Abstract:
An implantable cardiac device including a depletable power source, having an initial energy capacity, and method provide precise recommended replacement time and end of life indications for the depletable power source. The recommended replacement times and end of life times are based upon the actual remaining energy of the depletable power source. The recommended replacement time and end of life time are stored in memory for later transmission by a telemetry circuit to a nonimplantable receiver for read out or display.

Description:
FIELD OF THE INVENTION 
     The present invention is generally directed to an implantable cardiac device which is powered by a depletable power source. The present invention is more particularly directed to such a device which provides precise recommended replacement time and absolute replacement time (sometimes referred to as “end-of-life”) indications based upon actual remaining energy capacity of the depletable power source. 
     BACKGROUND OF THE INVENTION 
     Implantable cardiac devices are well known in the art. Such devices include implantable monitors which monitor heart activity over extended periods of time for diagnostic purposes, implantable pacemakers which both monitor heart activity and apply stimulation pulses to a heart when required to maintain normal sinus rhythm, and implantable cardioverter-defibrillators which monitor heart activity for tachyarrhythmias and apply shocking stimulation pulses to a heart to return the heart to normal sinus rhythm. Implantable cardioverter-defibrillators also commonly include pacemaker functionality. 
     Since implantable cardiac devices are implanted beneath the skin of a patient, they are powered by a depletable power source, such as a battery. When the remaining battery energy capacity falls below a certain lower limit corresponding to an absolute replacement time, sometimes referred to as “end-of-life” (EOL), the device must be replaced. Further, prior to EOL, as for example 90 days prior to EOL, the battery will reach a remaining energy capacity corresponding to the recommended replacement time (RRT). An RRT indication is generally provided to alert the patient&#39;s physician that EOL is imminent and is timed relative to EOL to afford the physician sufficient time to schedule replacement of the device before EOL is reached. 
     Most present RRT and EOL indicators utilize battery voltage and/or impedance and their correlation to the remaining battery capacity to provide information as to when RRT and EOL will be reached. Unfortunately, the tolerances in the battery voltage measurements as well as battery impedance have too wide a variance to provide an adequate time margin for replacement after an indication of RRT. Many implantable devices are thus explanted based upon these inadequate measurements. These indicators generally start providing remaining battery capacity information only during the final 20 percent of battery life because significant changes in these factors do not occur during the initial 80 percent of the battery life. This has resulted in conflicting RRT indications in the field. 
     Further, in some instances, follow-up times may be relatively long. This can result in an implantable cardiac device entering not only RRT but also even EOL without clinical assistance. 
     SUMMARY OF THE INVENTION 
     The present invention provides an implantable cardiac device and method which monitors actual remaining depletable power source energy capacity for making accurate RRT and/or EOL indications. The amount of current provided by the power source is constantly measured by a current monitor to derive a first average of current, provided by the power source since its initial hook-up or “beginning-of-life” (BOL), and a second average current, provided over a last 24 hour period. 
     The remaining energy capacity of the power source is determined by multiplying the first average current by the time since BOL to derive actual used capacity and then subtracting the actual used capacity from the initial capacity of the power source. An EOL date is then calculated by dividing the remaining energy capacity by the second current average to derive the remaining power source life and then adding the remaining power source life to a current date. The RRT date may then be determined by subtracting a fixed time period, such as 90 days, for example, from the EOL date. 
     The RRT and EOL dates provided are extremely accurate because they are based upon the actual remaining power source energy capacity. Further, a first alarm may be issued when the current date equals or exceeds the RRT date and a second alarm may be issued when the current date equals or exceeds the EOL date. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features and advantages of the present invention will be apparent from the following more particular description thereof, presented in conjunction with the accompanying drawings wherein: 
     FIG. 1 shows a simplified functional block diagram of an implantable cardioverter/defibrillator (ICD), which represents one type of implantable cardiac stimulation device with which the present invention may be used; 
     FIG. 2 shows a functional block diagram of an implantable dual-chamber pacemaker, which represents another type of implantable medical device with which the invention may be used; and 
     FIG. 3 is a flow diagram illustrating a method of providing precision RRT and EOL indications in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
     As indicated above, the present invention may be used with various types of implantable cardiac devices, including an implantable heart monitor, an implantable pacemaker configured to treat bradycardia and/or tachycardia, an implantable cardioverter/defibrillator (ICD), or a combined ICD and pacemaker. 
     To better understand the invention, it will first be helpful to have an understanding of the basic functions performed by an implantable cardiac device with which the invention may be used, e.g., an ICD device and/or a dual-chamber pacemaker. While a dual-chamber device has been chosen, this is for teaching purposes only. It is recognized that the present invention could be implemented in a single-chamber device, that one of skill in the art could readily adapt the dual-chamber device shown in FIG. 2 to perform single-chamber functionality, and that a single-chamber device is within the spirit of the invention. 
     In FIG. 1, there is shown a simplified functional block diagram of an ICD device  20 , and in FIG. 2, there is shown a simplified functional block diagram of a dual-chamber pacemaker  70 . It should also be noted that in some instances the functions of an ICD and a pacemaker may be combined within the same stimulation device. However, for teaching purposes, the devices will be described as separate stimulation devices. 
     It is the primary function of an ICD device to sense the occurrence of an arrhythmia, and to automatically apply an appropriate electrical shock therapy to the heart aimed at terminating the arrhythmia. To this end, the ICD device  20  as shown in the functional block diagram of FIG. 1, includes a control and timing circuit  22 , such as a microprocessor, state machine or other such control circuitry, that controls a high output charge generator  26 . The high output charge generator  26  generates electrical stimulation pulses of moderate or high energy (corresponding to cardioversion or defibrillation pulses, respectively), e.g., electrical pulses having energies of from 1-10 joules (moderate), or 11-40 joules (high), as controlled by the control/timing circuit  22 . 
     Such moderate or high energy pulses are applied to the patient&#39;s heart through at least one lead  30  having at least two defibrillation electrodes, such as coil electrodes  38  and  40 . The lead  30  preferably also includes at least one electrode for pacing and sensing functions, such as electrode  32 . Typically, the lead  30  is transvenously inserted into the heart so as to place the coil electrodes  38  and  40  in the apex of the heart and in the superior vena cava, respectively. While only one lead is shown in FIG. 1, it is to be understood that additional defibrillation leads and electrodes may be used as desired or needed in order to efficiently and effectively apply the shock treatment generated by the high voltage generator  26  to the patient&#39;s heart  28 . 
     The ICD  20  also includes a sense amplifier  42  that is coupled to at least one sensing electrode  32 . It is the function of the sense amplifier  42  to sense the electrical activity of the heart  28 , as is known in the art, to generate an electrogram including R-waves which occur upon depolarization, and hence contraction, of ventricular tissue; and P-waves which occur upon the depolarization, and hence contraction, of atrial tissue. Thus by sensing R-waves and/or P-waves through the sense amplifier  42 , and by providing the control/timing circuit  22  with the generated electrogram, the control/timing circuit  22  is able to make a determination as to the rate and regularity of the patient&#39;s heartbeat. Such information, in turn, allows the control/timing circuit  22  to determine whether the heart  28  of a patient is experiencing an arrhythmia, and to apply appropriate stimulation therapy. 
     The control/timing circuit  22  further has a memory circuit  44  coupled thereto wherein the operating parameters used by the control/timing circuit  22  are stored. Such operating parameters define, for example, the amplitude of each shock energy pulse to be delivered to the patient&#39;s heart  28  within each tier of therapy, as well as the duration of these shock pulses. The memory  44  may take many forms, and may be subdivided into as many different memory blocks or sections (addresses) as needed to store desired data such as RRT and/or EOL times and control information. 
     Advantageously, the operating parameters of the implantable device  20  may be non-invasively programmed into the memory  44  through a telemetry circuit  46 , in telecommunicative contact with an external programmer  48  by way of a suitable coupling coil  50 . The coupling coil may serve as an antenna for establishing a radio frequency (RF) communication link  52  with the external programmer  48 ; or the coil  50  may serve as a means for inductively coupling data to and from the telemetry circuit  46  from and to the external programmer  48 , as is known in the art. Further, such telemetry circuit  46  advantageously allows status information relating to the operation of the ICD  20  as contained in the control/timing circuit  22  or memory  44 , such as RRT and/or EOL times determined in accordance with the present invention to be sent to the external programmer  48  through the established link  52 . 
     The control/timing circuit  22  includes appropriate processing and logic circuits for analyzing the electrogram generated by the sense amplifier  42  and determining if such signals indicate the presence of an arrhythmia. Typically, the control/timing circuit  22  is based on a microprocessor, or similar processing circuit, which includes the ability to process or monitor input signals (data) in a prescribed manner, e.g., as controlled by program code stored in a designated area or block of the memory  44 . The details of the design and operation of the control/timing circuit  22  are not critical to the present invention. Rather, any suitable control/timing circuit  22  may be used that carries out the functions described herein. The use, design, and operation of microprocessor-based control circuits to perform timing and data analyses functions is known in the art. 
     The ICD  20  further includes a depletable power source or battery  43 . The battery provides operating power to all of the circuits of the ICD. Coupled to the battery  43  is a current monitor  45 . The current monitor, in accordance with the present invention, continuously senses the total current drawn from the battery  43  by the circuits of the ICD  20 . The monitor  45  may include, for example, a resistance which provides a voltage drop proportional to the used current. The voltage drop may then be applied to an analog-to-digital converter to provide a digital representation of the used current to the control circuit  22 . As will be seen subsequently, the processor  22  uses the current measurement to determine a first average current which is the average current used since BOL and a second average current which is the average current used over a last 24 hour period. 
     In FIG. 2, a simplified block diagram of the circuitry needed for a dual-chamber pacemaker  70  is illustrated. The pacemaker  70  is coupled to a heart  28  by way of leads  74  and  76 , the lead  74  having an electrode  75  that is in contact with one of the atria of the heart, and the lead  76  having an electrode  77  that is in contact with one of the ventricles of the heart. The leads  74  and  76  are electrically and physically connected to the pacemaker  70  through a connector  73  that forms an integral part of the housing wherein the circuits of the pacemaker are housed. 
     The connector  73  is electrically connected to a protection network  79 , which network  79  electrically protects the circuits within the pacemaker  70  from excessive shocks or voltages that could appear on the electrodes  75  and/or  77  in the event such electrodes were to come in contact with the high voltage signal, e.g., from a defibrillation shock. 
     The leads  74  and  76  carry stimulation pulses to electrodes  75  and  77  from an atrial pulse generator (A-PG)  78  and a ventricular pulse generator (V-PG)  80 , respectively. Further, electrical signals from the atria are carried from the electrode  75  through the lead  74 , to the input terminal of an atrial channel sense amplifier (P-AMP)  82 ; and electrical signals from the ventricles are carried from the electrode  77  through the lead  76 , to the input terminal of a ventricular channel sense amplifier (R-AMP)  84 . Similarly, electrical signals from both the atria and ventricles are applied to the inputs of an IEGM (intracardiac electrogram) amplifier  85 . The amplifier  85  is typically configured to detect an evoked response from the heart  28  in response to an applied stimulus, thereby aiding in the detection of “capture.” 
     The dual-chamber pacemaker  70  is controlled by a control system  86  that typically includes a microprocessor programmed to carry out control and timing functions. The control system  86  receives the atrial electrogram signal from the atrial amplifier  82  over signal line  88 . Similarly, the control system  86  receives the ventricular electrogram signal from the ventricular amplifier  84  over signal line  90 , and the output signals from the IEGM amplifier  85  over signal line  91 . The control system  86  also generates trigger signals that are sent to the atrial pulse generator  78  and the ventricular pulse generator  80  over signal lines  92  and  94 , respectively. These trigger signals are generated each time that a stimulation pulse is to be generated by the respective pulse generator  78  or  80 . The atrial trigger signal is referred to simply as the “A-trigger” and the ventricular trigger signal is referred to as the “v-trigger.” 
     As shown in FIG. 2, the pacemaker  70  further includes a memory circuit  100  that is coupled to the control system  86  over a suitable data/address bus  102 . This memory circuit  100  allows certain control parameters, used by the control system  86  in controlling the operation of the pacemaker, to be programmably stored and modified, as required, in order to customize the pacemaker&#39;s operation to suit the needs of a particular patient. Further, data sensed during the operation of the pacemaker may be stored in the memory  100  for later retrieval and analysis. That data may include RRT and/or EOL times as determined in accordance with the present invention. 
     As with the memory  44  of the ICD device  20  shown in FIG. 1, the memory  100  of the pacemaker  70  (FIG. 2) may take many forms, and may be subdivided into as many different memory blocks or sections (addresses) as needed in order to allow desired data and control information to be stored. Preferably, the memory  100  is capable of storing a relatively large amount of sensed data as a data record, which data record may then be used to guide the operation of the device. That is, the operating mode of the pacemaker may be made dependent, at least in part, on past performance data. For example, an average atrial rate may be determined based on the sensed atrial rate over a prescribed period of time. This average rate may then be stored and updated at regular intervals. Such stored rate may then be compared to a present atrial rate and, depending upon the difference, used to control the operating mode of the pacemaker. Other parameters, of course, in addition to (or in lieu on atrial rate, may be similarly sensed, stored, averaged (or otherwise processed), and then used for comparison purposes against one or more currently sensed parameters. Advantageously, modem memory devices allow for the storage of large amounts of data in this manner. 
     A clock circuit  103  directs an appropriate clock signal to the control system  86 , as well as to any other needed circuits throughout the pacemaker  70  e.g., to the memory  100 , by way of clock bus  105 . 
     A telemetry/communications circuit  104  is further included in the pacemaker  70 . This telemetry circuit  104  is connected to the control system  86  by way of a suitable command/data bus  106 . In turn, the telemetry circuit  104 , which is included within the implantable pacemaker  70  may be selectively coupled to an external programming device  108  by means of an appropriate communication link  110 , which communication link  110  may be any suitable electromagnetic link, such as an RF (radio frequency) channel, a magnetic link, an optical link, and the like. Advantageously, through the external program  108  and the communication link  110 , desired commands may be sent to the control system  86 . Similarly, through this communication link  110  with the programmer  108  data commands, either held within the control system  86 , as in a data latch, or stored within the memory  100 , may be remotely received from the programmer  108 . Similarly, data initially sensed through the lead  74  or  76  and processed by the microprocessor control circuits  86 , or other data measured within or by the pacemaker  70 , such as RRT and/or EOL times may be stored and uploaded to the programmer  108 . In this manner, non-invasive communications can be established with the implanted pacemaker from a remote, non-implanted location. 
     The pacemaker  70  additionally includes a depletable power source such as a battery  93 . The battery  93  provides operating power to all of the circuits of the pacemaker  70 . The battery  93  is coupled to a current monitor  95 . The current monitor may be identical to the current monitor  45  of FIG.  1 . Responsive to the digital representation of the used current, the processor  86  determines the first and second average currents as previously described. 
     It is noted that the pacemaker  70  in FIG. 2 is referred to as a dual-chamber pacemaker because it interfaces with both the atria and the ventricles of the heart. Those portions of the pacemaker  70  that interface with the atria, e.g., the lead  74 , the atrial pulse generator  78 , and corresponding portions of the control system  86 , are commonly referred to as the “atrial channel.” Similarly, those portions of the pacemaker  70  that interface with the ventricles, e.g., the lead  76 , the ventricular sense amplifier  84 , the ventricular pulse generator  80 , and corresponding portions of the control system  86 , are commonly referred to as the “ventricular channel.” 
     As needed for certain applications, the pacemaker  70  may further include at least one sensor  112  that is connected to the control system  86  of the pacemaker  70  over a suitable connection line  114 . A common type of sensor is an activity sensor, such as piezoelectric crystal, that is mounted to the case of the pacemaker. Other types of sensors are also known, such as sensors that sense the oxygen content of blood, respiration rate, pH of blood, body motion, and the like. The type of sensor used is not critical to the present invention. The pacemaker  70  further includes magnet detection circuitry  87 , coupled to the control system  86  over signal line  89 , as is well known in the art. 
     As with the ICD device  20  of FIG. 1, the telemetry or communications circuit  104  may be of conventional design. Similarly, the external programmer  108  may be of any suitable design in the art. Likewise, the memory circuit  100  and the circuits utilized in the atrial and ventricular channels may all be of common design as is known in the pacing art. The present invention is not concerned with the details of the circuitry utilized for each of these pacing elements. Rather, it is concerned with the manner in which all of these pacing elements cooperate with each other in order to provide a particular pacing mode of operation. Such cooperation is controlled by the control system  86 . 
     The control system  86  may be realized using a variety of different techniques and/or circuits. The preferred type of control system  86  is a microprocessor-based control system. It is noted, however, that the control system  86  could also be realized using a state machine. Indeed, any type of control circuit or system could be employed for the control system  86 . The present invention is likewise not concerned with the details of the control systems  22  and  86 . Rather it is concerned with the end result achieved by the control system. That is, so long as the control system  86  controls the operation of the pacemaker (or other medical device)so that the desired functions are achieved as set forth herein, e.g., by following the steps described below in the flow chart of FIG. 3, it matters little what type of control system is used. Those of skill in the implantable medical device art, given the teachings presented herein, should thus be able to fashion numerous different types of control systems or circuits that achieve the desired device control. 
     Representative of the types of control systems that may be used with the invention is the microprocessor-based control system described in U.S. Pat. No. 4,940,052 entitled MICROPROCESSOR CONTROLLED RATE-RESPONSIVE PACEMAKER HAVING AUTOMATIC RATE RESPONSE THRESHOLD ADJUSTMENT. Reference is also made to U.S. Pat. Nos. 4,712,555 and 4,944,298, wherein a state machine type of operation for a pacemaker is described; and U.S. Pat. No. 4,788,980, wherein the various timing intervals used within the pacemaker and their inter-relationships are more thoroughly described. The &#39;052, &#39;555, &#39;298, and &#39;980 patents are incorporated herein by reference. 
     Referring now to FIG. 3, it is a flow diagram which illustrates a method for providing precision RRT and EOL indications in accordance with a preferred embodiment of the present invention. The operative steps illustrated in FIG. 3 may be performed by either the processor  22  of FIG. 1 or the processor  86  of FIG.  2 . 
     The method initiates at a block  120  wherein T o , the battery hook-up date or beginning-of-life (BOL) date, is stored along with C i , the initial energy capacity of the battery. Following block  120 , the method advances to block  122  wherein the current date is stored. Next, in block  124 , the first current average (I avg ) and the second current average (I 24 ) are determined. The first current average (I AVG ) keeps track of the average current drawn from the battery since its BOL. The second current average (I 24 ) keeps track of the most recent average current, e.g., during the last 24-hour period. As a result, the first current average is an average energy depletion rate since the battery BOL and the second current average is the average energy depletion rate over the last 24-hour period. 
     The method then advances to block  126  wherein the time since BOL, or the energy depletion time. is determined. Block  126  is performed by subtracting the current date set in block  122  from the battery hook-up date set in step  120 . The process then advances to block  128  wherein the actual used energy capacity of the battery is determined. Block  128  is preferably performed by multiplying the first current average which, as previously described, is the average battery current since BOL, by the time since BOL determined in step  126 . With the used energy capacity of the battery determined in block  128 , the process then advances to block  130  wherein the actual remaining energy capacity of the battery is determined. Block  130  is preferably performed by subtracting the used battery energy determined in step  128  from the initial battery capacity set in block  120 . 
     Once the remaining battery energy capacity is determined, the method advances to block  132  wherein the remaining power source life, or remaining time to EOL, is determined. Block  132  is preferably performed by dividing the remaining battery energy capacity determined in block  130  by the second current average which, as previously described, is the average current drawn from the battery over a last 24 hour period. This renders the remaining time to EOL in day units. 
     Once the remaining time to EOL is determined, the method advances to block  134  wherein the EOL date, the date of power source depletion, is determined. In accordance with this preferred embodiment, block  134  is performed by adding the remaining time to EOL determined in block  132  to the current date originally set in block  122 . This provides an EOL date. The EOL date of block  134  is then used to update the EOL date stored in memory in accordance with block  136 . 
     After the EOL date is determined and updated in memory, the method advances to a block  138 . Here, it is determined if the current date equals or exceeds the EOL date. If it is, the method advances to block  142  which resets any RRT alarm (which may have been set in block  152 , as described below) and issues an EOL alarm at regularly scheduled intervals. The EOL alarm is an alarm which is perceptible and used to advise the patient or physician that the EOL date has arrived. Once the EOL alarm has been triggered, the device will continue pacing without updating the EOL or RRT dates at block  143 . The duration of the EOL alarm may be independently controlled, such as by a latch, to maintain the EOL alarm for some preset period of time without effecting the processing illustrated in FIG.  3 . 
     If the current date has not exceeded the EOL date, then the method advances to block  144  wherein the RRT date is determined. In accordance with this preferred embodiment, the RRT date is determined by subtracting a fixed period of time, such as 90 days, from the EOL date determined in step  134 . This will provide an RRT date which precedes the EOL date by 90 days, for example. Once the RRT date is determined, it is then updated in memory in accordance with block  146 . 
     Next, the method advances to block  148  where it is determined if the current date equals or exceeds the RRT date. If it does, the process proceeds to block  152  to cause the RRT alarm to issue at regular intervals. The RRT alarm alerts the patient that the RRT date has been reached. Again, the duration of the RRT alarm may be independently controlled, such as by a latch, without disturbing the processing of the method illustrated in FIG.  3 . 
     Once the RRT alarm is issued or if it is determined in block  150  that the EOL alarm has already issued, the method then advances to block  154  which determines if a new cardiac cycle has begun. The method pauses until a new cycle begins at which time the process returns to block  122 . Hence, as can be seen herein, the process steps  122  through  154  are repeated each cardiac cycle. Only block  120  is not repeated since block  120  relates to initial battery parameters which do not change over time. 
     The RRT and EOL alarms may take many different forms. As known in the art, the RRT and EOL alarms may be audible alarms, utilizing different pitches or audible tone sequences to enable discernment between the RRT and EOL alarms. Alternatively, the alarms may be in the form of a vibration caused by a vibrator within the implantable device. Other forms of alarms are also known and may be utilized in practicing the present invention. 
     While the invention has been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the invention. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein.