Abstract:
Methods for estimating a remaining service life of an implantable medical device (IMD) battery employ calculations using a characteristic discharge model of the battery; the calculations require measurements of battery voltage and time. Systems employing the methods may include an external device coupled to the IMD, for example, via a telemetry communications link, wherein a first portion of a computer readable medium included in the IMD is programmed to provide instructions for the measurement, or tracking, of time and the measurement of battery voltage, and a second portion of the computer readable medium included in the external device is programmed to provide instructions for carrying out the calculations when the voltage and time data is transferred via telemetry from the IMD to the external device.

Description:
TECHNICAL FIELD 
       [0001]    The present invention pertains to implantable medical devices (IMDs) and more particularly to systems and methods for estimating the remaining service life of an IMD battery. 
       BACKGROUND 
       [0002]    A number of commercially available programmable IMDs, for example, cardiac pacemakers and defibrillators, electrical signal monitors, hemodynamic monitors, nerve and muscle stimulators and infusion pumps, include electronic circuitry and a battery to energize the circuitry for the delivery of therapy and/or for taking physiological measurements for diagnostic purposes. It is common practice to monitor battery life within an IMD so that a patient in whom the IMD is implanted should not suffer the termination of therapy, and or diagnostic benefit, from that IMD when the IMD battery runs down. Several methods for deriving estimates of remaining battery life, which employ monitoring schemes that require periodic measurements of battery voltage and either, or both of, battery impedance and current drain, have been described in the art, for example, in commonly assigned U.S. Pat. No. 6,671,552. Although the previously described methods can provide fairly accurate estimates of remaining battery life, there is still a need for methods that employ simplified monitoring schemes in which fewer measurements are taken. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    The following drawings are illustrative of particular embodiments of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements. 
           [0004]      FIG. 1  is a schematic of an exemplary system in which embodiments of the present invention may be employed. 
           [0005]      FIG. 2  is a block diagram of an exemplary system architecture. 
           [0006]      FIG. 3  is a representation of an exemplary hybrid cathode discharge model, which is plotted as battery voltage versus depth of discharge for various current drains, according to exemplary embodiments of the present invention. 
           [0007]      FIG. 4  is an equation defining the discharge model, from which the plots of  FIG. 2  may be derived. 
           [0008]      FIG. 5  is a flow chart outlining some methods of the present invention. 
           [0009]      FIG. 6  is a chart including an exemplary array of times defining remaining battery service life. 
           [0010]      FIG. 7  is a plot depicting an accuracy of exemplary longevity predictions made according to some methods of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides practical illustrations for implementing exemplary embodiments of the present invention. 
         [0012]      FIG. 1  is a schematic of an exemplary system in which embodiments of the present invention may be employed.  FIG. 1  illustrates an IMD  12  and an endocardial lead  14  implanted within a patient  10 ; lead  14  electrically couples IMD  12  to a heart  18  of patient  10  in order that therapy, for example, pacing pulses, may be delivered from IMD  12  to heart  18 .  FIG. 2  is a block diagram of an exemplary system architecture of IMD  12  for initiating and controlling pacing therapy delivery, for processing physiological signals sensed by lead  14 , and for initiating and tracking device-related measurements. The exemplary system is described in greater detail in the aforementioned commonly assigned U.S. Pat. No. 6,671,552, salient portions of which are hereby incorporated by reference. It should be noted that the scope of the present invention is not limited to the type of therapy delivered; for example, IMD  12  may be implanted in a different location than that shown in  FIG. 1  and/or may include additional or alternate components for providing additional or alternate therapies, for example, an infusion pump for delivery of therapeutic agents, and/or a capacitor and associated high voltage circuitry for delivery of defibrillation pulses. Furthermore, embodiments of the present invention may be employed by systems including IMDs that only function as monitors, for example, electrocardiography and hemodynamic monitors. 
         [0013]      FIG. 2  illustrates IMD  12  including a battery  136  coupled to power supply circuitry  126  for powering the operation of IMD  12 ; circuitry  126  is also shown controlled by a microcomputer-based system  102  to measure battery voltage and return a value for each measured voltage. In addition to providing control and timing for the function of IMD  12 , system  102  includes means for storing sensed physiologic parameters as well as device specific data. According to embodiments of the present invention, system  102  is pre-programmed to measure battery voltage at particular points in time after an initial measurement is made when IMD  12  is implanted in patient  10 . Time from implant is tracked by IMD  12 , for example, by a piezoelectric crystal  132  coupled to a system clock  122 , according to the illustrated embodiment, so that each battery voltage measurement is stored with an associated time. Those skilled in the art will understand that each point in time may be a range of seconds in duration, for example, up to approximately 10 seconds, in which case each associated voltage measurement is actually an average over the range of seconds. 
         [0014]      FIGS. 1 and 2  further illustrate IMD  12  including a telemetry antenna  28  coupled to telemetry circuitry  124 , which is controlled by system  102  and receives and transmits data therefrom and thereto. Antenna  28  may be coupled by a telemetry communications link to an external telemetry antenna  24  of an external device  26 , to facilitate uplink and downlink data transmissions  20 ,  22  between IMD  12  and external device  26 , which may be activated by closure of a magnetic switch  130  by an external magnet  116 . It should be noted that other communication interfaces may be incorporated. External device  26  may perform as both a monitor and programmer for IMD  12 , or just as a monitor. Telemetry transmission schemes and associated components/circuitry for systems including IMDs are well known to those skilled in the art. 
         [0015]    According to preferred embodiments of the present invention, at the time of implant and at subsequent check-ups, a clinician uplinks each stored battery voltage measurement and its associated time of measurement, via telemetry, to external device  26 , which includes pre-programmed instructions for using the voltage and time data in performing iterative calculations to determine an estimated time of remaining service life of battery  136 . Alternately, system  102  may be pre-programmed with the instructions to perform the calculations and determine the estimated remaining service life, which estimated remaining life may be uplinked to external device  26  for display. Methods of the present invention for determining the estimated remaining battery service life rely upon a known characteristic discharge model for the battery, in conjunction with tracked time since implant, and will be described in greater detail below. 
         [0016]      FIG. 3  is a representation of an exemplary hybrid cathode discharge model, which is plotted as battery voltage versus depth of discharge for various current drains, according to exemplary embodiments of the present invention; and  FIG. 4  is an equation defining the discharge model from which the plots of  FIG. 3  may be derived. According to exemplary embodiments of the present invention, battery  136  is a Li/CF x -CSVO battery having a lithium anode, a cathode comprising approximately 27% by wt. CSVO, approximately 63% by wt. CF x , approximately 7% by wt. PTFE, and approximately 3% by wt. carbon black, and an electrolyte of 1 M LiBF 4  in a blend of approximately 60 vol % gamma-butyrolactone and approximately 40 vol % of 1,2 dimethoxyethane. With reference to  FIGS. 3 and 4  it may be appreciated that, according to the model, battery voltage (mV in  FIG. 4  to indicate units of millivolts) is a function of utilization, or depth of discharge (DOD in  FIG. 3  and % U in  FIG. 4 ) and current drain, which is expressed in micro amps (μA) in  FIG. 3 , and as average current density, j (current divided by cathode area, which denoted as “A” in the exemplary code presented below), in the equation of  FIG. 4 . The model was empirically derived according to discharge data (voltage, millivolts, versus capacity, milliamp hours, for average current drains of 10, 20, 40, 80, 160, 320 and 640 μA) collected from the discharge testing of a group of hybrid cathode battery cells having the exemplary chemistry defined above. The model, being composed of a continuous function that is the sum of four sigmoids and an inverse linear function, defines mean performance over a range of current densities between approximately 2 μA/cm 2  and approximately 120 μA/cm 2 , and is valid for 8:1 hybrid cathode medium-rate design batteries which include cathodes having a thickness of approximately 0.2635 cm. The remaining values for a&#39;s, b&#39;s, c&#39;s and d&#39;s in the equation of  FIG. 4  are constants describing a linear dependence on the natural log (ln) of current density, j, wherein ‘s’ and ‘i’ stand for slope and intercept, respectively. which according to the exemplary battery described above, the constants have the following values: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 a1i = 1539.638808 
                 c2s = −0.327193718 
               
               
                   
                 a1s = 96.51332057 
                 a3i = 579.5959788 
               
               
                   
                 b1i = 263.2151899 
                 a3s = −68.2329044 
               
               
                   
                 b1s = 45.95491553 
                 b3i = 111.2942791 
               
               
                   
                 c1i = 99.79527187 
                 b3s = −8.397220729 
               
               
                   
                 c1s = −0.763492632 
                 c3i = −17.4660755 
               
               
                   
                 d1i = −0.80075693 
                 c3s = 0.371829129 
               
               
                   
                 d1s = −0.147524143 
                 a4i = 513.8243731 
               
               
                   
                 a2i = 178.5774773 
                 a4s = −105.4823468 
               
               
                   
                 a2s = −16.76898322 
                 b4i = 137.4776252 
               
               
                   
                 b2i = 91.57887975 
                 b4s = −10.57044628 
               
               
                   
                 b2s = −2.012539503 
                 c4i = −34.14648953 
               
               
                   
                 c2i = −0.877895093 
                 c4s = 8.214314006 
               
               
                   
                 a5i = 0.005599606 
                 b5i = 0.006570709 
               
               
                   
                 a5s = −0.00058946 
                 b5s = 0.0000958809 
               
               
                   
                   
               
             
          
         
       
     
         [0017]    The depth of discharge (DOD) is defined as discharged capacity, ΔQ, divided by the initial capacity, Q max  of the battery (multiplied by 100 for a percentage), and a simplified expression of battery voltage is as follows: 
         [0000]        V=f (Δ Q,I ), 
         [0000]    wherein I is current drain; an average current drain may be expressed as: 
         [0000]    
       
      
       I 
       avg 
       =ΔQ/Δt,  
      
     
         [0000]    wherein Δt is elapsed time. Thus, it may be appreciated that, given an initial current drain of the battery, prior to commencement of battery service at implant, given the initial capacity of the battery, and given a measured battery voltage at tracked points in time, during battery service, iterative calculations of battery voltage at each tracked point in time, per the equation shown in  FIG. 4 , may be performed, wherein an estimated average current drain (evolved at each subsequent point in time from the initial current drain) is incremented until the calculated voltage converges on the measured voltage at each tracked point in time. With reference to the plot of  FIG. 3 , given the time of a particular voltage measurement, there is a single DOD value, for a given average current drain, that will yield the measured battery voltage. The following is a Visual Basic code of a “root-finder” algorithm, which includes the above described iterative calculation, for carrying out methods of the present invention: 
         [0000]                                Function DOD3(V As Double, dt As Double, DODlast As Double,       llast As Double) As Double       Dim lest As Double, lmax As Double, lmin As Double, Vcalc As Double,       dQest As Double       Dim DODest As Double       Qmax = 1327       A = 4.522       lest = llast + 0.000001       lmax = 0.09       lmin = 0.005       Qlast = DODlast * Qmax / 100       n = 0       Do        n = n + 1        dQest = lest * dt        DODest = 100 * (Qlast + dQest) / Qmax        Vcalc = mV(lest * 1000 / A, DODest) / 1000        If Vcalc &gt; V Then         lmin = lest         lest = 0.5 * (lmax + lest)        Else         lmax = lest         lest = 0.5 * (lmin + lest)        End If       Loop Until ((Abs(Vcalc − V) &lt; 0.0001) Or       ((lmax − lest) &lt; 0.0001 * lmax) Or (n = 1000))       If n = 100 Then        DOD3 = DODlast        Else        DOD3 = DODest       End If       End Function                    
The above algorithm uses the bisection method, but those skilled in the art will appreciate that alternate “root finder” algorithms, for example, using Newton&#39;s method or the secant method, may be employed by embodiments of the present invention.
 
         [0018]      FIG. 5  is a flow chart outlining some methods of the present invention. Steps  402 ,  404 ,  406 ,  408 ,  410  and  412  of  FIG. 5  correspond to the exemplary algorithm detailed in the above code, wherein iterative calculations are performed by incrementing an estimated average current drain (lest), per step  412 , and estimating a corresponding DOD (DODest), per step  404 , until a difference between the calculated battery voltage (Vcalc), per step  406 , and the measured battery voltage (V), per step  401 , is small enough (i.e. less than 0.0001 volt, per the above code) to affirm that Vcalc is converged on V at step  410 . At each subsequent point in time, represented by step  422 , when a voltage measurement is taken, per step  401 , the iterative calculation starts with the incremented estimate of average current drain that corresponds to the converged calculated voltage at the preceding point in time (llast). Although not detailed in the chart, the above code instructs that llast be initially incremented by 0.000001 milliamp (0.001 μA) for the start of each iterative calculation. Thus, each iterative calculation initially uses the final incremented estimated average current drain from the previous iterative calculation. Battery voltage measurements for iterative calculations may be individual measurements scheduled at any time increment, or, preferably averages of measurements taken over intervals, either consistent or variable, ranging from approximately two weeks to approximately 10 weeks. Individual voltage measurements may constitute a daily average of multiple measurements, for example, eight measurements, over a day. As previously described, the battery voltage measurements may be stored in IMD  12  ( FIGS. 1-2 ) until a time of a scheduled patient check up, when a telemetry link is established to uplink the voltage measurements and associated points in time to external device  26  where the iterative calculation is performed for each point in time. 
         [0019]    According to alternate methods of the present invention, a discharge model, for example, the equation shown in  FIG. 4 , may be re-arranged to define current as a function of voltage and time, so that the above described iterative calculations are not required, and a DOD may be estimated based on average current drain calculated directly from measured voltage the corresponding elapsed amount of time. Furthermore, it should be noted that for a battery chemistry impacted by temperature variation and in an application wherein temperature varies, a temperature-corrected discharge model may be employed and temperature measured in addition to voltage. 
         [0020]      FIG. 5  further illustrates step  420  in which a remaining service life, which corresponds to the last estimated DOD, is determined. The remaining service life, according to preferred embodiments of the present invention, is defined as the time remaining before a start of a period of time known as the recommended replacement time (RRT); the RRT provides a safety factor to assure that the battery will not become completely depleted (100% DOD) prior to the patient and/or clinician receiving a signal or warning that the battery life is nearing an end, sometimes called an end of life (EOL) indicator. According to some embodiments of the present invention, a DOD of less than 100% and greater than approximately 85% corresponds to a time when an EOL indicator is provided, for example via an audible signal emitted, for example, from a transducer  128  of IMD  12 , shown in  FIG. 2  or via a report generated by external device  26  during a telemetry session between IMD  12  and external device  26 . 
         [0021]      FIG. 6  is a chart including an exemplary array of times, in units of months, remaining before the start of the RRT for each DOD listed along the left hand side of the array. The times, otherwise known as longevity predictions, were derived using the discharge model equation of  FIG. 4 , wherein voltage was calculated at 0.5% increments of DOD, for each of the current drains listed across the top of the array. The times, or longevity predictions, associated with each current drain and the increments of DOD included in the chart, were calculated from the discharge model using a battery voltage of approximately 2.6 volts for the start of RRT; referring back to  FIG. 3 , it can be seen that 2.6 volts approximately corresponds with the increasingly rapid decline in battery voltage toward the end of the life of the battery, where the start of RRT is preferably defined. It should be noted that the discharge curves of  FIG. 3  are for the exemplary battery chemistry, previously defined, and any voltage value corresponding to a relatively steep part of the discharge curve near the end of life could be selected. Because of sources of variability associated with deriving these longevity predictions, the predictions are given in terms of minimum and maximum values, which correspond to 5% and 95% confidence limits, respectively, for example, calculated via Monte Carlo simulations using normal distributions of cathode mass and battery cell voltage, and using a uniform distribution for error in voltage readings. According to certain embodiments of the present invention, a chart including an array, similar to that illustrated in  FIG. 6 , is programmed, preferably into external device  26 , along with instructions for determining the remaining battery service life, i.e. time to RRT. By referencing the array with the last incremented estimated current drain (step  412  of  FIG. 5 ) and the last estimated DOD (step  404  of  FIG. 5 ), which resulted in a converged calculated voltage (step  410  of  FIG. 5 ), and using interpolation, if necessary, the time to RRT may be determined to be within the corresponding range defined by the chart. 
         [0022]      FIG. 7  is a plot depicting an accuracy of exemplary battery longevity predictions made according to some methods of the present invention. Values of predicted months, determined via the methods described herein, versus actual measured months to the start of RRT (battery voltage of 2.6 volts at start of RRT) are plotted for two life test battery samples, SN  3 , SN  11  and SN  6 . The samples were discharged on a constant 86.6 ohm load so that the current drain declined as the battery voltage declined. Although future current drain may change, the methods incorporate an assumption that the most recent estimated average current drain will continue into the future. However, with reference to  FIG. 7 , it may be appreciated that the predictions are generally conservative, estimating a fewer number of months to the start of RRT, and that the predictions become more accurate as the battery comes closer to complete depletion (100% DOD), where the slope of the characteristic discharge curves ( FIG. 3 ) becomes steeper. 
         [0023]    In the foregoing detailed description, the invention has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims. For example, although examples have been provided herein for a particular battery type and associated cathode discharge model, it should be recognized that systems and methods of the present invention may be employed for any battery type for which voltage can be modeled as a function of current drain and DOD.