Pacemaker

Heart

History

Modern technology

Resources

The rhythmic, regular beating of the heart is controlled by a natural pacemaker—a small patch of cells at the top of the right atrium called the sinoatrial (SA) node that sends rhythmic electric impulses along specific conducting fibers to the heart muscles, stimulating them to contract and relax in a regular sequence. When the heart muscles fail to receive the pacemaker’s signals, the heart ceases pumping blood. Within a few minutes, the patient faints, and within a few more minutes, dies—unless the heart muscles can be stimulated to resume contracting.

An artificial pacemaker is designed to help a damaged heart beat normally; it is programmed to send an electrical impulse to stimulate the heart muscle if it does not sense a normal heart beat within a specific amount of time. Dual-chamber pacemakers, the most common type, use leads to sense and pace activity in both the atrium (upper chamber) and ventricle (lower chamber). Single-pass pacemakers use only one lead and sense only one chamber, usually the ventricle. Faster and more easily implanted than the dual lead type, this pacemaker is indicated only in certain instances.

Heart

The heart is a unique organ that must function continuously to pump blood supplying oxygen to the body. It speeds up during special times of need, as when an individual is running or doing stressful work. It slows at night or during sleep when the demand for blood decreases.

This tiny pump, about the size of a fist, squeezes approximately 2.5 fl oz (75 ml) of blood out into the body with each beat. At a normal heart rhythm, this adds up to about 10 pt (5 l) of blood each minute. The heart pumps 2,500 gal (9500 l) of blood each day, and more than 100 million gal (400 million l) of blood in an average lifetime. The SA node regulates every heartbeat in time and intensity.

The SA node may function irregularly over time or even stop functioning, which will interfere with the performance of the heart. There are other electrically active tissues that will issue regulatory signals if the SA node stops generating an electrical current. The heartbeat will slow considerably under guidance of the next layer of tissue. An abnormally slow heartbeat is called bradycardia. The heartbeat may also become irregular, developing an arrhythmia. On the other hand, the SA node may become overactive, causing the heart to race at an abnormally high speed, a condition called tachycardia.

History

English surgeon W.H. Walshe first suggested using electric impulses to restart the heart in 1862. Nearly a century later, Harvard University-educated American cardiologist Paul Maurice Zoll believed he could use the heart’s responsiveness to electrical stimulation to treat cases of blockages of the heart, what is commonly called heart block. His first attempt, passing an electrode down the esophagus, failed, but in 1952 he developed an external pacemaker, passing an electric shock to the heart through electrodes placed on a patient’s chest. In October 1952, Zoll’s pacemaker was used to maintain a heartbeat in a man suffering from congestive heart failure; after two days, the patient’s own heart took over again.

Zoll’s machine, while effective, had inherent limitations: The shocks were painful to the chest muscles, and the machine—and thus the movement of the patient—were restricted to the nearest electrical outlet. Researchers envisioned an implantable pacemaker, and American inventor Wilson Greatbatch had dreamed of building one since he first heard of heart block in

1951. The restrictive size of vacuum tubes and storage batteries, however, made it impossible. As transistors became widely available in the late 1950s, Greatbatch mentioned his idea to Dr. William Chardack of the Buffalo, New York, Veterans Administration Hospital and, with Chardack’s encouragement, put together an implantable pacemaker in three weeks, working in the barn behind his home. After two years of animal testing, in 1960, Chardack and his associates implanted the first pacemakers in the chest wall of a human patient. Also in 1960, Ake Senning and his colleague, Elmqvist, designed an implantable pacemaker with an external coil and internal receiver.

The early pacemakers were designed to regulate every single heartbeat. It took over the function of the SA node; from the time of implantation, the patient’s heartbeat was directed by the pacemaker at a preset speed (usually about 70 to 72 beats per minute). Thus the patient’s capacity for exercise was limited because no matter what conditions he was under, his heart maintained the same rate of beating. It would not speed up to provide additional oxygen needed by the tissues when the patient exercised. Since then, however, a great deal of progress has been made.

Modern technology

Modern pacemakers are much improved over these designs. They are lightweight and relatively easy to install, and their lithium batteries last up to ten years as opposed to the mercury-zinc battery’s life of twenty months. The first generation of pacemakers sent signals at a preset rate; researchers Ken Anderson and Dennis Brumwell of Medtronic, Inc., in Minneapolis, Minnesota, advanced pacemaker technology immensely when they invented activity-responsive pacing in 1981 by using piezoelectric crystals that reacted to differing levels of body exertion. Medtronic’s Activitrax^®, introduced in 1985, was the first pacemaker to adjust pacing rate to exercise level. Several even more advanced pacemakers became available in the late 1980s, among them Medtronic’s Legend^®. These devices can be reprogrammed while they are still implanted, using radio-frequency signals to reset the pacemaker’s microprocessor. They also store information about cardiac events, and some models can even transmit that information over the telephone, directly from the patient’s chest to the doctor’s office.

Implantable cardioverter defibrillators (ICDs) designed by Guidant Corporation and other manufacturers primarily for other purposes, including the treatment of heart rhythms that are abnormally fast and life threatening (e.g., atrial fibrillation), can function as pacemakers for patients affected by these arrhythmias.

In a long-term study of malfunction rates of pacemakers and ICDs, researchers from the U.S. Food and Drug Administration (FDA), Beth Israel Deaconess Medical Center (Boston, Massachusetts), and Harvard Medical School (Boston, Massachusetts), found that from 1990 to 2002, 2.25 million pacemakers and 415,780 ICDs were implanted in the United States. The FDA estimates that the number of pacemakers implanted annually increased from about 95,000 in 1990 to more than 267,000 in 2002.

Current models of pacemakers monitor the heart to determine the heart rate and do not interfere with the heart function unless the heart rate drops below a predetermined speed (usually 66 to 68 beats per minute). Only then will the pacemaker deliver an electrical signal to drive the heart until the pacemaker determines that the SA node is again on track. The mechanical device then ceases its signals and returns to monitoring the heart rate. This is called demand pacing.

Current pacemakers weigh less than one ounce (25 g), are about the size of a quarter, and pace the upper and lower chamber as needed.

Some patients are at risk of a form of arrhythmia called fibrillation, which is a completely uncoordinated, quivering, nonfunctional heartbeat. If not corrected quickly, fibrillation can cause death. Since 1985, pacemakers have been available to monitor the speed of the

KEY TERMS

Arrhthymia —Any abnormal rhythm of the heart, which can be too rapid, too slow, or irregular in pace.

Atria —The two upper chambers of the heart.

Demand pacing —A type of pacemaker that responds to the needs of the body rather than uniformly pacing heartbeats at a single rate.

Ventricles —The two lower chambers of the heart; also the main pumping chambers.

heart and deliver an appropriate electrical shock to the heart muscle if it begins to fibrillate. The device can deliver a low-level pacing shock, an intermediate shock, or a jolting, defibrillating shock if necessary.

Surgeons prefer to implant pacemakers in the shoulder because the procedure can be carried out under local anesthetic. The wire from the pacemaker is inserted into one of the large veins in the shoulder and fed down into the heart, through the right atrium and into the ventricle where it is attached to the heart muscle. If the wire cannot be fed through veins that are too small or diseased, the pacemaker can be implanted in the abdomen.

Doctors must see patients with pacemakers frequently to check the battery power and make sure the circuitry is intact. Leads may become disconnected, the wire may break, or scarring may form around the electrode, all of which can render the pacemaker useless. Patients should avoid sources of electromagnetic radiation, including security scanning devices at airports and diagnostic tests using magnetic resonance imaging (MRI), both of which can turn off the pacemaker. Some states prohibit a person from driving an automobile for a period of time after he/she has received a pacemaker if he/she has previously experienced unconsciousness as a result of arrhythmia.

See also Circulatory system.

Resources

BOOKS

Elenbogen, Kenneth A., and Mark A. Wood, eds. Cardiac Pacing and ICDs. Malden, MA: Blackwell Pub., 2005.

Fuster, Valentin, ed. Hurst’s The Heart. New York: McGraw-Hill, Medical Publications Division, 2004.

Rosendorff, Clive, ed. Essential Cardiology: Principles and Practice. Totowa, NJ: Humana Press, 2005.

Serge, Barold. Cardiac Pacemakers Step By Step: An Illustrated Guide. Elmsford, NY: Futurea, 2004.

Zipes, Douglas, ed. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. Philadelphia, PA: W.B. Saunders, 2005. Saunders, 2005.

Larry Blaser

Pacemakers

Definition

A pacemaker is an implantable electronic device that delivers electrical stimulation to the heart to help regulate its beat.

Purpose

Pacemakers are used to correct abnormal rhythms of the heart, most notably, brachycardia, an abnormally slow heartbeat. Normal heartbeat is 60 to 100 beats per minute (bpm) and brachycardia occurs anywhere below 60. One cause of brachycardia is when the natural pacemaker of the heart, the sinoatrial (SA) node, does not function. Known as sick sinus syndrome, signals from the node are always slow or do not accelerate to accommodate exercise or stress. Considered a part of the normal aging process, this syndrome results in a heartbeat that is too slow to circulate enough blood to meet the needs of the body. Symptoms include fatigue, activity intolerance, or even unconsciousness (also known as syncope ). Pacemakers cure this condition by providing the needed electrical stimulus when the SA node does not work.

Pacemakers can also be used to treat a condition known as heart block. This problem occurs when the electrical connection between the upper chambers of the heart (atria) and lower chambers of the heart (ventricles) either fails or is significantly slowed. The area of the heart where this signal travels is called the atrio-ventricular (AV) node. The ventricles, without other stimulus, will produce their own beat of about 20 to 40 bpm, which is insufficient to support the body. Accordingly, patients with this problem feel tired and can lose consciousness. A pacemaker can treat this condition by keeping the heart rate within the normal range.

Patients that have brachycardia or heart block are at high risk for developing a tendency to have very fast, very inefficient contractions of the atria known as atrial defibrillation. A pacemaker that senses this abnormal rhythm and can switch to a mode of firing that brings it under control has been developed. Once the defibrillation has stopped, the pacemaker automatically switches back to its usual mode of function.

Description

The two main parts of a pacemaker are the pulse generator and the leads. The pulse generator is made of a computer chip, other electronic circuitry, and a lithium battery, all enclosed in a titanium case about the size of three to four stacked fifty-cent pieces. There can be one or two leads that carry the electrical impulse produced by the generator to the heart. The generator works by sensing whether the heart is firing at the right rate and supplying the electrical signal needed to start the heartbeat if it is not. The leads are flexible, double insulated wires that are placed within the heart chambers so that the needed signal is supplied to the area of the heart as needed. The leads can be unipolar, where the implanted tip is the negative pole (the positive is the pacemaker case) or bipolar where both the negative and positive poles are in the tip. Because the electrical signal has to travel across the chest with unipolar leads, pacemakers with leads of this type are more susceptible to outside interference.

If the pacemaker has one lead, it is known as a single chamber pacemaker. The lead can be placed in either the right atrium or the right ventricle. This type of device can be used only if the signal from the SA node or the AV node is the problem, and all other electrical conduction in the patient's heart is working correctly. Patients with this type of pacemaker can sometimes feel an uncomfortable neck throbbing, chest fullness or faintness when the device fires, known as pacemaker syndrome. Because of this problem, and the general ability to pump a greater volume of blood, some patients are treated with a dual chamber system.

The dual chamber pacemaker has two leads, one that is implanted in the right atrium, and one in the right ventricle. These pacemakers are also called sequentially pacing because the electrical signal is produced in a sequence—first to the atrium, then to the ventricle. The signal generators in dual chamber systems evaluate the heart's own electrical production in both chambers and produce their own signal when either or both become inadequate.

A third type of pacemaker is a rate-responsive system. These devices have the ability to sense physical activity and alter the heart rate to accommodate it. The responsiveness of this system results from one or more types of sensors. Some conditions that are sensed include motion, depth and rate of breathing, and blood temperature. As any of these conditions increase, the pacemaker speeds the rate of firing. Rate-responsive pacemakers most closely mimic the way the heart works naturally.

To help treat patients who have atrial fibrillation, pacemakers have been developed that can switch how they work to treat the rapid abnormal heart beat, then return to the normal function.

Operation

Installing a pacemaker is a relatively minor surgical procedure that generally takes about an hour. It is often performed by an electrophysiologist, a specialized cardiologist, or surgeon. Under local anesthesia, a small incision is made under the collarbone, then the lead or leads are threaded through the subclavian vein into the heart's right side. Fluroscopy, a type of x ray that involves projecting an image on a fluorescent screen, is used to guide the process and requires the wearing of a lead apron during operation. Often, right-handed patients have their pacemaker put in their left side and vice versa to speed return to normal activities.

Once the leads are in place, tests are performed to make sure the placement provides the needed connection for pacing. If the signals from the leads on the heart are too weak, the tip may have been placed in dead heart tissue and may need to be repositioned. The connection can be attached to the surface of the heart by a small corkscrew, known as active fixation, or a tined tip, known as passive fixation. With either passive or active fixation, a layer of fibrin (a blood protein) matures the lead connection within six weeks of the installation.

Next, the pulse generator is embedded into a pocket under the skin of the chest and the leads are connected. At this point the pulse generator has to be checked to make sure it is functioning correctly using a pacemaker system analyzer (PSA), a computer which checks the device is working correctly. If all checks out, the skin is sutured in place and a dressing placed over the wound.

Fine tuning of the pacemaker settings will occur in the recovery room using a programmer, a special computer equipped with a wand that is placed on the patient's chest over the pacemaker. The programmer and the pacemaker communicate in a method similar to a television remote control. Two important variables in this programming are the pacemaker's capture and sensing. Capture refers to the voltage and pulse width of the electrical signal the device will deliver. The programming is set to ensure that the capture is set high enough that two to three times the threshold (minimum) voltage necessary is delivered, called the margin of safety. However, the capture should not be so high as to unnecessarily drain the battery and require earlier replacement of the device.

Sensing involves the ability of the pacemaker to detect signals coming from the patient's heart and to shut itself off until a predetermined interval passes without a signal. Pacemakers see the heart signals much like an implanted electrocardiography unit. Poor sensing is what causes the pacemaker syndrome often seen with single chamber pacemakers. For proper sensing, the leads need to be adjusted so that the intra-cardial signals are seen at the highest voltage possible. This allows the sensitivity of the pacemaker to be set at a lower level. If the sensitivity has to be turned up too much, chest muscle activity could interfere with the heart signal.

Most patients stay in the hospital for one to two days after implantation, but some can leave the same day.

Safety

Once the pacemaker is installed environmental conditions can affect the functioning of the unit. These include:

• strong electromagnetic fields, such as those used in arc-welding
• contact sports
• shooting a rifle from that shoulder
• cell phones used on that side of the body
• some medical tests such as magnetic resonance imaging (MRI)

Environmental conditions often erroneously thought to affect pacemakers include:

• microwave ovens (The waves only affect old, unshielded pacemakers.)
• airport security (Although metal detector alarms could be set off—patients should carry a card stating they have a pacemaker implanted.)

Maintenance

In general, if the condition of the patient's heart, drug intake, and metabolic condition remain the same, the pacemaker requires only periodic checking every two months or so for battery strength and function. This is done by placing a special device over the pacemaker that allows signals to be sent over the telephone to the doctor, a process called trans-telephonic monitoring.

If changes in medications or physical condition occur, the doctor can adjust the pacemaker settings using a programmer, which involves placing the wand above the pacemaker and remotely changing the internal settings.

Drugs taken by the patient and metabolic conditions affect both capture and sensing thresholds. For example, drugs such as ephedrine or glucocosteroids cause lower thresholds, while some anti-arrhythmics cause higher thresholds. Hyperoxia (an excess of oxygen in the system) and hypocapnia (a deficiency of carbon dioxide) are two metabolic conditions that can lower thresholds and acidosis (an accumulation of acid in the body) or alkalosis (an accumulation of base in the body) can cause higher thresholds. Reprogramming of the pacemaker can accommodate the new capture and sensing values needed.

When the periodic testing indicates that the battery is getting low, an elective pacemaker replacement operation is scheduled. The entire signal generator is replaced because the batteries are sealed within the case. The leads can often be left in place and reattached to the new generator. Batteries usually last about six to eight years.

Health care team roles

Electrophysiologists are specially trained cardiologists who study and treat problems with the heart conduction system. They are often the type of physician that will implant the pacemaker system and oversee the programming or reprogramming of the device. They are assisted in the operating room by specially trained nurses, who can help with the testing of the pacemaker, and the anesthesiologist, who is responsible for numbing the area of the incision and keeping the patient comfortable. Pacemaker manufacturers often send representatives to be present for the implantation and initial programming.

The maintenance of the pacemaker can be overseen by the electrophysiologist or cardiologist and their staff, which can include specially trained cardiac medical assistants as well as nurses.

Training

The training for pacemakers and their use occurs during medical training (medical or nursing school) and on the job. Physicians, nurses, and other allied health professionals can also receive training in pacemakers as part of their continuing education courses. Such training often focuses on a particular aspect of pacemaker use, such as diagnosing problems in persons having pacemakers implanted, the installation of transient pacing, or the treatment of fibrillation or heart failure with pacemakers.

KEY TERMS

Atrial fibrillation— An abnormal rhythm of the heart characterized by rapid, nonproductive contractions of the atria.

Brachycardia— A diseased condition of the heart characterized by an abnormally slow heartbeat.

Dual chamber— A type of pacemaker having two leads that are placed in the right atria and the right ventricle.

Electrophysiologist— A specially trained physician or cardiologist specializing in the treatment and study of disorders of the heart's electrical conduction system.

Fluroscopy— A special type of x ray where images are projected on a fluorescent screen. Used to guide lead placement in pacemaker installations.

Single chamber— A type of pacemaker having one lead that is placed either in the right atria or the right ventricle.

Subclavian vein— The large vein in the chest that pacemaker leads are threaded through to be implanted in the right atria or ventricle.

Trans-telephonic monitoring— A method of checking on the function and battery strength of a pacemaker involving a special device that allows signals from an implanted pacemaker to be communicated to health care personnel using the telephone lines.

Resources

BOOKS

Gersh, Bernard J., ed. Mayo Clinic Heart Book. New York: William Morrow and Company, Inc., 2000.

PERIODICALS

Van Orden Wallace, Carol J. "Diagnosing and Treating Pacemaker Syndrome." Critical Care Nursing 21 (2001): 24-35.

ORGANIZATIONS

American Heart Association. 7272 Greenville Avenue, Dallas, Texas 75231. (800) AHA-USA1. 〈http://www.americanheart.org〉.

OTHER

Shea, Julie B. "Pacemakers" Treatment of Heart Disorders. North American Society of Pacing and Electrophysiology. 2000. 〈http://www.naspe.org/your_heart/treatments/pacemaker.html〉. (April 8, 2001).

Pacemakers

Definition

A pacemaker is an implantable electronic device that delivers electrical stimulation to the heart to help regulate its beat.

Purpose

Pacemakers are used to correct abnormal rhythms of the heart, most notably, brachycardia, an abnormally slow heartbeat. Normal heartbeat is 60 to 100 beats per minute (bpm) and brachycardia occurs anywhere below 60. One cause of brachycardia is when the natural pacemaker of the heart, the sinoatrial (SA) node, does not function. Known as sick sinus syndrome, signals from the node are always slow or do not accelerate to accommodate exercise or stress . Considered a part of the normal aging process, this syndrome results in a heartbeat that is too slow to circulate enough blood to meet the needs of the body. Symptoms include fatigue, activity intolerance, or even unconsciousness (also known as syncope). Pacemakers cure this condition by providing the needed electrical stimulus when the SA node does not work.

Pacemakers can also be used to treat a condition known as heart block. This problem occurs when the electrical connection between the upper chambers of the heart (atria) and lower chambers of the heart (ventricles) either fails or is significantly slowed. The area of the heart where this signal travels is called the atrioventricular (AV) node. The ventricles, without other stimulus, will produce their own beat of about 20 to 40 bpm, which is insufficient to support the body. Accordingly, patients with this problem feel tired and can lose consciousness. A pacemaker can treat this condition by keeping the heart rate within the normal range.

Patients that have brachycardia or heart block are at high risk for developing a tendency to have very fast, very inefficient contractions of the atria known as atrial defibrillation. A pacemaker that senses this abnormal rhythm and can switch to a mode of firing that brings it under control has been developed. Once the defibrillation has stopped, the pacemaker automatically switches back to its usual mode of function.

Description

The two main parts of a pacemaker are the pulse generator and the leads. The pulse generator is made of a computer chip, other electronic circuitry, and a lithium battery, all enclosed in a titanium case about the size of three to four stacked fifty-cent pieces. There can be one or two leads that carry the electrical impulse produced by the generator to the heart. The generator works by sensing whether the heart is firing at the right rate and supplying the electrical signal needed to start the heartbeat if it is not. The leads are flexible, double insulated wires that are placed within the heart chambers so that the needed signal is supplied to the area of the heart as needed. The leads can be unipolar, where the implanted tip is the negative pole (the positive is the pacemaker case) or bipolar where both the negative and positive poles are in the tip. Because the electrical signal has to travel across the chest with unipolar leads, pacemakers with leads of this type are more susceptible to outside interference.

If the pacemaker has one lead, it is known as a single chamber pacemaker. The lead can be placed in either the right atrium or the right ventricle. This type of device can be used only if the signal from the SA node or the AV node is the problem, and all other electrical conduction in the patient's heart is working correctly. Patients with this type of pacemaker can sometimes feel an uncomfortable neck throbbing, chest fullness or faintness when the device fires, known as pacemaker syndrome. Because of this problem, and the general ability to pump a greater volume of blood, some patients are treated with a dual chamber system.

The dual chamber pacemaker has two leads, one that is implanted in the right atrium, and one in the right ventricle. These pacemakers are also called sequentially pacing because the electrical signal is produced in a sequence—first to the atrium, then to the ventricle. The signal generators in dual chamber systems evaluate the heart's own electrical production in both chambers and produce their own signal when either or both become inadequate.

A third type of pacemaker is a rate-responsive system. These devices have the ability to sense physical activity and alter the heart rate to accommodate it. The responsiveness of this system results from one or more types of sensors. Some conditions that are sensed include motion, depth and rate of breathing, and blood temperature. As any of these conditions increase, the pacemaker speeds the rate of firing. Rate-responsive pacemakers most closely mimic the way the heart works naturally.

To help treat patients who have atrial fibrillation, pacemakers have been developed that can switch how they work to treat the rapid abnormal heart beat, then return to the normal function.

Operation

Installing a pacemaker is a relatively minor surgical procedure that generally takes about an hour. It is often performed by an electrophysiologist, a specialized cardiologist, or surgeon. Under local anesthesia , a small incision is made under the collarbone, then the lead or leads are threaded through the subclavian vein into the heart's right side. Fluroscopy, a type of x ray that involves projecting an image on a fluorescent screen, is used to guide the process and requires the wearing of a lead apron during operation. Often, right-handed patients have their pacemaker put in their left side and vice versa to speed return to normal activities.

Once the leads are in place, tests are performed to make sure the placement provides the needed connection for pacing. If the signals from the leads on the heart are too weak, the tip may have been placed in dead heart tissue and may need to be repositioned. The connection can be attached to the surface of the heart by a small corkscrew, known as active fixation, or a tined tip, known as passive fixation. With either passive or active fixation, a layer of fibrin (a blood protein) matures the lead connection within six weeks of the installation.

Next, the pulse generator is embedded into a pocket under the skin of the chest and the leads are connected. At this point the pulse generator has to be checked to make sure it is functioning correctly using a pacemaker system analyzer (PSA), a computer which checks the device is working correctly. If all checks out, the skin is sutured in place and a dressing placed over the wound.

Fine tuning of the pacemaker settings will occur in the recovery room using a programmer, a special computer equipped with a wand that is placed on the patient's chest over the pacemaker. The programmer and the pacemaker communicate in a method similar to a television remote control. Two important variables in this programming are the pacemaker's capture and sensing. Capture refers to the voltage and pulse width of the electrical signal the device will deliver. The programming is set to ensure that the capture is set high enough that two to three times the threshold (minimum) voltage necessary is delivered, called the margin of safety. However, the capture should not be so high as to unnecessarily drain the battery and require earlier replacement of the device.

Sensing involves the ability of the pacemaker to detect signals coming from the patient's heart and to shut itself off until a predetermined interval passes without a signal. Pacemakers see the heart signals much like an implanted electrocardiography unit. Poor sensing is what causes the pacemaker syndrome often seen with single chamber pacemakers. For proper sensing, the leads need to be adjusted so that the intra-cardial signals are seen at the highest voltage possible. This allows the sensitivity of the pacemaker to be set at a lower level. If the sensitivity has to be turned up too much, chest muscle activity could interfere with the heart signal.

Most patients stay in the hospital for one to two days after implantation, but some can leave the same day.

Safety

Once the pacemaker is installed environmental conditions can affect the functioning of the unit. These include:

• strong electromagnetic fields, such as those used in arc-welding
• contact sports
• shooting a rifle from that shoulder
• cell phones used on that side of the body
• some medical tests such as magnetic resonance imaging (MRI)

Environmental conditions often erroneously thought to affect pacemakers include:

• microwave ovens (The waves only affect old, unshielded pacemakers.)
• airport security (Although metal detector alarms could be set off—patients should carry a card stating they have a pacemaker implanted.)

Maintenance

In general, if the condition of the patient's heart, drug intake, and metabolic condition remain the same, the pacemaker requires only periodic checking every two months or so for battery strength and function. This is done by placing a special device over the pacemaker that allows signals to be sent over the telephone to the doctor, a process called trans-telephonic monitoring.

If changes in medications or physical condition occur, the doctor can adjust the pacemaker settings using a programmer, which involves placing the wand above the pacemaker and remotely changing the internal settings.

Drugs taken by the patient and metabolic conditions affect both capture and sensing thresholds. For example, drugs such as ephedrine or glucocosteroids cause lower thresholds, while some anti-arrhythmics cause higher thresholds. Hyperoxia (an excess of oxygen in the system) and hypocapnia (a deficiency of carbon dioxide) are two metabolic conditions that can lower thresholds and acidosis (an accumulation of acid in the body) or alkalosis (an accumulation of base in the body) can cause higher thresholds. Reprogramming of the pacemaker can accommodate the new capture and sensing values needed.

When the periodic testing indicates that the battery is getting low, an elective pacemaker replacement operation is scheduled. The entire signal generator is replaced because the batteries are sealed within the case. The leads can often be left in place and reattached to the new generator. Batteries usually last about six to eight years.

Caregiver concerns

Electrophysiologists are specially trained cardiologists who study and treat problems with the heart conduction system. They are often the type of physician that will implant the pacemaker system and oversee the programming or reprogramming of the device. They are assisted in the operating room by specially trained nurses, who can help with the testing of the pacemaker, and the anesthesiologist, who is responsible for numbing the area of the incision and keeping the patient comfortable. Pacemaker manufacturers often send representatives to be present for the implantation and initial programming.

KEY TERMS

Atrial fibrillation —An abnormal rhythm of the heart characterized by rapid, nonproductive contractions of the atria.

Dual chamber —A type of pacemaker having two leads that are placed in the right atria and the right ventricle.

Electrophysiologist —A specially trained physician or cardiologist specializing in the treatment and study of disorders of the heart's electrical conduction system.

Fluroscopy —A special type of x ray where images are projected on a fluorescent screen. Used to guide lead placement in pacemaker installations.

Single chamber —A type of pacemaker having one lead that is placed either in the right atria or the right ventricle.

Subclavian vein —The large vein in the chest that pacemaker leads are threaded through to be implanted in the right atria or ventricle.

Trans-telephonic monitoring —A method of checking on the function and battery strength of a pacemaker involving a special device that allows signals from an implanted pacemaker to be communicated to health care personnel using the telephone lines.

The maintenance of the pacemaker can be overseen by the electrophysiologist or cardiologist and their staff, which can include specially trained cardiac medical assistants as well as nurses.

Training

The training for pacemakers and their use occurs during medical training (medical or nursing school) and on the job. Physicians, nurses, and other allied health professionals can also receive training in pacemakers as part of their continuing education courses. Such training often focuses on a particular aspect of pacemaker use, such as diagnosing problems in persons having pacemakers implanted, the installation of transient pacing, or the treatment of fibrillation or heart failure with pacemakers.

Resources

BOOKS

Gersh, Bernard J., ed. Mayo Clinic Heart Book. New York: William Morrow and Company, Inc., 2000.

PERIODICALS

Van Orden Wallace, Carol J. “Diagnosing and Treating Pacemaker Syndrome.” Critical Care Nursing 21 (2001): 24–35.

ORGANIZATIONS American Heart Association. 7272 Greenville Avenue, Dallas, Texas 75231. (800) AHA-USA1. http://www.americanheart.org.

OTHER

Shea, Julie B. “Pacemakers” Treatment of Heart Disorders. North American Society of Pacing and Electrophysiology. 2000. http://www.naspe.org/your_heart/treatments/pacemaker.html. (April 8, 2001).

Michelle L. Johnson M.S., J.D.

Pacemaker

The pacemaker is an electronic biomedical device that can regulate the human heartbeat when its natural regulating mechanisms break down. It is a small box surgically implanted in the chest cavity and has electrodes that are in direct contact with the heart. First developed in the 1950s, the pacemaker has undergone various design changes and has found new applications since its invention. Today, pacemakers are widely used, implanted in tens of thousands of patients annually.

Background

The heart is composed of four chambers, which make up two pumps. The right pump receives the blood returning from the body and pumps it to the lungs. The left pump gets blood from the lungs and pumps it out to the rest of the body. Each pump is made up of two chambers, an atrium and a ventricle. The atrium collects the incoming blood. When it contracts, it transfers the blood to the ventricle. When the ventricle contracts, the blood is pumped away from the heart.

In a normal functioning heart, the pumping action is synchronized by the pacemaker region of the heart, or sinoatrial node, which is located in the right atrium. This is a natural pacemaker that has the ability to create electrical energy. The electrical impulse is created by the diffusion of calcium ions, sodium ions, and potassium ions across the membrane of cells in the pacemaker region. The impulse created by the motion of these ions is first transferred to the atria, causing them to contract and push blood into the ventricles. After about 150 milliseconds, the impulse moves to the ventricles, causing them to contract and pump blood away from the heart. As the impulse moves away from each chamber of the heart, that section relaxes.

Unfortunately, the natural pacemaker can malfunction, leading to abnormal heartbeats. These arrhythmias can be very serious, causing blackouts, heart attacks, and even death. Electronic pacemakers are designed to supplement the heart's own natural controls and to regulate the beating heart when these break down. It is able to do this because it is equipped with sensors that constantly monitor the patient's heart, and a battery that sends electricity, when needed, through lead wires to the heart itself to stimulate the heart to beat.

In addition to outer units, artificial pacemakers can be permanently implanted in a patient's chest. This is done by first guiding the lead through a vein and into a chamber of the heart, where the lead is secured. Fluoroscopic imaging helps facilitate this process. The pacemaker itself is next placed in a pocket, which is formed by surgery just above the upper abdominal quadrant. The lead wire is then connected to the pacemaker, and the pocket is sewn shut. This is a vast improvement over early methods, which required opening the chest cavity and attaching the leads directly to the outer surface of the heart.

History

The idea of using an electronic device to provide consistent regulation of the beating heart was not initially obvious to the early developers of the pacemaker. The first pacemaker, developed by Paul Zoll in 1952, was a portable version of a cardiac resuscitator. It had two lead wires that could be attached to a belt worn by the patient. It was plugged into the nearest wall socket and delivered an electric shock that stimulated the heart of a patient having an attack. This stimulation would usually be enough to cause the heart to resume its normal function. While moderately effective, this early pacemaker was primarily used in emergency situations.

Through 1957 and 1960 significant improvements were made to Zoll's original invention. In an attempt to reduce the amount of voltage needed to restart the heart and increase the length of time electronic pacing could be accomplished, C. Walton Lillehei made a pacemaker that had leads attached directly to the outer wall of the heart. Later, in 1958, a battery was added as the power source, making the pacemaker truly portable, which allowed patients to be mobile. This also enabled patients to use the pacemaker continuously instead of only for emergencies. Lillehei's pacemaker was external. William Chardack and Wilson Greatbatch invented the first implantable pacemaker. It was implanted in a living patient in 1960.

The modern technique for putting a pacemaker into a patient's heart was developed by Seymour Furman. Instead of cutting open the chest cavity, he used a method of inserting the leads into a vein and threading them up into the ventricles. With the leads inside the heart, even lower voltages were needed to regulate the heartbeat. This increased the length of time a pacemaker could be inside a person. Although his method was not widely used initially, by the late 1960s most cardiac specialists had switched to Furman's endocardial pacemakers. Since then improvements have been made in their design, including smaller pacemaker devices, longer lasting batteries, and computer controls.

Raw Materials

The materials used to construct pacemakers must be pharnacologically inert, nontoxic, sterilizable, and able to function in the environmental conditions of the body. The various parts of the pacemaker, including the casing, microelectronics, and the leads, are all made with biocompatible materials. Typically, the casing is made of titanium or a titanium alloy. The lead is also made of a metal alloy, but it is insulated by a polymer such as polyurethane. Only the metal tip of the lead is exposed. The circuitry is usually made of modified silicon semiconductors.

Design

Many types of pacemakers are available. The North American Society of Pacing and Electrophysiology (NASPE) has classified them by which heart chamber is paced, which chamber is sensed, how the pacemaker responds to a sensed beat, and whether it is programmable. Despite this vast array of models, all pacemakers are essentially composed of a battery, lead wires, and circuitry.

The primary function of a pacemaker battery is to store enough energy to stimulate the heart with a jolt of electricity. Additionally, it also provides power to the sensors and timing devices. Since these batteries are implanted into the body, they are designed to meet specific characteristics. First, they must be able to generate about five volts of power, a level that is slightly higher than the amount required to stimulate the heart. Second, they must retain their power over many years. A minimum time frame is four years. Third, they must have a predictable life cycle, allowing the doctor to know when a replacement is required. Finally, they must be able to function when hermetically (airtight) sealed. Batteries have two metals that form the anode and cathode. These are the battery components through which charge is transferred. Some examples include lithium/iodide, cadmium/nickel oxide, and nuclear batteries.

Pacemaker leads are thin, insulated wires that are designed to carry electricity between the battery and the heart. Depending on the type of pacemaker, it will contain either a single lead, for single chamber pacemakers, or two leads, for dual chamber pacemakers. With the constant beating of the heart, these wires are chronically flexed and must be resistant to fracture. There are many styles of leads available, with primary design differences found at the exposed end. Many of the leads have a screw-in tip, which helps anchor them to the inner wall of the heart.

The circuitry is the control center of the pacemaker. Located here are heart monitoring sensors, voltage regulators, timing circuits, and externally programmable controls. The circuitry is composed primarily of resistors, capacitors, diodes, and semiconductors. Modern pacemaker circuitry is a vast improvement over earlier models. With the application of semiconductors, circuit boards have become much smaller. They also require less energy, produce less heat, and are highly reliable.

The Manufacturing
Process

Pacemakers are sophisticated electronic devices. Therefore, some manufacturers rely on outside suppliers to provide many of the component parts. The construction of a pacemaker is not a linear process but an integrated one. Component parts such as the battery, leads, and the circuitry are constructed individually, then pieced together to form the final product.

Making the battery

• 1 The primary type of battery used in pacemakers is a lithium/iodine cell. One method used by manufacturers to make these batteries involves first mixing together the iodine and a polymer such as poly2-vinylpyridine (PVP). They are heated together, forming a molten charge-transfer complex. This liquid is then poured into a half moon-shaped, preformed cell which contains the other components of the battery, including the lithium anode (positive charge) and a cathode collector screen. The iodine/polymer blend solidifies as it cools to form the cathode. After the cathode is formed, the battery is hermetically sealed to prevent moisture from entering.

Making the leads

• 2 The leads are typically composed of a metal alloy. The wire is made by an extrusion process in which the metal is heated until it is molten, then pushed through an appropriately sized opening. It is cut, then bundled with many other wires and treated with a polymeric insulator such as polyurethane. One end of the lead wires is fashioned with a shaped tip, and the other is fitted with a pacemaker connector.

Making the motherboard

• 3 The motherboard contains all the electrical circuitry of the pacemaker, including the semiconductor chips, resistors, capacitors, and other devices. Using a complex method known as hybridization, these components are combined to form a single complex circuit. Construction begins with a small board (less than 0.32 sq in [2 sq cm]) which has the electronic configuration mapped out. The appropriate components are put in place on the board. They are then affixed using a minimum number of soldering welds.

Final assembly and packaging

• 4 When all of the component pieces are available, final assembly takes place. The circuitry is connected to the battery, and both are inserted into the metal casing. The casing used for a pacemaker is typically formed using titanium or a titanium alloy. It is constructed in multiple pieces that are sealed together after the other pacemaker components are introduced. A fitting is also affixed to the casing, providing a connecting point for the leads.
• 5 The finished devices are then put into final packaging along with accessories. After being exhaustively tested, they are then sent out to distributors and finally to doctors.

Quality Control

The quality of each pacemaker is ensured by making visual and electrical inspections throughout the entire production process. These tests will detect most flaws. Since the batteries must be absolutely reliable, they are specially manufactured and exhaustively tested, thereby increasing the associated costs tremendously. The functionality of each finished pacemaker is also tested before it is sent out for sale. Many of these tests are done under varying environmental conditions, such as excessive humidity and stress.

Manufacturers set their own quality standards for the pacemakers that they produce. However, standards and performance recommendations are required by various medical organizations and governmental agencies. In the United States, pacemakers are classified as Class III biomedical devices, which means they require pre-market approval from the United States Food and Drug Administration (FDA).

The Future

With the increasing numbers of senior citizens in the United States, it is anticipated that a greater percentage of the population will require pacemakers. As research efforts continue, future devices promise to be longer lasting, more reliable, and more versatile. Advances in battery technology, such as using radioactive isotopes for power, will undoubtedly improve the longevity of implanted pacemakers. Developments in microelectronics should provide even smaller devices which are less prone to environmental interferences. A late-breaking development in the field is the application of cardiac pacemaking technology to the brain. In this system, scientists connect the lead wires to a specific site on the brain and stimulate it as needed to regulate heartbeat. This device has been shown to be particularly effective in calming the tremors associated with Parkinson's disease.

Where to Learn More

Books

Banbury, Catherine. Surviving Technological Innovation in the Pacemaker Industry, 1959-1990. Garland Pub., 1997.

Ellenbogen, Kenneth, ed. Clinical Cardiac Pacing. Saunders, 1995.

Fox, Stuart. Human Physiology. WCB Publishers, 1990.

Moses, H., J. Schneider, B. Miller, and G. Taylor. A Practical Guide to Cardiac Pacing. Little, Brown and Co., 1991.

Periodicals

Jeffrey, Kirk. "Many Paths to the Pacemaker." Invention & Technology, Spring 1997, pp. 28-39.

—PerryRomanowski

Pacemakers

Definition
Demographics
Description
Diagnosis/Preparation
Aftercare
Risks
Normal results
Morbidity and mortality rates

Definition

A pacemaker is a surgically implanted electronic device that regulates a cardiac arrhythmia.

Pacemakers are most frequently prescribed when the heartbeat decreases under 60 beats per minute at rest (severe symptomatic bradycardia). They are also used in some cases to slow a fast heart rate over 120 beats per minute at rest (tachycardia).

Demographics

The population for pacemaker implant is not limited by age, sex, or race. Over 100,000 pacemakers are implanted per year in the United States. The occurrence is more frequent in the elderly with over 85% of implants received by those over age 65. A history of myocardial infarction (heart attack), congenital defect, or cardiac transplant also increases the likelihood of pacemaker implant.

Description

Approximately 500,000 Americans have an implantable permanent pacemaker device. A pacemaker implantation is performed under local anesthesia in a hospital by a surgeon assisted by a cardiologist. An insulated wire called a lead is inserted into an incision above the collarbone and guided through a large vein into the chambers of the heart. Depending on the configuration of the pacemaker and the clinical needs of the patient, as many as three leads may be used in a pacing system. Current pacemakers have a double, or bipolar, electrode attached to the end of each lead. The electrodes deliver an electrical charge to the heart to regulate heartbeat. They are positioned on the areas of the heart that require stimulation. The leads are then attached to the pacemaker device, which is implanted under the skin of the patient’s chest.

Patients undergoing surgical pacemaker implantation usually stay in the hospital overnight. Once the procedure is complete, the patient’s vital signs are monitored and a chest x ray is taken to ensure that the pacemaker and leads are properly positioned.

Modern pacemakers have sophisticated programming capabilities and are extremely compact. The smallest weigh less than 13 grams (under half an ounce) and are the size of two stacked silver dollars. The actual pacing device contains a pulse generator, circuitry programmed to monitor heart rate and deliver stimulation, and a lithium iodide battery. Battery life typically ranges from seven to 15 years, depending on the number of leads the pacemaker is configured with and how much energy the pacemaker uses. When a new battery is required, the unit can be exchanged in a simple outpatient procedure.

A temporary pacing system is sometimes recommended for patients who are experiencing irregular heartbeats as a result of a recent heart attack or other acute medical condition. The implantation procedure for the pacemaker leads is similar to that for a permanent pacing system, but the actual pacemaker unit housing the pulse generator remains outside the patient’s body. Temporary pacing systems may be replaced with a permanent device at a later date.

Diagnosis/Preparation

Patients being considered for pacemaker implantation will undergo a full battery of cardiac tests, including an electrocardiogram (ECG), electrophysiological study, or both, to fully evaluate the brady-cardia or tachycardia.

The symptoms of fatigue and lightheadedness that are characteristic of bradycardia can also be caused by

KEY TERMS

Electrocardiogram (ECG)— A recording of the electrical activity of the heart. An ECG uses externally attached electrodes to detect the electrical signals of the heart.

Electrophysiological study— A test that monitors the electrical activity of the heart in order to diagnose arrhythmia. An electrophysiological study measures electrical signals through a cardiac catheter that is inserted into an artery in the leg and guided up into the atrium and ventricle of the heart.

Embolism— A blood clot, air bubble, or clot of foreign material that blocks the flow of blood in an artery. When an embolism blocks the blood supply to a tissue or organ, the tissue the artery feeds dies (infarction). Without immediate and appropriate treatment, an embolism can be fatal.

Magnetic resonance imaging (MRI)— An imaging technique that uses a large circular magnet and radio waves to generate signals from atoms in the body. These signals are used to construct images of internal structures.

a number of other medical conditions, including anemia. Certain prescription medications can also slow the heart rate. A doctor should take a complete medical history and perform a full physical work-up to rule out all non-cardiac causes of bradycardia.

Patients are advised to abstain from eating six to eight hours before the surgical procedure. The patient is usually given a sedative to help him or her relax for the procedure. An intravenous (IV) line will also be inserted into a vein in the patient’s arm before the procedure begins in case medication or blood products are required during the insertion.

Aftercare

After an implant without complications the patient can expect a hospital stay of one to five post-procedure days. Pacemaker patients should schedule a follow-up visit with their cardiologist approximately six weeks after the surgery. During this visit, the doctor will make any necessary adjustments to the settings of the pacemaker. Pacemakers are programmed externally with a handheld electromagnetic device. Pacemaker batteries must be checked regularly. Some pacing systems allow patients to monitor battery life through a special telephone monitoring service that can read pacemaker signals.

WHO PERFORMS THIS PROCEDURE AND WHERE IS IT PERFORMED?

Pacemaker implants are performed by a cardiologist who has completed medical school and an additional internship and residency program. Additional training as an electrophysiologist may be acquired by the physician during the residency program. Specific training by the pacemaker manufacturer may also be acquired. Hospitals performing these procedures have access to cardiac catheterization facilities or operating rooms equipped with portable fluoroscopy units.

Patients with cardiac pacemakers should not undergo a magnetic resonance imaging (MRI) procedure. Devices that emit electromagnetic waves (including magnets) may alter pacemaker programming or functioning. A 1997 study found that cellular phones often interfere with pacemaker programming and cause irregular heart rhythm. However, advances in pacemaker design and materials have greatly reduced the risk of pacemaker interference from electromagnetic fields.

Risks

Because pacemaker implantation is an invasive surgical procedure, internal bleeding, infection, hemorrhage, and embolism are all possible complications. Infection is more common in patients with temporary pacing systems. Antibiotic therapy given as a precautionary measure can reduce the risk of pacemaker infection. If infection does occur, the entire pacing system may have to be removed.

The placing of the leads and electrodes during the implantation procedure also presents certain risks for the patient. The lead or electrode could perforate the heart or cause scarring or other damage. The electrodes can also cause involuntary stimulation of nearby skeletal muscles.

A complication known as pacemaker syndrome develops in approximately 7% of pacemaker patients with single-chamber pacing systems. The syndrome is characterized by the low blood pressure and dizziness that are symptomatic of bradycardia. It can usually be corrected by the implantation of a dual-chamber pacing system.

QUESTIONS TO ASK THE DOCTOR

• How many pacemaker implants has the physician performed?
• What type of pacemaker will be implanted, univentricular or biventricular, and how many of the specific procedure has the physician performed?
• How long will the expected hospital stay be?
• What precautions should be taken in the weeks following discharge from the hospital?
• What precautions will need to taken in day to day activities following pacemaker implant?
• When can normal daily, such as driving, exercise and work, activities be initiated?
• What will indicate that the pacemaker is failing and when should emergency care be sought?
• How long will the battery function and when should treatment to replace the device be sought?
• Is there special documentation I will need for air travel during security screenings?
• Will there be notification of manufacturer recalls?

Normal results

Pacemakers that are properly implanted and programmed can correct a patient’s arrhythmia and resolve related symptoms.

Morbidity and mortality rates

In the United States, patients experience complications in 3.3% and 3.8% of cases, with those over 65 years of age demonstrating a slightly higher complication rate of 6.1%. The most common complications include lead dislodgement, pneumothorax (collapsed lung), and cardiac perforation. The risk of death is less then 0.5% throughout the course of the hospital stay.

Resources

BOOKS

Libby, P. et al. Braunwald’s Heart Disease. 8th ed. Philadelphia: Saunders, 2007.

PERIODICALS

Gregoratas, Gabriel, et al. “ACC/AHA Guideline Update for Implantation of Pacemakers and Antiarrhythmia Devices.” Circulation 106(2002): 1175–209.

ORGANIZATIONS

American Heart Association. 7320 Greenville Ave. Dallas, TX 75231. (214) 373-6300. http://www.americanheart.org.

Paula Anne Ford-Martin

Allison J. Spiwak, MSBME

Packed cell volume seeHematocrit

Packed red blood cell volume seeHematocrit