts. 
   During duration  2111 , the initial amplitude register contains 0 and the final amplitude register contains 70 ( 46   16 ). The clock periods per step is 10. During duration  2111 , waveform  2105  starts at 0 volts and ascends 0.5 volts every 100 microseconds until the amplitude value is 3.5 volts. During duration  2113 , the initial amplitude register contains 30 ( 1 E 16 ). The clock periods per step is 20 (corresponding to 200 microseconds). During duration  2113 , waveform  2107  starts at 1.5 volts and ascends to 2.5 volts in one step. 
   Finite state machine  2000  (as shown in  FIG. 20 ) supports wave shaping with ws_ 1  state  2005 , ws_ 2  state  2007 , and ws_ 3  state  2009 . With wave shaping enabled, state  2005 ,  2007 , or  2009  is entered from pwa setup state  2003  through transitions  2057 ,  2055 , and  2059 , respectively. The pwa state is not executed when wave shaping is enabled. In the embodiment, the synthesis associated with any phase ( 2150 ,  2160 ,  2170 ) may be circumvented. For example, ws_ 1  state  2005  may enter ws_ 2  state  2007  through transition  2061 , may enter ws_ 3  state  2009  through transition  2063 , or may enter delay_ 2  state  2013  through transition  2065 . 
   Other embodiments of the invention may support a different number of phases than is utilized in the exemplary embodiment. Also, other embodiments may utilize wave shaping for other portions of waveform  2101  (e.g. a pwb pulse  2129 ). 
     FIG. 22  shows a first apparatus that supports wave shaping as shown in  FIG. 19  according to an embodiment of the present invention. Output voltage V out    2203  corresponds to phase  2150 ,  2160 , or  2170 . A digital to analog converter (DAC)  2201  generates V out    2203  in accordance to a digital input  2209 . Input  2209  is obtained from register  2205 . Register  2205  receives a digital input  2211  from waveform controller  1001 . Input  2211  is stored in register  2205  when clocked by clk_step  2207 , which occurs at a rate of updating phases  2150 ,  2160 , or  2170  (corresponding to a “step”). Waveform controller  1001  updates digital input  2211  in order to cause V out    2203  to equal a desired value during phases  2150 ,  2160 , or  2170  in accordance with an initial output amplitude, a final output amplitude, an amplitude step size, and a step time duration parameters. 
   In a variation of the embodiment, DAC  2201  determines a voltage drop across a regulator (e.g.  1401 ,  1403 ,  1405 , or  1407 ). The value of the stimulation waveform (with a voltage amplitude) is approximately a voltage input to the regulator minus the voltage drop (as determined by DAC  2201 ). Consequently, digital input  2211  is determined by subtracting an approximate value of the stimulation waveform from the input voltage to the regulator. 
     FIG. 23  shows a second apparatus that supports wave shaping as shown in  FIG. 19  according to an embodiment of the present invention. An output V out    2301  corresponds to phases  2150 ,  2160 , and  2170  in  FIG. 21. V   out    2301  is the output of an analog adder  2303  having inputs  2305  and  2307 . Input  2305  is obtained from a gate  2309  in which a step voltage V 1    2311  is gated by a gate control  2313  in accordance with a step time duration. With apparatus  2300 ,
   V out= V out+ V in 
     FIG. 24  shows a logic flow diagram  2400  representing a method for supporting wave shaping according to an embodiment of the present invention. Step  2401  determines whether wave shaping is activated. If not, process  2400  is exited in step  2403 . In such a case, pwa stimulation pulse  1923  is generated as an essentially flat pulse over time duration  1909 . If wave shaping for an i th  phase of the pwa pulse is activated, step  2405  is executed. 
   In step  2405 , an initial output voltage V start , a final output voltage V final , a step size V i , a step duration t 1  and a phase time duration T 1  are determined. In step  2407 , V out  is equal to V start . Step  2409  determines if the step time duration t 1  has expired. If so, V out  is incremented by the step size V 1  in step  2411 . If V out  equals the final output voltage V final  in step  2413 , the output voltage V out  remains constant until the end of the phase duration T 1  in step  2415 . If V out  is not equal to the final output voltage V final  and the phase time duration T 1  has not expired (as determined in step  2417 ), step  2409  is repeated in order to update V out  for another step time duration t 1 . 
   Other embodiments of the invention may support wave shaping of a current amplitude of waveform  2101 . In such cases, a voltage amplitude may be converted into a current amplitude by driving a resistor that is associated with a regulator (e.g.  1401 ,  1403 ,  1405 , and  1407 ). 
   Simultaneous Delivery of a Plurality of Independent Therapy Programs. 
     FIG. 25  shows a stimulation arrangement that is associated with an implantable neuro stimulator according with prior art such as that disclosed in U.S. Pat. No. 5,895,416. Lead  2501  comprises a plurality of electrodes including cathode  2503 , cathode  2505 , and anode  2507 . Anode  2507  provides a common reference for either a voltage amplitude pulse or a current amplitude pulse through cathodes  2503  and  2505 . Waveforms  2511  and  2513  are applied to cathodes  2503  and  2505 , respectively. Waveform  2511  differs from waveform  2513  by amplitude scaling; however, component time durations are the same for waveform  2511  and waveform  2513 . Moreover, the waveforms serve to treat the same neurological condition in a specific portion of the body. 
     FIG. 26  shows a stimulation electrode arrangement that is associated with INS  200  according to an embodiment of the present invention. INS  200  stimulates leads  2601  and  2603 . Lead  2601  comprises electrodes  2605 - 2619 , and lead  2603  comprises electrodes  2621 - 2635 . The basic “unit” of therapy is a “therapy program” in which amplitude characteristics, pulse width, and electrode configuration are associated with a pulse train for treatment of a specific neurological conduction in a specific portion of the body. Multiple therapy programs may therefore be used to either treat distinct neurological conditions or treat the same neurological condition but in distinct areas of the body. The pulse train may comprise a plurality of pulses (voltage or current amplitude) that are delivered essentially simultaneously to the electrode configuration. 
   In  FIG. 26 , four therapy programs (program  2637 , program  2639 , program  2641 , and program  2643 ) are configured and activated. In the embodiment, thirty two therapy programs may be defined in which one to four therapy programs may be activated to form a therapy program set. (Other embodiments may support a different number of therapy programs and a different size of the therapy program set.) 
   Additional therapy programs (not directly accessible by the patient) may be provided for any number of reasons including, for example and without limitation, to treat neurological conditions in distinct parts of the body, to treat distinct neurological conditions, to support sub-threshold measurements, patient notification, and measurement functions. For example, a patient notification program is used to define an output pulse train for patient notification such as some type of patterned stimulation that can be discernable by the patient. The patient notification program may be activated by a low battery (battery  1467 ) condition. A lead integrity measurement program defines a pulse train to executing lead (e.g.  2601  and  2603 ) integrity measurements. 
   In  FIG. 26 , the therapy program set comprises therapy programs  2637  (program  1 ),  2639  (program  2 ),  2641  (program  3 ), and  2643  (program  4 ). Each therapy program comprises four waveforms C 1 , C 2 , C 3 , and C 4  that are generated by regulators  1401 ,  1403 ,  1405 , and  1407 , respectively. Table 2 illustrates the configuration of the program set as shown in FIG.  26 . Stimulation pulses are applied to cathodes  2607 - 2617  of lead  2601  and to cathodes  2623 - 2633  of lead  2603 , while anodes  2605 ,  2619 ,  2621 , and  2635  serve as common references. 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               EXAMPLE OF THERAPY PROGRAM SET 
             
           
        
         
             
                 
               Lead 1 (2601) 
               Lead 2 (2603) 
             
           
        
         
             
               Electrode 
               1 
               2 
               3 
               4 
               5 
               6 
               1 
               2 
               3 
               4 
               5 
               6 
             
             
                 
             
             
               program 
                 
                 
               C1 
               C2 
                 
                 
                 
                 
               C3 
               C4 
                 
                 
             
             
               1 (2637) 
             
             
               program 
                 
                 
               C1 
               C2 
                 
                 
                 
                 
               C3 
               C4 
             
             
               2 (2639) 
             
             
               program 
                 
                 
                 
                 
               C1 
               C2 
                 
                 
                 
                 
               C3 
               C4 
             
             
               3 (2641) 
             
             
               program 
               C1 
               C2 
                 
                 
                 
                 
               C3 
               C4 
             
             
               4 (2643) 
             
             
                 
             
           
        
       
     
   
   With therapy program  2637  (program  1 ), stimulation pulses  2655 ,  2657 ,  2651 , and  2653  are applied to cathodes  2611 ,  2613 ,  2627 , and  2629 , respectively. With therapy program  2639  (program  2 ), stimulation pulses  2665 ,  2667 ,  2661 , and  2663  are applied to cathodes  2611 ,  2613 ,  2627 , and  2629 , respectively. The pulse characteristics of a regulator (e.g.  1401 ,  1403 ,  1405 ,  1407 ) may vary from one therapy program to another. For example, pulse  2655  and pulse  2665  are generated by regulator  1401 ; however, pulse  2655  and pulse  2665  may have different characteristics in order to obtain a desired therapeutical effect. 
   With therapy program  2641  (program  3 ), stimulation pulses  2675 ,  2677 ,  2671 , and  2673  are applied to cathodes  2615 ,  2617 ,  2631 , and  2633 , respectively. With therapy program  2643  (program  4 ), stimulation pulses  2685 ,  2687 ,  2681 , and  2683  are applied to cathodes  2607 ,  2609 ,  2623 , and  2625 , respectively. 
   Thus, embodiments of the INDEPENDENT THERAPY PROGRAMS IN AN IMPLANTABLE MEDICAL DEVICE are disclosed. One skilled in the art will appreciate that the present invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.