Glossary of Terms

The Heart Rhythm Society brings you an online dictionary of common medical terms related to heart, and rhythm problems.



Angiotensin Converting Enzyme (ACE) Inhibitors
ACE inhibitors are a group of medications that help relax blood vessels. They are used to treat high blood pressure and heart failure. For people with heart failure, ACE inhibitors have been shown to prolong life and minimize symptoms.

Antitachycardia Pacing
Antitachycardia pacing is a method of treating ventricular tachycardia (an abnormally fast heartbeat) by stimulating the heart with a preset rapid series of electrical pulses, reestablishing a normal heartbeat.

The aorta is the largest artery in the body. It carries blood from the left side of the heart to every part of the body.

Heart rhythm disorders (arrhythmias) are problems that affect the electrical system, or "wiring," of the heart muscle. Heart arrhythmias are very common and millions of people will experience an abnormal heart rhythm some time during their lives. Most are not serious.

Asynchrony is a condition in which the heart fails to maintain a normal time sequence between atrial and ventricular contraction.

Atherosclerosis is a form of arteriosclerosis in which the inner layers of artery walls become thick and irregular due to deposits of fat, cholesterol and other substances. This buildup is sometimes called plaque. As the interior walls of arteries become lined with layers of these deposits, the arteries become narrowed, and the flow of blood through them is reduced.

Atrial Fibrillation (AF or Afib)
More than 2 million people in the United States have atrial fibrillation, making it a very common heart rhythm disorder. In Afib, the heartbeat is irregular and rapid, sometimes beating as often as 300 times a minute, about four times faster than normal. Although it isn't life threatening, Afib can lead to other rhythm problems, chronic fatigue and congestive heart failure. Chances of having a stroke are five times higher for those with Afib. There are a number of effective treatments to control Afib (for consistency) and/or reduce the risk that it will cause serious health problems.

Atrial Flutter (AFL)
Atrial flutter is similar to Atrial Fibrillation because it too is characterized by a rapid heartbeat. Instead of many disorganized signals, however, atrial flutter is caused by a single electrical wave that circulates very rapidly in the atrium, about 300 times a minute, leading to a very fast, steady heartbeat.

Atrioventricular (AV) Node
The atrioventricular node is a cluster of cells located in the wall between the right and left atria, just above the ventricles. This part of the heart’s electrical pathway helps carry signals from the atria to the ventricles.

Automatic External Defibrillator (AED)
External defibrillators deliver brief, high-energy shocks through paddles or electrode patches applied to the patient's chest. The display on an AED gives simple instructions to rescuers - and has a built-in safeguard. The defibrillator will override attempts to deliver a shock until sensors in the machine confirm the victim is in cardiac arrest.


Beta-blockers are prescription drugs that reduce heart rate and the oxygen needs of the heart by inhibiting the effects of the hormone epinephrine, which constricts blood vessels.

Bradycardia is characterized by an abnormally slow heart rhythm (less than 60 beats per minute). A normal heart contracts about 100,000 times each day, at a rate of 60 to 100 times a minute. The weak pace may mean the heart doesn’t beat often enough to ensure blood flow. Heart block (or Atrioventricular Block) and Sick Sinus Syndrome are forms of bradycardia.


Cardiac Resynchronization Therapy (CRT)
Cardiac resynchronization relies on electrical leads to correct difficulty of the normal electrical impulse in conducting through the heart, which is commonly diagnosed in patients experiencing heart failure. The leads electrically stimulate heart muscle to synchronize 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.

Cardiomyopathy is a group of disorders in which the heart muscle is weakened and cannot pump blood efficiently. Diseased heart function affects the lungs, liver, and body systems.

Catheter Ablation
In this procedure, one or more flexible, thin tubes (catheters) are 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.

Coronary Artery Disease (CAD)
CAD occurs when the blood vessels become clogged. This condition is also called coronary heart disease or ischemic heart disease.


Defibrillation is the termination of a fast, erratic, and often fatal heart rate by delivering a high-energy electrical shock to restore the heart’s normal rhythm. A shock is administered through electrodes placed on the chest (external defibrillation) or in the heart (internal defibrillation).

Dyspnea is difficult or labored respiration, resulting in shortness of breath. This term is often associated with lung or heart disease.


Echocardiogram (Echo)
An echocardiogram is a non-invasive test that records a sound wave picture of the heart and gives information about the heart valves and the function of the heart’s muscular walls, including measurement of ejection fractions.

Edema is the build-up of abnormal fluid in body tissues characterized by swelling throughout various areas of the body and shortness of breath. Edema can be a sign of heart failure.

Ejection Fraction (EF)
The ejection fraction (EF) is a measure of the proportion (or fraction) of blood that is expelled by the ventricle with each contraction, or heartbeat. The healthy heart pumps out 55 percent or more of the blood in the left ventricle with each beat. If the ejection fraction falls below 55 percent, it is an indication that the heart muscle is weakened.

Electrophysiology (EP) Test
The electrophysiology test is a test that observes and measures the electrical impulses of the heart. An EP study uses programmed stimulation (small amount of energy delivered to the heart) to initiate abnormal heart rhythms, determine the origin of arrhythmias, and assess the risk of sudden cardiac arrest.

An embolism is a sudden obstruction of a blood vessel by an embolus, an abnormal particle circulating in the blood.


Fibrillation is an erratic, chaotic quivering of the myocardium during which the heart cannot effectively pump blood. Fibrillation of the atrium is called atrial fibrillation (Afib); in the ventricle it is called ventricular fibrillation (VF). Ventricular fibrillation is typically fatal if not corrected within several minutes.

A flutter is a condition in which the contractions in the upper or lower chamber of the heart become extremely rapid (but regular). A patient may or may not feel the flutter.


Heart Attack (Myocardial Infarction)
A heart attack occurs when arteries are clogged to the point of decreasing or stopping the flow of blood to the heart muscle. The lack of oxygen damages or kills heart muscle, causing the heart attack. Recognizing symptoms and getting prompt emergency treatment can eliminate, prevent or limit the amount of heart muscle damage.

Heart Block
Heart block occurs when electrical impulses generated in the upper chambers of the heart are not properly transmitted to the lower chambers. The heart then beats too slowly, reducing the oxygen that gets to the body and brain.

Heart Failure
Heart failure, or cardiomyopathy, occurs when the heart muscle is too weak to effectively pump blood through the body. Early diagnosis and treatment can stop or slow progression of heart failure. There are two types of heart failure: diastolic, the inability of the heart to relax, thus preventing blood from entering the heart’s pumping chambers, and systolic, the inability of the heart to pump enough blood into circulation.

Heart Valve Problems
Heart valve problems can be inherited or can develop and can wreak havoc on the heart’s ability to push blood from chamber to chamber. Medication and surgery are treatment options.

Hypertension (High Blood Pressure)
Hypertension is the chronic increase in blood pressure above its normal range.

Hypotension (Low Blood Pressure)
Hypotension is the chronic decrease in blood pressure below its normal range.


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 cardiacarrest. 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.

Infarct or Infarction
An infarct is an area of deadened tissue resulting from obstructed blood circulation that normally feeds and nourishes tissue.

Inotropes are medications that increase the heart’s ability to contract.

Ischemia is the insufficient flow of blood to tissue, usually due to constriction or obstruction of an artery.


Left Ventricular Hypertrophy
Left ventricular hypertrophy is a thickening of the muscle wall of the left ventricle.

Long QT Syndrome (LQTS)
Long QT syndrome is a disorder of the electrical system. It can be inherited, acquired after taking certain medications, or caused by a combination of heredity and medications. People with LQTS are susceptible to ventricular fibrillation.


The Myocardium is the middle and thickest layer of the heart. It contracts to pump blood out of the heart and then relaxes as the heart refills with returning blood.


Necrosis is the death of a portion of tissue affected by local injury (e.g., loss of blood supply).


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.

The sensations of feeling the heartbeat are called “palpitations.”

Peripheral Artery Disease
Peripheral artery disease is a clogging of the arteries in the legs or, less often, in the arms.

Premature Contractions
Extra, early or "skipped" beats are the most frequent cause of irregular heart rhythms. These can start in the upper or lower chambers of the heart.


Revascularization is the reopening of an artery at the area of a previous angioplasty or stenting procedure.


Sick Sinus Syndrome (SSS)
SSS is not a specific disease, but a group of signs or symptoms that indicate the heart’s natural electrical pacemaker, the sinoatrial node, is not functioning properly. In SSS, the heart rate can switch back and forth between a slow rate (bradycardia) and a fast rate (tachycardia). A permanent pacemaker, sometimes in combination with medication, is the primary treatment. SSS affects about 3 out of every 10,000 people. It becomes more common as we age.

Sinus Rhythm
Sinus rhythm refers to the normal rhythm of the heart initiated in the sinoatrial node.

Sinus Tachycardia
A harmless rhythm, sinus tachycardia is a normal increase in heart rate that happens with fever, excitement and exercise. It does not require treatment except in rare cases when an underlying problem, such as anemia or hyperthyroidism, should be treated.

Stenosis is the narrowing or constriction of the diameter in a blood vessel. It can reoccur (this is called restenosis) following corrective surgery on a blood vessel.

Although not true "heart" disorders, strokes are a related condition. While some strokes occur when a blood vessel bursts, most strokes happen for the same reasons as a heart attack, clogged or blocked vessels. All strokes pose serious health threats. Atrial fibrillation is a risk factor for stroke.

Sudden Cardiac Arrest (SCA)
In SCA, the heart abruptly and unexpectedly ceases to function (cardiac arrest). The most common cause of SCA is a heart rhythm disorder (arrhythmia) called ventricular fibrillation (VF). VF is an "electrical problem" in the heart. Suddenly, the electrical signals that regulate the pumping action of the lower chambers of the heart (ventricles) become rapid and chaotic. The normal rhythmic contractions of the ventricles stop, and the heart can't pump blood to the rest of the body. The brain is starved of oxygen, and the individual loses consciousness in seconds. Without immediate emergency help, death follows within minutes of an episode of ventricular fibrillation.

Supraventricular Tachycardia (SVT)
The most common type of SVT is atrial fibrillation, an irregular and rapid heartbeat in the upper chambers of the heart (or atria).

Syncope (Fainting)
Fainting, or feeling as if one might faint, can be caused by serious heart rhythm disorders and needs to be evaluated carefully. Sometimes the cause is not heart related, for instance when low blood sugar is to blame, but still can be dangerous. No matter what the cause, fainting can be dangerous simply because of the potential for injuries from falling.


An abnormally fast heart rhythm, or tachycardia can prove dangerous because the racing interferes with the heart’s ability to contract properly. As a result, a victim may suffer a range of symptoms, from lightheadedness to sudden cardiac arrest.


Ventricular Fibrillation (VF)
Sudden cardiac arrest, caused by VF, poses the greatest threat and accounts for half of all cardiac deaths. SCA is due to an electrical circuitry problem. It is not a the same as a heart attack, or myocardial infarction, which is a circulatory problem caused by clogged blood vessels that cut off the supply of blood to the heart. In VF, the heartbeat is rapid and chaotic, which causes the lower heart chambers, or ventricles, to go into a spasm. Sometimes, however, a heart attack can lead to VF. VF is abrupt and happens without any warning and it halts all heart functioning. The lack of oxygen throughout the body, and especially to the brain, is deadly.

Ventricular Tachycardia (VT)
Characterized by a very fast heart rate, VT usually is seen in the setting of other serious heart disease. Occasionally, it occurs in people with normal hearts. It usually requires prompt treatment, sometimes with medication. Sometimes it is treated with radiofrequency ablation or surgery. Often people with VT are protected by implantation of a defibrillator. Because VT can lead to ventricular fibrillation (see above) it is considered a serious condition that warrants aggressive monitoring and treatment.