An Implantable Cardioverter Defibrillator (ICD) is a small electrical device that monitors the rhythm of your heartbeat. When it detects an abnormal rhythm (arrhythmia) it works to restore the normal heartbeat.

An Implantable Cardioverter Defibrillator (ICD) is a small electrical device that monitors the rhythm of your heartbeat. When it detects an abnormal rhythm (arrhythmia) it works to restore the normal heartbeat.

An ICD contains a battery-powered electronic circuit consisting of a pulse generator, and 2 (up to 3) electrode leads which are passed through a vein into your heart. Most modern ICDs have 3 main functions –

It can act like a pacemaker when the heart is beating too fast or too slow, delivering a series of electrical impulses.

If your heart beat is too fast it can deliver a burst of even faster beats to try to restore normal rhythm. This is known as “anti-tachycardia pacing” or ATP

When your heart is beating too slowly the device works like a regular pacemaker giving your heart the extra beats. This is known as “bradycardia pacing”

It may deliver one or more electrical shocks to remedy more serious abnormalities or if ATP does not bring the heart beat back to normal. This is known as defibrillation.

A new less invasive subcutaneous-ICD (S-ICD) (under the skin) may be suitable for certain select patients. It recognises and treats abnormal heart rhythms the same as a regular ICD. The generator is usually placed under the skin under your arm on the lower left side of the chest and the leads are threaded under the skin and up along the breast bone. They are not connected directly into the heart.


Why is it done

ICDs are generally implanted in people who have had a life threatening heart rhythm problem (arrhythmia) or possibly even have had a cardiac arrest or are considered at risk of developing a serious heart rhythm problem and may have the device inserted as a precaution.

Most people have an ICD because their heart has been damaged by a heart attack and may have already had a life threatening arrhythmia and are at risk of having it again.

Certain inherited heart conditions put people at risk of developing life threatening arrhythmias. These include cardiomyopathy (abnormal heart muscle disease), LQTS (inherited heart rhythm disorder) and rare conditions like Brugada Syndrome.

Another type of heart condition that puts some people at risk of developing dangerous rhythm problems is heart failure.

…ICDs treat life threatening heart rhythm problems but they do not prevent a heart attack…

What are the risks

Most people have no problems having an ICD implanted. Like any procedure to the heart there are some associated risks. There is always a small risk of getting an infection at the site and your doctor or nurse will advise you on what to look out for. There may be some bleeding or swelling at the site, particularly if you are taking blood thinners but some pressure applied to the wound site will usually stop this. Bruising is common and may spread around your chest and arm.

Serious complications when having the device fitted are rare.

Other possible risks include Pneumothorax (small air leak from the lungs into the chest); Pericarditis (infection of the lining of the heart); Pericardial effusion (collection of fluid around the heart); Technical problems with the leads.

Occasionally some people experience inappropriate shocks when the ICD incorrectly identifies the heart rhythm as abnormal and delivers a shock when it is not needed.

How it is done (what happens)

Before your doctor decides if you need an ICD, you’ll have several tests done to find out more about your heart function and your risk of life threatening arrhythmias. These could include: ecg, echo, holter monitor, exercise stress test, angiogram, electrophysiology study.

You are usually asked to fast (have nothing to eat or drink) for about 6 hours before having the ICD implanted. Check with your cardiac team about taking your usual medications. If you are taking any blood thinners e.g.Warfarin/Xarelto/Pradaxa/Apixaban, you may need to stop taking these 3-5 days beforehand, so make sure you follow your doctor’s instructions.

ICDs are usually implanted under the skin in the upper left side of the chest near to the shoulder. The leads are threaded through a large vein into the chambers of the heart. The procedure is carried out in the cath lab and is often done using local anesthetic so you will be awake but should not feel anything. The procedure can take a few hours to complete particularly if 3 leads are inserted.

When the ICD is in place your doctor will test it and programme it to treat your specific heart rhythm problem. This may involve giving you some extra sedation/anaesthetic, as it may require speeding up your heart into a disorganised rhythm and ensuring the ICD delivers a shock to correct it back to a normal rhythm.


You are usually kept in hospital on bed rest over night. You will be advised about excessive shoulder movements which can dislodge the leads and will need to avoid heavy lifting or pulling with the affected arm for the first 4 – 6 weeks. This allows the ICD and the leads to “bed in” or settle into place.

After the procedure, a chest x-ray will be taken to check the position of the device and the leads and make sure there is no problem with the lungs. The wound will be covered with a dressing for a few days. You will receive detailed information on how to care for the ICD site going home and follow-up appointments.


When the ICD delivers therapy you may experience a number of different sensations. You may feel an abnormal heart rhythm before the ICD treats the rhythm disturbance.

Once the ICD has delivered its treatment and the heart rhythm has been restored you may wish to rest for a short while. You do not need to go to the hospital right away. Phone your hospital to tell them you’ve had a shock and they will arrange to check your ICD at your convenience. If you still feel unwell after the ICD has delivered a shock or the ICD gives you a series of shocks, you should contact the emergency services immediately.

Everyone’s experience of ICD treatment will be different. The intensity and frequency of treatment will vary from person to person. Some people experience many shocks in a year while others may never feel the treatment.


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