Smaller than the size of a palm, an ICD packs a lot of power into a little space. It sends electrical pulses to the heart when rhythms get dangerously out of control, effectively halting racing beats and protecting against Sudden Cardiac Death.

Pager-sized pacemakers rest just under the collarbone to stimulate hearts that beat too slowly.

Each individual heartbeat is actually a collection of several muscle movements spurred into action by electrical impulses.

The first electrical signal comes from the heart’s own natural pacemaker, the sinoatrial node, comprised of electrically active cells and located in the upper right heart chamber. This node sends a steady stream of electrical signals along a pathway through the heart’s upper chambers. The signals then travel to the electrical bridge — the atrioventricular node — between the upper and lower chambers and, finally, move to the lower chambers.

A problem at any point in the electrical pathway can wreak havoc with the regular beating of a heart. Luckily, an artificial pacemaker — a small, battery-operated device — can take over the role of the heart’s own electrical system, if necessary.

What is a pacemaker?

Although it weighs just about an ounce, a pacemaker contains a powerful battery, electronic circuits and computer memory that together generate electronic signals. The signals, or pacing pulses, are carried along thin, insulated wires, or leads, to the heart muscle. The signals cause the heart muscle to begin the contractions that cause a heartbeat.

A pacemaker is implanted just below the collarbone in a procedure that takes about two hours. It is programmed to stimulate the heart at a pre-determined rate, and settings can be adjusted at any time. Routine evaluation, sometimes even via telephone, ensures the pacemaker is working properly and monitors battery life, which generally runs from five to ten years.


Full-contact sports can damage a pacemaker.

Magnetic Resonance Imaging (MRI) tests employ powerful magnets to create images. If you are scheduled for an MRI test, you should make certain your doctor knows you have a pacemaker.

Electronic security systems, such as those common in airports, can pose problems if exposure is prolonged. Passing through such systems, even several times, does not pose a threat. However, patients with pacemakers should avoid the “wand” used in some security checks.

Microwave ovens, cellular phones and electric blankets do not, as myth might have it, affect pacemaker functioning.

When is a pacemaker needed?

The most common reason for a pacemaker is a heartbeat that slows to an unhealthy rate, or bradycardia. A pacemaker resets the heart rate to an appropriate pace, ensuring adequate blood and oxygen are delivered to the brain and other parts of the body.

Types of pacemakers

Three basic types exist to serve different purposes:
Single-Chamber Pacemakers — In a single-chamber pacemaker, only one wire (pacing lead) is placed into a chamber of the heart. Sometimes it is the upper chamber, or atrium. Other times it is the lower chamber, or ventricle.

Dual-Chamber Pacemakers — In dual-chamber pacemakers, wires are placed in two chambers of the heart. One lead paces the atrium and one paces the ventricle. This approach more closely matches the natural pacing of the heart. This type of pacemaker can coordinate function between the atria and ventricles.

Rate-Responsive Pacemakers — These have sensors that automatically adjust to changes in a person’s physical activity.

Other devices — Some devices, such as implantable cardioverter defibrillators (ICDs), designed primarily for other purposes, can function as pacemakers in certain situations.


In the largest clinical trial of pacemakers conducted in the United States, researchers found that patients with sick sinus syndrome benefit from dual-chamber pacemakers. They are less likely to develop atrial fibrillation or mild forms of heart failure compared to people who are treated with single-chamber pacemakers.

When are pacemakers used?

Pacemakers may be prescribed for a number of conditions, including:
Bradycardia — a condition in which the heart beats too slowly, causing symptoms such as fatigue, dizziness or fainting spells. Bradycardia may be caused by the wear and tear of age or by conditions such as sick sinus syndrome (SSS) or heart block.

Atrial fibrillation — a common heart rhythm disorder in which the upper chambers of the heart beat rapidly and chaotically. Sometimes people with atrial fibrillation can also have slow rhythms. Medicines used to control atrial fibrillation may result in slow rhythms which are treated by pacemakers.

Heart failure — a condition in which the heartbeat is not sufficient to supply a normal volume of blood and oxygen to the brain and other parts of the body. A special pacemaker can be carefully programmed to increase the force of muscle contractions in the heart. This is called “biventricular pacing” or “resynchronization” therapy.

Syncope — a condition best known as the common faint, is usually not serious. Some patients faint when their heart rhythm becomes very slow. For a small percentage of people who experience severe and frequent fainting spells, a pacemaker may prevent the heart rate from slowing to the point of fainting.


The underlying cause of an arrhythmia provides the basis for selecting the best treatment. Treatments fall into several main categories along a continuum from the least to most invasive. In general, the least invasive treatment that effectively controls the arrhythmia is the treatment of choice. Options include lifestyle changes, medication, devices, ablation procedures, and surgery, including the implantation of pacemakers and defibrillators.

Lifestyle Changes

Since other heart disorders increase the risk of developing arrhythmias, lifestyle changes often are recommended. In addition, improving health can lesson the symptoms of arrhythmias and other heart disorders as well as prove beneficial to overall patient health.


Medications can control abnormal heart rhythms or treat related conditions such as high blood pressure, coronary artery disease, heart failure and heart attack. Drugs also may be administered to reduce the risk of blood clots in patients with certain types of arrhythmias.

Electronic Devices

By delivering a controlled electric shock to the heart, defibrillators or cardioverters “shock” the heart back into a normal heart rhythm.


The first step in treating any arrhythmia is determining the underlying cause of the problem. Then, patients and clinicians can discuss treatment options factoring in the circumstances of each individual case. The following are some of the considerations when determining appropriate treatment:

  • The nature and severity of the heart rhythm disorder and its symptoms
  • Underlying diseases and other conditions that contribute to the disorder or affect the patients health or quality of life
  • The patient’s age, overall health and personal and family medical history
  • Medications and other therapies that the patient may be using for other conditions

Sometimes the devices are external, such as in an emergency situation. Often, the electronics are implanted in the patient’s chest.

Implanted Cardioverter Defibrillators (ICDs) — ICDs are 99 percent effective in stopping life-threatening arrhythmias and are the most successful therapy to treat ventricular fibrillation, the major cause of sudden cardiac death. ICDs continuously monitor the heart rhythm, automatically function as pacemakers for heart rates that are too slow, and deliver life-saving shocks if a dangerously fast heart rhythm is detected.

Pacemakers — Devices that “pace” the heart rate when it is too slow (bradycardia) can take over for the heart’s natural pacemaker, the sinoatrial node, when it is functioning improperly. Pacemakers monitor and regulate the rhythm of the heart and transmit electrical impulses to stimulate the heart if it is beating too slowly.

Devices for Heart Failure — The U.S. Food and Drug Administration (FDA) recently approved a special type of pacemaker for certain patients with heart failure. In Cardiac Resynchronization Therapy, an implanted device paces both the left and right ventricles (lower chambers) of the heart simultaneously. This resynchronizes muscle contractions and improves the efficiency of the weakened heart.

Catheter Ablation

In this procedure, one or more flexible, thin tubes (catheters) arc guided via x-ray into the blood vessels and directed to the heart muscle. A burst of radiofrequency energy destroys very small areas of tissue that give rise to abnormal electrical signals.


Although surgery is sometimes used to treat abnormal heart rhythms, it is more commonly elected to treat other cardiac problems, such as coronary artery disease and heart failure. Correcting these conditions may reduce the likelihood of arrhythmias.


In cardiac ablation, a form of energy renders a small section of damaged tissue inactive. This puts an end to arrhythmias that originated at the problematic site.

Normally, electricity flows throughout the heart in a regular, measured pattern. This normally operating electrical system is the basis for heart muscle contractions.

Sometimes, the electrical flow gets blocked or travels the same pathways repeatedly creating something of a “short circuit” that disturbs normal heart rhythms. Medicine often helps. In some cases, however, the most effective treatment is to destroy the tissue housing the short circuit. This procedure is called cardiac ablation.

Cardiac ablation is just one of a number of terms used to describe the non-surgical procedure. Other common terms are: cardiac catheter ablation, radiofrequency ablation, cardiac ablation, or simply ablation.

The ablation process

Like many cardiac procedures, ablation no longer requires a full frontal chest opening. Rather, ablation is a relatively non-invasive procedure that involves inserting catheters —narrow, flexible wires — into a blood vessel, often through a site in the groin or neck, and winding the wire up into the heart. The journey from entry point to heart muscle is navigated by images created by a fluoroscope, an x-ray-like machine that provides continuous, “live” images of the catheter and tissue.

Once the catheter reaches the heart, electrodes at the tip of the catheter gather data and a variety of electrical measurements are made. The data pinpoints the location of the faulty electrical site. During this “electrical mapping,” the cardiac arrhythmia specialist, an electrophysiologist, may sedate the patient and instigate some of the very arrhythmias that are the crux of the problem. The events are safe, given the range of experts and resources close at hand, and are necessary to ensure the precise location of the problematic tissue.

Once the damaged site is confirmed, energy is used to destroy a small amount of tissue, ending the disturbance of electrical flow through the heart and restoring a healthy heart rhythm. This energy may take the form of radiofrequency energy, which cauterizes the tissue, or intense cold, which freezes, or cryoablates the tissue. Other energy sources are being investigated.

Patients rarely report pain, more often describing what they feel as discomfort. Some watch much of the procedure on monitors and occasionally ask questions. After the procedure, a patient remains still for four to six hours to ensure the entry point incision begins to heal properly. Once mobile again, patients may feel stiff and achy from lying still for hours.

When is ablation appropriate

Many people have abnormal heart rhythms (arrhythmias) that cannot be controlled with lifestyle changes or medications. Some patients cannot or do not wish to take life-long antiarrhythmic medications and other drugs because of side effects that interfere with their quality of life.

Most often, cardiac ablation is used to treat rapid heartbeats that begin in the upper chambers, or atria, of the heart. As a group, these are know as supraventricular tachycardias, or SVTs. Types of SVTs are:

  • Atrial Fibrillation
  • Atrial Flutter
  • AV Nodal Reentrant Tachycardia
  • AV Reentrant Tachycardia
  • Atrial Tachycardia

Less frequently, ablation can treat heart rhythm disorders that begin in the heart’s lower chambers, known as the ventricles. The most common, ventricular tachycardia, may also be the most dangerous type of arrhythmia because it can cause sudden cardiac death.

For patients at risk for sudden cardiac death, ablation often is used along with an implantable cardioverter device (ICD). The ablation decreases the frequency of abnormal heart rhythms in the ventricles and therefore reduces the number of ICD shocks a patient may experience.

For many types of arrhythmias, catheter ablation is successful in 90-98 percent of cases — thus eliminating the need for open-heart surgeries or long-term drug therapies.

EP Study

Highly trained specialists perform EP studies in a specially designed EP Lab outfitted with advanced technology and equipment.

Normally, electricity flows throughout the heart in a regular, measured pattern. This electrical system brings about heart muscle contractions. A problem anywhere along the electrical pathway causes an arrhythmia, or heart rhythm disturbance. By accurately diagnosing the precise cause of an arrhythmia, it is possible to select the best possible treatment.

Why an EP study

While electrocardiograms (EGGs) are important tests of the heart’s electrical system, they are brief tests that record only the events that occur while the tests are running. Arrhythmias, by their very nature, are unpredictable and intermittent, which makes it unlikely that an EGG or electrocardiogram will capture the underlying electrical pathway problem. Even tests that stretch over longer time lengths, such as Holter monitoring, may not capture an event.

During an EP study, a specially trained cardiac specialist may provoke arrhythmia events and collect data about the flow of electricity during actual events. As a result, EP studies can help locate the specific areas of heart tissue that give rise to the abnormal electrical impulses that cause arrhythmias. This detailed electrical flow information provides valuable diagnostic and, therefore, treatment information.


An electrophystologic, or EP study provides information that is key to diagnosing and treating arrhythmias. Although it is more invasive than an electrocardiogram (ECG) or echocardiogram, and involves provoking anhythmias, the test produces data that makes it possible to:

  • Diagnose the source of arrhythmia symptoms
  • Evaluate the effectiveness of certain medications in controlling the heart rhythm disorder
  • Predict the risk of a future cardiac event, such as Sudden Cardiac Death
  • Assess the need for an implantable device (a pacemaker or ICD) or treatment procedure (radiofrequency catheter ablation)

EP studies most often are recommended for patients with symptoms indicative of heart rhythm disorders or for people who may be at risk for Sudden Cardiac Death.

An overview of the procedure

While ECGs are non-invasive, an EP study is somewhat invasive. The study is performed after giving local anesthesia and conscious sedation (twilight sleep) to keep the patient as comfortable as possible. The procedure involves inserting a catheter — a narrow, flexible tube — attached to electricity-monitoring electrodes, into a blood vessel, often through a site in the groin or neck, and winding the catheter wire up into the heart. The journey from entry point to heart muscle is navigated by images created by a fluoroscope, an x-ray-like machine that provides continuous, “live” images of the catheter and heart muscle.

Once the catheter reaches the heart, electrodes at its tip gather data and a variety of electrical measurements are made. These data pinpoint the location of the faulty electrical site. During this “electrical mapping,” the cardiac arrhythmia specialist, an electrophysiologist, may instigate, through pacing (the use of tiny electrical impulses), some of the very arrhythmias that are the crux of the problem. The events are safe, given the range of experts and resources close at hand and are necessary to ensure the precise location of the problematic tissue.

Once the damaged site or sites are confirmed, the specialist may administer different medications or electrical impulses to determine their ability to halt the arrhythmia and restore normal heart rhythm. Based on this data, as well as information garnered before the study, sometimes the specialist will proceed to place an implantable cardioverter device (ICD) or a pacemaker or will perform radiofrequency ablation. In any case, the information proves useful for diagnosis and treatment.


Each heart has its own normal rhythm brought about by the seamless flow of electrical impulses throughout the organ. This electrical flow begins in the heart’s natural “pacemaker” (also know as the sinoatrial node or sinus node) in the upper right heart chamber, the right atrium. The electricity flows through the upper chambers (atria), crosses the bridge between upper and lower chambers (atrioventricular node) and travels to the lower chambers (ventricles.)

The passage of electricity culminates in a carefully coordinated contraction of heart muscle that pushes blood through the human body. Problems in the precise flow of electricity are the cause of arrhythmias.

Throughout the procedure, the patient is sedated but awake and remains still. Patients rarely report pain, more often describing what they feel as discomfort. Some watch the procedure on monitors and occasionally ask questions. Others sleep. The procedure usually takes about two hours. The patient remains still for four to six hours afterward to ensure the entry point incision begins to heal properly. Once mobile again, patients may feel stiff and achy from lying still for hours.

Who performs the test and where

Since potentially dangerous arrhythmias are provoked during an EP study, it’s crucial that specialized staff are present to handle all situations. A physician electrophysiologist, with advanced training in the diagnosis and treatment of heart rhythm problems, performs the EP study. The electrophysiologist leads a team of specially trained health care professionals, technicians and nurses, who assist during the procedure. The team performs the EP study in an electrophysiology laboratory, or EP lab, a well-equipped, controlled clinical environment usually located within a hospital or clinic. As a result, the test is quite safe and complications are rare.

Cardiac Resynchronization

Cardiac resynchronization relies on electric leads to correct an arrhythmia commonly diagnosed in patients experiencing heart failure. The leads electrically stimulate heart muscle to synchronized the contractions of the heart’s two lower chambers, or ventricles. Only when the lower chambers beat in harmony can they contract with enough force to push blood carrying oxygen through the body.

More than 22 million people worldwide suffer from congestive heart failure (CHF), a potentially debilitating disease. Until recently, lifestyle changes, medication and, sometimes, heart surgery were the only treatment options. Patients with severe symptoms, however, received little, if any, relief from such approaches. To make matters worse, up to 40 percent of patients with CHF also have an arrhythmia that further reduces the heart’s ability to beat properly.

Cardiac resynchronization therapy (CRT) is an innovative new therapy that can relieve CHF symptoms by improving the coordination of the heart’s contractions.

CRT builds on the technology used in pacemakers and implantable cardioverter devices. CRT devices also can protect the patient from slow and fast heart rhythms.

Overview of a heart beat

The heart is comprised of four chambers: two upper atria, and two lower ventricles. An electrical system controls the synchronized pumping action of these chambers.

The normal heartbeat originates in a section of the right atrium known as the sinoatrial, or SA node. The electrical signal from the sinoatrial node spreads through both atria causing them to contract and squeeze blood into the ventricles. The electrical signal then passes through an electrical bridge known as the atrioventricular or AV node. After a split second delay, the signal continues to the ventricles by way of a specialized network known as the left and right bundle branches. The bundle branches separate to the left and right ventricles, which enables the electrical signal to stimulate both ventricles simultaneously This coordinated contraction, or squeezing, of the ventricles is necessary for optimal pumping of blood to the body and lungs.


Damaged heart muscle can became so weak that it can no longer pump effectively, leading to cardiomyopathy and CHF.

Coronary artery disease and heart attacks are the most frequent causes of CHF, but inherited disorders, viral infections and toxins, such as alcohol, also can cause heart muscle damage. Symptoms of CHF typically include shortness of breath, swelling of the feet and legs, abdominal swelling, fatigue, exercise intolerance, diminished appetite and depression.

Most often, medications aim to control CHF symptoms, such as the build up of excess fluid that causes leg swelling and makes it difficult to breath. Medications can reduce fluid retention, strengthen the heart’s squeezing ability and relax blood vessels, thereby reducing the resistance to blood flow and easing the heart’s workload.

In addition, lifestyle changes, such as low-salt diets and exercise, can help control symptoms.

Uncoordinated contractions

When there is a delay in electrical signal transmission through the left bundle branch, this causes left bundle branch block (LBBB). Because the electrical signal to the left ventricle is delayed, the right ventricle begins to contract a fraction of a second before the left ventricle, instead of simultaneously. The result is an asynchronous, or uncoordinated contraction of the ventricles and a mis-timing in the contraction pattern of the left atrium and ventricle. Other conduction abnormalities, such as right bundle branch block (RBBB), also may contribute to less efficient contraction of the heart. This further reduces the pumping ability of the already weakened heart muscle.

Cardiac Resynchronization Therapy

The concept behind CRT is quite simple. Resynchronization restores the normal coordinated pumping action of the ventricles by overcoming the delay in electrical conduction caused by bundle branch block. This is accomplished by means of a special type of cardiac device.

These powerful, “built-in” devices have enormous potential to improve the quality of life and probably survival for patients with heart failure.

The CRT device

Pacemakers are typically used to prevent symptoms due to an excessively slow heartbeat. The pacemaker continuously monitors the heartbeat and, when necessary, delivers tiny, imperceptible electrical signals to stimulate the heartbeat. Most pacemakers have two electrode wires, or leads, one in the right atrium and one in the right ventricle. This ensures the pacemaker will maintain the normal coordinated pumping relationship between the upper and lower chambers of the heart.


The ideal candidate for a CRT device is someone with:

Some CRT candidates also have a high risk of Sudden Cardiac Death. For these patients, a special CRT device can stop potentially life-threatening rapid heartbeats by delivering an electrical shock known as defibrillation. This device incorporates a standard implantable cardioverter defibrillator (ICD) with a CRT pacemaker creating a “CRT-D” device. (The “D” refers to defibrillation.)

  • Moderate to severe CHF symptoms, despite lifestyle changes and medication
  • A weakened and enlarged heart muscle
  • A significant electrical delay in the lower pumping chambers (bundle branch block)

The wires that carry the electrical signals connect to an electrical pulse generator placed under the skin in the upper chest. In addition to the two leads (right atrium and right ventricle) used by a common pacemaker, the CRT device has a third lead that is positioned in a vein on the surface of the left ventricle.

This allows the CRT device to simultaneously stimulate the left and right ventricles and restore a coordinated, or “synchronous,” squeezing pattern. This is sometimes referred to as “bi-ventricular pacing” because both ventricles are electrically stimulated (paced) at the same time. This reduces the electrical delay and results in a more coordinated and effective heart beat.

CRT results

The response to CRT can vary greatly among patients. Clinical studies involving more than 2000 patients worldwide demonstrate modest improvements in exercise tolerance, CHF severity, and quality of life in most patients. Improvement may happen quickly, but sometimes it can take several months.


Smaller than the size of a palm, an lCD packs a lot of power into a little space. It sends electrical pulses to the heart when rhythms get dangerously out of control, effectively halting racing beats and protecting against Sudden Cardiac Death.

Almost everyone has seen a physician on television, paddles in hand, yelling “Clear!”, then applying those paddles to the chest of a patient to shock him “back to life”. As dramatic as the scene may be, defibrillation, or shock, can be the only way to stop certain deadly heart arrhythmias before they kill.

For those who are at high risk of the deadliest forms of arrhythmias —ventricular tachycardia and ventricular fibrillation — an internal “shocking” device may provide the best defense against sudden cardiac arrest. Such a device, known as an implantable cardioverter defibrillator (ICD), is considered effective in fighting cardiac arrest over 90 percent of the time, an astounding success for a condition that few survived as recently as 15 years ago.

About ICDs

Implantable cardioverter defibrillators (ICDs) are small devices, about the size of a pager, that are placed below the collarbone. Via wires, or leads, these devices continuously monitor the heart’s rhythm. If the heart beats too quickly, the ventricles will not have enough time to fill with blood and will not effectively pump blood to the rest of the body Left unchecked, the rapid heartbeat could cause death. To intervene, the ICD issues a lifesaving jolt of electricity to restore the heart’s normal rhythm and prevent sudden cardiac death.


Cardiac arrest, or sudden cardiac death (SCD), happens when a heart rhythm disturbance prevents the heart from operating properly and delivering blood to the brain and other vital organs.

A heart attack occurs when a partial or complete vessel blockage interferes with the ability of blood to flow to the heart, and heart muscle dies. Cardiac arrest, or sudden cardiac death (SCD), is NOT a heart attack, but a prior heart attack can put someone at risk for SCD.

ICDs also can act as pacemakers when a heart beat that is too slow (bradycardia) is detected.

Most ICDs keep a record of the hearts activity when an abnormal heart rhythm occurs. With this information, the electrophysiologist, a specialist in arrhythmias, can study the hearts activity and ask about other symptoms that may have occurred. Sometimes the lCD can be programmed to “pace” the heart to restore its natural rhythm and avoid the need for a shock from the ICD. Pacing signals from the ICD are not felt by the patient; shock signals are, and have been described as a kick in the chest.

When is ICD therapy the right choice?

In the simplest terms, anyone who has had or is at a high risk of having ventricular tachycardia. fibrillation or sudden cardiac arrest is a candidate for an ICD.


Sudden rapid heartbeats originating in the ventricles are the most dangerous arrhythmias. Ventricular tachycardia, a rapid yet steady beat is dangerous in its own right. It can turn into ventricular fibrillation or VF, which is characterized by irregular and chaotic rapid heartbeats. Because the fibrillating heart muscle cannot contract and pump blood to the brain and vital organs, VF is the number one cause of sudden cardiac death. Without immediate emergency treatment of an electric shock to restore normal rhythm, an individual loses consciousness within seconds and dies within minutes.

Many people have both coronary artery disease (the primary cause of heart attacks) and an arrhythmia (a heart rhythm disorder). They are at particular risk for sudden cardiac death and may be candidates for ICDs, even though they have no noticeable symptoms of an abnormal heart rhythm.

A cardiac arrhythmia specialist should evaluate cardiac patients if they have experienced any of the following:

  • A prior cardiac arrest
  • Ventricular tachycardia (VT) which is an episode of rapid heartbeat originating from the lower chambers of the heart
  • Ventricular fibrillation (VF), which is similar to VT but is characterized by a heartbeat that is too rapid and is irregular or chaotic
  • Ejection fractions of less than 35 to 40 percent. An ejection fraction (EF) is the proportion, or fraction, of blood pumped by the heart with each beat. A normal heart pumps out a little more than half the heart’s volume of blood with each beat, making a normal EF 55 percent or higher
  • Patients at a high risk for sudden cardiac death (SCD) because of an inherited heart abnormality

Heart Rhythm Society
1400 K Street, NW
Suite 500
Washington, DC 20005
Phone (202) 464-3400
Fax (202) 464-3401