Today’s permanent pacemakers and implantable cardioverter defibrillators (ICDs) are truly marvels of medical science. The complex timing cycles and fundamental algorithms that are programmed into these “small wonders” are the result of extensive research - as well as some trial-and-error over the years.
Devices have two main parts to their generator: a header and a can. The header is made of a clear plastic and contains pin ports, set screws, metal contacts, wires, a radiopaque manufacturer marker, and a small suture hole. All of these parts are visible through the transparent housing.
The can, on the other hand, is composed of a titanium shell (see Figure 1). This metal case does not allow visualization of the contents within. This article will look at the components contained inside a device can. These components include the battery, pacing circuit, sensing circuit, timing circuit, rate-adaptive sensor, memory, telemetry antenna, and reed switch.
BATTERY
In the attempt to make devices as small as possible, space inside the can is prime real estate. Because electricity is the most valuable commodity for an implantable device, the battery occupies about 50% of a pacemaker’s volume and about 25% of the space within an ICD can (50% of an ICD’s can space is inhabited by the high-energy capacitors used for defibrillation) (see Figure 2). Many energy sources have been tested for implantable devices, including nuclear power, photoelectric cells, and even the harnessing of the body’s energy. Today’s pacemakers are powered by lithium iodine batteries, while ICDs utilize lithium silver vanadium oxide batteries as their source.
PACING CIRCUIT
A fully-charged pacemaker battery has a charge of about 2.8 volts. If we were to pace the heart with this amount of energy for each beat, the battery would deplete quickly. However, if half of this energy was delivered with each beat (1.4 volts), the battery life would be doubled. The pacing circuit within a device allows the output voltage to be adjusted in 0.1-volt increments through the use of a voltage amplifier. In this way, the minimum energy needed to pace safely can be utilized to maximize battery life.
SENSING CIRCUIT
A pacemaker should only pace when the heart rate is too slow. A device “sees” intrinsic heart electricity through the sensing circuit. This circuit collects electrical information from the intracardiac leads, filters it, amplifies it, and compares it to some reference value. This reference value, called sensitivity, serves as a benchmark for incoming signals: any signals smaller than this size are ignored by the device. Smaller signals may originate from another heart chamber, nearby skeletal muscles, or external sources. Larger signals are sensed, and the device “decides” what to do in response to these signals.
All pacing devices have a fail-safe mechanism built into the sensing circuit. This noise-reversion circuitry ensures pacing is not withheld in the presence of electromagnetic interference (EMI). This is especially important for patients that are pacemaker-dependent. Any inhibition of pacing in these patients causes ventricular standstill.
ICDs have a second safety mechanism built into their sensing circuit to prevent the large defibrillation energy from traveling up the intracardiac lead and into the delicate electronics of the can (called a Zener diode).
TIMING CIRCUIT
Programming a pacemaker’s rate limits adjusts the commands for the timing circuit. This component acts like a stopwatch, utilizing a crystal oscillator as a time reference. Following each sensed or paced beat, the timing circuit begins counting down to time zero (i.e. from 1000 msec to 0 msec for 60 beats per minute). If time zero is reached and no intrinsic beat is sensed, the device delivers a paced beat. If an intrinsic beat is sensed before time zero, the countdown is reset and starts over. This beat-by-beat vigilance is driven by the many complex algorithms programmed into the timing and logic control board. All pacing “decisions” are made on this board.
RATE-ADAPTIVE SENSOR
As pacemakers evolved, there was an evident need for adjusting pacemaker rates based on patient activity level. Patients had no way to influence their pacing rate. This was especially problematic for very active patients. Rate-adaptive sensors were created to address this issue.
The first attempt at a rate-adaptive sensor solution was the activity sensor. This quartz sensor was mounted to the inside of the can and increased the rate when the crystal was stressed by vibrations or pressure. Unfortunately, patients sleeping on the same side as their pacemaker experienced inappropriate rate increases due to pressure on the can. This type of sensor was also limited because it only sensed up-and-down motion. Bicycle riding and swimming are two examples of activities that were poorly sensed by the activity sensor.
The next rate-adaptive sensor developed was the accelerometer. Learning from the activity sensor’s design flaw, this quartz crystal sensor is mounted to the circuit board, not the metal shell. A small weight is attached to the opposite end of the crystal, resembling a diving board. Inertia causes the board to flex and generate an electrical current proportional to the amount of flex (i.e. amount of activity). This sensor can “see” up-and-down AND back-and-forth motion. Most devices have an accelerometer as part of their rate-adaptive sensor.
The other part of most modern rate-adaptive sensors is comprised of minute-ventilation circuitry. Because minute-ventilation is proportional to metabolic demand, an increase in breaths per minute is a good indicator of increased activity. This circuitry continuously measures the impedance from the can to one intracardiac lead. This transthoracic impedance increases and decreases with inspiration and expiration, respectively. The impedance change reflects the down-and-up movement of the heart within the chest cavity as the diaphragm moves beneath it. More impedance cycles per minute trigger increases in the pacing rate through this sensor.
MEMORY
Current devices have two types of memory. ROM (read-only memory) houses the commands for the pacing, sensing, and timing circuits. RAM (random access memory) stores intracardiac electrograms and other clinical data. This data becomes very important in patient management, allowing the physician to customize the device for the patient and have access to a continuous rhythm monitor. RAM also makes it possible to upload new software to implanted devices non-invasively.
TELEMETRY ANTENNA
All devices have an antenna built into their cans, allowing for non-invasive telemetry between the device and the device programmer (a portable, proprietary computer used to “talk” to devices). This telemetry antenna allows for two-way communication of all clinical data and programming instructions for the device. The company-specific nature of the device programmers ensures a device is not inadvertently reprogrammed (called phantom programming).
REED SWITCH
The last can component, the reed switch, serves two purposes in pacemakers and ICDs. The first purpose is shared by both types of devices. To guard against phantom programming, the reed switch must be closed to initiate device telemetry for most devices. Closing the switch is accomplished by placing a magnet near the device.
The second purpose of the reed switch differs depending on the type of device. In pacemakers, a magnet closes the reed switch and forces the device to pace asynchronously (constantly, without regard to any intrinsic rhythm). Trans-telephonic monitoring, where a pacemaker patient can have their device checked over the phone line, utilizes this magnet-dependent feature. Based on the pacing rate and patterns evoked by the magnet, information regarding the battery life and pacing status can be obtained. Clinicians can also use the magnet response to determine the manufacturer of a pacemaker, which is helpful in choosing a device programmer when the patient does not know their device’s company name.
In ICDs, the second purpose of the reed switch is to disable the tachycardia detection circuitry. This feature is especially important for ICD patients undergoing operative procedures where electrocaudery will be used. If an ICD were to detect this electrical signal, it would inappropriately “treat” it as ventricular fibrillation (VF) with a high-energy shock.
All of these components fit neatly into a modern device generator. Every part contributes vitally to the whole, culminating in a modest-looking little metal device capable of life-saving feats. A can’s outward simplicity masks an inner complexity that deserves respect.
REFERENCES
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Barold SS, Stroobandt RX, Sinnaeve AF. Cardiac pacemakers step by step: an illustrated guide. Malden, Massachusetts: Blackwell Publishing Company, Inc., 2004.
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Ellenbogen KA, Wood MA. Cardiac pacing and ICDs. 3rd ed. Malden, Massachusetts: Blackwell Publishing Company, Inc., 2002.
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Hayes DL, Lloyd MA, Friedman PA, eds. Cardiac pacing and defibrillation: a clinical approach. Armonk, New York: Futura Publishing Company, Inc., 2000.
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Mallela VS, Ilankumaran V, Rao NS. Trends in cardiac pacemaker batteries. Indian Pacing and Electrophysiology Journal, 2004;4:201-12.
