Pacemaker Technology
Permanent pacemaker technology
The cardiac pacemaker is an electric circuit in which a battery provides electricity that travels through a conducting wire through the myocardium, stimulates the heart to beat (“capturing” the heart), and then goes back to the battery, thus completing the circuit.
Definitions of Terms
Some simplified definitions and principles of electronics must be appreciated to discuss pacemaker technology.
Coulomb (C) is the unit of charge and is either positive or negative. One negative coulomb represents the charge of approximately 6.24 × 1018 electrons.
Ampere is the unit of electric current and represents a charge moving at the rate of 1 coulomb per second. It is often abbreviated amp. Because the current in a pacemaker is low, the units are usually in thousandths of an ampere, or milliamperes (mA). Current is abbreviated i or I.
Volt (V) is the unit of “electric pressure” or electromotive force that causes current to flow. Voltage can be thought of as the difference in potential energy between two points with an unequal electron population.
Resistance (R) is the opposition, present to varying degrees in all matter, to the flow of electric current.
Ohm (abbreviated ω) is the unit of resistance. One ohm is the resistance that results in a current of 1 ampere when a potential of 1 volt is placed across the resistance.
Ohm’s law states that voltage (V) is equal to current (i) times resistance (R): V= iR. Impedance is a complex quantity having the dimensions of ohms. Whereas resistance applies only to idealized circuits with constant voltage and current, and no capacitors, impedance is the proper term for the opposition to current flow in the pacing system. Complex mathematic methods exist for computing impedance, which are beyond the scope this book. For our purposes,
Ohm’s law is adequate to describe the relationship among current, voltage, and impedance. In this text, the terms impedance and resistance will be used interchangeably, and the abbreviation R will be used for resistance or impedance.
Ohm’s law is adequate to describe the relationship among current, voltage, and impedance. In this text, the terms impedance and resistance will be used interchangeably, and the abbreviation R will be used for resistance or impedance.
Joule (J) is the fundamental unit of work or energy. In electric terms, in the pacing system, the energy released can be expressed as voltage × current × time = energy (in joules).
Watt (W) is the unit of power. It is the rate at which work is done. One watt is 1 J per second, or voltage × current = watt.
Basic Principles of Pacing
An electric circuit must consist of a complete, closed loop for current to flow through it.
Conductors are materials that have a relatively large number of free electrons and therefore pass an electric current well.
Insulators have few free electrons and therefore pass an electric current poorly.
A capacitor is a device made of two conductors separated by an insulator; it is used to store electrical charges.
Farad is the unit of capacitance. It is equal to a capacitor having a potential difference of 1 volt between its plates when it is charged with 1 coulomb. Capacitors are found in pacemakers, but play a particularly important role in ICDs, which require a reasonable charge built up in the capacitor in order to deliver a significant shock to cardiovert or defibrillate a patient. In pacing technology the unit is often used in microfarads.
A highly simplified circuit is shown in Figure 2-1. A battery is used to generate a force of 5 V. The circuit contains a 500 ω resistor and the current, from
Ohm’s law, is 0.01 amp (or 10 mA). Figure 2-1 represents an oversimplification of the electric events during a pacing spike. In reality, the voltage, current, and impedance of a pacemaker system change throughout the delivery of the pacemaker spike.
Ohm’s law, is 0.01 amp (or 10 mA). Figure 2-1 represents an oversimplification of the electric events during a pacing spike. In reality, the voltage, current, and impedance of a pacemaker system change throughout the delivery of the pacemaker spike.
 Figure 2-1 Simple Electric Circuit. This closed loop circuit shows electrons flowing from the negative end of the circuit, through a wire, through a resistor, and back to the positive end of the battery. The driving force is 5 V, the resistance is 500 ω, and the current is 10 mA. (Note that we are showing the actual direction of electron flow in this and the subsequent figures. In standard electric circuit diagrams, current is traditionally and arbitrarily described as flowing from positive to negative.) When this legend was first written, a typical impedance within a pacemaker system was 500 ohms. The trend has been to engineer higher resistance of about 800 or 900 ohms in a system. The greater resistance still allows excellent pacing and reduces battery drain (less current used per pacemaker spike). |
Pacemaker Power Sources
General Characteristics of Pacemaker Batteries
The ideal pacemaker battery should be able to generate approximately 5 Volts, which exceeds the voltage normally required to stimulate the myocardium in patients, thus providing an adequate safety margin. The battery also should be capable of being sealed hermetically. Many recalls of pacemakers by manufacturers have been caused by moisture intrusion into the unit, so hermetic sealing is important. Although hermetic means airtight, we use the term in a stricter sense. Hermeticity, as defined by the pacing industry, is an extremely low rate of helium gas leakage from the sealed pacemaker container. The ideal pacemaker battery should have a low rate of self-discharge, meaning that it does not lose power when it is not being used, even over a period of several years. The battery should fail in a predictable manner so that an indicator such as rate change can be designed into the circuitry to warn the physician of impending battery failure and the need for replacement.
One of the most important characteristics of a pacemaker battery is its longevity in clinical use. A simple way to compare specific batteries is to note the number of electrons the battery can deliver over its lifetime. Because this number is large, the battery capacity is expressed in ampere-hours, which is the rate of delivery of electrons integrated over time. The ampere-hour rating depends on both the chemicals used in the battery and the physical size of the battery. Typical ampere-hour ratings for commercially used batteries are 1 to 3 ampere-hours. Theoretically, a battery with a rating of 3 ampere-hours is superior to a battery with a rating of 1 or 2 ampere-hours, but this type of comparison is oversimplified. The voltage at which the battery operates is important, the size of the battery is important clinically, and the theoretically deliverable energy may be more than the actual deliverable energy when a battery is in a patient and in use over a period of years. Thus, although the ratings are potentially useful, the possibility of an inappropriate comparison exists.
The main purpose of the pacemaker’s battery power is to stimulate the heart with the pacemaker spike. In demand units, the pacemaker must sense the patient’s relatively weak QRS signal requiring amplification. Sensing is not a passive process and does require a small amount of power. A small amount of power also is required for the timing device in the pacemaker.
How a Pacemaker Battery Works
Because the most commonly used power source in the United States is now the lithium iodine battery, we will use it as an example of how a battery works. All batteries consist of three basic components: a material that gives off electrons (the anode), a material that takes up electrons (the cathode), and an ionic conductor (the electrolyte) that separates the anode and cathode. The electrode reactions of the battery may be represented as follows:
2Li → 2Li+ 2e– (anode reaction)
2Li+ + 2e– + I2 → 2LiI (cathode reaction)
2Li + I2 → 2LiI (overall reaction)
where Li = lithium, e = electron, I = iodine, and LiI = lithium iodide.
In this example, the lithium at the anode ionizes (loses an electron) and migrates as a positively charged ion through the electrolyte toward the cathode. The electrolyte is the lithium iodide that is formed continuously by the reaction between lithium and iodine. Electrons are left behind at the anode, which therefore becomes negatively charged relative to the other electrode (cathode). If the two electrodes then are connected by a conductive pathway (for example, a pacing wire in a patient), electrons can flow from one end of the battery to the other. The definitions of anode and cathode for a battery and for current flow in a circuit are opposite. The anode of a battery is the negative end of the battery, whereas in an electric circuit, the same negative electrode is called the cathode. Because this is rather confusing, after this brief discussion of batteries, we will use these terms only as they apply to the total circuit and not to the battery.
The LiI formed during the use of the battery is a solid that gradually increases the separation between the lithium and the iodine in the battery. This separation slowly causes the voltage of the battery to drop, even though both lithium and iodine remain in the battery. The battery does not run down because of depletion of chemicals; instead, the internal resistance of the battery rises, causing the voltage to drop. This concept is of possible clinical relevance because companies have made pacemakers capable of giving noninvasive readings of the internal resistance of the lithium iodine battery as an index of battery depletion, although modern devices now just report out expected degrees of battery depletion on telemetry.
Types of Pacemaker Batteries
The lithium anode battery has become the most commonly used pacing power source. Several types are either in use or under investigation. The lithium iodine battery generates 2.8 V at body temperature. Use of a voltage doubler in the circuit can raise the pacing voltage to approximately 5.0 (the doubler is not 100% efficient). Estimating the life of the lithium iodine battery is
difficult because modifications have been made in their manufacture, and current drain varies considerably in individual patients. A realistic estimate of the life of contemporary lithium iodine batteries is 4 to 10 years. It should be emphasized that battery life does not equal pacemaker life, because problems with circuitry or the lead system may cause a pacemaker to fail despite a functioning battery. One major influence on battery life is its physical size, a consideration that is sometimes clinically important. A small, thin lithium iodine generator with an estimated life of approximately 4 years (assuming complete pacing) may be reasonable in an elderly, thin patient who has a short life expectancy or in whom the pacemaker is expected to be used only intermittently. A larger, heavier unit with an estimated life of 8 years or longer is more reasonable in a patient who has a longer life expectancy and in whom generator size is not a major concern.
difficult because modifications have been made in their manufacture, and current drain varies considerably in individual patients. A realistic estimate of the life of contemporary lithium iodine batteries is 4 to 10 years. It should be emphasized that battery life does not equal pacemaker life, because problems with circuitry or the lead system may cause a pacemaker to fail despite a functioning battery. One major influence on battery life is its physical size, a consideration that is sometimes clinically important. A small, thin lithium iodine generator with an estimated life of approximately 4 years (assuming complete pacing) may be reasonable in an elderly, thin patient who has a short life expectancy or in whom the pacemaker is expected to be used only intermittently. A larger, heavier unit with an estimated life of 8 years or longer is more reasonable in a patient who has a longer life expectancy and in whom generator size is not a major concern.
The more frequent use of DDD (dual chamber) pacemakers, which in many patients leads to two pacemaker spikes per heartbeat instead of one, drains a battery more rapidly. Also, some of the sensor technology used for rate-responsive pacing leads to earlier battery depletion. These factors complicate the estimate of battery life.
The lithium iodine battery has become so widely used that there is a tendency to equate the lithium iodine battery with all pacemaker batteries. A different type of lithium battery that is also valuable, however, is the lithium vanadium silver pentoxide battery, which is used in the implantable defibrillator. This type of battery, rather than a lithium iodine battery, is used for the implantable defibrillator because of the need for rapid discharge from the battery to supply relatively high-power requirements for repeated shocks to the heart. The lithium iodine discharge would be inappropriately slow.
The zinc mercuric oxide battery was the power source most commonly used in the early years after pacemakers were introduced and is of historical interest. The battery has a voltage of only 1.35 V; therefore, five or six batteries usually were placed in series to provide adequate voltage for pacing. Compared with the currently used lithium batteries, the zinc mercuric oxide battery had a relatively short life of 1 to 5 years, it could not be hermetically sealed because it produces gas with use, and it was capable of sudden failure. Because of these problems, zinc mercury oxide batteries are no longer implanted.
Nuclear-powered pacemakers were first used clinically around 1970 and a few thousand were implanted. Despite a long life expectancy, the nuclear battery has disadvantages compared with the lithium battery and is no longer used. Nuclear batteries cost more than lithium batteries, controversy existed over what constituted safe and acceptable radiation exposure to the patient, and regulations about follow-up of the patient were necessarily strict to minimize the chance of environmental contamination with radioactive material.
Cadmium nickel oxide batteries are externally rechargeable batteries that require recharging every few weeks. They were used in the past, but are no longer implanted.
Pacemaker Circuitry
The microprocessor-based technology that revolutionized the computer industry also revolutionized the pacemaker industry. The semiconductors used in pacemakers allow handling of complex information in a small space at a relatively low cost with little expenditure of energy, little generation of heat, and a high degree of reliability. One of the reasons that semiconductor circuitry is more complex and more reliable than the older “discrete component” technology is that few welded metal-to-metal connections are required. In a circuit, the fewer welded connections present, the more reliable it is. If a modern multiprogrammable pacemaker were constructed, using older transistor technology, it would be at least as large as a television set.
Some of the terms used in describing pacemaker circuitry are not widely familiar and are briefly explained below.
The abbreviation CMOS is used often to describe the pacemaker’s circuitry and stands for complementary metallic oxide semiconductor. When an area in a semiconductor that tends to accept electrons is next to an area that tends to donate electrons, they are complementary; electrons can flow in a unidirectional current at low voltage with little generation of heat. The complex CMOS technology is extremely compact and operates at low energy. In the future, other types of semiconductor technology may be used in pacemakers.
Large-scale integration (LSI) is a nonspecific term that refers to the technology that produces high-density circuits with the capacity to have thousands of components in an area of a few square millimeters.
The semiconductor chip or chips are only one portion of the pacemaker circuitry, which also contains resistors, capacitors, and other components. The process of combining these components into a single complex circuit is referred to as hybridization. Figure 2-2 depicts a hybrid circuit.
Other terms used in describing pacemaker circuits include digital technology and analog technology. In digital technology, information is processed by turning switches on or off. It is reliable and energy efficient and may be used in the timing circuit and programming circuits. In analog technology, information is processed by regulating the amount of current or voltage in a system; for example, the sensing circuit may use analog technology to sense the amplitude of the patient’s QRS complex.
The general principles of the programmable circuitry of an idealized pacemaker are discussed in Chapter 4.
The Pacemaker Lead
The pacing lead conducts electricity from the pacemaker generator to the heart (and, in the bipolar system, back to the other pole of the pacemaker generator to complete the circuit). Because the heart beats approximately 40 million times per year, the lead must be resistant to fracture in order to withstand this chronic flexure. Usually the wire (or wires, in the bipolar system) is made of a
metal alloy to allow good conductivity, is fatigue resistant, is coiled to increase flexibility, and is multifilar to provide redundancy within the lead.
metal alloy to allow good conductivity, is fatigue resistant, is coiled to increase flexibility, and is multifilar to provide redundancy within the lead.
 Figure 2-2 Hybrid Circuit of a Programmable Pacemaker. This circuit is connected to a lithium battery and the entire system is hermetically sealed in a metal covering. It then can be connected to a fitting for the pacing wire connector, thus forming the complete pacemaker generator. (CMOS, complementary metallic oxide semiconductor.) |
The wire must be insulated from the body, and if the lead is bipolar, both wires must be insulated from each other, usually with Silastic or polyurethane; only the metal tip or electrode is exposed. Some clinical differences exist between Silicone and polyurethane coating. Polyurethane tends to allow a smaller size and be more slippery (important for implanting multiple leads). Some polyurethane leads have demonstrated stress cracking on the surface, but current modifications have corrected this and stress cracking does not appear to be of clinical significance. The Silastic leads tend to be somewhat larger and less slippery, but do have proven durability. Newer types of insulation are overcoming these differences.
Numerous styles of permanent leads are available. A schematic description of an old transvenous lead is shown in Figure 2-3. This lead has separate connectors for the wires that go to the tip electrode and the ring electrode. The wires are side by side and insulated from each other and from the body. The exposed metal tips appear stippled. Figure 2-4 demonstrates a bipolar lead with a coiled three-strand wire (for redundancy if one strand should fracture) going to the exposed tip and a separate wire going to the metal band or ring electrode approximately 1 cm behind the tip. The end of the lead has tines to facilitate entrapment of the lead in the trabeculae of the right ventricle or in the right atrial appendage. This lead is shown purely for didactic purposes; such a lead is no longer made. It does, however, demonstrate the principle that the two wires to the band and tip electrode are insulated from each other within the lead (it is more difficult to illustrate this concept with the modern coaxial leads).
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