Power Sources and Capacitors for Pacemakers and Implantable Cardioverter-Defibrillators

Introduction

This chapter is about batteries and capacitors used to power pacemakers, defibrillators, and other similar implantable devices. Batteries are active components that convert chemical energy into electrical energy, whereas capacitors are passive and temporarily store energy, often to increase the available power (rate of energy delivery) in an electrical circuit. The purpose of this chapter is to communicate useful information that will help clinicians manage patients who have implanted medical devices that require electrical power.

The battery is conceptually different from the other components of an implantable medical device. In principle, the other components are designed to last indefinitely. However, for current cardiac devices such as pacemakers and defibrillators, the available chemical energy of the battery is consumed during its use. Eventually, the output of the battery becomes insufficient to operate the electronics and must be replaced. At present, batteries for implantable cardiac devices are part of the device and the entire pulse generator must be replaced to renew the battery. However, this is not true of all implantable pulse generators. Many neurologic stimulators now use rechargeable batteries. In the future, more implantable devices may also come to use rechargeable batteries, so the energy powering the device can be renewed.

Capacitors that are used to intermittently boost the power capability of electronic circuits are of principal interest here. The large capacitors in an implantable cardioverter-defibrillator (ICD) allow the device to deliver a therapeutic, high-voltage, high-energy shock to the heart over a period of a few milliseconds, something the battery itself could not do.

We believe that understanding the properties and limitations of the components used to power implantable devices will lead to a better understanding of the devices themselves and of how to best manage patients with these devices.

Batteries

Basic Function and Electrochemistry of Batteries

Energy Storage in Batteries

A battery converts chemical energy into electrical energy. The source of this energy is the electrochemical reactions that occur within the battery. The type and amount of the materials participating in these reactions determine the deliverable energy of a battery.

Chemical Reactions

During a spontaneous chemical reaction, substances react to form more stable products. For example, in combustion, a fuel combines with oxygen from air to form products like water, carbon dioxide, and other species. The fuel is oxidized during this process (meaning it loses electrons). Oxygen from air is reduced (meaning it gains electrons). The exact amounts of each that react are determined by the stoichiometry of the reaction. Equations 8-1 to 8-4 are examples of different types of reactions in which oxidation and reduction occur. They are called redox reactions because electrons are transferred from one reactant to another.

2 H_{2} + O_{2} \to 2 H_{2} O (formation of water)

$2 H_{2} + O_{2} \to 2 H_{2} O (formation of water)$

{CH}_{4} + 2 O_{2} \to {CO}_{2} + 2 H_{2} O (combustion of methane)

${CH}_{4} + 2 O_{2} \to {CO}_{2} + 2 H_{2} O (combustion of methane)$

4 Fe + 3 O_{2} \to 2 {Fe}_{2} O_{3} (rusting of iron)

$4 Fe + 3 O_{2} \to 2 {Fe}_{2} O_{3} (rusting of iron)$

2 Li + I_{2} \to 2 LiI (discharge of a lithium / iodine pacemaker battery)

$2 Li + I_{2} \to 2 LiI (discharge of a lithium / iodine pacemaker battery)$

Chemical reactions like these occur spontaneously because the products are in a lower energy state than the reactants. The difference in energy appears as heat in the case of combustion or rusting. A battery or galvanic cell is designed to convert much of this energy difference into electrical energy rather than heat. A battery operates because the electrons transferred during a redox reaction are conducted from one terminal of the battery through the external circuit and back to the battery through its other terminal. The maximum work these electrons can do outside the battery is related to the difference in a thermodynamic quantity called the free energy of the reaction.

Major Components of Batteries

Figure 8-1 shows a simple battery. The major parts of a battery are the anode, the cathode, and the electrolyte. The anode and cathode must be physically separated and both must be in contact with the electrolyte.

Figure 8-1, Schematic Representation of Sealed Battery.

Anode and Cathode

The two battery components involved in the electrochemical reaction during discharge are the anode and the cathode. The anode, lithium in most medical batteries, furnishes electrons to the external circuit, whereas the cathode, usually an oxide or halogen rich compound, receives the electrons.

The anode is defined as the electrode at which electrochemical oxidation occurs, and the cathode as the electrode at which electrochemical reduction occurs. This is true in all electrochemical cells. In a spontaneous galvanic cell such as a battery, the anode is electrically negative and the cathode positive. This brings up a seeming inconsistency. In nonspontaneous electrochemical reactions, such as those that occur at pacing electrodes, electrochemical reactions are driven by an externally applied voltage. In this case oxidation (loss of electrons) will occur at the positive electrode and reduction (gain of electrons) at the negative electrode. Thus for pacing electrodes, the anode is positive and the cathode is negative. The polarities are reversed, but the underlying electrochemical processes defining anode and cathode, namely, oxidation and reduction, respectively, are the same. The terminology is entirely consistent.

Electrolyte

The anode and the cathode must be separated from each other so they cannot directly react with one another, but they also need to be connected by an ionically conducting medium called the electrolyte. As the battery discharges, it furnishes electrons at one terminal, pushes them through an external circuit, and receives them back at a second terminal. The electrolyte allows electrical charge, as ions, to flow within the battery, completing the circuit. The electrolyte must conduct ions but not electrons. If the electrolyte conducted electrons, the battery would be shorted internally just as if the terminals were connected by a metal wire. Batteries with lithium anodes, like those used to power pacemakers and ICDs, must use nonaqueous electrolytes because lithium readily reacts with water. Most lithium-based batteries employ a mixture of organic ethers and esters as solvents for the electrolyte. A dissolved lithium salt is used to make the electrolyte conductive. For example, lithium/manganese dioxide batteries, in consumer products, often use electrolytes containing a lithium salt dissolved in dimethoxyethane ether and propylene carbonate ester.

Separator

The separator is a structural member of the battery that keeps the anode and cathode materials physically apart, thus preventing shorting of the battery. In batteries with liquid electrolytes, the separator is usually a porous polymer film that is immersed in and permeated by the electrolyte. In the case of lithium/iodine batteries, traditionally used for pacemakers, the separator and electrolyte are one and the same, namely, the growing layer of solid, ionically conductive lithium iodide discharge product.

Current Collector

The current collector makes the connection between the positive or negative terminal of the battery and its respective active electrode material inside the cell. A current collector is usually a wire connected to a screen, or grid, which is embedded in the anode or cathode material. The current collector may also serve as a structural member of the battery to provide physical integrity and strength to that electrode. Some medical batteries use the case of the cell as the current collector for one of the electrodes. Batteries are termed “case-negative” if the anode is in contact with the case and “case-positive” if the cathode is in contact with the case.

Sealing of Batteries

Batteries used to power implantable pacemakers and ICDs are well sealed, most often in hermetically welded containers with glass electrical feedthroughs to make the electrical connections between the inside and the outside of the battery. Sealing is necessary to prevent any interchanges of materials between the battery and its surroundings. Medical batteries are typically considered hermetically sealed if the leak rate out of the battery for a test gas, usually helium, is less than 1 × 10 ⁻⁷ cm ³ /sec at one atmosphere pressure difference between the inside and the outside of the battery.

Classification of Batteries

Batteries may be classified in many ways. Categories may include application, functional characteristics, chemistry, or the physical state of a certain component. One fundamental distinction is between primary and secondary batteries.

Primary Batteries

Primary batteries can only be used once. They are not designed to be recharged, and in fact, it is dangerous to attempt to recharge them. A familiar example of a primary battery is the alkaline zinc/manganese dioxide flashlight battery. Most batteries used to power modern implantable medical devices are primary batteries that use lithium anodes because such batteries have very high energy densities.

Secondary Batteries

Secondary batteries are designed to be repetitively discharged and recharged. Familiar examples of secondary batteries include lead-acid batteries used in automobiles and lithium-ion batteries widely used in portable electronic devices, home appliances, power tools and even some electric vehicles.

Lithium-ion batteries are the most relevant secondary medical battery. They have found application as supplemental power sources in some left ventricular assist devices, and they have become widely used to power implantable neurologic stimulators for control of chronic pain. Although they are not currently in development for pacemakers and ICDs, features such as more frequent telemetry could cause manufacturers to reconsider lithium-ion battery use in cardiac applications in the future.

Functional Characteristics of Batteries

Capacity

The fundamental unit of battery capacity is the coulomb (C) or ampere-second (A-sec). This is the amount of charge delivered by one ampere (A) of current in 1 second. In the context of implantable devices, it is customary to express battery capacity in terms of ampere-hours (A-hr), which represents the charge carried by a current of one ampere flowing for 1 hour. One ampere-hour is equivalent to 3600 coulombs. Batteries for implantable medical devices can range in capacity from a small fraction of an ampere-hour to 7 ampere hours. As capacity has a strong correlation to useful service life of a device, smaller capacity batteries are used in devices with shorter implant durations, smaller capacity needs for therapy and electronics, or in devices that can be recharged. Many papers have been written about ways of determining and specifying the capacity of a medical battery. Various methods produce numbers ranging from upper-limit theoretical values that can never be achieved in the field to very realistic values that are based on detailed models or accelerated testing. More sophisticated methods account for limited availability of the components within the battery, kinetic limitations on the electrochemical reaction, and losses to side-reactions over time.

Estimating the deliverable capacity of implantable medical batteries is made especially difficult because of their long service life. The time frame for the operation of most implantable medical devices is so long (5-10 years) that real-time measurements of capacity are not practical. Therefore accelerated tests and models are typically used to estimate the amount of deliverable capacity in these batteries. Technology in this area is now well developed, and it is possible to make highly accurate projections of deliverable battery capacity under a range of usage conditions.

Energy and Energy Density

The fundamental unit of energy is the joule (J). This is the energy given to one coulomb (1 C) of charge that is accelerated by a difference in potential of one volt (1 V). One joule is also the energy transferred by one watt (1 W) of power in one second. Just as battery capacity is often measured in ampere-hours, battery energy is often expressed in watt-hours (W-hr) instead of joules. One watt-hour is equivalent to 3600 J.

An important battery parameter in the design of an implantable device is its energy density, a quantity that can be expressed on either a mass or a volume basis. For medical applications, volume is usually more important than mass, so ratings based on volumetric energy density are most commonly used. The time integral of the product of voltage and current divided by the total volume of the battery is its energy density. Modern batteries for implantable devices have energy densities as high as one W-hr/cm ³ , including the case.

Stoichiometry and Cell Balance

The specific amount of anode and cathode materials that will react is determined by the stoichiometry of the battery reaction. A cell that contains exactly the required ratio of anode and cathode materials is said to be a balanced cell. However, most medical batteries are not designed with exactly the stoichiometric ratio of the active cathode and anode in order to provide predictable end-of-service characteristics. In many cases the anode (lithium) is in excess so the voltage decrease near the end-of-service life is not too abrupt.

Cell Voltage and Current

The open-circuit voltage of a battery can be calculated from the thermodynamic free energy for the discharge reaction. This is the voltage that will be measured when there are no kinetic limitations, a condition that occurs only when an insignificant amount of current is being drawn from the battery. With the onset of current flow, the voltage at the battery terminals will be smaller than the open-circuit value. Both chemistry and battery design determine the relationship between voltage and current drawn from the battery. For example, a lead-acid battery for automotive use is constructed of very conductive materials and is designed with large, high surface area electrodes so that extremely high currents can be drawn from it to run an engine's starter motor. On the other hand, a transistor radio battery is designed with small electrodes because relatively low currents are typically needed to power small, portable electronic devices. A typical current-voltage relationship is shown in Figure 8-2 .

Figure 8-2, Typical Load Voltage Versus Current Drain Plot for a Battery.

In Figure 8-2 the load voltage approaches the open-circuit voltage as the current approaches zero. At the other extreme, the maximum (short-circuit) current is observed when the load voltage approaches zero.

Internal Resistance and Impedance

Electrical impedance and resistance are important battery properties that play a significant role in the clinical performance of many implantable devices. The terms impedance and resistance are often used interchangeably, but are not precisely the same. Both are terms for the change in voltage per unit change in current in an electrical circuit, but they are measured under different conditions. Impedance is the more general term, encompassing effects of resistance, capacitance, inductance, and other circuit elements on the relationship between voltage and current. The resistive component of impedance is measured using direct current methods. Alternating current and transient methods are used to measure the additional components of impedance besides resistance.

For simple, resistive, electrical circuit elements, Ohm's law, V = IR, accurately describes a linear relationship between voltage drop, V, and the corresponding change in current, I, with resistance, R, as the proportionality constant. However, a battery is a complex electrochemical device with several time-dependent and/or nonlinear processes operating in series/parallel combinations. Different processes may dominate at different current levels, depths of discharge and time. Consequently, the relationship between current and voltage for a battery is, in general, nonlinear, even at very low currents. Even though this relationship is sometimes characterized by a quantity called R _DC , Ohm's law should only be applied to batteries with caution.

Nonideal Battery Behavior

The previous discussion has focused on the principles and the nomenclature of battery operation. It is also important to understand some of the things that limit a battery's ability to power an implantable device.

Polarization

Polarization is any process that causes the voltage at the terminals of a battery to drop below its open-circuit value when it is providing current. The internal resistance of the battery is one important cause. This is well illustrated in Figure 8-3 for the lithium/iodine battery, but the same is true for all batteries to some degree.

Figure 8-3, Load Voltage Versus Capacity Plot.

This figure shows the curves for discharge voltage versus capacity at four constant current discharge rates. The differences in these curves are mainly caused by the voltage drop associated with the internal resistance of the battery. Other contributing elements of voltage loss when a battery provides current are concentration polarization, which is associated with concentration gradients that may develop in the electrolyte or the active electrode materials, and activation polarization, which is associated with the kinetics of the electron-transfer reactions at the electrode/electrolyte interfaces.

When current is drawn from a battery, all of these processes occur to some extent. The net effect of these kinetic limitations is always observed as a decrease in the voltage at the terminals of the battery. In general, neither concentration polarization nor electron-transfer polarization conforms to Ohm's Law.

Self-Discharge and Other Parasitic Reactions

Self-discharge is the spontaneous discharge of a cell or battery by an internal chemical reaction rather than through useful electrochemical discharge. We have all seen the effects of self-discharge at one time or other. The flashlight that does not work when needed after prolonged storage is a good example. One mechanism by which self-discharge can occur involves a slow direct reaction between the anode and the cathode. This can occur if one or both of the active electrode materials are very slightly soluble in the electrolyte. Other self-discharge processes may involve reactions between either the anode or the cathode and another substance in the battery, such as the solvent in the electrolyte. These are often called parasitic reactions. A typical example of this would be a reaction between the anode and the electrolyte solvent to form a passive film on the lithium anode or to form a gas that pressurizes the battery case. These parasitic reactions are usually very slow, but because medical batteries are expected to operate for many years, their accumulated effects can be appreciable. Some parasitic reactions, such as the reaction between lithium and electrolyte, may not become apparent for a long period of time because implantable batteries are typically designed with an excess of lithium. Although it is hard to measure the very slow rate of self-discharge or other parasitic reactions, techniques such as microcalorimetry, which can detect the small amounts of heat that are involved, have been used for this purpose. The heat generated can be used to calculate the rate of self-discharge by applying thermodynamic principles. Assessment of self-discharge is an important element in the creation of accurate predictive models of battery performance that can be used both in device design and longevity estimation.

Use of Batteries in Implantable Cardiac Rhythm Management Devices

Implantable Battery Design Requirements

Implantable medical devices must be thought of as a system. The most important requirement in battery selection for implantable devices is high reliability. Other significant factors include the desired longevity of the device (directly related to battery energy density, circuit design, and overall device size) and an appropriate indication of impending battery depletion (end-of-service warning). The basic considerations when designing a battery for an implantable medical device include the current variations that can be expected from the circuit as the device provides its service for individual patients. Once the range of these application requirements are defined, the current, voltage, and capacity requirements of the battery can be determined. An important limitation is the physical space, both volume and shape, within the device that can be allotted to the battery. Once this is determined, the required energy density can be calculated, and various battery systems and designs can be considered and evaluated. To put these decisions about the battery in perspective, one needs to realize that the battery occupies about a third the volume of an ICD. The circuitry occupies another third, and the capacitors fill the remaining volume. In a pacemaker, the battery and circuitry each occupy about half the volume.

Power Requirements

An important parameter for a device is the peak power requirement. For example, the rate of energy consumption differs markedly for pacemakers and defibrillators. Pacemakers use very small amounts of energy when they stimulate the heart, on the order of 15 µJ. Defibrillators, on the other hand, deliver as much as 40 J when they give a defibrillation shock. A battery optimized for a pacemaker could never come close to supplying energy at the rate required to power a defibrillator. Likewise, a defibrillator battery is not an optimum choice to power a pacemaker, although it could easily supply the current needed. This last statement is true because the high-power design of a defibrillator battery will have a significantly lower energy density than that of a pacemaker battery, by as much as a factor of two. Thus if a defibrillator battery was used primarily for pacing, and everything else was equal, it would need to be twice as big as an optimized pacemaker battery to obtain the same longevity. Optimizing a battery for longevity and power becomes more complicated when a device performs multiple functions such as both bradycardia pacing and defibrillation. For example, up to now, the lithium/iodine battery has been the dominant power source for implantable cardiac pacemakers, which typically have peak power demands on the order of 100 to 200 µW. Under these conditions, the lithium/iodine battery can maintain an adequate voltage even when its internal resistance reaches several thousand ohms. On the other hand, an implantable cardiac defibrillator may have peak power requirements approximately 10,000 times greater than those of a pacemaker. Under such a high power demand the voltage of a lithium/iodine battery would drop to almost zero and the power delivered to the device would be almost nil.

In recent years the distinction between a need for high- and low-rate batteries has become somewhat more blurred because features like distance (“wireless”) telemetry and multisite pacing need both more current and more capacity to operate. The result is that battery designers have been challenged to develop more medium-rate batteries that can deliver more power than pacemaker batteries of the past while still having a high energy density.

Average Versus Instantaneous Current Drain

Implantable medical devices are often characterized by their average current drain. However, average current drain does not tell the entire story because certain events, such as therapy delivery and telemetry, may require temporary current excursions that can be very different from the average value. Sometimes the effect of the instantaneous current demand can be mitigated by the use of a capacitor that buffers the battery to allow short bursts of power that may be more than two orders of magnitude higher than it could deliver directly. This allows the use of battery designs with reduced anode and cathode surface areas and improved volumetric efficiency. The same principle involving a battery plus a capacitor is used to deliver the defibrillation therapy to the heart from an ICD even though the magnitudes of the current and voltage are much greater.

Shape, Size, and Mass Constraints

Finally, all device requirements (longevity, end-of-service indication, peak power, and so on) must be balanced against device volume, shape, and mass. Volume and thickness are particularly important for safe implantation in children and for esthetic reasons in many adults.

The ratio of volume to surface area in the battery is an important parameter. The performance of a battery of fixed volume will vary substantially depending on its electrode surface area-to-volume ratio. The operating current and longevity demanded of the battery determine both the minimum areas and the amounts of the anode and the cathode needed. Increasing the electrode area-to-volume ratio increases current capability, but this ratio must not be made too large or the battery will be too costly, have a diminished energy density, and most likely have greater complexity (a cause for reliability concerns).

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