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DEFIBRILLATOR WITH SWITCHABLE
POWER SOURCE AND PATIENT WARNING
SYSTEM CARDIAC DEVICE
CROSS-REFERENCE TO RELATED
This application relates to copending U.S. patent application Ser. No. 09/545,044, filed Apr. 7, 2000, titled "Hybrid Battery Network for Implantable Medical Device" to Fayram, which is assigned to the same assignee as the present invention, and which is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates generally to implantable medical devices possessing a multiple cell power source. More particularly, this invention relates to an implantable cardioverter defibrillator that provides charge monitoring of a multiple cell power source so that if the charge of a cell is depleted, a patient warning signal is provided and the power source circuitry is reconfigured to rely only on cells with an acceptable remaining charge.
BACKGROUND OF THE INVENTION
In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. Disruption of the heart's natural pacemaker and conduction system, as a result of aging or disease, can be successfully treated using implantable cardiac simulation devices, including pacemakers and implantable cardioverter defibrillator. A pacemaker generally delivers rhythmic electrical pulses to the heart to maintain a normal rhythm in patients having bradycardia, which is too slow of heart rate, or other conduction abnormalities. An implantable cardioverter defibrillator, commonly referred to as an "ICD", is capable of recognizing tachycardia or fibrillation and delivering electrical therapy to terminate such arrhythmias. ICD's are often configured to also perform pacemaking functions as well.
Special difficulties arise in providing an adequate energy source for ICD's, particularly those that are also intended to perform pacemaking functions. Cardioversion-defibrillator typically requires a few high-power electrical shocks to be generated relatively infrequently. For cardioversion, the shocks are typically at about two joules. For defibrillation, the shocks are typically at about twenty joules. Pacemaking functions, in contrast, may require that numerous relatively low-power electrical shocks be generated frequently. Pacing shocks are typically on the order of micro-joules. Energy is also required to monitor the heart for the purposes of detecting when cardioversion, defibrillation or pacing is required. Monitoring causes a continuous low-power current draw of about ten microamperes.
Also, energy may be required to reform whatever capacitor is used in connection with delivering defibrillation shocks. In this regard, aluminum electrolytic capacitors, which are commonly employed, typically must be charged to full voltage every couple of months to prevent degradation. Whether energy is actually required to perform capacitor reformation depends upon whether the patient receives relatively frequent defibrillation shocks. The ICD's of patients that do not receive at least one defibrillation shock
every month or two will require a periodic capacitor reformation cycle. The ICD's of patients that do receive at least one defibrillation shock every month or two, however, do not typically require capacitor reformation cycles because
5 reformation is achieved automatically during the generation of the defibrillation shocks.
To accommodate these various energy requirements, some ICD designs employ two power cells, a high-power cell and a low-power cell. Exemplary high-power cells
1° include manganese dioxide cells and silver vanadium oxide cells. Exemplary low-power cells include carbon monofluoride cells and lithium iodine cells. In one possible example, the high-power cell provides energy for capacitor reformation and for cardioverter defibrillator functions and the
15 low-power cell provides energy for the pacemaking and monitoring functions. In other possible examples, the highpower cell provides energy for capacitor reformation, cardioverter defibrillator functions and pacemaking functions and the low-power cell provides energy only for the moni
20 taring functions.
The simplest ICD devices having separate high-power and low-power cells performed only monitoring, pacing and defibrillation functions and were configured to always draw energy for defibrillation from the high-power cell and to
25 always draw energy for pacing and monitoring from the low-power cell, i.e. the power cells are non-switchable. However, the actual energy drawn from the low- and highpower sources varies considerably from patient to patient. For example, some patients require frequent pacing but little
30 or no defibrillation whereas other patients require relatively frequent defibrillation but little or no pacing. Still others require neither pacing nor defibrillation but merely require continuous monitoring.
35 As a result of the wide variations in actual energy usage, circumstances can arise within ICD's having non-switchable power sources wherein one power cell becomes quickly depleted thereby necessitating early replacement of the ICD even though the other power cell retains considerable energy
4Q and could otherwise continue to provide energy for the ICD. For example, circumstances can arise wherein the ICD must be replaced because the low-power cell has been depleted from frequent pacing or from a long period of continuous monitoring even though the high-power cell has abundant
45 energy and could otherwise continue to power all ICD functions.
One design proposes a device that switches from the low-power cell to the high-power cell if the low-power cell becomes depleted, wherein energy for pacing and defibril
50 lation is drawn from a silver vanadium oxide cell, and energy for monitoring is drawn from a lithium iodine cell. Thus, energy for monitoring is switched from the lithium iodine cell to the silver vanadium oxide cell if the lithium iodine cell becomes depleted.
55 Although this latter design represents an improvement over non-switchable systems, substantial room for further improvement remains. As an example, with that design, if the patient requires a considerable amount of pacing and a considerable amount of defibrillation therapy, the silver
go vanadium oxide cell will become quickly depleted, thereby necessitating early replacement of the device even though the lithium iodine cell retains considerable energy reserves.
In such circumstances, it would be preferable to switch the device, while the silver vanadium oxide cell still retains
65 sufficient energy for defibrillation, to draw energy for the pacemaking functions from the lithium iodine cell to thereby slow the depletion of the silver vanadium oxide cell by more
effectively using the remaining energy of the lithium iodine cell. However, this prior design does not provide for switching from the high-power cell to the low-power cell and thereby may result in a premature replacement of the ICD.
Moreover, even the manner by which this prior design 5 operates to switch from the low-power cell to the highpower cell could be improved. In this regard, this prior design merely operates to determine whether the low-power cell has become completely depleted and, if so, switches completely and immediately to the high-power cell. Further 1° improvement can be gained by gradually adjusting the relative amounts of energy drawn from the two power cells in an optimal manner.
In addition, in any battery-powered device, the device performance will eventually become compromised if one 15 battery cell discharges below a functional level prior to device replacement. Particularly in the case of ICDs, such an event could be life-threatening to the patient. An elective or recommended replacement indicator provided by manufacturers of ICDs is used to indicate the recommended time for 20 replacing the ICD. However, replacement indicators are not always accurate and have been found to underestimate the remaining battery life. While device replacement prior to the end of the battery life is crucial, improving the cost effectiveness of ICD therapy by extending the implant time is 25 also a goal.
The cost effectiveness and safety of ICD therapy, therefore, can continue to be improved with improvements in battery technology. Hence, it would be desirable to provide a battery system that safely maximizes the implant time and that optimizes energy delivery from a power source to the ICD circuitry. Such capabilities would be desirable in an ICD regardless of the number of battery cells used or the battery chemistry implemented.
The present invention addresses these needs by providing an implantable cardioverter defibrillator (ICD) equipped with a multiple cell power source and switching circuitry for 4Q reconfiguring the power source when any cell charge precipitously decreases. The reconfigured power source selects the remaining cell or cells that retain an acceptable charge for powering monitoring and output functions of the ICD.
Furthermore the present invention provides that, upon 45 detection of a reduced cell charge, a patient warning is issued to alert the patient that medical attention should be sought for determination of a safe device replacement time.
The foregoing and other features of the present invention are realized by providing an implantable, multichamber 50 cardiac stimulation device equipped with pacing, cardioversion and defibrillation capabilities powered by a multiple cell power source. A preferred embodiment of the stimulation device includes a power source equipped with two batteries for powering the circuitry of the device, one battery 55 having a higher resistance and greater energy density than the other battery; a control system for controlling the operation of the device; a set of leads that connects cardiac electrodes to the stimulation device for receiving cardiac signals and for delivering atrial and ventricular stimulation 60 pulses; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; a data acquisition system, such as an A/D converter, for sampling cardiac signals; and pulse generators for generating atrial and ventricular stimulation pulses. 65
The stimulation device further includes memory for storing operational parameters for the control system, such as
stimulation parameter settings and timing intervals. The stimulation device also includes a telemetry circuit for communicating with an external programmer.
The power source is further equipped with switching circuitry such that battery cells may be selectively connected to the power source output for providing current to the sensing, output and control functions of the ICD. In a preferred embodiment, the lower energy density cell, preferably a lithium silver vanadium oxide cell, provides current flow to the circuitry of the stimulation device.
The higher energy density cell, preferably a lithium carbon monofluoride cell, provides current flow to the low density cell to maintain its charge. A current sensor provided between the high and low density cells detects if the current flow between the two cells deviates from a normal range. Upon detection of an unacceptable current flow, the control system opens or closes the appropriate switches such that the discharged cell is disconnected from the power supply output circuit The remaining cell continues to power all device functions.
The stimulation device is further provided with a patient warning system so that when a power cell is found by a control program to have a charge low enough to require reconfiguration of the power source circuitry, a low-battery warning signal is issued to the patient. The patient warning is preferably an audible buzzer or a twitch stimulation applied to excitable tissue surrounding the implanted device causing a sensation perceptible by the patient. The patient has been advised to seek medical attention upon perceiving the low-battery alarm such that an appropriate device replacement time may be scheduled.
The methods of the present invention thus improve the safety of the ICD by excluding a discharged power cell from the power supply circuitry so that remaining cells power all device functions adequately and by providing a patient warning of a low battery condition allowing the patient to seek medical attention well before device function becomes seriously impaired. The methods of the present invention may be advantageously applied using the most recent battery technology, which may use two or more cells of varying battery chemistries.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features of the present invention and the manner of attaining them will be described in greater detail with reference to the following description, claims, and drawings, wherein reference numerals are reused, where appropriate, to indicate a correspondence between the referenced items, and wherein:
FIG. 1 is a simplified, partly cutaway view illustrating an implantable stimulation device in electrical communication with at least three leads implanted into a patients heart for delivering multi-chamber stimulation and shock therapy;
FIG. 2 is a functional block diagram of the multi-chamber implantable stimulation device of FIG. 1, illustrating the basic elements that provide pacing stimulation, cardioversion, and defibrillation in four chambers of the heart;
FIG. 3 is a schematic diagram of a switchable power supply circuitry used in the stimulation device of FIG. 2, according to the present invention; and
FIG. 4 is a flow chart providing an overview of the operations included in one embodiment of the present invention for using the switchable power supply circuitry of FIG. 3.
DETAILED DESCRIPTION OF PREFERRED
The following description is of a best mode presently contemplated for practicing the invention. This description is not to be taken in a limiting sense but is made merely for 5 the purpose of describing the general principles of the invention. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. The present invention is directed at providing an improved switchable power 10 supply and patient alarm in an implantable cardiac stimulating device possessing pacemaking, cardioversion and defibrillation capabilities.
A cardiac stimulation device will thus be described in conjunction with FIGS. 1 and 2, in which the features included in the present invention could be implemented. It is recognized, however, that numerous variations of such a device exist in which the methods included in the present invention could be implemented without deviating from the 2Q scope of the present invention.
FIG. 1 illustrates a stimulation device 10 in electrical communication with a patient's heart 12 by way of three leads 20, 24 and 30 suitable for delivering multi-chamber stimulation and shock therapy. To sense atrial cardiac signals 25 and to provide right atrial chamber stimulation therapy, the stimulation device 10 is coupled to an implantable right atrial lead 20 having at least an atrial tip electrode 22, which typically is implanted in the patient's right atrial appendage. The right atrial lead 20 may also have an atrial ring electrode 30 23 to allow bipolar stimulation or sensing in combination with the atrial tip electrode 22.
To sense the left atrial and ventricular cardiac signals and to provide left-chamber stimulation therapy, the stimulation device 10 is coupled to a "coronary sinus" lead 24 designed 35 for placement in the "coronary sinus region" via the coronary sinus ostium so as to place a distal electrode adjacent to the left ventricle and additional electrode(s) adjacent to the left atrium. As used herein, the phrase "coronary sinus region" refers to the venous vasculature of the left ventricle, 40 including any portion of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus.
Accordingly, the coronary sinus lead 24 is designed to: 45 receive atrial and ventricular cardiac signals; deliver left ventricular pacing therapy using at least a left ventricular tip electrode 26 for unipolar configurations or in combination with left ventricular ring electrode 25 for bipolar configurations; deliver left atrial pacing therapy using at least a left 50 atrial ring electrode 27, and shocking therapy using at least a left atrial coil electrode 28.
The stimulation device 10 is also shown in electrical communication with the patient's heart 12 by way of an implantable right ventricular lead 30 having, in this 55 embodiment, a right ventricular tip electrode 32, a right ventricular ring electrode 34, a right ventricular (RV) coil electrode 36, and a superior vena cava (SVC) coil electrode 38. Typically, the right ventricular lead 30 is transvenously inserted into the heart 12 so as to place the right ventricular 60 tip electrode 32 in the right ventricular apex so that the RV coil electrode 36 will be positioned in the right ventricle and the SVC coil electrode 38 will be positioned in the right atrium and/or superior vena cava. Accordingly, the right ventricular lead 30 is capable of receiving cardiac signals, 65 and delivering stimulation in the form of pacing and shock therapy to the right ventricle.
FIG. 2 illustrates a simplified block diagram of the multi-chamber implantable stimulation device 10, which is capable of treating both fast and slow arrhythmias with stimulation therapy, including cardioversion, defibrillation, and pacing stimulation. While a particular multi-chamber device is shown, this is for illustration purposes only, and one of skill in the art could readily duplicate, eliminate or disable the appropriate circuitry in any desired combination to provide a device capable of treating the appropriate chamber(s) with cardioversion, defibrillation and pacing stimulation.
The stimulation device 10 includes a housing 40 which is often referred to as "can", "case" or "case electrode", and which may be programmably selected to act as the return electrode for all "unipolar" modes. The housing 40 may further be used as a return electrode alone or in combination with one or more of the coil electrodes 28, 36, or 38, for defibrillation shocking purposes. The housing 40 further includes a connector having a plurality of terminals 42, 43, 44, 45, 46, 48, 52, 54, 56, and 58 (shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the corresponding terminals). As such, to achieve right atrial sensing and stimulation, the connector includes at least a right atrial tip terminal (AR TIP) 42 adapted for connection to the atrial tip electrode 22. The connector may also include a right atrial ring terminal (AR RING) 43 for connection to the right atrial ring electrode 23.
To achieve left chamber sensing, pacing, and shocking, the connector includes at least a left ventricular tip terminal (Vj, TIP) 44, a left ventricular ring terminal (VL RING) 45, a left atrial ring terminal (AL RING) 46, and a left atrial shocking coil terminal (AL COIL) 48, which are adapted for connection to the left ventricular tip electrode 26, the left ventricular ring electrode 25, the left atrial ring electrode 27, and the left atrial coil electrode 28, respectively.
To support right ventricular sensing, pacing and shocking, the connector further includes a right ventricular tip terminal (yk TIP) 52, a right ventricular ring terminal (VR RING) 54, a right ventricular shocking coil terminal (RV COIL) 56, and an SVC shocking coil terminal (SVC COIL) 58, which are adapted for connection to the right ventricular tip electrode 32, right ventricular ring electrode 34, the RV coil electrode 36, and the SVC coil electrode 38, respectively.
At the core of the stimulation device 10 is a programmable microcontroller 60 that controls the various modes of stimulation therapy. The microcontroller 60 typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy, and may further include RAM or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, the microcontroller 60 includes the ability to process or monitor input signals (data) as controlled by a program code stored in a designated block of memory.
FIG. 2 illustrates an atrial pulse generator 70 and a ventricular pulse generator 72 that generate stimulation pulses for delivery by the right atrial lead 20, the right ventricular lead 30, and/or the coronary sinus lead 24 via a switch 74. It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial pulse generator 70 and the ventricular pulse generator 72 may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The atrial pulse generator 70 and the ventricular pulse generator 72 are controlled by the microcontroller 60 via
appropriate control signals 76 and 78, respectively, to trigger or inhibit the stimulation pulses.
The microcontroller 60 further includes timing control circuitry 79 which is used to control the timing of such stimulation pulses (e.g. pacing rate, atrio-ventricular (AV) 5 delay, atrial interchamber (A—A) delay, or ventricular interchamber (V—V) delay, etc.), as well as to keep track of the timing of refractory periods, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc. 1°
The switch 74 includes a plurality of switches for connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, the switch 74, in response to a control signal 80 from the microcontroller 60, determines the polarity of the 15 stimulation pulses (e.g. unipolar, bipolar, cross-chamber, etc.) by selectively closing the appropriate combination of switches. Atrial sensing circuits 82 and ventricular sensing circuits 84 may also be selectively coupled to the right atrial lead 20, coronary sinus lead 24, and the right ventricular lead 20 30, through the switch 74, for it detecting the presence of cardiac activity in each of the four chambers of the heart.
Accordingly, the atrial and ventricular sensing circuits 82 and 84 may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. The switch 74 determines the "sensing polarity" of the cardiac signal by selectively closing the appropriate switches. In this way, the clinician may program the sensing polarity independent of the stimulation polarity. ^
Each of the atrial sensing circuit 82 or the ventricular sensing circuit 84 preferably employs one or more low power, precision amplifiers with programmable gain and automatic gain or sensitivity control, bandpass filtering, and a threshold detection circuit, to selectively sense the cardiac 3J signal of interest. The automatic sensitivity control enables the stimulation device 10 to deal effectively with the difficult problem of sensing the low amplitude signal characteristics of atrial or ventricular fibrillation
The outputs of the atrial and ventricular sensing circuits 40 82 and 84 are connected to the microcontroller 60 for triggering or inhibiting the atrial and ventricular pulse generators 70 and 72, respectively, in a demand fashion, in response to the absence or presence of cardiac activity, respectively, in the appropriate chambers of the heart. The 45 atrial and ventricular sensing circuits 82 and 84, in turn, receive control signals over signal lines 86 and 88 from the microcontroller 60, for controlling the gain, threshold, polarization charge removal circuitry, and the timing of any blocking circuitry coupled to the inputs of the atrial and 50 ventricular sensing circuits 82 and 84.
For arrhythmia detection, the stimulation device 10 includes an arrhythmia detector 77 that utilizes the atrial and ventricular sensing circuits 82 and 84 to sense cardiac signals, for determining whether a rhythm is physiologic or 55 pathologic. As used herein "sensing" refers to the process of noting an electrical signal. "Detection" refers to the step of confirming that the sensed electrical signal as the signal being sought by the detector. As an example, "detection" applies to the detection of both proper rhythms (i.e., "R 60 wave" or "R wave") as well as improper dysrhythmias including arrhythmia and bradycardia (e.g., detection of the absence of a proper rhythm.)
The timing intervals between sensed events (e.g. P-waves, R-waves, and depolarization signals associated with fibril- 65 lation which are sometimes referred to as "F-waves" or "Fib-waves") are then classified by the arrhythmia detector
77 by comparing them to a predefined rate zone limit (e.g. bradycardia, normal, low rate ventricular tachycardia, high rate ventricular tachycardia, and fibrillation rate zones) and various other characteristics (e.g. sudden onset, stability, physiologic sensors, and morphology, etc.), in order to determine the type of remedial therapy that is needed (e.g. bradycardia pacing, anti-tachycardia stimulation, cardioversion shocks or defibrillation shocks, collectively referred to as "tiered therapy").
Cardiac signals are also applied to the inputs of a data acquisition system 90, which is depicted as an analog-todigital (A/D) converter for simplicity of illustration. The data acquisition system 90 is configured to acquire intracardiac electrogram (EGM) signals, convert the raw analog data into digital signals, and store the digital signals for later processing and/or telemetric transmission to an external device 102. The data acquisition system 90 is coupled to the right atrial lead 20, the coronary sinus lead 24, and the right ventricular lead 30 through the switch 74 to sample cardiac signals across any pair of desired electrodes.
Advantageously, the data acquisition system 90 may be coupled to the microcontroller 60 or another detection circuitry, for detecting an evoked response from the heart 12 in response to an applied stimulus, thereby aiding in the detection of "capture". In the embodiment shown in FIG. 2, the microcontroller 60 includes an automatic capture detector 65 that searches for an evoked response signal following a stimulation pulse during a "detection window" set by timing control circuitry 79. The microcontroller 60 enables the data acquisition system 90 via control signal 92 to sample the cardiac signal that falls in the capture detection window. The sampled signal is evaluated by automatic capture detector 65 to determine if it is an evoked response signal based on its amplitude, peak slope, morphology or another signal feature or combination of features. The detection of an evoked response during the detection window indicates that capture has occurred.
The microcontroller 60 is further coupled to a memory 94 by a suitable data/address bus 96, wherein the programmable operating parameters used by the microcontroller 60 are stored and modified, as required, in order to customize the operation of the stimulation device 10 to suit the needs of a particular patent. Such operating parameters define, for example, stimulation pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each stimulation pulse to be delivered to the patient's heart 12 within each respective tier of therapy.
The stimulation device 10 additionally includes a power source that is illustrated as a battery 110 that provides operating power to all the circuits shown in FIG. 2. For the stimulation device 10, which employs shocking therapy, the battery 110 must be capable of operating at low current drains for long periods of time, preferably less than 10 fiA, and also be capable of providing high-current pulses when the patent requires a shock pulse, preferably, in excess of 2 A, at voltages above 2 V, for periods of 10 seconds or more.
The battery 110 preferably has a predictable discharge characteristic so that elective replacement time can be detected. In accordance with the present invention, the battery 110 includes at least two cells. A preferred embodiment of the power source and method for use will be described in greater detail in conjunction with FIGS. 3 and 4.
A patient warning signal generator 64 is included in the microcontroller 60 such that a patient may be alerted to a