Patent Publication Number: US-11040206-B2

Title: System and method for delivering sub-threshold and super-threshold therapy to a patient

Description:
CLAIM OF PRIORITY 
     This application is a continuation of U.S. application Ser. No. 14/295,735, filed Jun. 4, 2014, which application claims the benefit of priority under 35. U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 61/832,083, filed on Jun. 6, 2013, each of which is herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present inventions relate to tissue modulation systems, and more particularly, to programmable neuromodulation systems. 
     BACKGROUND OF THE INVENTION 
     Implantable neuromodulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients. 
     These implantable neuromodulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and an implantable neuromodulation device (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the neuromodulation lead(s) or indirectly to the neuromodulation lead(s) via a lead extension. The neuromodulation system may further comprise a handheld external control device (e.g., a remote control (RC)) to remotely instruct the neuromodulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. 
     Implantable neuromodulation devices are active devices requiring energy for operation, and thus, the neuromodulation system oftentimes include an external charger to recharge a neuromodulation device, so that a surgical procedure to replace a power depleted neuromodulation device can be avoided. To wirelessly convey energy between the external charger and the implanted neuromodulation device, the charger typically includes an alternating current (AC) charging coil that supplies energy to a similar charging coil located in or on the neuromodulation device. The energy received by the charging coil located on the neuromodulation device can then be stored in a rechargeable battery within the neuromodulation device, which can then be used to power the electronic componentry on-demand. 
     Electrical modulation energy may be delivered from the neuromodulation device to the electrodes in the form of an electrical pulsed waveform. Thus, electrical energy may be controllably delivered to the electrodes to therapeutically modulate neural tissue. The configuration of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode configuration, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode configuration represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, width, and rate of the electrical pulses (which may be considered electrical pulse parameters) provided through the electrode array. Each electrode configuration, along with the electrical pulse parameters, can be referred to as a “modulation parameter set.” 
     With some neuromodulation systems, and in particular, those with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the neuromodulation device, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode configurations). 
     As briefly discussed above, an external control device can be used to instruct the neuromodulation device to generate electrical pulses in accordance with the selected modulation parameters. Typically, the modulation parameters programmed into the neuromodulation device can be adjusted by manipulating controls on the handheld external control device to modify the electrical modulation energy provided by the neuromodulation device system to the patient. Thus, in accordance with the modulation parameters programmed by the external control device, electrical pulses can be delivered from the neuromodulation device to the electrode(s) to modulate a volume of tissue in accordance with a set of modulation parameters and provide the desired efficacious therapy to the patient. The best modulation set will typically be one that delivers modulation energy to the volume of tissue that must be modulated in order to provide the therapeutic benefit (e.g., treatment of pain), while minimizing the volume of non-target tissue that is modulated. 
     However, the number of electrodes available, combined with the ability to generate a variety of complex electrical pulses, presents a huge selection of modulation parameter sets to the clinician or patient. For example, if the neuromodulation system to be programmed has an array of sixteen electrodes, millions of modulation parameter sets may be available for programming into the neuromodulation system. Today, neuromodulation systems may have up to thirty-two electrodes, thereby exponentially increasing the number of modulation parameters sets available for programming. 
     To facilitate such selection, the clinician generally programs the neuromodulation device through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neuromodulation device to allow the optimum stimulation parameters to be determined based on patient feedback or other means and to subsequently program the neuromodulation device with the optimum modulation parameter sets. 
     For example, in order to achieve an effective result from conventional SCS, the lead or leads must be placed in a location, such that the electrical modulation energy (in this case, electrical stimulation energy) creates a sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient. The paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient&#39;s body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. When electrical leads are implanted within the patient, the computerized programming system, in the context of an operating room (OR) mapping procedure, may be used to instruct the neuromodulation device to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient. 
     Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the neuromodulation device, with a set of modulation parameters that best addresses the painful site. Thus, the navigation session may be used to pinpoint volume of activation (VOA) or areas correlating to the pain. Such programming ability is particularly advantageous for targeting the tissue during implantation, or after implantation should the leads gradually or unexpectedly move that would otherwise relocate the stimulation energy away from the target site. By reprogramming the neuromodulation device (typically by independently varying the stimulation energy on the electrodes), the volume of activation (VOA) can often be moved back to the effective pain site without having to re-operate on the patient in order to reposition the lead and its electrode array. When adjusting the volume of activation (VOA) relative to the tissue, it is desirable to make small changes in the proportions of current, so that changes in the spatial recruitment of nerve fibers will be perceived by the patient as being smooth and continuous and to have incremental targeting capability. 
     Although alternative or artifactual sensations are usually tolerated relative to the sensation of pain, patients sometimes report these sensations to be uncomfortable, and therefore, they can be considered an adverse side-effect to neuromodulation therapy in some cases. Because the perception of paresthesia has been used as an indicator that the applied electrical energy is, in fact, alleviating the pain experienced by the patient, the amplitude of the applied electrical energy is generally adjusted to a level that causes the perception of paresthesia. It has been shown, however, that the delivery of sub-threshold electrical energy (e.g., high frequency pulsed electrical energy and/or low pulse width electrical energy) can be effective in providing neuromodulation therapy for chronic pain without causing paresthesia. 
     Although high-frequency modulation therapies have shown good efficacy in early studies, one notable drawback is the relatively high energy requirement to achieve high-frequency modulation in contrast to lower frequency stimulation techniques. In particular, the amount of energy required to generate an electrical waveform is proportional to the frequency of the electrical waveform. Thus, neuromodulation devices that generate relatively low frequency modulation energy typically need to be recharged only once every 1-2 weeks, whereas neuromodulation devices that generate relatively high frequency modulation energy may require a daily or more frequent recharge. 
     In order to conserve energy during high-frequency modulation therapy, it is suggested that the delivery of the high-frequency modulation energy to the patient be alternately bursted on and off, with the hope that the therapy provided when the high-frequency modulation energy is bursted on will have a lasting effect during the period during which the high-frequency modulation energy has been bursted off. However, continued therapy during the periods that the high-frequency modulation energy has been bursted off is not guaranteed. 
     There, thus, is a need to ensure that therapy is continued during a high-frequency modulation regimen while still minimizing energy consumption. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present inventions, a neuromodulation system is provided. The neuromodulation system comprises a plurality of electrical terminals configured for being respectively coupled to a plurality of electrodes, and modulation output circuitry configured for delivering a sub-threshold electrical energy (e.g., a pulse train having a pulse rate greater than 1500 Hz and/or a pulse width less than 500 μs) to a first set of the electrical terminals, and for delivering a super-threshold electrical energy (e.g., a pulse train having a pulse rate less than 1500 Hz and/or a pulse width greater than 500 μs) to a second set of the electrical terminals. The sub-threshold level may be referred to as a patient-perception threshold, which may be referred to as a boundary below which a patient does not sense delivery of the electrical energy and above which the patient does sense delivery of the electrical energy. For example, in a spinal cord modulation system, the patient-perception threshold may be a boundary below which a patient does not experience paresthesia. The first and second electrical terminals may be the same as each other or different from each other. 
     The neuromodulation system further comprises control circuitry configured for controlling the modulation output circuitry in an automated manner that alternately cycles the sub-threshold electrical energy on during high-energy consumption therapy periods and off during low-energy consumption therapy periods, and that alternately cycles the sub-threshold electrical energy on during the low energy consumption therapy periods and off during the high energy consumption therapy periods. The high-energy consumption therapy periods may be interleaved with the low-energy consumption therapy periods, e.g., in a non-overlapping manner. 
     The neurostimulation system optionally comprises a user interface configured for receiving input from a user. In one case, the control circuitry may be configured for modifying at least one of the sub-threshold electrical energy and the super-threshold electrical energy in response to the input from the user. In another case, the control circuitry is configured for storing a time schedule in the memory in response to the input from the user, and defining the high-energy consumption therapy periods and low-energy consumption therapy periods in accordance with the time schedule. The neurostimulation system optionally comprises a sensor configured for detecting a patient activity level, in which case, the control circuitry is configured for defining the high-energy consumption therapy periods during times when the patient activity level is relatively high, and defining the low-energy consumption therapy periods during times when the patient activity level is relatively low. 
     The neuromodulation system may further comprise memory configured for storing a predetermined tissue modulation regimen, wherein the control circuitry is configured for controlling the modulation output circuitry to automatically cycle each of the sub-threshold electrical energy and the super-threshold electrical energy on and off in accordance with the predetermined tissue modulation regimen. The predetermined tissue modulation regimen may define a sub-threshold modulation program and a super-threshold modulation program, in which case, the modulation output circuitry may be configured for delivering the sub-threshold electrical energy to the first electrical terminal set in accordance with the sub-threshold modulation program, and delivering the super-threshold electrical energy to the first electrical terminal set in accordance with the super-threshold modulation program. The neuromodulation system may further comprise a casing containing the plurality of electrical terminals, the modulation output circuitry, and the control circuitry. 
     In accordance with another aspect of the present invention, a method of providing therapy to a patient is provided. The method comprises delivering a sub-threshold electrical energy (e.g., a pulse train having a pulse rate greater than 1500 Hz and/or a pulse width less than 500 μs) to a first tissue region (e.g., spinal cord tissue) of the patient, and delivering a super-threshold electrical energy (e.g., a pulse train having a pulse rate less than 1500 Hz and/or a pulse width greater than 500 μs) to a second tissue region (e.g., spinal cord tissue) of the patient. The first and second tissue regions may be the same as each other or different from each other. The sub-threshold electrical energy is alternately cycled on during high-energy consumption therapy periods and off during low-energy consumption therapy periods, thereby providing the therapy (e.g., alleviation of chronic pain) to the patient, and the super-threshold electrical energy is alternately cycled on during the low energy consumption therapy periods and off during the high energy consumption therapy periods, thereby supplementing the therapy provided to the patient. The high-energy consumption therapy periods may be interleaved with the low-energy consumption therapy periods, e.g., in a non-overlapping manner. 
     An optional method comprises receiving input from a user. In one case, at least one of the sub-threshold electrical energy and the super-threshold electrical energy is modified in response to the input from the user. In another case, a time schedule is stored in response to the input from the user, and the high-energy consumption therapy periods and low-energy consumption therapy periods are defined in accordance with the time schedule. Another optional method further comprises detecting a patient activity level, and defining the high-energy consumption therapy periods during times when the patient activity level is relatively high, and defining the low-energy consumption therapy periods during times when the patient activity level is relatively low. Still another method further comprises storing a predetermined tissue modulation regimen, wherein each of the sub-threshold electrical energy and the super-threshold electrical energy are cycled on and off in accordance with the predetermined tissue modulation regimen. The predetermined tissue modulation regimen may define a sub-threshold modulation program and a super-threshold modulation program, in which case, the sub-threshold electrical energy is delivered to the first tissue region in accordance with the sub-threshold modulation program, and the super-threshold electrical energy is delivered to the second tissue region in accordance with the super-threshold modulation program. 
     Other and further aspects and features of the invention will be evident from reading the following detailed description of the preferred embodiments, which are intended to illustrate, not limit, the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the design and utility of preferred embodiments of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate how the above-recited and other advantages and objects of the present inventions are obtained, a more particular description of the present inventions briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a plan view of a Spinal Cord Modulation (SCM) system constructed in accordance with one embodiment of the present inventions; 
         FIG. 2  is a plan view of the SCM system of  FIG. 1  in use with a patient; 
         FIG. 3  is a profile view of an implantable pulse generator (IPG) and percutaneous leads used in the SCM system of  FIG. 1 ; 
         FIG. 4  is a plot of monophasic cathodic electrical modulation energy; 
         FIG. 5 a    is a plot of biphasic electrical modulation energy having a cathodic modulation pulse and an active charge recovery pulse; 
         FIG. 5 b    is a plot of biphasic electrical modulation energy having a cathodic modulation pulse and a passive charge recovery pulse; 
         FIG. 6  is a timing diagram illustrating the sub-threshold therapy and super-threshold therapy being cycled on and off by the IPG of  FIG. 3 ; 
         FIG. 7  is a timing diagram illustrating a sub-threshold electrical pulse train and a super-threshold electrical pulse train being cycled on and off by the IPG of  FIG. 3 ; 
         FIG. 8  is a block diagram of the internal components of the IPG of  FIG. 3 . 
         FIG. 9  is front view of a remote control (RC) used in the SCM system of  FIG. 1 ; and 
         FIG. 10  is a block diagram of the internal components of the RC of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The description that follows relates to a spinal cord modulation (SCM) system. However, it is to be understood that the while the invention lends itself well to applications in SCM, the invention, in its broadest aspects, may not be so limited. Rather, the invention may be used with any type of implantable electrical circuitry used to stimulate tissue. For example, the present invention may be used as part of a pacemaker, a defibrillator, a cochlear stimulator, a retinal stimulator, a stimulator configured to produce coordinated limb movement, a cortical stimulator, a deep brain stimulator, peripheral nerve stimulator, microstimulator, or in any other neural stimulator configured to treat urinary incontinence, sleep apnea, shoulder sublaxation, headache, etc. 
     Turning first to  FIG. 1 , an exemplary SCM system  10  generally includes a plurality (in this case, two) of implantable neuromodulation leads  12 , an implantable pulse generator (IPG)  14 , an external remote controller RC  16 , a clinician&#39;s programmer (CP)  18 , an external trial modulator (ETM)  20 , and an external charger  22 . 
     The IPG  14  is physically connected via one or more percutaneous lead extensions  24  to the neuromodulation leads  12 , which carry a plurality of electrodes  26  arranged in an array. In the illustrated embodiment, the neuromodulation leads  12  are percutaneous leads, and to this end, the electrodes  26  are arranged in-line along the neuromodulation leads  12 . The number of neuromodulation leads  12  illustrated is two, although any suitable number of neuromodulation leads  12  can be provided, including only one. Alternatively, a surgical paddle lead in can be used in place of one or more of the percutaneous leads. As will be described in further detail below, the IPG  14  includes pulse generation circuitry that delivers electrical modulation energy in the form of a pulsed electrical waveform (i.e., a temporal series of electrical pulses) to the electrode array  26  in accordance with a set of modulation parameters. 
     The ETM  20  may also be physically connected via the percutaneous lead extensions  28  and external cable  30  to the neuromodulation leads  12 . The ETM  20 , which has similar pulse generation circuitry as the IPG  14 , also delivers electrical modulation energy in the form of a pulse electrical waveform to the electrode array  26  accordance with a set of modulation parameters. The major difference between the ETM  20  and the IPG  14  is that the ETM  20  is a non-implantable device that is used on a trial basis after the neuromodulation leads  12  have been implanted and prior to implantation of the IPG  14 , to test the responsiveness of the modulation energy delivered to the patient. Thus, any functions described herein with respect to the IPG  14  can likewise be performed with respect to the ETM  20 . 
     The RC  16  may be used to telemetrically control the ETM  20  via a bi-directional RF communications link  32 . Once the IPG  14  and neuromodulation leads  12  are implanted, the RC  16  may be used to telemetrically control the IPG  14  via a bi-directional RF communications link  34 . Such control allows the IPG  14  to be turned on or off and to be programmed with different modulation parameter sets. The IPG  14  may also be operated to modify the programmed modulation parameters to actively control the characteristics of the electrical modulation energy output by the IPG  14 . The CP  18  provides clinician detailed modulation parameters for programming the IPG  14  and ETM  20  in the operating room and in follow-up sessions. 
     The CP  18  may perform this function by indirectly communicating with the IPG  14  or ETM  20 , through the RC  16 , via an IR communications link  36 . Alternatively, the CP  18  may directly communicate with the IPG  14  or ETM  20  via an RF communications link (not shown). The clinician detailed modulation parameters provided by the CP  18  are also used to program the RC  16 , so that the modulation parameters can be subsequently modified by operation of the RC  16  in a stand-alone mode (i.e., without the assistance of the CP  18 ). 
     The external charger  22  is a portable device used to transcutaneously charge the IPG  14  via an inductive link  38 . Once the IPG  14  has been programmed, and its power source has been charged by the external charger  22  or otherwise replenished, the IPG  14  may function as programmed without the RC  16  or CP  18  being present. 
     For purposes of brevity, the details of the CP  18 , ETM  20 , and external charger  22  will not be described herein. Details of exemplary embodiments of these devices are disclosed in U.S. Pat. No. 6,895,280, which is expressly incorporated herein by reference. 
     As shown in  FIG. 2 , the neuromodulation leads  12  are implanted within the spinal column  42  of a patient  40 . The preferred placement of the neuromodulation leads  12  is adjacent, i.e., resting upon, the spinal cord area to be stimulated. Due to the lack of space near the location where the neuromodulation leads  12  exit the spinal column  42 , the IPG  14  is generally implanted in a surgically-made pocket either in the abdomen or above the buttocks. The IPG  14  may, of course, also be implanted in other locations of the patient&#39;s body. The lead extension  24  facilitates locating the IPG  14  away from the exit point of the neuromodulation leads  12 . As there shown, the CP  18  communicates with the IPG  14  via the RC  16 . 
     Referring now to  FIG. 3 , the external features of the neuromodulation leads  12  and the IPG  14  will be briefly described. One of the neuromodulation leads  12   a  has eight electrodes  26  (labeled E 1 -E 8 ), and the other neuromodulation lead  12   b  has eight electrodes  26  (labeled E 9 -E 16 ). The actual number and shape of leads and electrodes will, of course, vary according to the intended application. The IPG  14  comprises an outer case  44  for housing the electronic and other components (described in further detail below), and a connector  46  to which the proximal ends of the neuromodulation leads  12  mates in a manner that electrically couples the electrodes  26  to the electronics within the outer case  44 . The outer case  44  is composed of an electrically conductive, biocompatible material, such as titanium, and forms a hermetically sealed compartment wherein the internal electronics are protected from the body tissue and fluids. In some cases, the outer case  44  may serve as an electrode. 
     As will be described in further detail below, the IPG  14  includes a battery and pulse generation circuitry that delivers the electrical modulation energy in the form of one or more electrical pulse trains to the electrode array  26  in accordance with a set of modulation parameters programmed into the IPG  14 . Such modulation parameters may comprise electrode combinations, which define the electrodes that are activated as anodes (positive), cathodes (negative), and turned off (zero), percentage of modulation energy assigned to each electrode (fractionalized electrode configurations), and electrical pulse parameters, which define the pulse amplitude (measured in milliamps or volts depending on whether the IPG  14  supplies constant current or constant voltage to the electrode array  26 ), pulse duration (measured in microseconds), pulse rate (measured in pulses per second), and burst rate (measured as the modulation on duration X and modulation off duration Y). 
     Electrical modulation will occur between two (or more) activated electrodes, one of which may be the IPG case  44 . Modulation energy may be transmitted to the tissue in a monopolar or multipolar (e.g., bipolar, tripolar, etc.) fashion. Monopolar modulation occurs when a selected one of the lead electrodes  26  is activated along with the case of the IPG  14 , so that modulation energy is transmitted between the selected electrode  26  and case. Bipolar modulation occurs when two of the lead electrodes  26  are activated as anode and cathode, so that modulation energy is transmitted between the selected electrodes  26 . For example, electrode E 3  on the first lead  12   a  may be activated as an anode at the same time that electrode E 11  on the second lead  12   a  is activated as a cathode. Tripolar modulation occurs when three of the lead electrodes  26  are activated, two as anodes and the remaining one as a cathode, or two as cathodes and the remaining one as an anode. For example, electrodes E 4  and E 5  on the first lead  12   a  may be activated as anodes at the same time that electrode E 12  on the second lead  12   b  is activated as a cathode 
     The modulation energy may be delivered between a specified group of electrodes as monophasic electrical energy or multiphasic electrical energy. As illustrated in  FIG. 4 , monophasic electrical energy takes the form of an electrical pulse train that includes either all negative pulses (cathodic), or alternatively all positive pulses (anodic). 
     Multiphasic electrical energy includes a series of pulses that alternate between positive and negative. For example, as illustrated in  FIGS. 5 a  and 5 b   , multiphasic electrical energy may include a series of biphasic pulses, with each biphasic pulse including a cathodic (negative) modulation pulse (during a first phase) and an anodic (positive) charge recovery pulse (during a second phase) that is generated after the modulation pulse to prevent direct current charge transfer through the tissue, thereby avoiding electrode degradation and cell trauma. That is, charge is conveyed through the electrode-tissue interface via current at an electrode during a modulation period (the length of the modulation pulse), and then pulled back off the electrode-tissue interface via an oppositely polarized current at the same electrode during a recharge period (the length of the charge recovery pulse). 
     The second phase may have an active charge recovery pulse ( FIG. 5 a   ), wherein electrical current is actively conveyed through the electrode via current or voltage sources, or the second phase may have a passive charge recovery pulse ( FIG. 5 b   ), wherein electrical current is passively conveyed through the electrode via redistribution of the charge flowing from coupling capacitances present in the circuit. Using active recharge, as opposed to passive recharge, allows faster recharge, while avoiding the charge imbalance that could otherwise occur. Another electrical pulse parameter in the form of an interphase can define the time period between the pulses of the biphasic pulse (measured in microseconds). 
     More significant to the present inventions, the SCM system  10  is configured for alternately delivering a sub-threshold electrical pulse train and a super-threshold pulse train to the spinal cord tissue of the patient, and specifically, alternately cycling the sub-threshold electrical pulse train on during high-energy consumption therapy periods and off during low-energy consumption therapy periods, as illustrated in  FIG. 6 . Thus, during the high energy consumption therapy periods when the sub-threshold electrical pulse train is cycled on, the patient is actively provided with therapy in the form of pain relief. In order to supplement the sub-threshold when the sub-threshold electrical pulse train is cycled off, the SCM system  10  is capable of delivering a super-threshold electrical pulse train to the spinal cord tissue of the patient, and specifically, alternately cycling the super-threshold electrical pulse train on during the low energy consumption therapy periods and off during the high energy consumption therapy periods, as illustrated in  FIG. 6 . The SCM system  10  alternately cycles the sub-threshold electrical pulse train and super-threshold electrical pulse train on and off in an automated manner (i.e., without user intervention once the cycling function is initiated). 
     As shown in  FIG. 6 , the high-energy consumption therapy periods are interleaved with the low-energy consumption therapy periods. Although the present inventions should not be so limited in their broadest aspects, the high-energy consumption therapy periods and the low-energy consumption therapy periods do no overlap each other, such that when one therapy period terminates, the next therapy period is initiated. Alternatively, a high-energy consumption therapy period and a low-energy consumption therapy period may slightly overlap with each other, and therefore, blend in with each other. The significance is that there will be a time period when only the sub-threshold electrical pulse train is delivered to the spinal cord tissue, and a time period when only the super-threshold electrical pulse train is delivered to the spinal cord tissue. The sub-threshold electrical pulse train and the super-threshold electrical pulse train may be delivered to the same spinal cord tissue region or different spinal cord tissue regions. The significance is that delivery of the super-threshold electrical pulse train provides pain relief to the patient that may not otherwise be provided when the sub-threshold electrical pulse train has been cycled off. 
     The sub-threshold electrical pulse train is delivered to the spinal cord tissue of the patient in accordance with a set of modulation parameters that are designed to provide sub-threshold therapy to the patient. Likewise, the super-threshold electrical pulse train is delivered to the spinal cord tissue of the patient in accordance with a set of modulation parameters that are designed to provide super-threshold therapy to the patient. Some embodiments deliver a sub-threshold pulse train using a pulse rate greater than 1500 Hz, and some embodiments deliver a sub-threshold pulse train using a pulse rate greater than 2500 Hz. Some embodiments deliver a sub-threshold pulse train using a pulse width less than 500 μs, some embodiments deliver a sub-threshold pulse train using a pulse width less than 100 μs, and deliver a sub-threshold pulse train using a pulse width less than 500 μs. For example, as shown in  FIG. 7 , an exemplary sub-threshold pulse train may be delivered at a relatively low pulse amplitude (e.g., 2.5 ma), a relatively high pulse rate (e.g., greater than 1500 Hz, preferably greater than 2500 Hz), and a relatively low pulse width (e.g., less than 500 μs, or less than 100 μs, or less than 50 μs), and an exemplary super-threshold pulse train may be delivered at a relatively high pulse amplitude (e.g., 5 ma), a relatively low pulse rate (e.g., less than 1500 Hz, preferably less than 500 Hz), and a relatively high pulse width (e.g., greater than 100 μs, or greater than 200 μs, or greater than 500 μs). Although both the sub-threshold electrical pulse train and the super-threshold electrical pulse train are illustrated as a biphasic pulse train having an active charge recovery phase, it should be appreciated that they can be biphasic cathodic or anodic pulse trains having a passive charge recovery phase. 
     In the preferred embodiment, the sub-threshold electrical pulse train and super-threshold electrical pulse train are cycled on and off in accordance with a predetermined tissue modulation regimen stored in the SCM system  10 . The predetermined tissue modulation regimen may define a sub-threshold modulation program and a super-threshold modulation program, in which case, the sub-threshold electrical pulse train may be delivered to the spinal cord tissue in accordance with the sub-threshold modulation program, and the super-threshold electrical pulse train may be delivered to the spinal cord tissue in accordance with the super-threshold modulation program. 
     The SCM system  10  may modify one or both of the sub-threshold electrical pulse train and the super-threshold electrical pulse train (e.g., by modifying one of the modulation programs or otherwise the modulation parameters) in response to receiving input from the user. In an optional embodiment, the SCM system  10  may derive a set of super-threshold modulation parameters from a set of sub-threshold modulation parameters, or vice versa, using one of the techniques described in U.S. Provisional Patent Application Ser. No. 61/801,917, entitled “Systems and Methods for Delivering Sub-threshold Therapy to a Patient,” which is expressly incorporated herein by reference. 
     In one embodiment, the SCM system  10  stores a time schedule in response to receiving input from the user, and defines the high-energy consumption therapy periods and low-energy consumption therapy periods in accordance with the time schedule. The time schedule may define absolute times for the high-energy consumption therapy periods and low-energy consumption therapy periods (e.g., initiate high-energy consumption therapy period at 9:00 am; terminate the high-energy consumption therapy period and initiate the low-energy consumption therapy period at 11:00 am; terminate the low-energy consumption therapy period and initiate the high-energy consumption therapy period at 2:00 pm, etc.) or the time schedule may define relative times for the high-energy consumption therapy periods and low-energy consumption therapy periods (e.g., terminate high-energy consumption therapy period and initiate the low-energy consumption therapy period two hours after the high-energy consumption therapy period has been initiated; terminate the low-energy consumption therapy period and initiate the high-energy consumption therapy period three hours after the high-energy consumption therapy has been terminated, etc.). 
     In an optional embodiment, the SCM system  10  detects a patient activity level (expenditure of energy), and defines the high-energy consumption therapy periods during times when the patient activity level is relatively high, and defines the low-energy consumption therapy periods during times when the patient activity level is relatively low. In one technique, the physical activity level of the patient is estimated from the magnitude of time varying electrical parameter data measured from the electrodes  26  or data measured from other sensors (impedance, activity, accelerometer, etc.), as described in U.S. patent application Ser. No. 12/024,947, entitled “Neurostimulation System and Method for Measuring Patient Activity,” which is expressly incorporated herein by reference. In another technique, the physical activity level of the patient is estimated from a frequency that an orientation sensitive component implanted within the patient detects a change in orientation, as described in U.S. patent application Ser. No. 13/446,191, entitled “Sensing Device for Indicating Posture of Patient Implanted with a Neurostimulation Device, which is expressly incorporated herein by reference. 
     Turning next to  FIG. 8 , the main internal components of the IPG  14  will now be described. The IPG  14  includes modulation output circuitry  50  configured for generating electrical modulation energy in accordance with a defined pulsed waveform having a specified pulse amplitude, pulse rate, pulse width, pulse shape, and burst rate under control of control logic  52  over data bus  54 . Control of the pulse rate and pulse width of the electrical waveform is facilitated by timer logic circuitry  56 , which may have a suitable resolution, e.g., 10 μs. The modulation energy generated by the modulation output circuitry  50  is output via capacitors C 1 -C 16  to electrical terminals  58  corresponding to the electrodes  26 . The analog output circuitry  50  may either comprise independently controlled current sources for providing modulation pulses of a specified and known amperage to or from the electrodes  26 , or independently controlled voltage sources for providing modulation pulses of a specified and known voltage at the electrodes  26 . 
     Any of the N electrodes may be assigned to up to k possible groups or timing “channels.” In one embodiment, k may equal four. The timing channel identifies which electrodes are selected to synchronously source or sink current to create an electric field in the tissue to be stimulated. Thus, multiple timing channels can be utilized to concurrently deliver electrical current (by interlacing the pulses of electrical pulse trains together) to multiple tissue regions of the patient. Amplitudes and polarities of electrodes on a channel may vary, e.g., as controlled by the RC  16 . External programming software in the CP  18  is typically used to set modulation parameters including amplitude, pulse rate and pulse duration for the electrodes of a given channel, among other possible programmable features. 
     The N programmable electrodes can be programmed to have a positive (sourcing current), negative (sinking current), or off (no current) polarity in any of the k channels. Moreover, each of the N electrodes can operate in a multipolar (e.g., bipolar) mode, e.g., where two or more electrode contacts are grouped to source/sink current at the same time. Alternatively, each of the N electrodes can operate in a monopolar mode where, e.g., the electrodes associated with a channel are configured as cathodes (negative), and the case electrode (i.e., the IPG case) is configured as an anode (positive). 
     Further, the amplitude of the current pulse being sourced or sunk to or from a given electrode may be programmed to one of several discrete current levels, e.g., between 0 to 10 mA in steps of 0.1 mA. Also, the pulse duration of the current pulses is preferably adjustable in convenient increments, e.g., from 0 to 1 milliseconds (ms) in increments of 10 microseconds (μs). Similarly, the pulse rate is preferably adjustable within acceptable limits, e.g., from 0 to 50K pulses per second (pps). Other programmable features can include slow start/end ramping, burst modulation cycling (on for X time, off for Y time), interphase, and open or closed loop sensing modes. 
     The operation of this modulation output circuitry  50 , including alternative embodiments of suitable output circuitry for performing the same function of generating modulation pulses of a prescribed amplitude and duration, is described more fully in U.S. Pat. Nos. 6,516,227 and 6,993,384, which are expressly incorporated herein by reference. 
     The IPG  14  further comprises monitoring circuitry  60  configured for monitoring the status of various nodes or other points  62  throughout the IPG  14 , e.g., power supply voltages, temperature, battery voltage, and the like. The monitoring circuitry  60  may also be configured for measuring electrical parameter data from the electrodes  26  or other information from other sensors needed to determine the current activity level of the patient. The IPG  14  further comprises processing circuitry in the form of a microcontroller (μC)  64  that controls the control logic over data bus  66 , and obtains status data from the monitoring circuitry  60  via data bus  68 . The IPG  14  additionally controls the timer logic  58 . The IPG  14  further comprises memory  70  and oscillator and clock circuitry  72  coupled to the microcontroller  64 . The microcontroller  64 , in combination with the memory  70  and oscillator and clock circuitry  72 , thus comprise a microprocessor system that carries out a program function in accordance with a suitable program stored in the memory  70 . Alternatively, for some applications, the function provided by the microprocessor system may be carried out by a suitable state machine. 
     Thus, the microcontroller  64  generates the necessary control and status signals, which allow the microcontroller  64  to control the operation of the IPG  14  in accordance with a selected operating program and modulation program stored in the memory  70 . In controlling the operation of the IPG  14 , the microcontroller  64  is able to individually generate an electrical pulse train at the electrodes  26  using the modulation output circuitry  50 , in combination with the control logic  52  and timer logic  56 , thereby allowing each electrode  26  to be paired or grouped with other electrodes  26 , including the monopolar case electrode. In accordance with modulation parameters stored within the memory  70 , the microcontroller  64  may control the polarity, amplitude, rate, pulse duration and timing channel through which the modulation pulses are provided. 
     Thus, it can be appreciated that, under control of the microcontroller  64 , the modulation output circuitry  50  is configured for outputting a k number of individual electrical pulse trains respectively in a k number of timing channels to the electrical terminals  56 , with each electrical pulse train including bi-phasic pulses as shown in  FIGS. 5 a  and 5 b   . In the IPG  14 , up to four stimulation programs may be stored in the memory  70 , with each stimulation program having four timing channels. Thus, each modulation program defines four sets of modulation parameters for four respective timing channels. Of course, the IPG  14  may have less or more than four modulation programs, and less or more than four timing channels for each modulation program. The memory  70  also stores a time schedule, which as discussed above, defines the beginning and end of each of the high-energy consumption periods and low-energy consumption periods. The memory  70  may optionally store any threshold values to which the electrical parameter measurements or other measurements are compared to facilitate determining whether patient activity is relatively high or relatively low. 
     Significantly, at least one of these modulation programs will be a sub-threshold modulation program designed to treat chronic pain in a region of the patient (e.g., the lower back) and at least one of these modulation programs will be a super-threshold program designed to treat the chronic pain in the same region of the patient. The sub-threshold and super-threshold modulation programs, along with the time schedule, may be stored as a hybrid modulation program, or the sub-threshold and super-threshold modulation programs may be stored separately and accessed in accordance with the time schedule. 
     The microcontroller  64  accesses the sub-threshold modulation program and super-threshold modulation program from memory  70 , and controls the modulation output circuitry  50  in a manner that cycles the sub-threshold electrical pulse train on during the high-energy consumption time periods and off during the low-energy consumption time periods, and cycles the super-threshold electrical pulse train on during the low-energy consumption time periods and off during the high-energy consumption time periods. The microcontroller  64  can perform this cycling function based on the time schedule stored in the memory  70  or a determined patient activity level. 
     Although the sub-threshold electrical pulse train and super-threshold electrical pulse train are illustrated in  FIGS. 6 and 7  as being cycled on and off in a single timing channel, it should be appreciated that multiple sub-threshold electrical pulse trains and multiple super-threshold electrical pulse trains can be cycled on and off respectively during multiple timing channels. 
     Alternatively, in the same manner described in U.S. Provisional Patent Application Ser. No. 61/801,917, entitled “Systems and Methods for Delivering Sub-threshold Therapy to a Patient,” which is expressly incorporated herein by reference, only the sub-threshold modulation program is stored in memory  70 , in which case, the microcontroller  64  may derive the super-threshold modulation program from the stored sub-threshold modulation program, or only the super-threshold modulation program is stored in memory  70 , in which case, the microcontroller  64  may derive the sub-threshold modulation program from the stored super-threshold modulation program 
     The IPG  14  further comprises an alternating current (AC) receiving coil  74  for receiving programming data (e.g., the operating program, modulation programs including the parameters, and/or a time schedule) from the RC  16  (shown in  FIG. 1 ) in an appropriate modulated carrier signal, and charging and forward telemetry circuitry  76  for demodulating the carrier signal it receives through the AC receiving coil  74  to recover the programming data, which programming data is then stored within the memory  70 , or within other memory elements (not shown) distributed throughout the IPG  14 . 
     The IPG  14  further comprises back telemetry circuitry  78  and an alternating current (AC) transmission coil  80  for sending informational data sensed through the monitoring circuitry  60  to the RC  16 . The back telemetry features of the IPG  14  also allow its status to be checked. For example, when the RC  16  initiates a programming session with the IPG  14 , the capacity of the battery is telemetered, so that the external programmer can calculate the estimated time to recharge. Any changes made to the current stimulus parameters are confirmed through back telemetry, thereby assuring that such changes have been correctly received and implemented within the implant system. Moreover, upon interrogation by the RC  16 , all programmable settings stored within the IPG  14  may be uploaded to the RC  16 . Significantly, the back telemetry features allow raw or processed electrical parameter data (or other parameter data) previously stored in the memory  70  to be downloaded from the IPG  14  to the RC  16 , which information can be used to track the physical activity of the patient. 
     The IPG  14  further comprises a rechargeable power source  82  and power circuitry  84  for providing the operating power to the IPG  14 . The rechargeable power source  82  may, e.g., comprise a lithium-ion or lithium-ion polymer battery. The rechargeable battery  82  provides an unregulated voltage to the power circuitry  84 . The power circuitry  84 , in turn, generate the various voltages  86 , some of which are regulated and some of which are not, as needed by the various circuits located within the IPG  14 . The rechargeable power source  82  is recharged using rectified AC power (or DC power converted from AC power through other means, e.g., efficient AC-to-DC converter circuits, also known as “inverter circuits”) received by the AC receiving coil  134 . To recharge the power source  82 , an external charger (not shown), which generates the AC magnetic field, is placed against, or otherwise adjacent, to the patient&#39;s skin over the implanted IPG  14 . The AC magnetic field emitted by the external charger induces AC currents in the AC receiving coil  74 . The charging and forward telemetry circuitry  76  rectifies the AC current to produce DC current, which is used to charge the power source  82 . While the AC receiving coil  74  is described as being used for both wirelessly receiving communications (e.g., programming and control data) and charging energy from the external device, it should be appreciated that the AC receiving coil  74  can be arranged as a dedicated charging coil, while another coil, such as coil  80 , can be used for bi-directional telemetry. 
     It should be noted that the diagram of  FIG. 8  is functional only, and is not intended to be limiting. Those of skill in the art, given the descriptions presented herein, should be able to readily fashion numerous types of IPG circuits, or equivalent circuits, that carry out the functions indicated and described, which functions include not only producing a stimulus current or voltage on selected groups of electrodes, but also the ability to measure electrical parameter data at an activated or non-activated electrode. 
     Additional details concerning the above-described and other IPGs may be found in U.S. Pat. No. 6,516,227, U.S. Patent Publication No. 2003/0139781, and U.S. patent application Ser. No. 11/138,632, entitled “Low Power Loss Current Digital-to-Analog Converter Used in an Implantable Pulse Generator,” which are expressly incorporated herein by reference. It should be noted that rather than an IPG, the SCM system  10  may alternatively utilize an implantable receiver-modulator (not shown) connected to the modulation leads  12 . In this case, the power source, e.g., a battery, for powering the implanted receiver, as well as control circuitry to command the receiver-modulator, will be contained in an external controller inductively coupled to the receiver-modulator via an electromagnetic link. Data/power signals are transcutaneously coupled from a cable-connected transmission coil placed over the implanted receiver-modulator. The implanted receiver-modulator receives the signal and generates the modulation in accordance with the control signals. 
     Referring now to  FIG. 9 , one exemplary embodiment of an RC  16  will now be described. As previously discussed, the RC  16  is capable of communicating with the IPG  14 , CP  18 , or ETS  20 . The RC  16  comprises a casing  100 , which houses internal componentry (including a printed circuit board (PCB)), and a lighted display screen  102  and button pad  104  carried by the exterior of the casing  100 . In the illustrated embodiment, the display screen  102  is a lighted flat panel display screen, and the button pad  104  comprises a membrane switch with metal domes positioned over a flex circuit, and a keypad connector connected directly to a printed circuit board (PCB). In an optional embodiment, the display screen  102  has touchscreen capabilities. The button pad  104  includes a multitude of buttons  106 ,  108 ,  110 , and  112 , which allow the IPG  14  to be turned ON and OFF, provide for the adjustment or setting of modulation parameters within the IPG  14 , and provide for selection between screens. 
     In the illustrated embodiment, the button  106  serves as an ON/OFF button that can be actuated to turn the IPG  14  ON and OFF. The button  108  serves as a select button that can be actuated to switch the RC  16  between screen displays and/or parameters. The buttons  100  and  112  serve as up/down buttons that can be actuated to increment or decrement any of modulation parameters of the pulsed electrical train generated by the IPG  14 , including pulse amplitude, pulse width, and pulse rate. For example, the selection button  108  can be actuated to place the RC  16  in a “Pulse Amplitude Adjustment Mode,” during which the pulse amplitude can be adjusted via the up/down buttons  110 ,  112 , a “Pulse Width Adjustment Mode,” during which the pulse width can be adjusted via the up/down buttons  110 ,  112 , and a “Pulse Rate Adjustment Mode,” during which the pulse rate can be adjusted via the up/down buttons  110 ,  112 . Alternatively, dedicated up/down buttons can be provided for each modulation parameter. Rather than using up/down buttons, any other type of actuator, such as a dial, slider bar, or keypad, can be used to increment or decrement the modulation parameters. The selection button  108  can also be actuated to place the RC  16  in a “Scheduling Mode,” during which the beginning and ending of each of the high-energy consumption periods and low-energy consumption periods can be defined or adjusted. 
     Referring to  FIG. 10 , the internal components of an exemplary RC  16  will now be described. The RC  16  generally includes a controller/processor  114  (e.g., a microcontroller), memory  116  that stores an operating program for execution by the controller/processor  114 , as well as modulation programs defining modulation parameter sets; input/output circuitry, and in particular, telemetry circuitry  118  for outputting modulation programs and scheduling information to the IPG  14  or otherwise directing the IPG  14  to deliver modulation energy in accordance with the modulation parameters and scheduling information, and receiving status information from the IPG  14 ; and input/output circuitry  120  for receiving modulation control signals from the button pad  104  or other control elements and transmitting status information to the display screen  102  (shown in  FIG. 9 ). 
     Although, in the illustrated embodiment, the scheduling information is described as being transmitted from the RC  16  to the IPG  14 , it should be appreciated that the RC  16  may simply transmit control signals to the IPG  14  to cycle the super-threshold and sub-threshold therapies on and off in accordance with the time schedule stored in the memory  66 . Further details of the functionality and internal componentry of the RC  16  are disclosed in U.S. Pat. No. 6,895,280, which has previously been incorporated herein by reference. 
     Although particular embodiments of the present inventions have been shown and described, it will be understood that it is not intended to limit the present inventions to the preferred embodiments, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present inventions. Thus, the present inventions are intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope of the present inventions as defined by the claims.