Implantable Cardiac Defibrillator Defined

An implantable cardioverter defibrillator (ICD) is a small device that’s placed in the chest or abdomen. Doctors use the device to help treat irregular heartbeats called arrhythmias (ah-RITH-me-ahs).

An ICD uses electrical pulses or shocks to help control life-threatening arrhythmias, especially those that can cause sudden cardiac arrest (SCA).

SCA is a condition in which the heart suddenly stops beating. If the heart stops beating, blood stops flowing to the brain and other vital organs. SCA usually causes death if it’s not treated within minutes.

Abnormal heart rhythms (or arrhythmias) can cause your heart to beat too quickly, too slowly or in an irregular pattern. These heart rhythms can happen suddenly and unexpectedly and sometimes people die as a result.

Implantable cardioverter defibrillators (ICDs) are battery-powered devices that deliver an electrical shock to restore normal sinus rhythm when a life-threatening arrhythmia is detected.[1] They can also record abnormal rhythms, which is helpful once a shock has been delivered. They tend to be used in conjunction with anti-arrhythmic drugs. The first ICD was implanted in 1980. ICDs are:

  • Similar in size to a pacemaker.
  • Placed under the skin in the pectoral region.
  • Have a lead in the right ventricular apex.
  • ICD devices are being improved daily and newer devices can also function as a pacemaker.

An ICD can give your heart electric pulses or shocks to get your heart rhythm back to normal.

The ICD is inserted just under your collar bone. It looks similar to a pacemaker and is a little bigger than a matchbox. It is made up of:

  • a pulse generator – a battery powered electronic circuit
  • one or more electrode leads which are placed into your heart through a vein

A new type of ICD is suitable for some people. It’s called a subcutaneous (under the skin) ICD – or SICD for short. An SICD works in the same way as an ICD, but it’s inserted just under the skin of the chest (outside of the ribcage) and there are no leads placed into the heart. Your doctor will talk to you about this option if it’s right for you.

Risk factors for sudden cardiac death

These include:

  • Previous ventricular arrhythmia (ventricular tachycardia – VT)
  • Coronary artery disease
  • Familial cardiac conditions (for example, long QT syndrome)
  • Poor cardiac function (low ejection fraction)
  • NB: people who survive a first episode of a life-threatening VT are at high risk of further episodes.[1]


National Institute for Health and Care Excellence (NICE) guidance recommends that ICDs should be considered for patients in the following categories:[1]

Primary prevention

Previous myocardial infarction and either:

  • Left ventricular ejection fraction (LVEF) <35% and non-sustained VT on Holter monitoring and inducible VT; or
  • LVEF < 30% and broadened QRS duration (>120 milliseconds).
  • Familial conditions associated with sudden cardiac death (for example, long QT syndrome, hypertrophic cardiomyopathy, Brugada’s syndrome or arrhythmogenic right ventricular dysplasia). This may also include those who have had a surgical procedure for congenital heart disease.

Secondary prevention

Survived a VF- or VT-induced cardiac arrest.
Spontaneous sustained VT associated with haemodynamic compromise or syncope.
Sustained VT without syncope or cardiac arrest with LVEF <35%.
These are similar to the indications advocated in the USA.[2] The ICD leads are inserted via a vein, under local anaesthesia. During implantation the unit is tested under conscious sedation. ECG storage provides a retrievable record of the onset and termination of arrhythmias. Programming changes are made with a unit placed over the defibrillator.

Diagnostic Tests

Your doctor may recommend an ICD if he or she sees signs of a ventricular arrhythmia (or heart damage that would make one likely) on the following tests.

EKG (Electrocardiogram)

An EKG is a simple, painless test that detects and records the heart’s electrical activity. The test shows how fast the heart is beating and its rhythm (steady or irregular). An EKG also records the strength and timing of electrical signals as they pass through the heart.

A standard EKG only records the heartbeat for a few seconds. It won’t detect arrhythmias that don’t happen during the test.

To diagnose arrhythmias that come and go, your doctor may have you wear a portable EKG monitor. The two most common types of portable EKGs are Holter and event monitors.

Holter and Event Monitors

A Holter monitor records the heart’s electrical activity for a full 24- or 48-hour period. You wear one while you do your normal daily activities. This allows the monitor to record your heart for a longer time than a standard EKG.

An event monitor is similar to a Holter monitor. You wear an event monitor while doing your normal activities. However, an event monitor only records your heart’s electrical activity at certain times while you’re wearing it.

You may wear an event monitor for 1 to 2 months, or as long as it takes to get a recording of your heart during symptoms.


Echocardiography (echo) uses sound waves to create a moving picture of your heart. The test shows the size and shape of your heart and how well your heart chambers and valves are working.

Echo also can identify areas of poor blood flow to the heart, areas of heart muscle that aren’t contracting normally, and injury to the heart muscle caused by poor blood flow.

Electrophysiology Study

For this test, a thin, flexible wire is passed through a vein in your groin (upper thigh) or arm to your heart. The wire records the heart’s electrical signals.

Your doctor uses the wire to electrically stimulate your heart. This allows him or her to see how your heart’s electrical system responds. The electrical stimulation helps pinpoint where the heart’s electrical system is damaged.

Stress Test

Some heart problems are easier to diagnose when your heart is working hard and beating fast. During stress testing, you exercise to make your heart work hard and beat fast while heart tests, such as an EKG or echo, are done. If you can’t exercise, you may be given medicine to raise your heart rate.

Device details

Current ICD devices use tiered pacing, recognising cycle length, and can initiate the following appropriate therapy, all via single lead:

  • Anti-bradycardia pacing (like a normal pacemaker).
  • Pacing pulses (may cause brief palpitations or a feeling of dizziness) – adaptive bursts to end a VT.
  • Cardioversion shocks for persisting VT (if pacing pulses fail, low-energy cardioversion shocks are given to terminate VT with the minimum of pain).
  • Defibrillation shocks (high-energy shocks – feels like being kicked in the chest unless unconscious). Observers will notice the jolt. No harm comes to anyone touching the patient when they receive the shock.

How does an ICD work?

Your ICD constantly monitors your heart rhythm through the electrodes. If it notices a dangerous heart rhythm it can deliver the following treatments:

  • Pacing – a series of low-voltage electrical impulses (paced beats) at a fast rate to try and correct the heart rhythm
  • Cardioversion – one or more small electric shocks to try and restore the heart to a normal rhythm
  • Defibrillation – one or more larger electric shocks to try and restore the heart to a normal rhythm

Who needs an ICD?

You might need an ICD if:

  • you have already had a life threatening abnormal heart rhythm and are at risk of having it again
  • you haven’t had a life threatening heart rhythm, but you have had tests that show you are at risk of one in the future. This is usually because you have inherited certain faulty genes and may have a condition such as Cardiomyopathy, Long QT syndrome or Brugada Syndrome
  • you have another type of heart condition, such as heart failure, and have had or are at risk of having a life-threatening abnormal heart rhythm
  • you have had other treatments to correct your heart rhythm which have been unsuccessful.

How an ICD is inserted

How is an ICD fitted?
ICDs are inserted under local anaesthetic, but with sedation, so you will feel very sleepy. It can take anything from one hour to three or more hours to implant an ICD, and the time it takes will depend on the type of device you’re having. You will often (but not always) need to stay overnight in hospital and your ICD will be checked thoroughly before you leave.

Living with an ICD

It’s very important to have regular follow-up appointments at your ICD clinic, so you can have your ICD checked. Your appointments may be every 3 to 12 months, depending on the type of ICD you have and if it has delivered any treatment. You will need to have follow-up appointments for the rest of your life.

You may have to make some changes to your lifestyle, for instance if you drive or take part in contact sports. You also need to be aware of how some electronic devices, such as TENS machines or airport security systems, can affect your ICD.

Your doctor will discuss these with you, and you can find out more in our ICDs booklet.

Who can I talk to?

It’s natural to feel worried about having an ICD fitted, but it often helps to talk about your feelings with someone close to you or with a healthcare professional.

Source & More Info: NHLBI, and



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