Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-11-21
2004-07-06
Bockelman, Mark (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06760625
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of implantable medical devices and, in particular, to a cardiac stimulation device employing a CF
x
battery for improved performance and a system for monitoring charge level of the battery.
BACKGROUND OF THE INVENTION
Implantable medical devices are typically battery powered devices that are implanted within the patient's body to have therapy available to the patient on a continuous basis. Battery failure is a particular problem with these devices as replacement of batteries often requires invasive surgical procedures. One particularly common type of implantable medical device is an implantable cardiac stimulation device.
Implantable cardiac stimulation devices, such as pacemakers and intra-cardioverter defibrillators (ICD's), are employed to monitor cardiac activity and to provide therapy for patients with a variety of heart arrhythmias. Typically, these devices include sensors, that sense heart function and physiological parameters, and waveform generation and delivery systems, that provide electrical waveforms to the heart to correct arrhythmias and to ensure that more proper function of the heart is maintained. As the devices are implanted in a patient, it is desirable that the devices be as small and lightweight as possible in order to minimize impact on the patient.
Implantable cardiac stimulation devices are typically provided with batteries to power the monitoring and therapy delivery circuits. Due to the size constraints, the batteries used in implantable cardiac stimulation devices must be very small in size and yet able to provide power over a long period of time. Once the device is implanted, replacement of batteries typically involves invasive surgery. Hence, there is a strong desire to have small batteries that can provide significant power output to power the implantable device for extended periods of time. Known pacemaker devices typically use lithium iodine (Lil), commonly referred to as lithium batteries. Lithium batteries offer relatively high energy storage density and have known, predictable discharge characteristics.
While lithium batteries are commonly used for implantable cardiac stimulation devices, these batteries require additional circuitry that degrade device performance. Specifically, the performance characteristics of these batteries often require that additional circuitry be added to the device, thereby resulting in consumption of limited space in the implantable device and also consumption of limited power, or this additional circuitry has performance characteristics that limit the useful life of the implantable device.
For example,
FIG. 1
illustrates a high-level conventional pacemaker circuit diagram of the prior art. Lithium batteries are typically not capable of providing pacing pulses at increased energy levels. As is shown in
FIG. 1
, a typical lithium battery and a decoupling capacitor are often connected in parallel to address this problem. The decoupling capacitor is used to accumulate electrical charge between pacing pulse events to enable the pacemaker to periodically deliver a pulse of energy greater than a lithium battery is capable of providing directly. The decoupling capacitor is continuously charged by the lithium battery and partially discharged upon a pacing event.
However, known implementations of the decoupling capacitors of the requisite electrical properties occupy a large fraction of the overall volume of the pacemaker device. As pacemakers shrink in size due to product refinement, the size of the capacitor is becoming an increasingly larger proportion of the total pacemaker volume and is presenting a limitation to further reduction in the size and weight of pacemaker devices.
FIG. 1
illustrates another aspect of known pacemaker designs, in particular, a voltage tripler that is part of the control circuitry for the pacemaker. The control circuitry performs the basic timing and monitoring functions of the device and delivers the pacing pulses to the patient's heart. The voltage tripler increases the voltage delivered by the lithium battery in order to provide a sufficient potential for effectively stimulating the heart. A lithium battery will have an open circuit voltage of approximately 2.7 VDC in a fully charged condition and approximately 2 VDC near the end of its life and thus requires a voltage tripler to generate the more than 5 VDC required for an effective pacing pulse. However, the voltage tripler is a source of overall system inefficiency as each voltage multiplication incurs some degree of loss.
A further drawback to the lithium battery will be apparent considering the output voltage characteristics illustrated in
FIG. 2
, which shows a typical voltage vs. charge delivery graph for typical lithium batteries. Multi-chamber pacing is a feature of many pacemaker systems and comprises supplying pacing stimuli to two different sites in the heart as opposed to pacing a single site. Typical parameters for a single-chamber pacing system with a lithium iodide battery at beginning-of-life would be an open circuit voltage of approximately 2.7 V (almost 8.1 V after the voltage tripler) with an internal battery impedance or equivalent series resistance (ESR) of 300&OHgr; and a single lead of 500&OHgr; impedance. The voltage across the lead is regulated to be approximately 5 V and the pulsed current would be approximately 10 mA. Pulses are approximately 1 ms in duration and are applied every second, thus drawing a time averaged current of approximately 10 &mgr;A.
Similar multi-chamber pacing to two sites through two 500&OHgr; leads connected in parallel would draw a current pulse of approximately 20 mA and a time average current of 20 &mgr;A. However, as a lithium battery is discharged, the open circuit voltage drops while the ESR increases. After supplying approximately 900 mA-h, a typical lithium battery's output voltage decreases to approximately 2.4V and the ESR increases to approximately 10 k&OHgr;. Under these parameters, delivering to two leads with 20 &mgr;A average current pulls the battery output voltage down to 2.2 V (2.4 V−20E-6 A×10E3&OHgr;) and thus approximately 6.6 V after the tripler. These battery conditions give marginal performance even with the voltage tripling.
With further use, i.e., further discharge of the lithium battery, the open circuit voltage continues to decrease and the ESR continues to increase to approximately 30 k&OHgr;&mgr; at EOL. Thus, while a lithium battery in this condition still has considerable charge remaining, the internal impedance and voltage at which the charge is available render a lithium battery unsuitable for continued multi-chamber pacing. Because of this factor, approximately 30-50% of the total typical lithium iodide battery's capacity is not usable and is wasted.
Thus, the typical lithium/lithium iodine battery currently in use in many implantable cardiac stimulation devices generally requires additional components to deliver the power needed to provide therapy and also has a limited life span in some implementations. Limited life span, of course, requires more periodic follow up and also requires more frequent replacement of the device. As stated above, more frequent replacement of the device is undesirable as it typically requires invasive surgical procedures.
A further difficulty that occurs with lithium/lithium iodine batteries in implantable medical devices is that the internal configuration of the battery often limits telemetry transfer rates. The power that the lithium battery provides is generally not sufficient to support data transfer rates from implantable cardiac stimulation devices that are in excess of approximately 8 Kbaud of data. Typically, the decoupling capacitors are limited to only providing sufficient power to source the pacing pulses but do not have the capacity for providing sufficient charge to maintain voltage during a multi-minute, high speed transmission. This relatively low rate of data transfer therefore requires longer download per
Bockelman Mark
Pacesetter Inc.
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