Introduction

Implantable Cardioverter Defibrillators (ICDs) are small devices used to monitor the heart and treat any dangerous rhythms that could lead to sudden cardiac arrest.¹ It does this by sending an electrical impulse to the heart via a thin wire, known as the lead. The electrical impulse delivers a shock at a maximum of 80 joules (J), which should revert the heart into a regular rhythm, known as sinus rhythm.² 

How do ICDs work? 

ICDs are battery-powered devices, about the size of a large pocket watch, which are implanted in a pocket of skin under the left collarbone via a small surgical incision. Subcutaneous ICDs (S-ICD) are implanted on the left side, around 6 inches under the armpit; these will be discussed more in-depth later in this article.³ Both devices continuously monitor the heart to detect any significant increase in rate or change in rhythm, which could indicate the onset of an arrhythmia. 

Life-threatening arrhythmias 

An arrhythmia is caused by a disruption to the conduction system of the heart, which leads to the heart beating too fast, too slow or in an irregular manner.⁴ Arrhythmias become life-threatening when they cause haemodynamic instability, which means that the heart is not beating efficiently enough to sustain sufficient blood flow around the body. Life-threatening arrhythmias are known as ventricular arrhythmias, meaning that they originate in the lower chambers of the heart. There are two main types:

• Ventricular Tachycardia (VT)
• Ventricular Fibrillation (VF)

Both VT and VF can lead to sudden cardiac arrest which requires immediate life-saving intervention. An ICD provides the quickest form of intervention, given that it is continuously monitoring the heart, so it will detect and treat the arrhythmia faster than external defibrillation. You could compare it to having your own personal emergency response team with you at all times!⁵

How does an ICD respond to arrhythmias? 

Every ICD device is individually programmed according to a range of parameters, taking into account the patient's clinical history.⁶ The device has algorithms in place for when a malignant arrhythmia is detected, including additional therapies before administering a shock. Whilst a shock is sometimes the only option to terminate a life-threatening arrhythmia, it is considered a ‘last resort’ treatment, due to the physical and emotional effects it can have on patients.⁷ Previous studies have also shown that shock therapy has been linked to increased mortality and reduced quality of life,⁸ hence it is not used as the first line of treatment. 

Some ICDs can deliver a therapy known as anti-tachycardia pacing (ATP) prior to a shock, which delivers small, low-energy electrical impulses to the heart. ATP attempts to revert the heart to its normal rhythm without requiring a higher energy shock and it has been seen to effectively terminate 85-90% of VT episodes.⁹ For many patients, this therapy is painless and they don’t notice it happening, while others have described symptoms of lightheadedness and dizziness. 

The ICD uses configured ‘zones’ to determine whether ATP is likely to be effective in treating the abnormal heart rhythm, or if a high-energy shock is required immediately.⁶ These ‘zones’ are programmed according to different heart rates and allow the device to discriminate against non-life-threatening arrhythmias. This minimises the chances of the device delivering an inappropriate shock. 

Inappropriate shocks

An inappropriate shock is defined as a shock delivered during a non-life-threatening arrhythmia and can occur due to a range of different factors.¹⁰ Inappropriate shocks are most commonly delivered during an episode of supraventricular tachycardia (SVT); this occurs in the upper chambers of the heart and is not considered life-threatening, therefore does not require shock intervention.¹¹ The high heart rates that are seen during episodes of SVT can sometimes result in inappropriate shocks. 

Components of an ICD

Various parts make up an implantable cardioverter defibrillator:

Pulse generator:

• Size of large pocket watch 
• Contains battery and electrical circuits 
• Reads the electrical rhythm of the heart 
• Acts as the ‘brain’ of the device 

Wire/lead: 

• Can have 1, 2 or 3 leads, depending on the type of ICD 
• Inserted into the right lower chamber of the heart (known as the right ventricle) via central veins from the collarbone area 
• Connects the pulse generator to the heart 

Built-in pacemaker: 

• Transvenous ICDs have a built-in pacemaker 
• Used to take over the heartbeat when the intrinsic heart rate is too slow 
• Provides the ATP capabilities
• Subcutaneous ICDs (S-ICDs) do not have a built-in pacemaker 

Subcutaneous ICDs

Subcutaneous ICDs are implanted on the left side of the chest, approximately six inches beneath the armpit. They are considered to be a less invasive type of ICD device, because they only require one lead which sits underneath the skin, rather than running through the veins of the heart. This therefore reduces the infection risk post-procedure. However, they do not have the ability to pace the heart, thus ATP therapy is not an option. The device is larger than transvenous ICDs which can pose a greater cosmetic disadvantage to patients, but is generally well tolerated in terms of comfort.¹²  

Indications for ICD

Primary prevention

Primary prevention refers to the practice of intervening prior to a sudden cardiac arrest, in cases where there is substantial risk of it occurring. ICDs are indicated under this category in the following scenarios: 

• Patients who have an inherited heart condition with a high risk of sudden cardiac death, such as:
  • Long QT syndrome 
  • Brugada syndrome 
  • Arrhythmogenic right ventricular dysplasia 
• Patients who have undergone a surgical repair of congenital heart disease 

Secondary prevention 

Secondary prevention refers to the indication for an ICD implant in individuals who have already experienced a life-threatening arrhythmia. Implantation under these guidelines is recommended for patients who have:

• Survived a previous cardiac arrest 
• A history of sustained VT which causes loss of consciousness 
• A history of sustained VT without loss of consciousness, but in the presence of meeting the clinical criteria for heart failure 

Inherited heart conditions 

An inherited heart condition is an umbrella term used to describe a group of conditions which are passed on through genetics.¹³ They are caused by a fault, known as a mutation, in one or more genes, resulting in a variety of heart conditions. It is common for these conditions to present asymptomatically; as a result, many patients’ first presentation can be with sudden cardiac arrest. 

Fortunately, advances in cardiac diagnostics and genetic testing have increased the likelihood of these conditions being diagnosed before sudden cardiac arrest occurs, allowing for primary prevention via ICD implantation.¹⁴ It is also recommended that other members of a family should be genetically screened for inherited cardiac conditions, following the diagnosis of a relative.

Examples of inherited cardiac conditions are briefly outlined below. 

Brugada syndrome

Brugada syndrome is a genetic disorder caused by a disruption to specific channels in the heart, which are crucial in delivering messages that control how the heart beats.¹⁵ This disruption can lead to an abnormal heart rhythm, which can progress into life-threatening ventricular arrhythmias. This condition can be diagnosed with specific changes seen on an electrocardiogram (ECG), which is a non-invasive procedure involving stickers placed on the patient's chest to provide a trace of the heart's electrical activity. However, the changes seen on an ECG can be transient, thus a normal ECG reading does not rule out Brugada syndrome. 

Brugada syndrome can be asymptomatic, but some common symptoms include: 

• Feeling lightheaded and dizzy 
• Fainting or blacking out 
• Shortness of breath 
• The sensation of fluttering in the chest (known as palpitations) 

Long QT syndrome

Long QT syndrome is another condition caused by a disruption to the channels in the heart, which affects how the heart beats. This can be diagnosed by measuring a portion of the ECG, known as the QT section. Whilst it is often caused by a genetic mutation, the following factors can also raise the risk of developing long QT syndrome:¹⁶

• Medical conditions, such as:
  • Eating disorders 
  • Thyroid disease 
  • Severe diarrhoea or vomiting
• Medications, such as some antidepressants and antibiotics 
• Sex; more common in women 

Cardiomyopathies

Cardiomyopathy refers to a group of cardiac diseases which affect the muscular layer of the heart, known as the myocardium.¹⁷ This causes structural and electrical abnormalities in the heart, leading to progressive heart failure and sudden cardiac arrest. The risk of sudden cardiac arrest makes cardiomyopathies an indication of ICD implantation. 

There are three main types of cardiomyopathies:

• Hypertrophic Cardiomyopathy 
  • Unexplained increased thickness of the muscle layer of the heart in the left ventricle 
  • Most commonly caused by an inherited genetic mutation¹⁸
  • People often go undiagnosed due to lack of symptoms 
• Dilated Cardiomyopathy
  • Enlargement and dilation of the two lower chambers of the heart 
  • Impairs the heart's ability to effectively pump blood around the body 
  • Most people will experience symptoms, but some can remain asymptomatic 
  • The earlier a diagnosis and treatment is initiated, the better the prognosis¹⁹  
• Arrhythmogenic Cardiomyopathy 
  • Caused by disruption to the cells in the heart muscle leading to weakness of one or both of the lower chambers (ventricles) 
  • Typically presents in adults 

Implantation procedure

ICD implants are performed in a hospital setting and generally require an overnight stay. The procedure is done under local anaesthetic, so the patient is awake but won’t feel anything and they will typically be sedated making them feel relaxed and sometimes woozy! The patient is connected to equipment that monitors heart rate and rhythm, blood pressure and oxygen throughout the procedure. 

Following local anaesthetic, a small incision is made at the insertion site and a plastic tube, known as a sheath, is inserted into a vein. This allows the lead of the device to be advanced down the vein and into the heart. The process is performed under X-ray guidance.

Once the lead is in place, tests are done to verify the location and ensure it is working properly. When testing is complete, the device (pulse generator) is slipped under the skin at the incision site (under the left collarbone). The lead is then screwed into the pulse generator, and the skin incision is closed using stitches. A sterile dressing is then placed over the wound. 

After the procedure, the patient is transferred to a recovery room where they will be monitored and taken care of. 

Follow-up

Following an ICD implant, a patient will generally require an appointment 6-8 weeks later, where they will receive a full device check and an x-ray to ensure the lead has remained in the correct place in the heart. The patient will also be given an ICD identification card, which should be carried on them at all times. This will be useful in situations such as airports where, although most airport security detectors are safe to go through, the device may set off the metal detectors. 

Some ICDs can be connected to a remote monitoring system, which allows the device to send a signal via wifi or telephone, to a rhythm specialist who can analyse anything coming through. This can minimise the number of hospital visits required for check-ups, particularly when the device is coming to the end of its battery check as it can be continually monitored from home. 

FAQs 

What are the benefits of an ICD? 

An ICD will monitor your heart continuously and detect any dangerous rhythms, known as arrhythmias. If an arrhythmia is detected, the device will deliver a shock in order to revert the heart back to its normal rhythm. 

If you receive a transvenous ICD, the device will also have pacemaker capabilities, which means it can take over your heart rate if it is going too fast or slow. It will also have anti-tachycardia pacing abilities, which provides a painless intervention for reverting the heart out of dangerous arrhythmias, prior to having to deliver a shock. 

What are the risks?

ICDs have been known to occasionally deliver inappropriate shocks, meaning the device initiates a shock when it is not needed. This can cause increased levels of anxiety surrounding the device. 

There is a small risk (around 1.19%) of developing an infection at the surgical site of an ICD implant, which could lead to serious illness or even death.²⁰ However, the likelihood of this risk occurring is low, and infection risk is associated with any procedure requiring an open incision. 

The size of the pulse generator in an ICD has been decreasing over the years as the field has progressed, however, it can still be visible under the skin, particularly in slim patients. This is sometimes viewed as a cosmetic limitation. Despite the size, the comfort level has been reportedly well tolerated by patients. 

What does a shock feel like?

Patients have described a shock as “being kicked by a mule” or as an “earthquake”. Whilst it may be uncomfortable or painful, it is life-saving. Patients are often unconscious when the shock is administered, so they are unaware of it happening. 

How long does the battery last?

The battery will generally last between eight and fifteen years.  

What happens when the battery runs out?

When the battery is close to the end of its life, you will be booked in to have the pulse generator changed. This is a quick procedure under local anaesthetic, where the old device will be taken out and a new one will be placed in the same pocket under the skin. The lead will remain in place in the heart, making it a much faster procedure than the initial implant. 

Summary

Implantable cardioverter defibrillators are small devices used to monitor the heart rate and rhythm and deliver an internal shock to the heart during life-threatening rhythms. It uses between 1 and 3 leads placed in the veins of the heart, which connect to the device just below the left collarbone. Transvenous ICDs also have pacemaker capabilities, which allow them to treat slow heart rates, as well as providing an alternative treatment to dangerous arrhythmias, known as anti-tachycardia pacing, in an attempt to revert the heart to its normal rhythm.

Whilst there are risks associated with having an ICD, such as the potential for inappropriate shocks, it is a life-saving device. Progression in diagnostics of inherited cardiac conditions has allowed for the ability to treat conditions before any symptoms arise, reducing the chances of sudden cardiac arrest.