Patent Publication Number: US-2018043166-A1

Title: External Device for Determining an Optimal Implantable Medical Device for a Patient Using Information Determined During an External Trial Stimulation Phase

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of U.S. patent application Ser. No. 15/144,454, filed May 2, 2016 (now U.S. Pat. No. 9,789,322), which is a continuation of U.S. patent application Ser. No. 14/271,176, filed May 6, 2014 (now U.S. Pat. No. 9,327,135), which is a non-provisional application of U.S. Provisional Patent Application Ser. No. 61/831,037, filed Jun. 4, 2013, which are all incorporated by reference, and to which priority are claimed. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to implantable medical device systems, and in particular systems involving implantable stimulators. 
     BACKGROUND 
     Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system. 
     As shown in  FIG. 1 , a SCS system typically includes an Implantable Pulse Generator (IPG)  10 , which includes a biocompatible device case  12  formed of a conductive material such as titanium for example. The case  12  typically holds the circuitry and battery  14  necessary for the IPG to function. The IPG  10  is coupled to distal electrodes  16  designed to contact a patient&#39;s tissue. The distal electrodes  16  are coupled to the IPG  10  via one or more electrode leads (two such leads  18  and  20  are shown), such that the electrodes  16  form an electrode array  22 . The electrodes  16  are carried on a flexible body  24 , which also houses the individual signal wires  26  coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead  18 , labeled E 1 -E 8 , and eight electrodes on lead  20 , labeled E 9 -E 16 , although the number of leads and electrodes is application specific and therefore can vary. The leads  18 ,  20  contain proximal electrode contacts  29 , which couple to the IPG  10  using lead connectors  28  fixed in a non-conductive header material  30  such as an epoxy. 
     As shown in the cross-sections of  FIGS. 2A and 2B , an IPG  10  typically includes an electronic substrate assembly including a printed circuit board (PCB)  32 , along with various electronic components  34  mounted to the PCB. A telemetry coil  36  is used to transmit/receive data to/from an external controller (not shown). In these examples, the telemetry coil  36  is within the case  12 , as disclosed in U.S. Pat. No. 8,577,474, although it can also be placed in the header  30  in other examples. 
     IPGs can differ in the type of battery  14  employed.  FIG. 2A  shows an IPG  10   a  that contains a rechargeable secondary battery  14   a.  To facilitate recharging of battery  14   a,  the IPG  10   a  contains an additional charging coil  38 . As shown in  FIG. 3 , charging coil  38  allows an external charger  50  to provide power  90  to recharge the battery  14   a  when necessary. As one skilled in the art will understand, such means of charging battery  14   a  using an external charger  50  occurs transcutaneously via magnetic induction: the external charger  50  is turned on, and an AC current is generated in coil  52  in the external charger. This produces an AC magnetic field  90 , which induces an AC current in charging coil  38  in the IPG  10   a.  This current is rectified to a DC level in the IPG  10   a,  and used to recharge the battery  14   a.  Rechargeable batteries  14   a  can be formed using different chemistries, but lithium ion polymer batteries are popular for use in implantable medical devices, and produce voltages of about 4.2 Volts. 
       FIG. 2B , by contrast, shows an IPG  10   b  that contains a non-rechargeable primary battery  14   b,  i.e., one in which the electrochemical reaction is not reversible by passing a charging current therethrough. Because battery  14   b  is not rechargeable, there is no need for a charging coil (compare  38  in  FIG. 2A ) in IPG  10   b.  However, primary batteries use up the materials in one or both of their electrodes, and thus have a limited life span. Once the battery  14   b  is exhausted, it will be necessary to explant IPG  10   b  from the patient, so that the battery  14   b  can be replaced and the IPG  10   b  re-implanted, or so that a new IPG  10   b  with a fresh battery  14   a  can be implanted. Primary batteries  14   b  can be formed using different chemistries, but Lithium CFx batteries, or Lithium/CFx-SVO (Silver Vanadium Oxide) hybrid batteries are popular for use in implantable medical devices, and produce voltages of 1.2-3.2 Volts. 
     It is easy to assume that a patient should always be provided an implant with a rechargeable battery to permit charging when needed without the need of explantation, but there are also good reasons to prefer an implant with a non-rechargeable primary battery. Primary batteries are typically cheaper than rechargeable batteries, and may not suffer from reliability concerns inherent with rechargeable batteries. Moreover, use of a primary battery in an implant saves costs in other ways: the implant need not contain the overhead of a charging coil ( 38 ,  FIG. 2A ), and an external charger  50  ( FIG. 3 ) can be entirely dispensed with. Moreover, the patient is convenienced by having an implant with a primary battery, as she will not have to concern herself with charging it. The case of a primary-battery implant may also be smaller than a rechargeable-battery implant, which would also convenience the patent. 
     As the inventors recognize, a clinician currently has little guidance to know in advance whether a given patient would most likely benefit from having an implant with a rechargeable battery  14   a,  or from having an implant with a primary battery  14   b.  This disclosure provides solutions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an implantable medical device, specifically an Implantable Pulse Generator (IPG) in accordance with the prior art. 
         FIGS. 2A and 2B  show a rechargeable-battery IPG and a primary-battery IPG respectively in accordance with the prior art. 
         FIG. 3  shows an external charger used to recharge the battery in the rechargeable-battery IPG in accordance with the prior art. 
         FIG. 4  shows an external trial stimulator (ETS) used prior to implantation of an IPG, and an external controller in communication with the ETS, in accordance with the prior art. 
         FIG. 5  shows an improved external controller including a battery algorithm able to estimate battery performance parameters for a rechargeable-battery IPG and a primary-battery IPG during an external trial stimulation phase, and to optionally automatically recommend to a clinician which IPG would be best suited for implantation in the patient. 
         FIG. 6A  shows the user interface of the improved external controller including the option to run the battery algorithm, and  FIG. 6B  shows the output of that algorithm as provided to the clinician. 
         FIG. 7  shows a flow chart of one example of the battery algorithm. 
         FIG. 8  shows a modification in which the ETS provides current draw information to the battery algorithm in the improved external controller. 
         FIG. 9A and 9B  show a modification to the battery algorithm allowing the clinician to choose from more than one rechargeable-battery IPG and more than one primary-battery IPG. 
         FIG. 10  shows another modification to the battery algorithm allowing the clinician to generically choose from multiple IPGs, even if their battery types and circuitries don&#39;t match, or even if their circuitries don&#39;t match the circuitry in the ETS. 
     
    
    
     DETAILED DESCRIPTION 
     As noted earlier, it is not always clear to a clinician whether a given patient would benefit from having an implant with a rechargeable or primary battery. The inventors realize that, at least in the context of an IPG, information can be discerned from the external trial stimulation phase to assist the clinician in making this determination. 
     The external trial stimulation phase precedes actual implantation of the IPG, and is shown in  FIG. 4 . As shown, the patient at this stage has had electrode leads  18  and  20  implanted into her spinal cord  40  by the clinician. As is typical, the leads  18  and  20  are implanted on right and left sides of the spinal cord  40  to allow the IPG  10  flexibility to recruit and stimulate different nerves that may be causing the patient&#39;s pain or other symptoms. After implantation of the leads  18  and  20 , the proximal ends of the leads protrude through the patient&#39;s skin  25  via an opening  27 . 
     The patient is provided with an external trial stimulator (ETS)  70 , which is coupled to the implanted leads  18  and  20  via lead extensions  54  and  60 . These lead extensions  54  and  60  couple to the ETS  70  via connectors  52  and  58 , and couple to the leads  18  and  20  via lead acceptors  56  and  62 , which like lead connectors  28  ( FIG. 1 ) are designed to accept the proximal electrode contacts  29  of the leads  18  and  20  ( FIG. 1 ). As such, the lead extensions  54  and  60  contain at least the same number of wires as there are electrodes on each lead. The external trial simulator  70  can then be affixed to the patient in a convenient fashion for the duration of the external trial stimulation phase, which may last for two or so weeks for example. Typically, the external trial simulator  70  is placed into a belt to be worn by the patient. 
     The ETS  70  provides stimulation to the patient in much the same way as the IPG eventually will once it is implanted. Essentially, the ETS  70  mimics operation of the IPG, so that therapy can be tried and adjusted prior to actual implantation of the IPG. Therapy typically comprises a stimulation program (SP) specifying a number of stimulation parameters, such as which electrodes are selected for stimulation; whether such active electrodes are anodes or cathodes (c/ax); the magnitude of the stimulation (e.g., current; Ax) at the active electrodes; and the frequency (fx) and duration (dx) of stimulation at the active electrodes, assuming such stimulation comprises stimulation pulses as it typical. As shown in  FIG. 4 , the stimulation program is shown defining each of these stimulation parameters for each of the electrodes, but a stimulation program could be represented more simply. For example, if a particular electrode&#39;s amplitude is specified as Ax=0, then it will be understood that that electrode is not active, and that other parameters for that electrode (e.g., c/ax, fx, dx) are irrelevant or ignored. 
     The stimulation program used during the external trial stimulation phase can be modified wirelessly to try and determine effective therapy for the patient. As shown in  FIG. 4 , such control can be provided an external controller  110  which wirelessly  100  adjusts the stimulation program provided by the ETS  70  to the patient. The external controller  110  and the external trial stimulation  110  are thus both be provided with wireless telemetry means (e.g., antennas, transceiver circuitry) to facilitate such wireless communications  100 , which may take place using any suitable telemetry scheme and hardware. As shown, the external controller  110  may include a user interface for the clinician, including a display screen  82  and touchable buttons  84  which the clinician can use to modify the stimulation program and to transmit it to the ETS  70 . 
     The external controller  110  as shown is hand-held and portable, and thus is similar to external controllers used by patients to adjust their stimulation programs. See, e.g., U.S. Pat. No. 8,498,716; U.S. Patent Application Publication 2012/0101551. However, the external controller  110  can also take the form of a clinician&#39;s programmer of the type used by clinicians either in their offices or in an operating room environment. Generally speaking, the external controller  110  can comprise any suitable computer apparatus, such as a desk top computer, a lap top computer, a tablet computer, etc. As such, the user interface of the external controller  110  (e.g., display  82 ; buttons  84 ) can be separated from its computing and communication hardware, and thus the external controller  110  need not comprise an integrated device housing all necessary components as shown. Additionally, while it is convenient that the external controller  110  and ETS  70  communicate wirelessly  100 , they may also be physically linked by cabling. The ETS  70  usually contains a battery within its housing to provide the power necessary to implement the stimulation program and to provide the prescribed stimulation at the electrodes  16 . 
     If a suitable stimulation program relieving the patient&#39;s symptoms is determined during the external trial stimulation phase, this may suggest that implantation of an IPG is warranted. An IPG is thus implanted in the patient (typically in the patient&#39;s upper-buttock area), and the leads  18  and  20  are tunneled though the patient&#39;s tissue to connect the proximal electrode contacts  29  to the lead connectors  28  in the IPG&#39;s header  30 . Moreover, the stimulation program determined during the external trial stimulation phase can be wireless telemetered to the IPG (via telemetry coil  36 ;  FIGS. 2A and 2B ) after implantation, which should give the patient a basis for effective therapy, which can be wirelessly adjusted later by the patient using a patient external controller, as described above. 
     By contrast, if no suitable stimulation program is determined during the external trial stimulation phase, or if the patient experiences troubling side effects, this may suggest that neurostimulation will be ineffective for that patient. In this case, an IPG is not implanted, and the leads  18  and  20  are explanted. Alternatively, the implantation of the leads  18  and  20  may be adjusted by the clinician via further surgery, and the external trial stimulation period extended to see if a suitable stimulation program can then be determined. In short, external trial simulation allows the effectiveness of therapy to be vetted before subjecting the patient to the inconvenience of implantation of the IPG, and the possible need to explant the IPG if successful therapy cannot be achieved. 
     The inventors recognize that information gleaned during the external trial stimulation phase can assist a clinician in deciding whether a patient would best benefit from receiving an IPG  10   a  having a rechargeable battery ( FIG. 2A ), or an IPG  10   b  having a primary battery ( FIG. 2B ). 
       FIG. 5  shows a block diagram of the circuitry for an improved external controller  110  to assist the clinician in making this primary-battery versus rechargeable-battery determination. As shown, the external controller  110  includes a user interface  130 , such as the display  82  and buttons  84  mentioned earlier. Using the user interface  130 , the clinician can modify the stimulation program for a given patient. For example, as shown in  FIG. 6A , the clinician can select displayed options to either change specific stimulation parameters (electrode, polarity, amplitude, frequency, duration, etc.) for the stimulation program that is currently running, or can load another stimulation program previously stored in the external controller  110 . 
     Referring again to  FIG. 5 , the user interface  130  interfaces with a stimulation program module  140  to form, or retrieve from memory, the stimulation program resulting from these selections. Stimulation program module  140  can be associated with, or reside in, control circuitry  150  within the external controller  110 , which may comprise a microcontroller for example, or other assortments of logic blocks, including memory. Transceiver circuitry  160  wirelessly transmits the stimulation program SP 1  to the ETS  70  as described earlier. Alternatively, transceiver circuitry  160  need not be wireless and can send and/or transmit information to and from the ETS  70  via a wired connection, as also noted earlier. The ETS  70  then executes this stimulation program SP 1  to provide therapy to the patient, and when the patient reports good relief from symptoms, then this stimulation program SP 1  is assumed optimal, and would likely be stored within the external controller  110 . Storing optimal stimulation program SP 1  will be useful, as mentioned earlier, so that it can be wirelessly transmitted to the IPG  10   a  or  10   b  once it is eventually implanted in the patient. 
     After determining an optimal stimulation program SP 1  for the patient, the question remains whether the patient would best benefit from having a primary battery IPG  10   a  or a rechargeable battery IPG  10   b  implanted. To assist the clinician in making this determination, and referring again to  FIG. 6A , an option is provided to run a battery algorithm, which when selected (CMD;  FIG. 5 ) will present the clinician estimations regarding the power performance of both types of IPGs, as well as an optional automated recommendation as to which might be most appropriate for the patient. Battery algorithm may also run automatically when a stimulation program is changed for example, and without the need for selection at the user interface  130 . 
     When the clinician selects to run the battery algorithm (or it otherwise automatically operates), and referring again to  FIG. 5 , the battery algorithm  155  associated with the control circuitry  150  operates, which is also illustrated in flow chart form in  FIG. 7 . As a first step, the algorithm  155  receives the optimal stimulation program SP 1  ( FIG. 7, 200 ). A current draw module  165  then estimates the expected current draw I 1  resulting from that stimulation program, i.e., the current that the IPG would be expected to draw were it running the stimulation program SP 1  ( FIG. 7, 202 ). I 1  may also represent information indicative of the expected current draw, and could comprise an expected power draw (in Watts) for example. 
     To implement this function, current draw module  165  is programmed with information regarding the expected operation of the circuitry of the IPG to be implanted, which may be the same (apart from the battery) for IPGs  10   a  and  10   b,  and which also may be the same as the circuitry in the ETS  70  as noted earlier. Thus, current draw module  165  can determine I 1  based on the number of active electrodes, the amplitude (A) of the current at those electrodes, the duration (d) of the current pulses and their frequency (f)—i.e., the stimulation parameters of stimulation program SP 1 . Such information can be based on simulations of the circuitry in the IPG, or from experimental measurements taken from the circuitry of an example IPG. One skilled in the art will realize that how the current draw modules  165  determines I 1  can vary and can be based on a number of assumptions. For example, assume the current draw module  165  is programmed to understand that stimulation pulses with an amplitude A, a duration d, and a frequency f will draw 1 mA in the IPG. If the stimulation program SP 1  instead defines pulses with a combined amplitude of 0.6 A, duration 1.3 d, and frequency 0.7 f, the current draw module  165  can determine that I 1  would be 0.6*1.3*0.7*1 mA. One skilled will realize that there could be other ways for determining or estimating I 1  in current draw module  165 . 
     Once current draw I 1  is determined, the algorithm  155  then passes this value to a rechargeable battery module  170   a  and to a primary battery module  170   b.  These modules  170   a  and  170   b  are respectively programmed with information regarding the expected operation of battery  14   a  in rechargeable battery IPG  10   a  and battery  14   b  in primary battery IPG  10   b.  As the differences in batteries  14   a  and  14   b  warrant, modules  170   a  and  170   b  seek to determine different information for both types of IPGs relevant to the clinician&#39;s decision as to which should be implanted. 
     Rechargeable battery module  170   a  provides the clinician information regarding how often the rechargeable battery  14   a  in IPG  10   a  would need to be recharged using external charger  50  ( FIG. 2 ) assuming the IPG draws I 1  of current. More specifically, module  170   a  informs the clinician of the expected duration (D) of each charging session and the frequency (F) of such charging sessions. These parameters are relevant to consider because they impact the patient, who will ultimately be responsible for using the external charger  50  to charge the battery  14   a.    
     Determining charging duration D and frequency F can be based on simulation or experimental results regarding how efficiently or quickly the external charger  50  recharges battery  14   a.  For example, rechargeable battery module  170   a  can be programmed assuming a particular coupling or alignment between the external charger  50  and the IPG  10   a.  This assumption may comprise a “worst case” in which alignment between the external charger  50  and the IPG  10   a  is deemed suitable for charging the IPG  10   a &#39;s battery  14   a,  but is nonetheless not optimal. 
     Generally speaking, the frequency of charging (F) and the duration of charging (D) are inversely proportional, and both parameters are directly proportional to I 1 . As such, rechargeable battery module  170   a  can be programed with information of the relationship between frequency (F) and duration (D) at various current draws (I). The determined power draw I 1  can be used to “look up,” or interpolate, an appropriate relationship between F and D, as shown in the graph at the bottom left of  FIG. 5 . This graph shows the inverse relationship between F and D for the determined current draw I 1 , and further shows a window  182  of acceptable values for both F and D. Such acceptable values for window  182  are subject to designer preferences perhaps based on patient feedback; for example, a patient may not want to have to recharge their IPG  10   a  more than three times a week, and may desire that the duration of charging not be too short or too long. In the example shown, it is assumed that a patient would only desire to recharge IPG  10   a  between 1 and 3 times a week (i.e., 1/week&lt;F&lt;3/week), and that duration of charging should be between 10 and 30 minutes (i.e., 10 min&lt;D&lt;30 min). 
     With window  182  set in this manner, rechargeable battery module  170   a  thus seeks to determine logical values to report to the clinician regarding battery  14   a &#39;s expected recharging performance. Because charging is logically viewed by the patient as occurring a set number of times daily, it is generally desired to provide the clinician an integer value for the charging frequency F, which again could only comprise 1, 2, or 3 charging sessions per week in accordance with the constraints defined earlier for the window  182 . The rechargeable battery module  170   a  then assesses whether a charging duration D appears within window  182  at any of these values for F. In the example graph shown, only F 1 =2 provides a duration D 1  within the window  182 —specifically D 1 =13 minutes at F 1 =2 charging sessions per week—and thus such values comprise estimated battery performance parameters for rechargeable-battery IPG  10   a  (step  206 ;  FIG. 7 ). As such, this information will be reported to the clinician, as described in further detail later. 
     If more than one value for D appears in the window  182  for an acceptable value for F, then rechargeable battery module  170   a  may decide which would be the most appropriate to report to the clinician. As a default, and to provide an indication assumedly most convenience to the patient, the result having the lowest value for F (and its corresponding D) may be provided to the clinician. This is not strictly necessary, and instead the lowest value for D (and its corresponding F) could be provided instead. All values for F and D appearing within the window  182  could also be reported. 
     Primary battery module  170   b  by contrast is not concerned with recharging particulars, but is instead concerned with determining how long the primary battery  14   b  in IPG  10   b  can be expected to function given the estimated current draw I 1 . In this regard, the primary battery module  170   b  is programmed with the capacity (C) of the battery  14   b,  which is normally expressed in amp-hours. Knowing the capacity and the estimated current draw, the lifetime (LT 1 ) of the battery  14   b  can be estimated (LT 1 =C/I 1 ), which is shown graphically at the bottom right of  FIG. 5  (step  204 ;  FIG. 7 ). Alternatively, an index indicative of the lifetime of the battery  14   b  could also be determined and interpreted by the clinician. 
     Once these battery performance parameters (LT 1 , F 1 , D 1 ) have been determined by modules  170   a  and  170   b,  they can be provided to the clinician via the user interface  130  of the external controller  110  (step  208 ;  FIG. 7 ), as shown in  FIG. 6B . Modules  170   a  and  170   b,  like module  165 , are shown as part of the control circuitry  150  in the external controller  110 , and may comprise firmware or microcode in the control circuitry. However, this is not strictly necessary, and instead such modules can be stored in memory circuitry communicating with control circuitry  150 . 
     As also shown in  FIG. 6B , the battery algorithm  155  can optionally act further to provide a recommendation  156  to the clinician as to which IPG  10   a  or  10   b  would be best for the patient given the reported battery performance parameters. Recommendation  156  can be automatically generated in accordance with logic programmed into the algorithm  155 , and one simple, non-limiting example is shown in  FIG. 7 . In the example shown, algorithm  155  first determines whether the estimated lifetime LT 1  of the primary battery  14   b  meets a minimum time threshold, such as two years, which threshold might be chosen as a minimum acceptable time interval for explant of a depleted primary-battery IPG  10   b  and re-implantation of fresh IPG  10   b  (step  210 ). If this threshold is exceeded, the battery performance parameters of rechargeable-battery IPG  10   a  (F 1 , D 1 ) are assessed to see if any are within the window  182  (step  212 ) defined earlier. If not, a rechargeable-battery IPG  10   a  may not be acceptable as it would be inconvenient for the patient to charge, and hence the primary-battery IPG  10   b  (whose lifetime LT 1  is minimally acceptable) is recommended (step  214 ). If F 1  and D 1  are in the window  182 , suggesting that the rechargeable-battery IPG  10   a  is also acceptable, the lifetime of the primary-battery IPG  10   b  is again assessed to see if its lifetime exceeds a maximum time threshold, such as 5 years (step  218 ). This threshold might be chosen as a lifetime so significant that the primary-battery IPG  10   b  would necessarily be recommended (step  214 ), even if the rechargeable-battery IPG  10   a  might also be acceptable. By contrast, if the maximum lifetime threshold for the primary-battery IPG  10   b  is not exceeded (step  218 ), the rechargeable-battery IPG  10   a  may be chosen (step  204 ), which as well as being acceptable may be expected to last for well over five years. 
     Returning to step  210 , if the estimated lifetime LT 1  does not exceed the minimum time threshold, the battery performance parameters of rechargeable-battery IPG  10   a  (F 1 , D 1 ) are assessed to see if any are within the window  182  (step  216 ). If so, the rechargeable-battery IPG  10   a  may be chosen (step  204 ). If not, this may indicate that neither of the IPGs  10   a  or  10   b  are acceptable, perhaps because the estimated current draw I 1  is so significant that a primary-battery IPG  10   b  would be depleted too quickly, and that a rechargeable-battery IPG  10   a  would require an inordinate amount of recharging. Hence, the clinician may be notified of this fact (step  220 ), or may alternatively be suggested to consider a different IPG, perhaps one having a larger capacity primary or rechargeable battery. 
     The automated recommendation steps in the battery algorithm  155  could also include assessment of different factors, such as the life expectancy or age of the patient, which might play into or modify whether certain of the battery performance parameters are acceptable or can be tolerated. 
       FIG. 8  shows a modification to the above technique in which battery performance parameters are determined based on information sent or telemetered from the ETS  70 . In this alternative, the expected current draw I 1  is not estimated at the external controller  70  based on a stimulation program SP 1  as occurred earlier. Instead, the expected current draw I 1  is measured in the ETS  70  as it executes stimulation program SP 1 . This is a useful means for determining the current draw I 1  in the to-be-implanted IPG for a given stimulation program SP 1 , especially if the circuitry in the ETS is made to match the circuitry in IPGs  10   a  or  10   b,  as is often the case. 
     This can involve minor modification to the circuitry of the ETS  70 , which normally comprises a battery  250  and the load  270  that it powers. Load  270  includes all of the circuitry within the ETS  70 , including the biasing circuitry for driving currents to the electrodes pursuant to SP 1  (not shown), control circuitry  260 , and transceiver  295  for wirelessly communicating with the external controller  110 , all of which can again be the same as what is present in IPGs  10   a  and  10   b.  Added to the ETS  70  is circuitry designed to measure the current draw I 1  in ETS  70 , including switches  280 , a measuring resistor R, and a differential amplifier  290 . Other current draw measuring circuitry could be used as well; this is merely one example. 
     When the clinician selects to run the battery algorithm  155  ( FIG. 6A ), control circuitry  150  in the external controller  110  generates a command (CMD) which is wirelessly broadcast to the ETS  70 , which in turn measures the current drawn I 1 . This occurs in the depicted example by controlling switches  280   a  and  280   b.  During normal operation of the ETS  70  (i.e., when CMD hasn&#39;t been received at the ETS  70 ), switch  280   a  is closed and switch  280   b  is opened, and hence the ETS battery  250  can deliver the current draw I 1  to the load  270  so that it is powered normally. When command CMD is received, switch  280   b  is closed and switch  280   a  is opened, which places resistor R in line with the current draw I 1 . This causes a small voltage (V) to drop across resistor R, which is detected by the differential amplifier  290 . Because the value of R is known, the current draw I 1  can be determined (V/R) by control circuitry  260 , and telemetered back to the external condoler  110 . Note that the current draw in the ETS  70  (and the eventual IPG  10   a  or  10   b  that replaces it) may not be constant, particularly if the ETS is issuing pulses. As such, it may be necessary to measure the voltage drop over a significant time period, such as several pulse periods, to get an accurate or averaged understanding of the current draw, I 1 . 
     Once received by the external controller  110 , current draw I 1  can be processed by algorithm  155  as before to determine the battery performance parameters (LT 1 , F 1 , D 1 ), to display them to the clinician, and to recommend a proper IPG. Note in this alternative of the technique that current draw module  165  ( FIG. 5 ) can be omitted, because current draw I 1  is measured directly, and need not be inferred from the stimulation program SP 1 . 
     To this point in the disclosure it has been assumed that the disclosed technique has been used to assist the clinician in choosing for a given patient whether a rechargeable  10   a  or primary  10   b  IPG would be best for implantation. Further, it has been assumed that the operative circuitry in these IPGs  10   a  and  10   b  are the same (other than differences relating to their batteries  14  and  14   b ). However, the technique is not so limited, as depicted in  FIGS. 9A and 9B . These figures illustrate a modification to the battery algorithm  155 ′ in the external controller  110  in which more than two IPGs are selectable as possible candidates for implantation. 
     In  FIG. 9A , it is assumed that the clinician can choose from three different rechargeable-battery IPGs ( 10   a _ 1 ,  10   a _ 2 ,  10   a _ 3 ) each differing only in the capacity of their batteries ( 14   a _ 1 ,  14   a _ 2 ,  14   a _ 3 ), and three different primary-battery IPGs ( 10   b _ 1 ,  10   b _ 2 , and  10   b _ 3 ) each differing again only in the capacity of their batteries ( 14   b _ 1 ,  14   b _ 2 ,  14   b _ 3 ). It is further assumed that all IPGs otherwise have the same circuitry aside from their batteries, and that the IPGs with higher index numbers have higher battery capacities (e.g., battery  14   a _ 2  has a higher capacity than battery  14   a _ 1 , etc.). 
     In this example, current draw I 1  is again determined, and is sent to battery modules corresponding to each of the selectable IPGs. This yields additional battery performance parameters values (Fx, Dx, LTx) as shown. 
     Once these battery performance parameters are determined, the algorithm  155 ′ can make a recommendation  156 ′ of which IPG would be best suited for the patient, as shown in the flow chart of  FIG. 9B . (Earlier steps in algorithm  155 ′ should be clear from earlier discussions and aren&#39;t again discussed). The depicted automatic process for generating recommendation  156 ′ in  FIG. 9B  is quite simple for example purposes, but could of course be much more complicated given the greater number of battery performance parameters it considers. Step  300  simply determines the lowest-capacity primary battery IPG  10   b _ x  having an acceptable estimated lifetime LTx of greater than two years. This decision is based on the premise that a patient need not receive an IPG  10   b _ x  of a greater capacity than is needed; this should reduce IPG cost and further may benefit the patient who may receive the smallest IPG  10   b _ x  suitable for his needs. This minimum-capacity IPG  10   b _ x  is chosen, and the others which might have too low or too high a capacity are excluded from the recommendation. Similar step  302  essentially does the same assessment for the rechargeable-battery IPGs  10   a _ x , determining the lowest-capacity rechargeable battery IPG  10   a _ x  having acceptable power parameters Fx and Dx in the window  182  described earlier. This minimum-capacity IPG  10   a _ x  is chosen, and the others which might have too low or too high a capacity are excluded from the recommendation. Then a decision is made as to which of the two remaining IPGs  10   a _ x  and  10   b _ x  should be chosen, which in the illustrated example simply turns on whether the estimated lifetime of the primary-battery IPG  10   b _ x  is sufficiently long (&gt;5 yrs; step  304 ) to warrant choosing that implant (step  306 ), and if not choosing the rechargeable-battery IPG  10   a _ x  (step  308 ). 
     Algorithm  155 ′ may consider other factors when providing a recommendation  156 ′. For example, algorithm  155 ′ may not simply force a choice between the most logical of the rechargeable and primary-battery IPGs. Instead, other decisions may cause all of one type of IPG (e.g., all primary-battery IPGs  10   b _ x ) to be excluded from the recommendation, with the final choice for recommendation occurring between two of the rechargeable-battery IPGs  10   a _ x.    
     Note the battery algorithm  155 ′ can operate and be modified by wirelessly transmitting current draw I 1  from the ETS  70 , similar to what was described with reference to  FIG. 8 . 
       FIG. 10  shows another modification to the battery algorithm  155 ″, which like algorithm  155 ′ allows the clinician to choose from three different rechargeable-battery IPGs ( 10   a _ 1 ,  10   a _ 2 ,  10   a _ 3 ) and three different primary-battery IPGs ( 10   b _ 1 ,  10   b _ 2 , and  10   b _ 3 ). However, in this example, it is not assumed that the various IPGs have the same circuitry, nor is it assumed that the circuitry in the ETS  70  is the same as in any of the IPGs. In effect, algorithm  155 ″ allows the clinician to generically assess and choose between any number of different IPGs available for implantation regardless of how different they might be and regardless of how different they may be from the ETS  70 . 
     Because the circuitries in each of the IPGs may be different, a single current draw module  165  can&#39;t be used to predict a single current draw I 1  applicable to all of the IPGs for a given stimulation program, as their unique circuitries may draw different amounts of current when executing that program. Instead, and as shown, the stimulation program SP 1  is input to current draw modules  165   a _ x  and  165   b _ x  specifically programmed to estimate the current draw Ia_x or Ib_x of its corresponding IPG. Because the circuitry in each IPG may be different, the simulated or experimental data programmed into each current draw module  165  may be different. 
     For those IPGs having rechargeable batteries, the resulting Ia_x is input to a corresponding rechargeable battery module  170   a _ x , which as before is programmed with information regarding the expected operation of rechargeable batteries  14   a _ x  for each rechargeable battery IPG  10   a _ x , which again may include assumption about how efficiently such batteries can be recharged by the external chargers. Each rechargeable battery module  170   a _ x  outputs battery performance parameters Fx and Dx. 
     For the IPGs having primary batteries, the resulting Ib_x is input to a corresponding primary battery module  170   b _ x , which as before is programmed with information regarding the capacity of primary batteries  14   b _ x  for each IPG  10   b _ x . Each primary battery module  170   b _ x  outputs battery performance parameters LTx. 
     Once battery performance parameters Fx, Dx, and LTx are determined, they can be further considered by algorithm  155 ″ to provide a recommendation  156 ′, which may occur using the same steps of algorithm  155 ′ discussed earlier ( FIG. 9B ). Thus, with the external controller  110  properly programmed, algorithm  155 ″ allows the clinician to select the best IPG for the patient even though these IPGs may be different in their circuitries, their battery types (rechargeable versus primary), and their battery capacities. 
     While the external controller  110  and ETS  70  have been shown as separate, one skilled will understand that they could be combined into a single unit. That is, the ETS  70  could have its own user interface and be programmed with any of the disclosed battery algorithms to function as shown to assist a clinician. Moreover, one skilled will realize that while it is sensible that the disclosed technique operate on the external controller  110  normally used to communicate with the ETS  70 , this is not strictly necessary. Instead, the external device used to run the battery algorithm and to display the output of that algorithm need not be the same device used to send stimulation programs to the ETS  70 , so long as the external device running the battery algorithm is somehow otherwise made aware of necessary information from the external controller or the ETS. 
     Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.