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What is an ICD?

An ICD, or Implantable Cardioverting Defibrillator, is a device implanted in the body that can detect and terminate fast heart rhythms (Ventricular Tachyarrhythmias).

An ICD has two main parts:

  1. main body (battery and electrical circuitry)
  2. leads

The main body of the device is usually implanted under the skin in your shoulder region. The leads are connected to the device at one end and to your heart at the other end. The leads are able to transfer information about your heart rhythm to the device. The device will determine what rhythm your heart is in and will deliver therapy through the leads when necessary.

Why do I need an ICD?

You may have experienced or be susceptible to symptoms such as palpitations, dizziness, shortness of breath or blackouts. You may have had an Electrophysiology Study (EPS) to determine the cause of these symptoms. It has been found that you have experienced or are susceptible to serious arrhythmias that may cause these symptoms or, if left untreated, may be fatal. The function of the ICD is to treat these arrhythmias if you develop them.

Will the ICD prevent my symptoms?

The ICD can not prevent you from going into one of these fast heart rhythms. The device will recognise these fast abnormal rhythms and will deliver therapy to try to regulate your heart. You may or may not feel your usual symptoms before you receive therapy from the device. The device acts quickly, however many people will develop symptoms such as dizziness, palpitations or blackouts before the device has delivered therapy.

What types of therapy can these devices deliver?





Most ICDs are capable of delivering 4 types of therapy for your heart.

If your heart goes too slowly:

  1. the ICD can act as a pacemaker to prevent your heart dropping below a certain rate

If your heart is going too quickly:

  1. the ICD can deliver a sequence of very fast paced beats or “Anti-Tachycardia Pacing” (ATP)
  2. the ICD can deliver a low energy shock or “Cardioversion”
  3. the ICD can deliver a high energy shock or “Defibrillation”

What do I need to know about “shocks”?

A shock is delivered if your heart has gone into an arrhythmia and requires this type of therapy to regulate your heart. Most people describe shocks as feeling like a “kick in the chest by a horse”. The feeling you get from a “low energy” shock and a “high energy” shock may not vary. The feeling is momentary but very uncomfortable, however it is important to remember that this is a last resort for the device to regulate your heart from a potentially lethal arrhythmia.

ICDs are able to deliver several shocks in quick succession if necessary.

Any person touching you when a shock is delivered should not be harmed by the shock.

If you do have a shock, you should follow the directions detailed on your “Instructions for shock therapy” card. 


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About Us


Melbourne Heart Rhythm is the arrhythmia service at the Department of Cardiology at the Royal Melbourne Hospital. We are comprised of a team of highly trained and dedicated health care professionals specializing in heart rhythm management. We have a strong focus on patient education and patient centered care. Our arrhythmia specialists have an international reputation of excellence in arrhythmia research and clinical arrhythmia management.