Here’s a handy glossary created especially with ICD and heart-related terms. Let us know if you can’t find the term you're looking for.

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Accordion Bursts
Bursts from ATP with alternating incremental and decremental scanning.

ACE Inhibitors
Most heart failure patients are on angiotensin-converting enzyme (ACE) inhibitors. They can help control symptoms and may slow the advance of the disease. ACE inhibitors reduce the heart’s workload by making the blood vessels expand, which lowers blood pressure. They also reduce the tendency to retain salt and fluid.

Adaptive Rate
The ability of a device to increase or decrease its pacing rate in response to bodily needs, activity, or exercise.

Decision rule (for detection in the case of the ICD). For example, rhythm onset and stability applied to the cycle length sequence.

A Class III agent with broad antiarrhythmic action and extremely long half life.

Angiotensin II Receptor Blockers
Similar to ACE inhibitors, these are used to keep blood pressure from rising. They lower blood pressure without lowering the heart rate.

Antiarrhythmic Drugs
Medications designed to prevent or treat cardiac arrhythmias.

Anti-Tachycardia Pacing (ATP)
Often the first type of therapy used by an implantable heart device to treat a fast heart rhythm by delivering short bursts of pacing pulses.

An irregular heartbeat. This could be a rhythm that’s abnormally slow (bradycardia) or a rhythm that’s too fast (tachycardia). Read more about bradycardia and tachycardia.

See Coronary Artery Disease.

Atrial Fibrillation
Extremely rapid, irregular atrial impulses. This rapid rate, ranging from 300–600 beats per minute, does not allow the atria to pump blood effectively into the ventricles. It can also cause irregular rapid ventricular beats.

Atrial Flutter
A cardiac arrhythmia in which the atrial contractions are rapid (230-380 per minute), but regular.

Atrioventricular Node (AV node or AV junction)
Conduction tissue situated at the junction between the atria and ventricles. In the absence of any disease or drug effects, transmits impulses in 1-to-1 fashion from atria to ventricles during sinus rhythm (albeit with some physiologic delay). However, may conduct only intermittently during pathologically rapid atrial rhythms (e.g., atrial fibrillation).

One of the two upper chambers of the heart. The left atrium fills with oxygenated blood from the lungs. The right atrium fills with deoxygenated blood returning from the body.

Automated External Defibrillator (AED)
Portable devices used to electrically stimulate a fibrillating heart. Using an external defibrillator, strong electric shocks are passed between paddles and electrodes placed on a person’s chest.


Background Current
Also known as monitoring, sensing, overhead, or idle current. This is what the ICD circuitry consumes continuously in the absence of any therapy delivery.

These medications help control heart rate and reduce the heart’s tendency to beat faster. They are used to help the heart maintain a slower rate and lower blood pressure and are often used in combination with diuretics, digoxin and ACE inhibitors.

Blood Thinners
Used to reduce the risk of blood clots in the legs, lungs and heart.

Beats per minute.

(See also bradycardia, below) These are slow heart rhythms, which may arise from disease in the heart's electrical conduction system. Examples include sinus node dysfunction and heart block.

A condition in which the heart beats at less than 60 beats per minute. This heart rate may be too slow or irregular to meet the body's demands. (This is different than a physically fit person who may have a heart rate below 60.) Read more.


Calcium Channel Blockers
Sometimes used to help lower blood pressure and improve blood circulation in the heart.

Unit which measures capacitor charge storage ability for a given voltage.

Cardiac Catheterization
A heart procedure used to diagnose heart disease. A catheter (inserted into an artery in your arm or leg) is guided to your heart, contrast dye is injected and X-rays of the coronary arteries, heart chambers and valves are taken.

Cardiac Perforation
Penetration of the lead tip through the myocardium, clinically suspected and confirmed by chest x-ray, fluoroscopy, echocardiogram, or visual observation, which results in clinical symptoms, typically degradation of pacing/ICD lead electrical performance (high thresholds), chest pain, and

Cardiac Resynchronization Therapy (CRT)
Implantable device therapy for people with moderate to severe heart failure who also have ventricular dysynchrony. Helps the lower chambers of the heart (left and right ventricles) beat together again.

Specializes in the diagnosis, treatment and prevention of diseases of the heart and blood vessels.

A degenerative disease of the heart’s muscle tissue.

Cardiopulmonary Resuscitation (CPR)
The mechanical pushing of the heart (hands pushing on rib cage) and breathing (through the mouth) done in a rhythmic pattern. These actions keep blood and oxygen circulating through the body. CPR is often used as a first emergency response until an external defibrillator can be applied to restart the heart.

Cardiovascular Nurse
A nurse who works in a hospital area where patients with arrhythmias and other heart problems are monitored.

Cardiovascular Technician
Assists doctors with cardiovascular procedures and surgeries.

Conversion of an abnormal cardiac rhythm (VT or atrial tachyarrhythmia) to a normal one either by the use of medication or by the application of electric shock, as with a defibrillator. Shock strength is typically lower than that for VF.

Catheter (cath lab/electrophysiology(EP)) Lab Nurse
Participates in electrophysiology studies and surgeries for pacemakers and implantable cardioverter-defibrillators (ICDs).

Cell (electrical)
A single chamber “battery;” a battery is technically only 2 or more cells.

Ceramic Capacitor
A capacitor using a solid ceramic material for the dielectric.

Charge Time
The time required to bring the energy storage capacitors in an ICD to a full charge in order to deliver a therapeutic shock; normally ranges from 5-18 seconds. Equals to: (desired stored energy + unformed capacitor losses) / (battery power * inverter efficiency %)

Charging Circuit
Circuitry which converts the 3-6 Volts typically available from the battery to the much higher voltage (usually up to 750 Volts) for a defibrillation shock.

Chemotherapy drugs
Certain drugs used in chemotherapy may weaken the heart muscle and result in a low ejection fraction and heart failure.

Cardiovascular implantable electronic device (CIED)
Cardiovascular implantable electronic devices include the pacemaker (PM), implantable cardioverter-defibrillator (ICD), cardiac resynchronization device (CRT), implantable loop recorder (ILR) and implantable cardiovascular monitor (ICM). Pacemakers, ICD and CRT devices collectively have been termed cardiovascular implantable electronic devices (CIEDs).

Class IA agents
Sodium blocking anti-arrhythmic drugs which also slightly prolong cardiac action potential.

Class IB agents
Sodium blocking anti-arrhythmic drugs which also slightly shorten or do not affect cardiac action potential.

Class IC agents
Sodium blocking anti-arrhythmic drugs which primarily slow conduction without prolonging the cardiac action potential.

Class III agents
Potassium blocking anti-arrhythmic drugs which primarily prolong cardiac action potential.

Conduction Pathway
Pathways that conduct electrical impulses from the SA node through the atria, through the atrioventricular (AV) node and around the ventricles, thereby causing the heart to beat and pump blood throughout the body.

Conductor Fracture
A mechanical break within a lead conductor (includes connectors, coils, cables and/or electrodes) observed visually, electrically, or radiographically.

Congestive Heart Failure
A term often used to describe heart failure.

Coronary Artery Disease
Chronic condition in which a clogged artery prevents the heart from receiving sufficient blood.

Unit of charge equal to 6 x 10^18 electrons.

Crush Injury
Lead fractures caused by mechanical stresses in the region of the costoclavicular joint.


Pacing mode of dual chamber (atrial or ventricular) pacing and sensing, with inhibition of pacing in either chamber by a sensed rate above a programmed value.

An ICD therapy option used to treat ventricular fibrillation. Defibrillation consists of high-energy shock impulses.

Device interrogation
The use of telemetry to retrieve information from the ICD, its programmed parameters and data stored in its memory. These data may be retrieved and stored directly in an ICD's programmer, on a dedicated personal computer or retrieved and stored remotely on a server to be viewed on an Internet website.

Diagnostic Test
This is a test that is ordered by a physician to determine or rule out a condition. Read more.

Diuretics (water pills)
Medications prescribed for fluid retention and swelling of feet, legs and abdomen. Diuretics prompt the kidneys to filter more sodium and water from the blood. With less fluid in the body, the heart can pump and circulate blood with less effort. Additionally, diuretics can decrease fluid retention in the lungs, ankles, legs and other parts of the body.

A medication that increases the force of the heart’s contractions to relieve heart failure symptoms. Slows certain types of arrhythmia.

Shortness of breath, one of the classic symptoms of heart failure.

A condition in which the two lower chambers of the heart are not beating together as they do normally. In a normal heart, both sides beat together and are effectively "synchronized."


Echocardiogram (or “echo”)
A test that provides a measurement of how well your heart is pumping and is a key indicator of your heart’s function.

A physician who performs cardiovascular examinations using echocardiograms to produce a picture of a heart and great vessels using high-frequency sound waves.

Abnormal accumulation of excess fluid in the intercellular tissue spaces. Read more.

Ejection Fraction (EF)
A measurement of how much blood (what fraction or percentage of blood) the pumping chambers of the heart (the left and right ventricles), are able to pump out or eject and supply to the organs (brain, kidneys, liver, etc.) and muscles of your body. An Ejection Fraction of 50% or above is often considered normal.

Electrocardiogram (ECG)
A test that records the electrical activity of the heart, revealing evidence of previous heart attack, enlargement of the heart and abnormal rhythms.

Electrogram (EGM)
Intracardiac electrical signals corresponding to cardiac activity. Internal equivalent to surface ECG.

Electrophysiologist (EP)
A cardiologist with specialized training in the diagnosis and treatment of heart rhythm problems.

Electrophysiology (EP) Study
A study used to evaluate the electrical system of the heart. Read more.

Also known as Electromagnetic Interference, it is any magnetic or electrical interference from machines or devices which can interrupt the normal operation of a pulse generator.

A rare but serious infection of the lining of the heart (endocardium) or heart valves, which can cause the heart to work less efficiently.

External Defibrillation
An electric shock given to the heart through paddles placed on the chest in order to restart the electrical system of the heart. External defibrillation may be done using a manual external defibrillator or an automated external defibrillator (AED).

External Defibrillator
A portable device used to electrically stimulate a fibrillating heart. Using an external defibrillator, strong electric shocks are passed between paddles and electrodes placed on a person's chest.

External Loop Recorder
A device that monitors heart rhythm and rate for up to one month. During this test, the patient wears a device on the wrist or around the waist. When symptoms are experienced, the patient presses a button on the device to make a recording of the heart activity that just occurred.


Failure to Capture
Intermittent or complete failure to achieve cardiac stimulation (atrial or ventricular) at programmed output delivered outside of the cardiac refractory period. A sudden and significant increase in the pacing threshold value (elevated thresholds compared to previous measured value) at which 2:1 safety margin can no longer be achieved.

Failure to Sense (undersensing)
Intermittent or complete loss of sensing or failure to detect intended intrinsic cardiac signals (atrial or ventricular) during non-refractory periods at programmed sensitivity settings.

Follow-Up Nurse
Works in clinics or hospitals and checks the implantable device (pacemaker or ICD).



Heart Attack or Myocardial Infarction
When the heart muscle is damaged because blood is blocked from reaching it, a heart attack occurs. A heart attack is also called a myocardial infarction (MI) because the middle layer of the heart muscle (myocardium) is damaged or is non-functional (infarct).
Read more.

Heart Failure
Heart failure occurs when your heart muscle doesn't pump as much blood as the body needs. "Failure" doesn't mean that the heart has stopped pumping, just that it is failing to pump as effectively as it should. Heart failure is most often caused by a problem with the left ventricle of the heart.

Heart Failure Specialist
A cardiologist who focuses on the diagnosis and treatment of heart failure.

Heart Rhythm Team
A team of doctors, nurses, and other healthcare professionals who provide care for your heart condition and other medical conditions. See the Doctors and the Heart Care Team page for more information.

Heart's Electrical System
The heart has its own electrical system. Special tissues conduct electrical signals that travel along pathways through the heart to stimulate the heart to beat, that is, the contractions of the heart’s chambers pump the blood throughout the heart and into the body. See the How Does a Healthy Heart Work? page for more information.

Heart Valve
Valves within the human heart that open and close automatically to control the flow of blood into, through, and out of the heart. These valves open to ensure that blood flows into the heart chambers as needed, and they close to ensure that blood does not flow back into the heart after it has been pumped out to the lungs and body.

High Blood Pressure (Hypertension)
A condition where the heart is working harder than normal to force blood into the arteries and through the circulatory system. Over time, high blood pressure can cause damage to the heart and other organs.

Holter Monitor
A device that measures and records heart rhythm over 1-3 days. This test may be done when an ECG does not show the arrhythmia and it still is suspected to be the cause of symptoms. Patches with wires are placed on the chest. The wires are connected to a portable monitor that can be attached to a purse or belt.


Implantable Cardioverter Defibrillator (ICD)
An ICD is a small device placed inside the body that treats abnormal heart rhythms. It can deliver several types of therapies, including cardioversion, defibrillation, and anti-tachycardia pacing. An ICD is usually implanted in the upper chest or in the abdominal area.

Implantable cardiovascular monitor (ICM)
ICMs are implantable electronic devices that store cardiovascular physiologic data such as intracardiac pressure waveforms and other data in the device memory, but instead of focusing only on heart rhythm, the hemodynamic and cardiovascular physiologic information stored in these devices is used as an aid in managing patients with chronic cardiac diseases such as heart failure.

Industry employed allied professional (IEAP)
The IEAP has expertise with CIED technology and is employed by the CIED manufacturer. Although the IEAP may have formal credentials of a CIED nurse or EP lab technician and may be certified by the International Board of Heart Rhythm Examiners as a certified cardiac device specialist, there are limits on the roles and activities that these people can engage. The details are listed in Section 4 and quoted from the 2001 NASPE guidelines for the "Industry Employed Allied Professional."4Go

Insertable Loop Recorder
A device that continuously monitors the heart rhythm for up to 14 months. This small device is placed under the skin during an approximate 20-minute procedure using a local anesthetic. When a symptom is experienced, the patient places a hand-held activator over the recorder to record the heart activity. Later, a physician analyzes the stored information.

Insulation Breach
A disruption or break in lead insulation observed visually, electrically, or radiographically. Examples include: 1) proximal abrasions associated with lead-on-lead or lead-on-PG contact in the pocket, 2) mid-lead insulation damage caused by clavicular crush or insulation wear in the region of vein insertion, and 3) distal region wear due to lead-on-lead (intracardiac), lead-on-heart valve or lead-on-other anatomy contact.


The energy of a pulse of one volt and one ampere lasting one second. It is also about 1/4 of a calorie.



A thin, insulated wire with electrodes (electrical contacts) located near the tip.

Lead Dislodgement
Radiographic, electrical or electrocardiographic evidence of electrode displacement from the original implant site or electrode displacement that adversely affects pacing and/or lead performance.


Monitor Technician
Observes heart rhythm monitors on the post-operative hospital floor. Other duties may include performing electrocardiograms (ECGs) and administering patient follow-up care.

MRI (Magnetic Resonance Imaging) scan
A medical test used to produce images of the interior of the body. MRI uses strong magnetic fields and low-energy radio waves to create images. The strong magnets can interfere with the functioning of a pacemaker, ICD, or neurostimulator. If you have one of these devices, do not participate in Magnetic Resonance Imaging (MRI) tests without informing your physician about your implanted device.

Multiple Gated Acquisition Test (MUGA)
A small dose of radioactive material is injected intravenously into the bloodstream. A scanning device then reveals how much blood the heart is able to eject out of both the left and right ventricles.

Myocardial Infarction (MI)
See Heart Attack.

Infection of the heart’s muscle tissue, which inflames the heart and makes it unable to pump as efficiently.


North American Society of Pacing and Electrophysiology (NASPE)
An organization of physicians, scientists, and nurses throughout the world dedicated to the study and management of cardiac arrhythmias.

Nuclear Scan-Cardiolite
For this test, a radioactive substance is injected during exercise. Then a special image is taken of the heart to see how well blood is flowing through the heart.

Nuclear Stress Test
Also known as an isotope, thallium, Cardiolite or Myoview stress test, depending upon the method used. For this test, a radioactive substance is injected during exercise. Then a special image is taken of the heart to see how well blood is flowing through the heart.

A classification system developed by the New York Heart Association widely used to diagnose the functional class, or severity, of heart failure based on symptoms one experiences with exertion. NYHA Class I and II are considered mild, Class III is moderate to severe, and Class IV is considered severe heart failure.


Operating Room (OR) Nurse
Participates in surgeries for pacemakers and implantable cardioverter-defibrillators (ICDs).

Misinterpretation of cardiac or non-cardiac events as cardiac depolarization, e.g. T-waves, skeletal muscle potentials, and extra cardiac electromagnetic interference (EMI).


A pacemaker system is a two-part electrical system that includes a pulse generator (pacemaker) and one or two leads, or wires, which deliver impulses to the heart. The leads also carry signals back from the heart. By 'reading' these signals, the pulse generator is able to monitor the heart's activity and respond appropriately. A pacemaker helps to pace the heart when the natural rate is too slow (bradycardia) to pump enough blood to the body.

A mineral that helps control heart rhythm and is important to the nervous system and muscles. Diuretics remove potassium, so doctors may advise some patients to increase potassium intake.

Premature atrial contractions (PACs)
These are early extra beats that originate in the atria (upper chambers of the heart). They are harmless and do not require treatment.

Premature ventricular contractions (PVCs)
These are among the most common arrhythmias and occur in people with and without heart disease. This is the skipped heartbeat we all occasionally experience. In some people, it can be related to stress, too much caffeine or nicotine, or too much exercise. But sometimes, PVCs can be caused by heart disease or electrolyte imbalance. People who have a lot of PVCs, and/or symptoms associated with them, should be evaluated by a cardiologist. However, in most people, PVCs are usually harmless and rarely need treatment.

A device designed to receive telemetry from a family of CIEDs from a specific manufacturer, often used in the clinician's office. It will display and print the information to the operator and temporarily or permanently adjust (program) the behavior of the implanted device. Generally the programmer technology includes a specifically modified microcomputer and a programming wand or antenna to communicate with the CIED. The programmer magnet is often placed over the implanted device to collect information stored in it. It is usually fitted with a printer, storage devices such as hard drives and communication connections such as Ethernet, USB, WiFi, infrared and parallel and serial ports.

Pulmonary edema
Accumulation of fluid in the lungs usually due to heart failure.



Recommended Replacement Time.

ECG manifestation of ventricular depolarization. Also called QRS complex.

Radionuclide Angiogram (RNA) Test
See Multiple Gated Acquisition Test (MUGA).

Risk Assessment Quiz
A quiz designed to assess a person's risk for Sudden Cardiac Arrest.


Sinoatrial Node (SA node)
A group of cells located in the right atrium that sends out electrical signals which make the heart pump. The SA node is the heart’s natural pacemaker. These signals travel from the SA node, through the atrioventricular (AV) node, and then to the rest of the heart. The SA node also responds to the need for a faster heart rate. If a person is exercising or excited, the body will require greater blood circulation. A healthy SA node responds to these changes in the body and increases the heart rate accordingly.

Sinus Node (SA node)
See Sinoatrial Node.

A narrowing of an artery.

Stress Test
Reveals how well the heart functions during exercise. See the Diagnostic Tests page for more information.

Sudden Cardiac Arrest (SCA)
Sudden cardiac arrest is when the heart’s lower chambers (ventricles) suddenly develop a rapid, irregular rhythm (ventricular fibrillation) and the quivering ventricles cannot pump blood to the body.

Sudden Cardiac Death (SCD)
Natural death due to cardiac causes, noted by abrupt loss of consciousness and cardiac arrest within an hour of the onset of acute symptoms. Pre-existing heart disease may or may not have been known to be present.

Supraventricular Tachycardia (SVT)
A fast arrhythmia (over 100 beats per minute) that originates an electrical impulse in the atrium.

Superior vena cava, especially an electrode in this location.

Lithium silver vanadium oxide cell (used in most ICDs).

Supraventricular tachycardia.

A temporary loss of consciousness due to lack of blood to the main portion of the brain (the cerebrum), fainting.

Synchronized shock
Used for cardioversion and delivered to coincide with a sensed ventricular event (depolarization), thereby minimizing the likelihood of inadvertently inducing VF.

Portion of cardiac cycle during which the ventricles depolarize and contract.


Cardiac electric waveform corresponding to ventricular repolarization.

T-wave oversensing
Counting T-waves as ventricular depolarizations and hence deriving a falsely elevated heart rate. T-wave oversensing might result in inappropriate shocks.

Tachyarrhythmias are a fast or irregular heart rhythm, usually more than 100 beats per minute and as many as 400 beats per minute. Read more.

Telemetry is the wireless transmission and reception of data to and from the ICD. The word is derived from the Greek roots tele (meaning remote), and metron (meaning measure).

Tilt Table Test
This procedure attempts to simulate conditions that may cause arrhythmia. It enables a physician to understand how blood pressure, heart rate and rhythm respond to a change in position from lying down to standing. The patient is positioned on a table, given medication, and the table is tilted by varying degrees. The test typically lasts for a couple of hours.

Twiddler‘s Syndrome
First described in 1968, twiddler‘s syndrome refers to permanent malfunction of a pacemaker (or ICD) due to the patient‘s deliberate or subconscious spinning of the pulse generator in a capacious pocket. Not to be confused with Twitter Syndrome.



Valvular Heart Disease
One of the heart’s four valves may be narrowing (stenosis) or leaking, restricting blood flow through the heart. In certain cases, valvular endocarditis (infection of the heart valves) may cause damage to the valves, impairing blood flow.

Medications that cause the blood vessels to widen or relax so blood can flow more easily. ACE inhibitors are one type of vasodilator.

One of the two lower chambers of the heart that are responsible for pumping blood throughout the body. The right ventricle pumps blood depleted of oxygen back to the lungs where carbon dioxide is exchanged for oxygen. The left ventricle pumps oxygen-rich blood to the entire body.

Ventricular dysynchrony
A condition where the lower chambers of the heart do not beat at the same time due to a delay in the electrical conduction system.

Ventricular Fibrillation (VF)
A heart rhythm disorder that originates in the ventricles. It is characterized by an abnormally rapid heart rhythm that is also highly unstable and irregular. During VF, electrical signals are moving chaotically through the heart, preventing it from beating properly. This often results in fainting. If left untreated, it may result in sudden cardiac arrest.

Ventricular Tachycardia (VT)
A heart rhythm disorder originating in the ventricles. Rapid contractions prevent the heart from filling adequately with blood between beats. Patients may feel faint, become dizzy or collapse. It can be life-threatening if not treated.

Pacing mode of continuous ventricular pacing.

Unit of voltage which is electrical pressure analog and represents the force driving the current through its path.

Pacing mode of ventricular sensing and ventricular pacing with inhibition of pacing by sensed rate above a programmed value.