Power Systems for Implantable Pacemakers, Cardioverters, and Defibrillators

6 Power Systems for Implantable Pacemakers, Cardioverters, and Defibrillators



This chapter discusses batteries and capacitors used to power pacemakers, defibrillators, and similar implantable devices. Batteries are active components that convert chemical energy into electrical energy. Capacitors are passive and temporarily store energy, often to increase the available power (rate of energy delivery) in an electric circuit. This chapter provides practical information to help physicians 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 use rechargeable batteries so that the energy powering the device can be renewed.


While batteries transform chemical energy into electrical energy, capacitors act as energy storage devices. Capacitors that 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 few milliseconds, which the battery itself could not do. Understanding the properties and limitations of the components used to power implantable devices will help physicians not only better understand the devices, but also deliver the best care to their patients.



image Batteries



Basic Function and Electrochemistry





Major Components Of Batteries


Figure 6-1 shows a simple battery; the major parts are the anode, cathode, and electrolyte. The anode and cathode must be physically separated, and both must be in contact with the electrolyte.










Functional Characteristics Of Batteries






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-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. Figure 6-2 shows a typical current-voltage relationship in which the load voltage approaches the open-circuit voltage (OCV) 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 the same. Both are terms for the change in voltage per unit change in current in an electric 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 electric 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 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. Although this relationship is sometimes characterized by a quantity called RDC, Ohm’s law should only be applied to batteries with caution.



Non-Ideal Battery Behavior


In addition to the principles and the nomenclature of battery operation, it is also important to understand factors 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 6-3 for the lithium/iodine battery, but the same is true for all batteries to some degree. The differences in the curves for discharge voltage versus capacity at four rates of constant current discharge are mainly caused by the voltage drop associated with 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 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.




Batteries in Implantable Cardiac Rhythm Management Devices




Power Requirements


An important parameter for a device is the peak power requirement. For example, the rate of energy consumption differs greatly 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 40 J for 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. The high-power design of a defibrillator battery has a significantly lower energy density than that of a pacemaker battery, by a factor of as much as two. Thus, if a defibrillator battery was used primarily for pacing, and everything else were equal, it would need to be twice as large 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 of 100 to 200 µW. Under these conditions, the lithium/iodine battery, which is still used in many pacemakers, 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 an Li/I2 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-rate and low-rate batteries has become more blurred because features such as 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 medium-rate batteries that can deliver more power than pacemaker batteries of the past while still having a high energy density.





Size, Energy Density, and Current Drain


The relationship between battery size and average current is not one of direct proportionality. For example, decreasing the average current by 50% will not permit a 50% reduction in battery size without compromising longevity because of the inactive materials in a battery (e.g., case, electrolyte, current collectors). Likewise, the usable energy density is also a function of the current demand on the cell. As the current from the cell is increased, the resulting voltage drops significantly (see Fig. 6-3), which reduces the time during which the cell can provide current at or above the minimum voltage necessary to operate the electronic circuits. Thus, usable energy density, which is directly proportional to the area under the discharge (voltage vs. capacity) curve, is also reduced. For very-high-rate batteries such as those used to power ICDs, this is not such an issue because their internal resistance is extremely low.



The Battery and Longevity of Pulse Generator


Longevity is typically defined as the interval between device implantation and detection of the end-of-service indicator. Because therapy can vary substantially from patient to patient, the longevity requirement is typically linked to a specified set of nominal conditions and programmed parameters. The minimum battery capacity required to achieve the specified longevity can be calculated from the average current needed for this nominal set of conditions. The following equation relates the longevity of the pulse generator, L, to the deliverable capacity of the battery, Qdel, and the average pacing current, I:



(6-5) image



The unit of L is years, Qdel is in milliampere-hours (mA-hr), and I in milliamperes (mA). The conversion factor 8766 (365.25 days/yr × 24 hours/day) is needed because longevity is expressed in years, not hours.


The actual capacity that is built into the battery must be larger than Qdel, because additional capacity is needed to account for self-discharge and other parasitic losses of capacity (Qsd). More capacity must also be included to allow for an interval between the end-of-service indicator and the time when the battery can no longer power the device (QEOL). The total capacity (QTotal) is defined as follows:



image



The average current drain in Equation 6-5 depends on the characteristic of the pulse generator circuitry and the requirements for therapy. It has two main components: the static current drain, which powers the electronic components even when no therapy is delivered, and the therapeutic current. The trend throughout the evolution of implantable devices has been that current demands decrease as technology is improved, which leads to smaller batteries and pulse generators while maintaining relatively constant longevity. Some expect that this trend will continue, but the path to lower current often has a sawtooth profile because new features and therapeutic modalities temporarily increase the required current. For ICDs, the situation is more complicated because of the unpredictable mixture of bradycardia and tachyarrhythmia therapy delivery and the constant need to have a very high power capability in any device that may be required to deliver a defibrillation shock quickly.



Effect of Pulse Width on Pacing Current


Increasing the pacing rate, pulse width, or pulse amplitude increases the average pacing current. The average pacing current, excluding static current, is directly proportional to the pacing rate. Recall that the pacing pulse results from the discharge of a capacitor through the electrode-heart interface. This capacitor produces a pulse in which the current decays exponentially with time, as shown for two different pulse widths in Figure 6-4.



Thus, the time-dependent behavior of the current during the pacing pulse is given by the following equation:



(6-7) image



where VA is the amplitude at the beginning of the pacing pulse, RH is the impedance of the lead plus the heart (discussed later), C is the value of the capacitor that delivers the pacing pulse, and t is the time since the beginning of the pacing pulse. In Figure 6-4, A and B, the pulse width is tw and tw/2, respectively. The area under each current-time curve gives the total charge delivered during the pulse. Although the width of the pulse in B is half that of the pulse in A, the charge delivered by this pulse is considerably more than half that of the longer pulse. The exact ratio of the charge delivered in the two cases depends on the values of RH and C. Nevertheless, reducing the pulse width by a given fraction will always reduce the average pacing current by a substantially smaller fraction because of the exponentially decaying shape of the pacing stimulus current curve.



Effect of Pulse Amplitude on Pacing Current


The definition of pacing pulse amplitude may vary somewhat between manufacturers of implantable pulse generators. For our purposes, pulse amplitude is defined as the voltage delivered to the heart at the beginning of the pacing pulse (“leading edge” voltage). As stated earlier, the area under the current-time curve gives the charge delivered per pulse. Thus, doubling the amplitude doubles the current and the total charge delivered to the heart. Also, because the charge per pulse is doubled, it might seem that the average pacing current drawn from the battery would also be doubled. However, the impact on the pacing current is much larger than that, as seen from the following argument. The energy per pacing pulse is defined as follows:



(6-8) image



In Equation 6-8, VA is the average pacing stimulus output voltage of the pulse generator, IA is the average pacing current delivered to the heart, and tw is the pulse width. If we consider the lead-electrode-heart interface to be mainly resistive, Ohm’s law, I = V/R, can be substituted in Equation 6-8, which becomes the following:



(6-9) image



In Equation 6-9, RH

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Jun 4, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Power Systems for Implantable Pacemakers, Cardioverters, and Defibrillators

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