In the rapidly evolving field of treating cardiac arrhythmias, the importance of direct management of patients with implantable cardiac devices is growing. The devices have become increasingly complex, and understanding their algorithms and growing programming options is essential for physicians who implant and manage them. Written by experts and world authorities in the field, Pacemakers and Implantable Cardioverter Defibrillators: An Expert's Manual provides electrophysiologists, fellows in training, nurses, and cardiovascular technicians involved in day-to-day management of device patients with detailed information about the many device algorithms and interactions. *Heavily illustrated with over 300 figures and tables *Uniquely meets the day-to-day needs of all direct management professionals *Focuses in detail on algorithms *Describes device interactions, addressing every major manufacturer *Provides in-depth insight into pacing, including biventricular pacing *Discusses arrhythmia detection and device classification, testing, and therapy
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Pacemakers and Implantable Cardioverter Defibrillators
An Expert's Manual
By Amin Al-Ahmad, Kenneth A. Ellenbogen, Andrea Natale, Paul J. Wang
Cardiotext Publishing, LLCCopyright © 2010 Amin Al-Ahmad, Kenneth A. Ellenbogen, Andrea Natale, and Paul J. Wang
All rights reserved.
The Basics of Pulse Generator Design and Engineering
Hugh Thomas McElderry and John C. Evans
Pacemaker Pulse Generator
In its most simplified representation, the artificial pacemaker consists of the two major components: a pulse generator and the leads connecting the generator to the heart. The major functions of the pacemaker pulse generator are to power the device and to generate the impulses that go from the leads to the heart in accordance with specified programming. The major components of the pulse generator, responsible for these functions, are the battery and the small computer controlling the output of the device, a hybrid circuit consisting of a microprocessor, resistors, and many other components. The battery and hybrid circuit are packaged together in a casing made of titanium with a translucent epoxy header to allow for connection to leads (see Fig 1.1). Because leads are discussed elsewhere, this chapter focuses on the pulse generator, specifically its components and implications for the clinical care of patients with implanted cardiac pacemakers.
The first pacemakers consisted of approximately 10 discrete components in addition to the battery. These pulse generators were asynchronous, nonprogrammable single chamber devices. Although, the pacemakers of today serve the same primary role, the components to accomplish this goal have gone through several evolutionary changes. The hybrid circuit houses the integrated circuits and components needed for pacemaker functions. The integrated circuits contain data storage and memory elements, telemetry components, sensors, analog amplifiers, analog filters, analog-to-digital converters, and the hardware responsible for delivering the pacing pulse. Every manufacturer and model pulse generator is slightly different, but they all have some basic components in addition to the microprocessor that is responsible for the programming of the pacemaker.
Sensing intrinsic activity
The analog signal of the intrinsic cardiac electrical activity needs to be converted into a digital signal for processing. Before this is accomplished the intrinsic signal is amplified and filtered. It is then converted to a digital signal and processed. Intrinsic signals can have very small amplitudes and thus require amplification. Some pacemakers have the option of setting a sensitivity as low as 0.1 mV. This requires clean amplification as well as significant filtering in order to reliably sense signals at 0.1 mV without oversensing noise. The ideal filter would allow P and R waves to be sensed unobstructed, but would filter out all other signals such as myopotentials, EMI, other noncardiac signals, as well as undesirable cardiac signals such as T waves and far-field R waves. Band pass filters are implemented, which allow only the frequencies within a certain range to be seen. Lower frequency signals such as respiration artifact and higher frequency signals such as myopotentials are filtered out. The signal is then passed through an analog-to-digital converter, which measures the intrinsic signal as a series of discrete values. The microprocessor then analyzes these signals and acts on them based on the defined programming. Feed-through filters also play an important role in decreasing the effect of electromagnetic interference in pacing systems.
Random-Access Memory (RAM) and Read-Only Memory (ROM)
The code or series of instructions the microprocessor runs is called firmware. This code can either be stored in random-access memory (RAM), read-only memory (ROM), or a combination of both. RAM is simply an array of transistors in a device. It is both readable as well as writeable by the microprocessor and thus has the advantage that it can be changed and used for short-term storage. Despite the technology used in personal computers that implement a quantity of RAM in the order of magnitude of gigabytes, billions of bytes of data, current pacemakers have a quantity of RAM in the order of magnitude of kilobytes, just thousands of bytes of data. As RAM is simply an array of transistors, each transistor requires a steady amount of current to reliably store data. It is desirable to minimize this current drain.
The quantity of RAM in a pacemaker is limited by a number of factors. The RAM used in pacemakers has an extremely low leakage current, which allows for extended device longevity. There are also physical size constraints within the pulse generator that limit the amount of RAM that can be used. Newer pacemakers are trending to collect an increasing amount of diagnostics such as rate histograms, lifetime trends, and stored electrograms. This data is stored in RAM. The relatively slow telemetry speeds of inductive pacemakers can also limit the amount of RAM, as clinicians may not be willing to wait long periods of time to download all of the data in a large quantity of RAM.
Not only is the RAM used for storing diagnostic data, the code itself is often stored on RAM. The execution of the code also occurs from the RAM space, and is used for housing variables, function call trees, and general processor operation. Even if the code is stored in the read-only memory, ROM, the device will still contain some RAM for code execution and data storage.
ROM has been in used in some pacemakers and it provides the advantage as well as the disadvantage of being able to permanently store the code into ROM. The advantage of ROM code is that it is permanent, cannot be changed, and is more robust. This provides a higher level of stability and reliability than code stored in RAM.
Sometimes the architecture of the system is designed for the use of ROM code as well as allowing the code to be upgraded if needed. Through architectural design some ROM code links to data on a RAM-based system, thus allowing for the stability of the permanent storage of the code while still allowing some sections of the code to be upgraded. If a software glitch that compromises the patient's safety is discovered, it is desirable to correct the software glitch by upgrading the code rather than explanting and replacing the system.
Another advantage of ROM is that it is not susceptible to the rate of data errors that are common in RAM failures. All current pacemakers implement RAM, and thus they all are susceptible to RAM failures. A RAM failure is also sometimes known as a bit flip whereby the data stored in a transistor, either a 0 or 1, is actually represented by the incorrect value. Technology is now available to self-correct the majority of these errors. This level of error correction does not protect against something known as a hard error. This is when a value of a bit in RAM, either 0 or 1, cannot be changed. A hard error is rare in comparison to the frequency of bit flips.
Implantable medical devices such as pacemakers and defibrillators require periodic follow-up to ensure the system is working properly. Both at routine follow-up and when an issue is suspected, telemetry allows for communication between the device and a programmer. The system is checked to ensure adequate battery voltage, lead integrity, pacing threshold, and other parameters.
Until recently, all available pacemakers in the United States communicated with programmers by way of inductive or near-field telemetry. This technology is based on inductive coupling between two closely placed coils using the mutual inductance between these coils. The first coil and circuitry is sealed in the pacemaker "can" while the second coil is located in the "wand" which connects to the programmer.
The wand allows communication in real time to and from the pacemaker and the programmer. Intracardiac electrograms, measured battery voltage, lead impedance, and current consumption in the selected pacing programs are some of the parameters that can be displayed and documented via the programmer. The wand is also used to program features such as pacing mode and parameters.
In addition to inductive telemetry, many pacemakers also have radiofrequency (RF) telemetry technology, which makes telemetry from a base station possible. Although we are unaware of malicious attacks to implantable defibrillators that use RF, it has been shown that security and privacy vulnerabilities exist.
Transtelephonic monitoring (TTM) of pacemakers, which has been performed for decades, allows the early identification of pacing system failure or battery depletion. This modality was developed to monitor the function of pacing systems at a time when pulse generator and lead longevities were much less predictable.
The typical TTM system includes wristband electrodes, a magnet, and a device to transmit the recorded strip over a regular telephone line. Each transmission is initiated with a rhythm strip (electrocardiographic lead I), after which the patient is instructed to place the magnet in the skin overlying the pacemaker in order to record the "magnet rate." The rhythm strips transmitted allow the analysis of the atrial and ventricular rhythm and rate, appropriateness of sensing and capture, and the magnet rate which can indicate the proximity to elective replacement time. Some devices can perform automatic threshold tests with magnet placement.
Advances in technology have increased the desired requirements of pacemakers. As the quantity of diagnostic data stored in the pacemaker increases, the time it takes to telemeter this data to a programmer increases as well. Faster telemetry speeds to decrease this telemetry time and improve efficiency in the clinical setting are desired. Faster telemetry speeds are possible at a cost of increased energy demands on the device. RF telemetry offers increased speed as well as freedom to perform daily remote monitoring checks without requiring the patient to place a wand over the pacemaker utilizing inductive telemetry. RF is possible again, but at the cost of increased energy. Patient notifiers, either vibratory or audible, may also be desirable features but they increase current consumption. All current and future capabilities must function within these energy limitations imposed by the next major part of the pulse generator, the battery.
The battery in a pacemaker accounts for the majority of the size and weight of the device. Battery characteristics determine the functional life of a pacemaker, as well as the capability of certain pacemaker functions. Therefore, significant effort is put into improving and optimizing battery technology.
The ideal pacemaker battery would be small and lightweight, yet contain a large amount of energy. It would have a low self-discharge rate to promote longevity. The ideal battery would not generate any gaseous byproducts in the reaction, converting chemical to electrical energy, allowing it to be hermetically sealed within the pulse generator to ensure moisture does not invade the pacemaker and damage the device. The ideal battery would also have predictable and reliable performance to ensure that the clinician could accurately determine the elective replacement time.
The volume and the mass of the battery represent a significant portion of the total pulse generator. In the case of the pacemaker seen in Fig 1.1, the battery is 2.8 cc and is 25% of the total volume of the pulse generator. Advances in technology have allowed for more efficient digital circuitry, which has enabled a smaller battery to be utilized. A smaller battery in turn translates into a smaller pulse generator with similar longevity to previous generations.
The battery produces energy that is generated by its chemical components. Lithium has the highest specific energy and thus is often used in batteries where size is important. In 1972 the first lithium-based pacemaker battery was implanted. Since the late 1970s pacemakers have almost exclusively used lithium-based batteries. Over the years a number of different lithium-based chemistries have been used, but today virtually all pacemakers use lithium iodine chemistry. The lithium iodine battery has many of the characteristics of the ideal pacemaker battery, which have lead to its adoption over the last three decades.
A number of different battery chemistries were used prior to lithium batteries. The most popular battery in first-generation pacemakers was the zinc mercuric oxide battery. Over three million mercury cells were implanted between 1960 and 1976. Each cell has a voltage of 1.35 and thus five or six batteries were often placed in series to provide the required voltage for pacing. These cells also produced hydrogen gas, which prevented the pacemaker from being hermetically sealed. Because they could not be hermetically sealed, body fluids could leak into the device resulting in sudden failure. Another characteristic rendering the zinc mercuric oxide battery less than optimal is the limited longevity caused by a high rate of self-discharge at body temperature.
Nickel cadmium battery cells were also used in the first generations of devices. These batteries needed to be recharged every few weeks. To recharge the battery a wand containing a large coil was placed over the device. As an electric current passes through a coil of wire it creates an electromagnetic field. This electromagnetic field induces a current in a similar coil of wire located in the pacemaker. This induced current was used to charge the nickel cadmium batteries.
Nuclear-powered pacemakers have been used as well. The first nuclear-powered pacemaker was implanted in 1970. Plutonium-238 was the isotope utilized, with a half-life of 89 years. The radioactive decay created heat that was then converted into electricity. In the era of the zinc mercury batteries that seldom lasted more than 2 years, the nuclear-powered pacemaker had a longevity that was limited only by the amount of radioactive plutonium contained in the pacemaker. These pacemakers were not as common as the mercury-powered batteries, and in the 1970s fewer than 3000 nuclear-powered pacemakers had been implanted worldwide.
The common lithium iodine battery has a high internal resistance that increases as the cell is discharged. This can limit the speed at which the battery can supply electrons to the circuit and thus limit the instantaneous current. Pacemakers have traditionally operated in the microampere range, and thus this characteristic of the lithium iodine battery has not been a significant limitation. Although features such as RF telemetry and patient notifiers are used infrequently, they do require a significantly higher amount of current while they are being operated and would require different battery chemistry than the common lithium iodine systems. ICDs batteries commonly have a lithium silver vanadium oxide (SVO) chemistry and have the ability to deliver current on the ampere range.
Wilson Greatbatch has designed a QMR battery, which is a mid-rate battery offering a current supply in the mA range between that of current pacemaker and ICD battery chemistries. Lithium carbon monofluoride (CFx) is a battery chemistry that offers similar characteristics of the lithium iodine cells such as the relatively flat voltage discharge curve, and high energy density. It also has a low internal resistance and has been used in some drug-infusion pumps, neurostimulators, and pacemakers. An additional benefit of the CFx technology is that the case of the battery can be made of titanium, which not only provides a weight savings over stainless steel, but it is more MRI compatible.
Given that the calculation of the elective replacement time is primarily based on pacemaker battery, it is important for the battery voltage to slowly decay as the battery nears depletion. However, for energy supply to the electronics, it is desired for the battery voltage to remain constant. The lithium iodine battery provides the combination of both a steady voltage during the life of the battery as well as predictable voltage decay near the end of the life of battery. Figure 1.2 displays the voltage decay curve of a typical lithium iodine battery. The battery voltage is approximately 2.78 V at beginning of life. The voltage remains relatively constant over the life of the battery until its precipitous decline. Approximately 90% of the stored energy in the lithium iodide battery has been depleted once the battery voltage reaches 2.65 V. A battery voltage of 2.55 V relates to approximately 95% energy consumption. The predictable decay allows the physician to reliably anticipate end of battery life early enough for elective device replacement. In some low-voltage pulse generators the microprocessor is able to measure the amount of energy consumed over time. It may then use these values to more accurately predict the time of elective replacement based on the energy capacity in the battery and the measured energy consumption. Each specific battery, as well as pacemaker, may have different voltage indicators for the elective replacement indicator (ERI) as well as end of life (EOL). See Fig 1.2.
Excerpted from Pacemakers and Implantable Cardioverter Defibrillators by Amin Al-Ahmad, Kenneth A. Ellenbogen, Andrea Natale, Paul J. Wang. Copyright © 2010 Amin Al-Ahmad, Kenneth A. Ellenbogen, Andrea Natale, and Paul J. Wang. Excerpted by permission of Cardiotext Publishing, LLC.
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Table of Contents
ContentsAbout the Contributors, xi,
PART 1 PACEMAKER SYSTEMS,
1 Pacemaker Systems: The Basics of Pulse Generator Design and Engineering Hugh Thomas McElderry and John C. Evans, 3,
2 Pacemaker and Implantable Cardioverter Defibrillator Lead Design Andrea M. Russo, 19,
3 Pacemaker Timing Cycles: Ventricular Based and Atrial Based Fred Kusumoto, 65,
4 Algorithms for Minimizing Right Ventricular Pacing Michael O. Sweeney, 79,
5 Normal Pacemaker Sensing, Abnormal Sensing, and Automatic Sensing Carsten W. Israel, 117,
6 Pacemakers and Atrial Arrhythmias: Detection and Treatment David S. Kwon and Byron K. Lee, 147,
7 Miscellaneous Pacing Algorithms George H. Crossley and Robert Andrew Pickett, 157,
8 Automatic Threshold Measuring Algorithms Marco V. Perez, Henry H. Hsia, Paul C. Zei, Mintu P. Turakhia, Paul J. Wang, and Amin Al-Ahmad, 165,
9 Sensor-Driven Pacing Chu-Pak Lau, Chung-Wah Siu, and Hung Fat Tse, 179,
10 Interpretation of Device Diagnostics Jordan M. Prutkin and Robert W. Rho, 203,
PART 2 IMPLANTABLE CARDIOVERTER DEFIBRILLATORS,
11 Basic ICD Systems Jane Chen, 229,
12 ICD Lead Design Gautham Kalahasty and Kenneth A. Ellenbogen, 239,
13 Sensing in ICDs Sergio L. Pinski, 265,
14 ICD Therapy — ATP Karin K. M. Chia, Luigi Di Biase, Paolo Pieragnoli, Giuseppe Ricciardi, Laura Perrotta, Robert Canby, Amin Al-Ahmad, Mark S. Wathen, Andrea Natale, and Luigi Padeletti, 295,
15 ICD Therapy — Shock T. Jared Bunch, Jeffrey S. Osborn, and John D. Day, 309,
16 Discrimination Algorithms and Arrhythmia Detection Irina Suman Horduna and Paul Khairy, 317,
17 Pacing with an ICD System: What Are the Issues? Robert F. Rea, 361,
18 Assessing Defibrillation Efficacy at ICD Implant Charles D. Swerdlow and Paul J. DeGroot, 369,
PART 3 BIVENTRICULAR PACING SYSTEMS,
19 Basics of Biventricular Pacing Systems: Principles and Timing Cycles Ravi Ranjan and Joseph E. Marine, 399,
20 Programming to Avoid Phrenic Nerve Stimulation during Left Ventricular Pacing Paul J. Wang, Amin Al-Ahmad, Henry H. Hsia, Paul C. Zei, and Mintu P. Turakhia, 415,
21 AV and VV Optimization Jeffrey H. Chung, Robert J. Kim, and Kenneth M. Stein, 419,
22 Electromagnetic Interactions from Magnetic Resonance Imaging in Patients with Implantable Arrhythmia Devices Saman Nazarian and Henry R. Halperin, 433,