INTRODUCTION

Pacemakers are electronic devices that stimulate the heart with electrical impulses to maintain or restore a normal heartbeat. This topic review will discuss pacemakers, when they may be necessary or appropriate, the types of pacemakers that are available, and the precautions patients need to take after having a pacemaker placed.

THE HEART'S CONDUCTION SYSTEM AND "NATURAL PACEMAKER"

The heart has its own built-in electrical system, called the conduction system (figure 1). The conduction system sends electrical signals throughout the heart that determine the timing of the heartbeat and cause the heart to beat in a coordinated, rhythmic pattern. The conduction system stimulates precise contractions of the heart's chambers to ensure that blood is pumped effectively.

The electrical signals, or impulses, of the heart are generated by specialized tissue called the sinoatrial (SA) or sinus node (figure 1). The sinus node is sometimes called the heart's "natural pacemaker". Each time the sinus node generates a new electrical impulse; that impulse spreads out through the heart's upper chambers, called the right atrium and the left atrium (figure 2). This electrical impulse stimulates the atria to contract, pumping blood into the lower chambers of the heart (the right and left ventricles).

The electrical impulse then spreads to another area of specialized tissue located between the atria and the ventricles, the atrioventricular (AV) node. The AV node momentarily slows down the spread of the electrical impulse, to allow the left and right atria to finish contracting.

From the AV node, the impulse spreads into a system of specialized fibers called the bundle of His and the right and left bundle branches (figure 1). These fibers distribute the electrical impulse rapidly to all areas of the right and left ventricles, stimulating them to contract in a coordinated way. With this contraction, blood is pumped from the right ventricle to the lungs, and from the left ventricle throughout the body.

ARRHYTHMIAS

The heart's conduction system must function normally for the heart to beat properly and to pump blood effectively to meet the body's needs. Problems with the flow of electrical impulses in the heart are called arrhythmias, which is a general term meaning that there is an abnormality in the pattern of electrical conduction or electrical rhythm.

Bradyarrhythmias — Bradyarrhythmias are heart rhythm abnormalities that cause an abnormally slow heartbeat. Most bradyarrhythmias are due to one of two kinds of problems: sinus bradycardia or heart block.

Sinus bradycardia occurs when the heartbeat is too slow because the heart's "natural pacemaker" is operating too slowly. Although some people (for example, competitive athletes) may have a slow heartbeat as a result of good health, in others sinus bradycardia is an abnormal condition that requires treatment.

Heart block is a term for a delay or interruption in the heart's conduction system, causing the electrical impulses to travel too slowly or to be stopped. There are several kinds of heart block, classified according to location (where in the conduction system the block occurs) and degree (whether the block is mild, causing delayed conduction, or severe, causing conduction to stop).

• In first-degree atrioventricular (AV) block, all electrical impulses reach the ventricles from the atria, but are abnormally slowed as they pass through the AV node.
• In second-degree AV block, some atrial impulses fail to reach the ventricles ("dropped beats"), resulting in a slow or an irregular heart rate.
• In third-degree AV block, the most serious form, no atrial impulses are conducted to the ventricles. This condition is sometimes called complete heart block. For the heart to continue to beat, a separate electrical impulse (called an escape rhythm) may be generated in the ventricles. Without an escape rhythm, the ventricles (the chambers that pump blood throughout the body) stop beating.
• In right bundle branch block (RBBB), impulses are not conducted by the right bundle branch. Electrical impulses reach the right ventricle only by traveling through the heart muscle from the left ventricle. As a result, activation of the right ventricle is delayed.
• In left bundle branch block (LBBB), impulses are not conducted by the left bundle branch. Electrical impulses reach the left ventricle only by traveling through the heart muscle from the right ventricle. As a result, activation of the left ventricle is delayed.

Tachyarrhythmias — Tachyarrhythmias are heart rhythm abnormalities that cause an abnormally fast heartbeat. Two tachyarrhythmias that are sometimes treated with a pacemaker are atrial fibrillation and ventricular tachycardia.

Atrial fibrillation (AF) is a tachyarrhythmia originating in the atria. Electrical impulses appear at random in the atria and spread through the atrial muscle in an irregular, uncoordinated way. The atria "quiver" rather than contract normally. As a result, blood is not pumped effectively or regularly into the ventricles. Impulses to the ventricles may be conducted very rapidly, resulting in a rapid and irregular heart rate. (See "Patient information: Atrial fibrillation (Beyond the Basics)".)

Ventricular tachycardia (VT) is a tachyarrhythmia originating in the ventricles. A repetitive electrical impulse appears somewhere in the ventricles and spreads through the ventricular muscle. Usually, VT produces some effective ventricular contractions, but at a rapid rate. With very rapid VT, blood may not be pumped effectively, and cardiac arrest may result. Therefore, VT is a potentially dangerous tachyarrhythmia.

Arrhythmia symptoms — The symptoms of arrhythmias vary, depending upon the specific arrhythmia and other factors, especially if there is underlying heart disease. While some people may have no symptoms, others may have various symptoms and signs. Symptoms may include:

• Fainting episodes (syncope) (see "Patient information: Syncope (fainting) (Beyond the Basics)")
• Dizziness or lightheadedness (presyncope)
• Palpitations (a sensation of the heart pounding)
• Confusion
• Extreme fatigue
• Shortness of breath
• Impaired ability of the heart to pump enough blood to meet the body's needs (heart failure)

The decision to treat an arrhythmia with a pacemaker (or any other treatment) depends in part upon whether the person has symptoms or not as well as the severity of the symptoms.

Underlying causes — A variety of conditions can lead to the development of cardiac arrhythmias. Some of the more common causes include:

• Coronary artery disease, where there is a malfunction or damage of the heart due to narrowing or blockage of arteries supplying blood to heart muscle
• Damage from a heart attack and the development of scar tissue in the muscle of the heart
• Certain structural heart malformations present at birth (congenital heart defects)
• Inherited genetic abnormalities that are not necessarily associated with a structural problem of the heart, but may result in an arrhythmia (such as the long QT syndrome)
• Abnormalities in the control and regulation of the heartbeat by the nervous system, leading to fainting (called neurocardiogenic syncope)
• Diseases of heart muscle tissue, called cardiomyopathies (see "Patient information: Dilated cardiomyopathy (Beyond the Basics)" and "Patient information: Hypertrophic cardiomyopathy (Beyond the Basics)")
• Therapy with certain medications that may alter the heart's normal rhythm
• Normal aging of heart muscle

TEMPORARY AND PERMANENT PACEMAKERS

Artificial pacemakers are electronic devices that stimulate the heart with electrical impulses to maintain or restore a normal rhythm in people with slow heart rhythms. There are many situations in which an artificial pacemaker may be recommended.

Most commonly, a pacemaker is used for a slow heart rate (bradyarrhythmia) as described above. The decision to use such a device, as well as which specific type, will depend upon multiple factors, including:

• The exact nature and underlying cause of the arrhythmia
• Whether the condition is temporary or permanent
• The presence or absence of symptoms as described above
• The potential risk of complications from a pacemaker

How they work — An artificial pacemaker provides an electrical impulse (or "discharge") that can stimulate the heart, thus restoring or maintaining a regular heartbeat. Although various types of artificial pacemaker devices are available, they generally include the following components:

• A thin metal box or case called a pulse generator (picture 1), which contains the power source producing the electrical impulses of the pacemaker. In addition, the pulse generator contains a small computer processor that can be programmed to set the rate of the pacemaker, the pattern of pacing (see 'Types of pacemakers' below), the energy output, and various other parameters. The pulse generator for most modern permanent pacemakers weighs one to two ounces.
• Flexible insulated wires or leads carry electrical impulses from the generator to the heart muscle and relay information concerning the heart's natural activities back to the pacemaker. There may be several such wires, or leads, placed within the heart, most commonly in the right atrium and right ventricle.
• One or more electrodes at the tips of the leads transmit electrical impulses to the heart muscle when needed and also sense the heart's own electrical activity.

Types of pacemakers — A variety of types of pacemakers have been developed to restore or sustain a regular heartbeat in different ways.

• Demand pacemakers monitor the heart's natural electrical activity and discharge only when the heart's own rate is too slow or the heart misses a beat.
• Fixed-rate pacemakers (which are rarely used today) discharge impulses at a single, steady rate, regardless of the heart's own electrical activity.
• Rate-responsive pacemakers are designed to raise or lower the heart rate to help meet the body's needs during physical activity or rest. These devices also work on "demand".

Pacemakers may also be single, dual, or triple chambered:

• Single-chamber pacemakers have one lead to carry impulses to and from either the right atrium or right ventricle.
• A dual-chamber pacemaker usually has two leads, one to the right atrium and one to the right ventricle, which can allow a heart rhythm that more naturally resembles the normal activities of the heart.
• Triple-chambered pacemakers typically have one lead in the right atrium, one to stimulate the right ventricle, and one to stimulate the left ventricle. These devices are inserted in patients who have weakened heart muscle (which results in heart failure). These pacemakers "resynchronize" the ventricles and may improve the efficiency of the contraction of the heart, improving its blood flow.

Temporary pacemakers — Temporary pacemakers are intended for short-term use during hospitalization. They are used because the arrhythmia is expected to be temporary and eventually resolve, or because the person requires temporary treatment until a permanent pacemaker can be placed.

The pulse generator of a temporary pacemaker is located outside the body, and may be taped to the skin or attached to a belt or to the patient's bed.

Patients with temporary pacemakers are hospitalized and continuously monitored. Members of the healthcare team will perform regular examinations to monitor for any possible complications.

Permanent pacemakers — Permanent pacemakers are pacemakers that are intended for long-term use.

Indications — Specific guidelines have been established concerning the conditions when a permanent pacemaker is (1) definitely beneficial, useful, and effective, (2) may be indicated, or (3) is not useful or effective and, in some cases, may be harmful. Patients should speak with their healthcare provider concerning these guidelines and how they apply to their specific case.

As a general rule, permanent pacing is recommended for certain conditions that are chronic or recurrent and not due to a transient cause. Permanent pacing may be considered necessary or appropriate for certain people with symptomatic bradyarrhythmia or, less commonly, to help prevent or terminate tachyarrhythmia.

Implantation — The pacemaker is most commonly implanted into soft tissue beneath the skin in an area below the clavicle, which is known as prepectoral implantation; this is located under the skin and fat tissue but above the pectoral or breast muscle. The pacemaker leads are typically inserted into a major vein (transvenously) and advanced until the electrodes are secured within the proper region(s) of heart muscle. The other ends of the leads are attached to the pulse generator (figure 3).

Less commonly, the pulse generator is placed under the skin of the upper abdomen.

Generally the pacemaker is implanted in a sterile laboratory or operating room by a specialist (cardiologist, surgeon, or cardiac electrophysiologist) with experience in this procedure. Local anesthesia is used to make the procedure as pain-free as possible. In some cases, sedation or even general anesthesia may be used. The position of the pacemaker leads is usually checked using X-ray imaging (called fluoroscopy). The length of the procedure depends upon the type of device being placed.

Recovery from the procedure is rapid, but there may be some restrictions on arm movement and activities for the first few weeks. Lead dislodgement is more common in the first few weeks after implantation. The hospital stay is usually brief, and in some cases the procedure can be done as a day surgery. Uncommon but possible risks associated with permanent pacemaker implantation include collapsed lung (pneumothorax), infection, and bleeding.

Once implanted, pacemakers can be programmed to change the baseline heart rate, the upper heart rate at which the pacemaker will pace, and heart rate changes that should occur with exercise.

Follow-up care — People who have a permanent pacemaker will require periodic clinical check-ups, including certain tests such as ECGs, which record the electrical activity of the heart. In addition, the status of the pacemaker will be regularly checked or "interrogated" (often done remotely using a telephone or a secure web-based system) to provide information regarding the type of heart rhythm, the functioning of the pacemaker leads, the frequency of utilization of the pacemaker, the battery life, and the presence of any abnormal heart rhythms.

All contemporary devices are programmable with information and settings that can be altered and stored. Information is obtained by transmitting data from the pulse generator to a programmer, usually done during a follow-up office visit. However, with newer pulse generators it may be possible to obtain information about the pacemaker's performance by downloading data from the patient's device to the internet and then to the caregiver's office. Pacemaker activity can also be checked routinely via the telephone, using a telephone-transmitting device.

The pulse generators are usually powered by lithium batteries that function for an average of five to eight years before they need to be replaced. When the batteries start to wear out, they do so in a very slow and predictable fashion, allowing sufficient time to be detected and pulse-generator replacement planned. Replacing the pulse generator usually requires a simple procedure in which a skin incision is made over the old incision, the old generator is removed, and a new generator is implanted and joined with the existing leads.

The pacemaker leads are usually used indefinitely, unless a specific problem occurs (eg, the lead loses contact with the heart, the lead breaks, or the lead is not functioning properly). In such circumstances, the lead may require replacement. Often, the old lead is left in place but disconnected from the pulse generator, and a new lead is inserted. Removal of an old lead is feasible but difficult in most cases, because of the formation of scar tissue binding the lead to the blood vessels and heart muscle. Rarely, lead removal is necessary if the system becomes infected.

AVOIDING ELECTROMAGNETIC INTERFERENCE

Although contemporary pacemakers are less susceptible to interference than older models, electromagnetic energy can interfere in some cases. Thus, experts advise that people with pacemakers be aware of the following:

Household appliances — Pacemaker manufacturers do not recommend any special precautions when using normally functioning common household appliances such as microwave ovens, televisions, radios, toasters, and electric blankets.

Cellular phones — Due to the growing use of hand-held cellular phones, patients must be aware of their potential adverse effects. As examples:

• Evidence suggests that cellular phones do not cause interference with permanent pacemakers. While some older generation pacemakers and implantable cardioverter-defibrillators (ICDs) did occasionally experience interference from cellular telephones, clinical experience suggests that there is no significant interference between pacemakers or ICDs and modern wireless communication devices or portable media players.

Anti-theft systems — Electromagnetic anti-theft security systems are often found in or near the workplace, at airports, in stores, at courthouses, or in other high-security areas. Although interference with a pacemaker is possible, it is unlikely that any clinically significant interference would occur with the transient exposure associated with walking through such a field. Based upon several studies and observations, experts advise that patients with pacemakers should:

• Be aware of the location of anti-theft systems and move through them at a normal pace
• Avoid sitting or standing close to an anti-theft system

Metal detectors at airports — Similar to antitheft systems, metal detectors at airports can potentially interfere with pacemakers, although this is unlikely. Such exposure has been shown to cause interference in some cases and may be related to the duration of exposure and/or distance between the security system and the pacemaker. Metal detectors will likely be triggered by the presence of a pacemaker and therefore at places such as airports, it will be important for individuals with pacemakers to carry an identification card for their pacemaker, and airport personnel will likely prefer to do a manual search.

External electrical equipment — External electrical fields do not seem to cause a problem for most people with a pacemaker. However, in workplaces that contain welding equipment or strong motor-generator systems, because interference can inhibit pacing, it is recommended that a person with an implanted cardiac device remain at least two feet from external electrical equipment, verify that the equipment is properly grounded, and leave the immediate locale if lightheadedness or other symptoms develop.

Diagnostic or therapeutic procedures — Certain types of surgery and procedures may interfere with pacemakers. Most importantly, the use of electrocautery can inhibit pacemaker function. It is not uncommon therefore that a pulse generator may require specific reprogramming before the procedure and programming back to its baseline condition after the procedure. In some instances, a magnet is all that is required on the device to make sure that there is no problem with the device during the procedure. Such procedures include:

• Magnetic resonance imaging (MRI), which uses a strong magnetic field that is pulsed on and off at a rapid rate. For most patients with a pacemaker, this procedure is contraindicated.
• Transcutaneous electrical nerve/muscle stimulators (TENS), a method of pain control.
• Diathermy, which heats body tissues with high-frequency electromagnetic radiation or microwaves.
• Extracorporeal shock wave lithotripsy, the use of sound waves to break up gallstones and kidney stones.
• Therapeutic radiation for cancer or tumors, which can cause permanent pacemaker damage.
• Any surgery in which electrocautery is being used. The risks are greatest when the electrocautery is being performed close to the pulse generator.

Thus, doctors, dentists, and other healthcare providers should be informed about a person's pacemaker. If a procedure associated with pacemaker interference is contemplated, the possible benefits, risks, and alternatives should be considered and discussed, as appropriate. People with pacemakers should carry a medical identification card for emergencies.

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem. The manufacturer of your device likely also has a patient call line, and the number would be listed on your identification card.

This article will be updated as needed on our web site (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient level information — UpToDate offers two types of patient education materials.

The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.

Patient information: Pacemakers (The Basics)
Patient information: Implantable cardioverter-defibrillators (The Basics)
Patient information: Radiofrequency catheter ablation for the heart (The Basics)
Patient information: Cardiac resynchronization therapy (The Basics)
Patient information: Bradycardia (The Basics)
Patient information: Sick sinus syndrome (The Basics)
Patient information: Long QT syndrome (The Basics)

Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.

Patient information: Atrial fibrillation (Beyond the Basics)
Patient information: Syncope (fainting) (Beyond the Basics)
Patient information: Dilated cardiomyopathy (Beyond the Basics)
Patient information: Hypertrophic cardiomyopathy (Beyond the Basics)

Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.

Cardiac device interactions with electromagnetic fields
Cardiac resynchronization therapy in heart failure: Implantation and other considerations
Cardiac resynchronization therapy in heart failure: Indications
Dual chamber pacing system malfunction: Evaluation and management
Indications for permanent cardiac pacing
Infections involving cardiac implantable electronic devices
Modes of cardiac pacing: Nomenclature and selection
Pacing system malfunction: Evaluation and management
Temporary cardiac pacing
Treatment of the sick sinus syndrome

The following organizations also provide reliable health information.

• National Library of Medicine

     (www.nlm.nih.gov/medlineplus/pacemakersandimplantabledefibrillators.html, available in Spanish)

• National Heart, Lung & Blood Institute (NHLBI)

     (www.nhlbi.nih.gov/health/dci/Diseases/pace/pace_whatis.html)

• American College of Cardiology (ACC)

     (www.acc.org)

• American Heart Association

     (www.americanheart.org)

• Heart Rhythm Society

     (www.hrspatients.org/patients/treatments/pacemakers.asp)

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