Dr Matthew Webber
Heart Failure, Cardiac Devices and Advanced CMR Specialist

Implantable Cardioverter Defibrillator (ICD)

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What is an ICD?

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An implantable cardioverter defibrillator (ICD) is a small device (similar to a pacemaker) designed to detect and treat dangerous fast heart rhythms arising from the lower chambers of the heart (the ventricles). These abnormal rhythms can prevent the heart from pumping effectively and may lead to cardiac arrest if untreated.

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An ICD continuously monitors the heart rhythm. If a dangerous rhythm is detected, the device can automatically deliver treatment to restore a normal rhythm.

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This treatment may include:

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• Rapid pacing therapy to interrupt the abnormal rhythm

• A small electrical shock to reset the heart rhythm if necessary

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ICDs therefore act as a safety net, protecting patients at risk of life-threatening arrhythmias.

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Types of ICD

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There are two main types of ICD.

 

Transvenous ICD

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This is the most common.

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The generator is implanted beneath the skin below the collarbone in the same way the pacemaker is and one or more leads pass through the veins into the heart where they monitor the rhythm and deliver therapy if needed. They can also act in the same way as a pacemaker to protect the heart from slow heart rhythms to. 

 

Subcutaneous ICD (S-ICD)

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A subcutaneous ICD sits entirely outside the heart and blood vessels.

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The generator is placed under a muscle on the side of the chest and the lead runs just beneath the skin along the breastbone. Because the lead does not enter the heart or circulation, this system avoids some of the long-term risks associated with leads inside the heart. However, unlike transvenous ICDs, subcutaneous systems do not provide pacing therapy, so they are most suitable for patients who only require protection from dangerous fast rhythms.

 

What happens during the procedure?

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The implantation approach depends on the type of ICD.

 

Transvenous ICD

• Usually performed under local anaesthetic with sedation

• Procedure time typically around one hour

• Usually a day-case procedure

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Subcutaneous ICD

• Usually performed under general anaesthesia

• Procedure time typically around 45 minutes

 

Complications from ICD procedures are similar to that of permanent pacemakers. 

 

ICD shocks

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If the ICD detects a dangerous ventricular rhythm it may deliver a defibrillation shock to restore normal rhythm. Patients describe this sensation differently, but it is often described as a sudden strong thump in the chest. Although unpleasant, shocks are delivered only when the device detects a life-threatening rhythm and can be life-saving.

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Many patients with ICDs never experience a shock.

 

Inappropriate therapy

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ICDs use sophisticated algorithms to distinguish between dangerous ventricular rhythms and faster rhythms arising from the upper chambers of the heart. Occasionally the device may misinterpret a rhythm and deliver therapy when it is not required. This is known as inappropriate therapy.

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If this occurs, the device settings can usually be adjusted and the underlying cause treated to minimise the risk of it happening again.