Abstract:
A method and apparatus for determining estimated remaining longevity for an implantable stimulator. The device employs pre-calculated numbers of days for various combinations conditions of device usage parameters to determine remaining device longevity based upon identified actual conditions of device usage and employs the determined longevity to change longevity indicator states in the device. While between longevity state changes, the device the identified conditions of device usage and adjusts the determined longevity if the conditions of use change significantly. The indicator states may correspond to one or more of Recommended Replacement Time (RRT), Elective Replacement Indicator (ERI) or End of Service (EOS).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/912,216, filed on Dec. 5, 2013. The disclosure of the above application is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to implantable medical devices and, more particularly, to implantable medical devices. 
       BACKGROUND 
       [0003]    A variety of medical devices for delivering a therapy and/or monitoring a physiological condition have been used clinically or proposed for clinical use in patients. Examples include medical devices that deliver therapy to and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissue. Some therapies include the delivery of electrical signals, e.g., stimulation, to such organs or tissues. Some medical devices may employ one or more elongated electrical leads carrying electrodes for the delivery of therapeutic electrical signals to such organs or tissues, electrodes for sensing intrinsic electrical signals within the patient, which may be generated by such organs or tissue, and/or other sensors for sensing physiological parameters of a patient. Some medical devices may be “leadless” and include one or more electrodes on an outer housing of the medical device to deliver therapeutic electrical signals to organs or tissues and/or sense intrinsic electrical signals or physiological parameters of a patient. 
         [0004]    Medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of therapeutic electrical signals or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to a medical device housing, which may contain circuitry such as signal generation and/or sensing circuitry. In some cases, the medical leads and the medical device housing are implantable within the patient, while in other cases percutaneous leads may be implanted and connected to a medical device housing outside of the patient. Medical devices with a housing configured for implantation within the patient may be referred to as implantable medical devices. Leadless medical devices are typically implantable medical devices positioned within or adjacent to organs or tissues within a patient for delivery of therapeutic electrical signals or sensing. In some example, leadless implantable medical devices may be anchored to a wall of an organ or to tissue via a fixation mechanism. 
         [0005]    Implantable cardiac pacemakers or cardioverter-defibrillators, for example, provide therapeutic electrical signals to the heart, e.g., via electrodes carried by one or more medical leads or via electrodes on an outer housing of a leadless implantable medical device. The therapeutic electrical signals may include pulses for pacing, or shocks for cardioversion or defibrillation. In some cases, a medical device may sense intrinsic depolarizations of the heart, and control delivery of therapeutic signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate therapeutic electrical signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing stimulation to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting fibrillation. 
         [0006]    In general, implantable medical devices require a small housing form factor to enable an unobtrusive implantation within a patient. In the case of leadless implantable medical devices, the housing form factor must be extremely small to enable implantation within or adjacent to organs or tissue. For example, a leadless pacemaker may be implanted directly into a ventricle of the heart. Battery usage is always a concern when designing implantable medical devices, but this concern is increased for small form factor devices that can only accommodate a small battery canister. 
       SUMMARY 
       [0007]    The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
         [0008]    In traditional IPGs and ICDs, as a battery nears the end of its usable life, warnings are posted (RRT, ERI, EOS) and changes are made to a device&#39;s state (for example, at ERI the pacing mode and rate are changed to VVI 65 bpm). There are CENELAC specified minimum times between some of the state changes. Because of this, a combination of battery voltage/impedance trip points and a number of days at a state are used to advance these states. In some battery chemistries, the trip points occur on “plateaus” in the battery curve, making voltage/impedance an unreliable way to advance states. In this case, the number of days counters are the primary means of changing longevity states, with battery voltages as a backup only. 
         [0009]    For leadless medical devices such as pacemakers, the amount of longevity is changed dramatically by the amount of pacing and the amplitude of pacing. For this reason, the present invention employs a dynamic mechanism to calculate the number of days between longevity states. The present invention employs pre-calculated numbers of days for various combinations of device use (for example, the device use the pacing amplitude and pulse width, then calculate the pacing percentage, and “look up” how many days of longevity are expected between given longevity states). In addition, while between states, the device monitors to verify that the device was still being used in the same way, and adjust the number of days remaining if the use changes significantly. 
         [0010]    The invention employs pre-defined combinations of device use and remaining longevity to change longevity states in the device based on the device determined assessment of current device use and pre-calculated remaining longevity estimates. 
         [0011]    Device battery life is extremely variable based on things like Pacing impedance, Pacing Amplitude, Pacing Pulse Width, Pacing Rate and Pacing Percentage. In the case of some devices, the battery is such that several of the longevity state switches occur during a battery voltage plateau. Changing longevity states at the right time is critical to providing the CENELAC required number of days between longevity states while also maximizing the device longevity. The invention is particularly beneficial in this context. 
         [0012]    However, having the device constantly calculate exactly how much longevity is remaining is a burdensome level of complication. The present invention provides a balanced approach by having the device use device known parameters and a externally determined expected number of days between states for given conditions as a “look up table”. This method in some cases can result in nearly half a year of additional longevity versus the standard method of longevity state change 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a diagram illustrating an example therapy system comprising a leadless implantable medical device (IMD) that may be used to monitor one or more physiological parameters of a patient and/or provide therapy to the heart of a patient. 
           [0014]      FIG. 2  is a diagram illustrating another example therapy system comprising an IMD coupled to a plurality of leads that may be used to monitor one or more physiological parameters of a patient and/or provide therapy to the heart of a patient. 
           [0015]      FIG. 3  illustrates the IMD of  FIG. 1  in more detail 
           [0016]      FIG. 4  illustrates the IMD of  FIG. 2  in more detail 
           [0017]      FIG. 5  is a functional block diagram illustrating an example configuration of an IMD. 
           [0018]      FIG. 6  is a block diagram of an example external programmer that facilitates user communication with an IMD. 
           [0019]      FIG. 7  is a block diagram illustrating an example system that includes an external device, such as a server, and one or more computing devices that are coupled to an IMD and programmer via a network. 
           [0020]    The inter-relation of defined events related to the invention is illustrated in  FIG. 8 . 
           [0021]    The times of occurrences of these defined events relative to an exemplary battery discharge curve is illustrated in  FIG. 9   
           [0022]      FIG. 10  is a table illustrating an exemplary mechanism by which the present invention calculates estimated remaining battery life (Remaining Longevity Duration Value) in days. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  is a diagram illustrating an exemplary therapy system  10 A that may be used to monitor one or more physiological parameters of patient  14  and/or to provide therapy to heart  12  of patient  14 . Therapy system  10 A includes an implantable medical device (IMD)  16 A, which is coupled to programmer  24 . IMD  16 A may be an implantable leadless pacemaker that provides electrical signals to heart  12  via one or more electrodes (not shown in  FIG. 1 ) on its outer housing. Additionally or alternatively, IMD  16 A may sense electrical signals attendant to the depolarization and repolarization of heart  12  via electrodes on its outer housing. In some examples, IMD  16 A provides pacing pulses to heart  12  based on the electrical signals sensed within heart  12 . Patient  14  is ordinarily, but not necessarily, a human patient. 
         [0024]    In the example of  FIG. 1 , IMD  16 A is positioned wholly within heart  12  with one end proximate to the apex of right ventricle  28  to provide right ventricular (RV) pacing. Although IMD  16 A is shown within heart  12  and proximate to the apex of right ventricle  28  in the example of  FIG. 1 , IMD  16 A may be positioned at any other location outside or within heart  12 . For example, IMD  16 A may be positioned outside or within right atrium  26 , left atrium  36 , and/or left ventricle  32 , e.g., to provide right atrial, left atrial, and left ventricular pacing, respectively. Depending in the location of implant, IMD  16 A may include other stimulation functionalities. For example, IMD  16 A may provide atrioventricular nodal stimulation, fat pad stimulation, vagal stimulation, or other types of neurostimulation. In other examples, IMD  16 A may be a monitor that senses one or more parameters of heart  12  and may not provide any stimulation functionality. In some examples, system  10 A may include a plurality of leadless IMDs  16 A, e.g., to provide stimulation and/or sensing at a variety of locations. 
         [0025]      FIG. 1  further depicts programmer  24  in communication with IMD  16 A. In some examples, programmer  24  comprises a handheld computing device, computer workstation, or networked computing device. Programmer  24  includes a user interface that presents information to and receives input from a user. It should be noted that the user may also interact with programmer  24  remotely via a networked computing device. 
         [0026]    A user, such as a physician, technician, surgeon, electrophysiologist, other clinician, or patient, interacts with programmer  24  to communicate with IMD  16 A. For example, the user may interact with programmer  24  to retrieve physiological or diagnostic information from IMD  16 A. A user may also interact with programmer  24  to program IMD  16 A, e.g., select values for operational parameters of the IMD  16 A. For example, the user may use programmer  24  to retrieve information from IMD  16 A regarding the rhythm of heart  12 , trends therein over time, or arrhythmic episodes. 
         [0027]    As another example, the user may use programmer  24  to retrieve information from IMD  16 A regarding other sensed physiological parameters of patient  14  or information derived from sensed physiological parameters, such intracardiac or intravascular pressure, activity, posture, respiration, tissue perfusion, heart sounds, cardiac electrogram (EGM), intracardiac impedance, or thoracic impedance. In some examples, the user may use programmer  24  to retrieve information from IMD  16 A regarding the performance or integrity of IMD  16 A or other components of system  10 A, or a power source of IMD  16 A. As another example, the user may interact with programmer  24  to program, e.g., select parameters for, therapies provided by IMD  16 A, such as pacing and, optionally, neurostimulation. 
         [0028]    IMD  16 A and programmer  24  may communicate via wireless communication using any techniques known in the art. Examples of communication techniques may include, for example, low frequency or radiofrequency (RF) telemetry, but other techniques are also contemplated. In some examples, programmer  24  may include a programming head that may be placed proximate to the patient&#39;s body near the IMD  16 A implant site in order to improve the quality or security of communication between IMD  16 A and programmer  24 . 
         [0029]      FIG. 2  is a diagram illustrating another exemplary therapy system  10 B that may be used to monitor one or more physiological parameters of patient  14  and/or to provide therapy to heart  12  of patient  14 . Therapy system  10 B includes IMD  16 B, which is coupled to leads  18 ,  20 , and  22 , and programmer  24 . In one example, IMD  16 B may be an implantable pacemaker that provides electrical signals to heart  12  via electrodes coupled to one or more of leads  18 ,  20 , and  22 . In addition to pacing therapy, IMD  16 B may deliver neurostimulation signals. In some examples, IMD  16 B may also include cardioversion and/or defibrillation functionalities. In other examples, IMD  16 B may not provide any stimulation functionalities and, instead, may be a dedicated monitoring device. Patient  14  is ordinarily, but not necessarily, a human patient. 
         [0030]    Leads  18 ,  20 ,  22  extend into the heart  12  of patient  14  to sense electrical activity of heart  12  and/or deliver electrical stimulation to heart  12 . In the example shown in  FIG. 2 , right ventricular (RV) lead  18  extends through one or more veins (not shown), the superior vena cava (not shown), right atrium  26 , and into right ventricle  28 . RV lead  18  may be used to deliver RV pacing to heart  12 . Left ventricular (LV) lead  20  extends through one or more veins, the vena cava, right atrium  26 , and into the coronary sinus  30  to a region adjacent to the free wall of left ventricle  32  of heart  12 . LV lead  20  may be used to deliver LV pacing to heart  12 . Right atrial (RA) lead  22  extends through one or more veins and the vena cava, and into the right atrium  26  of heart  12 . RA lead  22  may be used to deliver RA pacing to heart  12 . 
         [0031]    In some examples, system  10 B may additionally or alternatively include one or more leads or lead segments (not shown in  FIG. 2 ) that deploy one or more electrodes within the vena cava or other vein, or within or near the aorta. Furthermore, in another example, system  10 B may additionally or alternatively include one or more additional intravenous or extravascular leads or lead segments that deploy one or more electrodes epicardially, e.g., near an epicardial fat pad, or proximate to the vagus nerve. In other examples, system  10 B need not include one of ventricular leads  18  and  20 . 
         [0032]    IMD  16 B may sense electrical signals attendant to the depolarization and repolarization of heart  12  via electrodes (described in further detail with respect to  FIG. 4 ) coupled to at least one of the leads  18 ,  20 ,  22 . In some examples, IMD  16 B provides pacing pulses to heart  12  based on the electrical signals sensed within heart  12 . The configurations of electrodes used by IMD  16 B for sensing and pacing may be unipolar or bipolar. 
         [0033]    IMD  16 B may also provide neurostimulation therapy, defibrillation therapy and/or cardioversion therapy via electrodes located on at least one of the leads  18 ,  20 ,  22 . For example, IMD  16 B may deliver defibrillation therapy to heart  12  in the form of electrical pulses upon detecting ventricular fibrillation of ventricles  28  and  32 . In some examples, IMD  16 B may be programmed to deliver a progression of therapies, e.g., pulses with increasing energy levels, until a fibrillation of heart  12  is stopped. As another example, IMD  16 B may deliver cardioversion or ATP in response to detecting ventricular tachycardia, such as tachycardia of ventricles  28  and  32 . 
         [0034]    As described above with respect to IMD  16 A of  FIG. 1 , programmer  24  may also be used to communicate with IMD  16 B. In addition to the functions described with respect to IMD  16 A of  FIG. 1 , a user may use programmer  24  to retrieve information from IMD  16 B regarding the performance or integrity of leads  18 ,  20  and  22  and may interact with programmer  24  to program, e.g., select parameters for, any additional therapies provided by IMD  16 B, such as cardioversion and/or defibrillation. 
         [0035]    In addition to the functions described with respect to IMD  16 A of  FIG. 1 , a user may use programmer  24  to retrieve information from IMD  16 B regarding the performance or integrity of leads  18 ,  20  and  22  and may interact with programmer  24  to program, e.g., select parameters for, any additional therapies provided by IMD  16 B, such as cardioversion and/or defibrillation. 
         [0036]      FIG. 3  is a diagram illustrating leadless IMD  16  of  FIG. 1  in further detail. In the example of  FIG. 3 , leadless IMD  16 A includes fixation mechanism  70 . Fixation mechanism  70  may anchor leadless IMD  16 A to a wall of heart  12 . For example, fixation mechanism  70  may take the form of multiple tines that may be inserted into a wall of heart  12  to fix leadless IMD  16 A at the apex of right ventricle  28 . Alternatively, other structures of fixation mechanism  70 , e.g., adhesive, sutures, or screws may be utilized. In some examples, fixation mechanism is conductive and may be used as an electrode, e.g., to deliver therapeutic electrical signals to heart  12  and/or sense intrinsic depolarizations of heart  12 . 
         [0037]    Leadless IMD  16 A may also include electrodes  72  and  74  at a tip of outer housing  78 . Electrodes  72  and  74  may be used to deliver therapeutic electrical signals to heart  12  and/or sense intrinsic depolarizations of heart  12 . Electrodes  72  and  74  may be formed integrally with an outer surface of hermetically-sealed housing  78  of IMD  16 A or otherwise coupled to housing  78 . In this manner, electrodes  72  and  74  may be referred to as housing electrodes. In some examples, housing electrodes  72  and  74  are defined by uninsulated portions of an outward facing portion of housing  78  of IMD  16 A. Other division between insulated and uninsulated portions of housing  78  may be employed to define a different number or configuration of housing electrodes. For example, in an alternative configuration, IMD  16 A may include a single housing electrode that comprises substantially all of housing  78 , and may be used in combination with an electrode formed by fixation mechanism  70  for sensing and/or delivery of therapy. 
         [0038]      FIG. 4  is a diagram illustrating IMD  16 B and leads  18 ,  20 ,  22  of therapy system  10 B of  FIG. 2  in greater detail. Leads  18 ,  20 ,  22  may be electrically coupled to a signal generator and a sensing module of IMD  16 B via connector block  34 . In some examples, proximal ends of leads  18 ,  20 ,  22  may include electrical contacts that electrically couple to respective electrical contacts within connector block  34  of IMD  16 B. In some examples, a single connector, e.g., an IS-4 or DF-4 connector, may connect multiple electrical contacts to connector block  34 . In addition, in some examples, leads  18 ,  20 ,  22  may be mechanically coupled to connector block  34  with the aid of set screws, connection pins, snap connectors, or another suitable mechanical coupling mechanism. 
         [0039]    Each of the leads  18 ,  20 ,  22  includes an elongated insulative lead body, which may carry a number of concentric coiled conductors separated from one another by tubular insulative sheaths. Bipolar electrodes  40  and  42  are located adjacent to a distal end of lead  18  in right ventricle  28 . In addition, bipolar electrodes  44  and  46  are located adjacent to a distal end of lead  20  in left ventricle  32  and bipolar electrodes  48  and  50  are located adjacent to a distal end of lead  22  in right atrium  26 . In the illustrated example, there are no electrodes located in left atrium  36 . However, other examples may include electrodes in left atrium  36 . 
         [0040]    Electrodes  40 ,  44 , and  48  may take the form of ring electrodes, and electrodes  42 ,  46 , and  50  may take the form of extendable helix tip electrodes mounted retractably within insulative electrode heads  52 ,  54 , and  56 , respectively. In some examples, one or more of electrodes  42 ,  46 , and  50  may take the form of pre-exposed helix tip electrodes. In other examples, one or more of electrodes  42 ,  46 , and  50  may take the form of small circular electrodes at the tip of a tined lead or other fixation element. Leads  18 ,  20 ,  22  also include elongated electrodes  62 ,  64 ,  66 , respectively, which may take the form of a coil. Each of the electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  62 ,  64 , and  66  may be electrically coupled to a respective one of the coiled conductors within the lead body of its associated lead  18 ,  20 ,  22 , and thereby coupled to respective ones of the electrical contacts on the proximal end of leads  18 ,  20 ,  22 . 
         [0041]    In some examples, as illustrated in  FIG. 4 , IMD  16 B includes one or more housing electrodes, such as housing electrode  58 , which may be formed integrally with an outer surface of hermetically-sealed housing  60  of IMD  16 B or otherwise coupled to housing  60 . In some examples, housing electrode  58  is defined by an uninsulated portion of an outward facing portion of housing  60  of IMD  16 B. Other division between insulated and uninsulated portions of housing  60  may be employed to define two or more housing electrodes. In some examples, housing electrode  58  comprises substantially all of housing  60 . 
         [0042]    IMD  16 B may sense electrical signals attendant to the depolarization and repolarization of heart  12  via electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  58 ,  62 ,  64 , and  66 . The electrical signals are conducted to IMD  16 B from the electrodes via conductors within the respective leads  18 ,  20 ,  22  or, in the case of housing electrode  58 , a conductor coupled to housing electrode  58 . IMD  16 B may sense such electrical signals via any bipolar combination of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  58 ,  62 ,  64 , and  66 . Furthermore, any of the electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  58 ,  62 ,  64 , and  66  may be used for unipolar sensing in combination with housing electrode  58 . 
         [0043]    In some examples, IMD  16 B delivers pacing pulses via bipolar combinations of electrodes  40 ,  42 ,  44 ,  46 ,  48  and  50  to produce depolarization of cardiac tissue of heart  12 . In some examples, IMD  16 B delivers pacing pulses via any of electrodes  40 ,  42 ,  44 ,  46 ,  48  and  50  in combination with housing electrode  58  in a unipolar configuration. 
         [0044]    Furthermore, IMD  16 B may deliver defibrillation pulses to heart  12  via any combination of elongated electrodes  62 ,  64 ,  66 , and housing electrode  58 . Electrodes  58 ,  62 ,  64 ,  66  may also be used to deliver cardioversion pulses to heart  12 . Electrodes  62 ,  64 ,  66  may be fabricated from any suitable electrically conductive material, such as, but not limited to, platinum, platinum alloy or other materials known to be usable in implantable defibrillation electrodes. 
         [0045]    The configuration of the systems illustrated in  FIGS. 1-4  are merely exemplary. In other examples, a system may include percutaneous leads, epicardial leads and/or patch electrodes instead of or in addition to the transvenous leads  18  and  22  illustrated in  FIG. 2 . Further, the IMD need not be implanted within patient  14 . In examples in which the IMD is not implanted in a patient, the IMD may deliver defibrillation pulses and other therapies to heart  12  via percutaneous leads that extend through the skin of patient  14  to a variety of positions within or outside of heart  12 . 
         [0046]    In addition, in other examples, a system may include any suitable number of leads coupled to IMD  16 B, and each of the leads may extend to any location within or proximate to heart  12 . For example, other examples of systems may include three transvenous leads located as illustrated in  FIGS. 2 and 4 , and an additional lead located within or proximate to left atrium  36 . Other examples of systems may include a single lead that extends from IMD  16 B into right atrium  26  or right ventricle  28 , or two leads that extend into a respective one of the right ventricle  26  and right atrium  26 . Any electrodes located on these additional leads may be used in sensing and/or stimulation configurations. 
         [0047]      FIG. 5  is a functional block diagram illustrating an example configuration of IMD  16 , which may be IMD  16 A of  FIGS. 1 and 3  or IMD  16 B of  FIGS. 2 and 4 . In the example illustrated by  FIG. 4 , IMD  16  includes a processor  80 , memory  82 , signal generator  84 , electrical sensing module  86 , telemetry module  88 , system clock  90 , reference clock  92 , clock calibrator  94 A, and power source  98 . Memory  82  may include computer-readable instructions that, when executed by processor  80 , cause IMD  16  and processor  80  to perform various functions attributed to IMD  16  and processor  80  herein. Memory  82  may comprise a computer-readable storage medium, including any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital or analog storage media. 
         [0048]    Processor  80  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. In some examples, processor  80  may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to processor  80  in this disclosure may be embodied as software, firmware, hardware or any combination thereof. IMD  16  also includes a sensing integrity module  90 , as illustrated in  FIG. 6 , which may be implemented by processor  80 , e.g., as a hardware component of processor  80 , or a software component executed by processor  80 . 
         [0049]    In the disclosed embodiments, the operation of the device according to the invention is accomplished by the processor  80  as defined by instructions stored in memory  82 . For purposes of the disclosed invention, the instruction set may correspond to the required sequence of operations as set forth in Exhibit A, attached hereto. 
         [0050]    Processor  80  controls signal generator  84  to deliver stimulation therapy to heart  12  according to operational parameters or programs, which may be stored in memory  82 . For example, processor  80  may control signal generator  84  to deliver electrical pulses with the amplitudes, pulse widths, frequency, or electrode polarities specified by the selected one or more therapy programs. 
         [0051]    Signal generator  84 , as well as electrical sensing module  86 , is electrically coupled to electrodes of IMD  16  and/or leads coupled to IMD  16 . In the example of leadless IMD  16 A of  FIG. 3 , signal generator  84  and electrical sensing module  86  are coupled to electrodes  72  and  74 , e.g., via conductors disposed within housing  78  of leadless IMD  16 A. In examples in which fixation mechanism  70  functions as an electrode, signal generator  84  and electrical sensing module  86  may also be coupled to fixation mechanism  70 , e.g., via a conductor disposed within housing  78  of leadless IMD  16 A. In the example of IMD  16 B of  FIG. 2 , signal generator  84  and electrical sensing module  86  are coupled to electrodes  40 ,  42 ,  48 ,  50 ,  56  and  62  via conductors of the respective lead  18  or  22 , or, in the case of housing electrode  58 , via an electrical conductor disposed within housing  60  of IMD  16 B. 
         [0052]    In the example illustrated in  FIG. 4 , signal generator  84  is configured to generate and deliver electrical stimulation therapy to heart  12 . For example, signal generator  84  may deliver pacing, cardioversion, defibrillation, and/or neurostimulation therapy via at least a subset of the available electrodes. In some examples, signal generator  84  delivers one or more of these types of stimulation in the form of electrical pulses. In other examples, signal generator  84  may deliver one or more of these types of stimulation in the form of other signals, such as sine waves, square waves, or other substantially continuous time signals. 
         [0053]    Signal generator  84  may include a switch module and processor  80  may use the switch module to select, e.g., via a data/address bus, which of the available electrodes are used to deliver stimulation signals, e.g., pacing, cardioversion, defibrillation, and/or neurostimulation signals. The switch module may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple a signal to selected electrodes. 
         [0054]    Electrical sensing module  86  monitors signals from at least a subset of the available electrodes in order to monitor electrical activity of heart  12 . Electrical sensing module  86  may also include a switch module to select which of the available electrodes are used to sense the heart activity. In some examples, processor  80  may select the electrodes that function as sense electrodes, i.e., select the sensing configuration, via the switch module within electrical sensing module  86 , e.g., by providing signals via a data/address bus. 
         [0055]    In some examples, electrical sensing module  86  includes multiple detection channels, each of which may comprise an amplifier. Each sensing channel may detect electrical activity in respective chambers of heart  12 , and may be configured to detect either R-waves or P-waves. In some examples, electrical sensing module  86  or processor  80  may include an analog-to-digital converter for digitizing the signal received from a sensing channel for electrogram (EGM) signal processing by processor  80 . In response to the signals from processor  80 , the switch module within electrical sensing module  86  may couple the outputs from the selected electrodes to one of the detection channels or the analog-to-digital converter. 
         [0056]    During pacing, escape interval counters maintained by processor  80  may be reset upon sensing of R-waves and P-waves with respective detection channels of electrical sensing module  86 . Signal generator  84  may include pacer output circuits that are coupled, e.g., selectively by a switching module, to any combination of the available electrodes appropriate for delivery of a bipolar or unipolar pacing pulse to one or more of the chambers of heart  12 . Processor  80  may control signal generator  84  to deliver a pacing pulse to a chamber upon expiration of an escape interval. Processor  80  may reset the escape interval counters upon the generation of pacing pulses by signal generator  84 , or detection of an intrinsic depolarization in a chamber, and thereby control the basic timing of cardiac pacing functions. The escape interval counters may include P-P, V-V, RV-LV, A-V, A-RV, or A-LV interval counters, as examples. The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used by processor  80  to measure the durations of R-R intervals, P-P intervals, P-R intervals and R-P intervals. Processor  80  may use the count in the interval counters to detect heart rate, such as an atrial rate or ventricular rate. 
         [0057]    The processor  80  also stores records of the following values in memory: a) cumulative lifetime brady pace counter; b) cumulative lifetime brady sense counter; c) programmed ventricular amplitude; and d) programmed ventricular pulse width. These values are used to determine the estimated remaining life of the pacemaker as described below. 
         [0058]    Telemetry module  88  includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as programmer  24  ( FIGS. 1 and 2 ). Under the control of processor  80 , telemetry module  88  may receive downlink telemetry from and send uplink telemetry to programmer  24  with the aid of an antenna, which may be internal and/or external. Processor  80  may provide the data to be uplinked to programmer  24  and receive downlinked data from programmer  24  via an address/data bus. In some examples, telemetry module  88  may provide received data to processor  80  via a multiplexer. 
         [0059]    The clocking system of IMD  16  includes system clock  90 , reference clock  92 , and clock calibrator  94 A. Each of the clocks described herein comprise oscillators that may operate at different frequencies with different accuracies and different power requirements. IMD  16  may require an extremely small housing form factor, especially in the case of leadless IMD  16 A of  FIGS. 1 and 3 . For example, leadless IMD  16  may have a form factor of less than 1 cubic centimeter. Due to the small form factor requirements, IMD  16  may only be able to accommodate a small battery canister such that current drain within IMD  16  must by extremely low. One aspect of reducing power in IMD  16  is to minimize current drain by the clocking system. 
         [0060]    A detailed description of the use of the clocking system to reduce power consumption is set forth in US Patent Publication No. US 20120109259 A1 incorporated herein by reference in its entirety 
         [0061]      FIG. 6  is a functional block diagram of an example configuration of programmer  24 . As shown in  FIG. 12 , programmer  24  includes processor  140 , memory  142 , user interface  144 , telemetry module  146 , and power source  148 . Programmer  24  may be a dedicated hardware device with dedicated software for programming of IMD  16 . Alternatively, programmer  24  may be an off-the-shelf computing device running an application that enables programmer  24  to program IMD  16 . In other examples, programmer  24  may be used to program IMD  16  of  FIG. 7  in a substantially similar manner as IMD  16  of  FIG. 6 . 
         [0062]    A user may use programmer  24  to select therapy programs (e.g., sets of stimulation parameters), generate new therapy programs, or modify therapy programs for IMD  16 . The clinician may interact with programmer  24  via user interface  144 , which may include a display to present a graphical user interface to a user, and a keypad or another mechanism for receiving input from a user. 
         [0063]    Processor  140  can take the form one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, and the functions attributed to processor  140  in this disclosure may be embodied as hardware, firmware, software or any combination thereof. Memory  142  may store instructions and information that cause processor  140  to provide the functionality ascribed to programmer  24  in this disclosure. Memory  142  may include any fixed or removable magnetic, optical, or electrical media, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM, or the like. Memory  142  may also include a removable memory portion that may be used to provide memory updates or increases in memory capacities. A removable memory may also allow patient data to be easily transferred to another computing device, or to be removed before programmer  24  is used to program therapy for another patient. Memory  142  may also store information that controls therapy delivery by IMD  16 , such as stimulation parameter values. 
         [0064]    Programmer  24  may communicate wirelessly with IMD  16 , such as using RF communication or proximal inductive interaction. This wireless communication is possible through the use of telemetry module  146 , which may be coupled to an internal antenna or an external antenna. An external antenna that is coupled to programmer  24  may correspond to the programming head that may be placed over heart  12 , as described above with reference to  FIG. 1 . Telemetry module  146  may be similar to telemetry module  88  of IMD  16  ( FIG. 6 ). 
         [0065]    Telemetry module  146  may also be configured to communicate with another computing device via wireless communication techniques, or direct communication through a wired connection. Examples of local wireless communication techniques that may be employed to facilitate communication between programmer  24  and another computing device include RF communication according to the 802.11 or Bluetooth specification sets, infrared communication, e.g., according to the IrDA standard, or other standard or proprietary telemetry protocols. In this manner, other external devices may be capable of communicating with programmer  24  without needing to establish a secure wireless connection. An additional computing device in communication with programmer  24  may be a networked device such as a server capable of processing information retrieved from IMD  16 . 
         [0066]      FIG. 7  is a block diagram illustrating an example system that includes an external device, such as a server  204 , and one or more computing devices  210 A- 210 N, that are coupled to the IMD  16  and programmer  24  (shown in  FIGS. 1 and 2 ) via a network  202 . In other examples, the system of  FIG. 13  may include IMD  116  of  FIG. 7  in a substantially similar manner as IMD  16  of  FIG. 6 . 
         [0067]    In this example, IMD  16  may use its telemetry module  88  to communicate with programmer  24  via a first wireless connection, and to communication with an access point  200  via a second wireless connection. In the example of  FIG. 13 , access point  200 , programmer  24 , server  204 , and computing devices  210 A- 210 N are interconnected, and able to communicate with each other, through network  202 . In some cases, one or more of access point  200 , programmer  24 , server  204 , and computing devices  210 A- 210 N may be coupled to network  202  through one or more wireless connections. IMD  16 , programmer  24 , server  204 , and computing devices  210 A- 210 N may each comprise one or more processors, such as one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, that may perform various functions and operations, such as those described herein. 
         [0068]    Access point  200  may comprise a device that connects to network  202  via any of a variety of connections, such as telephone dial-up, digital subscriber line (DSL), or cable modem connections. In other examples, access point  200  may be coupled to network  202  through different forms of connections, including wired or wireless connections. In some examples, access point  200  may be co-located with patient  14  and may comprise one or more programming units and/or computing devices (e.g., one or more monitoring units) that may perform various functions and operations described herein. For example, access point  200  may include a home-monitoring unit that is co-located with patient  14  and that may monitor the activity of IMD  16 . In some examples, server  204  or computing devices  210  may control or perform any of the various functions or operations described herein. 
         [0069]    In some cases, server  204  may be configured to provide a secure storage site for data that has been collected from IMD  16  and/or programmer  24 . Network  202  may comprise a local area network, wide area network, or global network, such as the Internet. In some cases, programmer  24  or server  206  may assemble data in web pages or other documents for viewing by trained professionals, such as clinicians, via viewing terminals associated with computing devices  210 A- 210 N. The illustrated system of  FIG. 13  may be implemented, in some aspects, with general network technology and functionality similar to that provided by the Medtronic CareLink® Network developed by Medtronic, Inc., of Minneapolis, Minn. 
         [0070]    In one or more examples, the functions described above may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media may include computer data storage media or communication media including any medium that facilitates transfer of a computer program from one place to another. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
         [0071]    The code may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
         [0072]    For purposes of understanding the invention the following definitions will be helpful: 
         [0073]    BOS—Beginning of Service 
         [0074]    When an individual IPG is first released by the manufacturer as fit for placing on the market. 
         [0075]    EOS—End of Service 
         [0076]    When the Prolonged Service Period (PSP) has elapsed and performance to design specifications cannot be assured. 
         [0077]    PSP—Prolonged Service Period 
         [0078]    Period beyond the Recommended Replacement Time (RRT) during which the IPG continues to function as defined by the manufacturer to prolong basic bradyarrhythmia pacing. 
         [0079]    PSL—Projected Service Life 
         [0080]    Period from the implantation of the IPG to the Recommended Replacement Time (RRT) under defined conditions. 
         [0081]    RRT—Recommended Replacement Time 
         [0082]    Time when the power source indicator reaches the value set by the manufacturer of the IPG for its recommended replacement. This indicates entry into the Prolonged Service Period (PSP). 
         [0083]    ERI—Elective Replacement Indicator 
         [0084]    ERI is not a CENELEC definition. It is a secondary indicator which is intended to inform the user that there is less than 90 days of device service remaining. 
         [0085]    Pre-RRT—Pre-Recommended Replacement Time 
         [0086]    Pre-RRT is not a CENELEC definition, and it is not shown to the user. It is an indicator that the battery voltage is transitioning from the first plateau to the second plateau. 
         [0087]    The occurrences of these events are stored in memory  82  and may be communicated to the user by means of telemetry to an associated device such as the programmer of  FIG. 6  as described above. Occurrence of these events may also be communicated remotely using the system of  FIG. 7  as described above 
         [0088]    The inter-relation of the above-defined defined events is illustrated in  FIG. 8 . 
         [0089]    The times of occurrences of these defined events relative to an exemplary battery discharge curve are illustrated in  FIG. 9 . 
         [0090]      FIG. 10  sets forth an exemplary look-up table relating pacing pulse with, pacing pulse programmed amplitude and pacing percentage to estimated battery life (Remaining Longevity Duration Value) in days. 
         [0091]    This look-up table will of course be different for each device in each type of device in which the invention is employed, depending on the current drain, battery capacity, battery chemistry, etc. of each device type. 
         [0092]    As an additional option, the static current drain for each individual device of a given type could be measured at production and the corresponding look up table for that device could be individually calculated and then written it to flash memory in the device. This would allow the longevity for each individual device to be maximized based on its own unique current drain. 
         [0093]    Additionally, while the look-up table of  FIG. 10  is based upon amplitude, pulse width and percentage of pacing, in alternative embodiments, additional factors such as pacing impedance could also be employed to provide a more accurate prediction of battery life. 
         [0094]    Operation of the device generally according to the invention is according to the following set of rules as set forth below. These rules may be embodied as a corresponding instruction set stored in memory  82 , executed by processor  80 . 
         [0095]    RRT Indicator 
         [0096]    The device records the time/date that RRT occurred. Once a number of consecutive daily battery measurements are at or below a Pre-RRT voltage trip threshold as discussed below, a configurable delay timer is started as indicated at “A” below. The RRT indicator is set once that timer expires or the battery voltage become less than or equal to a RRT voltage trip threshold for a programmable number of consecutive days. 
         [0097]    More specifically, the device sets the RRT indicator and records the Real Time Clock as a RRT Battery Voltage Detected Timestamp the first time either of the following conditions occurs: 
         [0098]    A. Remaining Longevity Duration determined as discussed below minus the number of days since Pre-RRT Battery Voltage was detected is &lt;=180 days. 
         [0099]    B. Three (3) consecutive daily Battery Voltage Measurements are less than or equal to Low Battery Voltage RRT Threshold where Holter Mode was not active during any of the 3 days. 
         [0100]    Pre-RRT Indicator 
         [0101]    The first time three (3) consecutive daily Battery Voltage Measurements are less than or equal to a Pre-RRT voltage trip threshold where Holter Mode was not active during any of the 3 days, the device sets a Pre-RRT Battery Voltage Detected flag and records the Real Time Clock in a Pre-RRT Battery Voltage Detected Timestamp. 
         [0102]    ERI Indicator 
         [0103]    The device sets the ERI indicator and record the Real Time Clock in ERI Battery Voltage Detected Timestamp the first time either of the following conditions occurs: 
         [0104]    A. Remaining Longevity Duration calculated as discussed below minus the number of days since Pre-RRT Battery Voltage was detected is &lt;=90 days. 
         [0105]    B. Three (3) consecutive daily battery voltage measurements are less than or equal to Low Battery Voltage ERI Threshold where Holter Mode was not active during any of the 3 days. 
         [0106]    EOS Indicator 
         [0107]    The device sets the EOS indicator and record the real time clock as an EOS Battery Voltage Detected Timestamp under either of the following conditions: 
         [0108]    A. &gt;=120 days have elapsed since the device set the ERI indicator and a POR occurs or 
         [0109]    B. Three consecutive battery voltage measurements are less than or equal to Low Battery Voltage EOS Threshold where holter mode was not active during any of the three days. 
         [0110]    As a result, the RRT, ERI and EOS indicators as discussed above may all be triggered responsive to a determined number of days elapsing since the Pre-RRT voltage was detected. This determined number of days since Pre-RRT was detected is based upon the Remaining Longevity Duration as initially calculated and then re-calculated as discussed below. 
         [0111]    Remaining Longevity Duration—Initial Value 
         [0112]    Responsive to Pre-RRT Battery Voltage being detected as above, the device: 
         [0113]    A. Determines the pacing percentage (cumulative lifetime brady pace counter/(cumulative lifetime brady pace counter+cumulative lifetime brady sense counter)); and 
         [0114]    B. Uses the pacing percentage, Programmed Ventricular Amplitude and Programmed Ventricular Pulse Width to set the Remaining Longevity Duration in days from Pre-RRT per  FIG. 10  (Remaining Longevity Duration Value) 
         [0115]    Remaining Longevity Duration Recalculation 
         [0116]    After Pre-RRT detected as above and before ERI has been reached, each day at midnight (prior to starting any Temporary Operation scheduled to start at midnight), if: 
         [0117]    A. The Telemetry State is Disconnect, AND 
         [0118]    B. The Emergency VVI Timer is not active, AND 
         [0119]    C. No Temporary Operation is in progress, 
         [0120]    THEN the device: 
         [0121]    A. Determines the pacing percentage (cumulative lifetime brady pace counter/(cumulative lifetime brady pace counter+cumulative lifetime brady sense counter)), AND 
         [0122]    B. Uses the pacing percentage, Programmed Ventricular Amplitude and Programmed Ventricular Pulse Width to determine the Potential Remaining Longevity Duration per  FIG. 10  in days from Pre-RRT, AND 
         [0123]    C. Resets Remaining Longevity Duration to the MIN(Potential Remaining Longevity Duration, previous Remaining Longevity Duration). 
         [0124]    Recalculation of Remaining Device Longevity therefore can only maintain or reduce amount of time remaining before triggering of the RRT/ERI/EOS indicators. In other words, once established, Remaining Longevity Duration may be only be reduced and this reduction means that the number of days between Pre-RRT and RRT/ERI/EOS is also reduced). Therefore, it is possible that once a change in Remaining Longevity Duration is identified, the number of days since Pre-RRT may already be larger than the newly changed Remaining Longevity Duration. In this case, RRT will be triggered immediately. 
         [0125]    Mode Change at ERI 
         [0126]    When ERI is detected, if the device is in a pacing mode, the device will change to standard ERI settings of WI pacing at 65 bpm. 
         [0127]    The external instrument (programmer) may re-program the parameters after the device has changed to the ERI values. 
         [0128]    ERI Parameter Change 
         [0129]    When ERI is detected and the programmed Brady Pacing Mode is not OOO or OVO, the device shall change the brady parameter values as follows: 
         [0130]    A. The programmed Brady Pacing Mode shall be set to VVI 
         [0131]    B. The programmed Brady Lower Rate shall be set to 65 BPM 
         [0132]    C. The programmed Brady Hysteresis Enable shall be set to OFF. Hysteresis is disabled to ensure that paces are delivered at 65 BPM 
         [0133]    Various examples of the disclosure have been described. These and other examples are within the scope of the following claims. 
         [0134]    For example, while the disclosed embodiment takes the form of a pacemaker, the invention is readily applicable to other stimulator types including implantable neuro-stimulators, and implantable cardioverters or in other implantable devices wherein battery drain may be variable over time as conditions of device use change. Further, while the invention is especially useful in the context of leadless implantable devices, it is correspondingly beneficial in the context of devices including one or more leads as discussed above.