Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-10-02
2003-12-30
Layno, Carl (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06671552
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of implantable medical devices (IMDs), and more particularly to a system and method for determining an estimate of remaining battery life of the IMD.
BACKGROUND OF THE INVENTION
At present, a wide variety of IMDs are commercially released or proposed for clinical implantation that are programmable in a variety of operating modes and are interrogatable using RF telemetry transmissions. Such medical devices include implantable cardiac pacemakers, cardioverter/defibrillators, cardiomyostimulators, pacemaker/cardioverter/defibrillators, drug delivery systems, cardiac and other physiologic monitors, electrical stimulators including nerve and muscle stimulators, deep brain stimulators, and cochlear implants, and heart assist devices or pumps, etc. Most such IMDs comprise electronic circuitry and an IMD battery that provides power to the electronic circuitry and that depletes in energy over time. Therefore, it is necessary to monitor the state of the battery in such IMDs so that the IMD can be replaced before the battery depletes to a state that renders the IMD inoperable.
Typically, certain therapy delivery and monitoring operational modes and parameters of the IMD are altered temporarily or chronically in a non-invasive (i.e. non-surgical) manner using downlink telemetry transmission from an external programmer of programming and interrogation commands (herein referred to as “downlink telemetry data”). Moreover, a wide variety of real time and stored physiologic data and non-physiologic data particular to the patient (referred to collectively herein as “patient data”) can be uplink telemetry transmitted by the IMD to the programmer in response to a downlink telemetry transmitted interrogation command. Other device specific data, including programmed operating modes and parameter values, device state data, and particularly the battery voltage and/or impedance, can also be uplink telemetry transmitted by the IMD to the programmer in response to a downlink telemetry transmitted interrogation command. Such device specific data and patient data are collectively referred to as “uplink telemetry data”.
Since it is often extremely critical for patients' well-being that IMDs do not cease operating, it is common for IMDs to monitor the level of battery depletion and to provide some indication when the depletion reaches a level at which the battery should be replaced. Pacing implantable pulse generators (IPGs) manufactured by Medtronic, Inc., for example, typically monitor battery energy and depletion and develop an “elective replacement indicator” (ERI) when the battery depletion reaches a level such that replacement will soon be needed to avoid further depletion to a battery “end of life” (EOL) condition. Operating circuitry in the pacing IPG typically responds to issuance of an ERI by switching or deactivating operating modes to lower power consumption in order to maximize the ERI-to-EOL interval. For example, internal diagnostic functions and advanced rate-response functions may be discontinued upon issuance of ERI. Additionally, pacing IPG may switch to a relatively low rate, demand pacing mode upon issuance of the ERI as described in commonly assigned U.S. Pat. Nos. 4,390,020, 5,370,668, and 6,016,448, for example. Moreover, the battery impedance, voltage and other indicators of the level of battery depletion can be interrogated during a telemetry session and uplink telemetry transmitted for display and analysis employing the programmer as described above.
In pacing IPGs that monitor battery depletion and provide an ERI, it is important that there be sufficient time between triggering of ERI and complete battery depletion (battery EOL), so that the pacemaker will continue to operate for at least some minimum amount of time after issuance of an ERI. In this way, the physician will have sufficient time to take appropriate action, e.g., to replace the device before battery EOL. At the same time, it is also important not to trigger ERI too early or due to transient faults, since it is desirable that the sudden operational changes associated with ERI not be made until it is actually necessary to do so. Consequently, efforts have been undertaken to avoid issuing an ERI when transient battery states occur that could trigger issuance of the ERI as set forth in the above-referenced '668 patent, for example, or to derive multi-level ERI indicators as set forth in the above-referenced '448 patent, for example.
In addition, efforts have been made to derive and provide the physician with a reliable estimate of remaining battery life between the ERI and EOL, sometimes characterized as an elective replacement time (ERT) or a recommended replacement time (RRT) as described in U.S. Pat. No. 5,620,474, for example.
Lithium-Iodine batteries are among the most commonly used power sources in pacing IPGs, and much has come to be known about their depletion characteristics. In particular, it is well known in the art that the output voltage from Lithium-Iodine batteries is relatively flat during early stages of depletion, but drops off rather sharply before EOL. This is due in part to the internal resistance of Lithium-Iodine batteries, which is relatively linear as a function of energy depletion until near EOL, at which time the resistance curve exhibits a “knee” where internal resistance begins to rise rapidly. The voltage, capacitance, and impedance characteristics of various Lithium-Iodine cells exhibited over their life times from beginning of life (BOL) are described further in commonly assigned U.S. Pat. No. 5,391,193 and in the above-referenced '668 and '448 patents, for example.
The Lithium-iodine battery impedance is history-dependent, i.e. the battery impedance at a point in time following a high rate of discharge of the battery differs from the battery impedance that would be exhibited at the same point in time at a lower rate of discharge. Thus, it is necessary to track the accumulated discharge or current drain that the battery is subjected to from BOL in order to predict the time to ERT or RRT with less uncertainty.
The prior art fairly consistently observes that it is necessary in some way to employ all of these factors in assessing the state of discharge of the Lithium-Iodine battery due to its discharge characteristics However, the use of voltage measurement alone was suggested in U.S. Pat. No. 4,313,079 whereby a battery depletion monitor employs a CMOS inverter to compare the battery voltage to a reference voltage. When the reference voltage exceeds the measured battery voltage, the inverter changes state to indicate battery depletion. However, the loaded terminal voltage of a Lithium-Iodine battery can vary significantly depending upon current consumption due to the internal impedance characteristics discussed above. Thus, if relatively little current is drawn from the Lithium-Iodine battery for a period of time when the battery is nearing but has not reached the ERI point, a sudden prolonged period of high demand on the battery may cause a situation in which too little time is available between the ERI and EOL of the battery. For a particular pacing IPG and lead combination in a given patient, there will be a variation in the effective load on the Lithium-Iodine battery, and a resulting variation in the overall current drain.
Accordingly, if ERI is predicated upon sensing the voltage of the Lithium-Iodine battery and detecting when it drops below a certain level, there can be very little assurance that the level chosen will correspond to the knee of the internal resistance curve. It is therefore necessary to select a high threshold voltage and that unduly shortens the useful life of the pacing IPG.
Many other approaches have been described in the prior art for estimating the remaining life until ERI from measured Lithium-Iodine battery voltage and impedance and also including data related to the operating history of the IMD, e.g., cumulative delivered pacing pulses in the case of pacemake
Busacker James W.
Crandall Kathleen U.
Jain Mukul
Kleckner Karen J.
Merritt Donald R.
Chapik Daniel G.
Layno Carl
Medtronic Inc.
Wolde-Michael Girma
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