Pacemaker and ICD Implantation: What These Heart Devices Do and When They Are Needed

The heart needs two systems to work properly.

The first is mechanical: the heart muscle must contract and pump blood.

The second is electrical: specialised cells generate and conduct electrical signals that coordinate each heartbeat.

When the electrical system becomes too slow, unreliable or dangerously abnormal, medication alone may not provide adequate protection.

In selected patients, an implanted cardiac device can provide electrical support.

The two devices most commonly discussed in this setting are:

• Pacemakers, which provide electrical stimulation when the heart rate is too slow or the normal conduction system cannot reliably maintain an adequate rhythm.
• Implantable cardioverter-defibrillators (ICDs), which can detect dangerous ventricular arrhythmias and deliver therapy to terminate them.

A third category is cardiac resynchronization therapy (CRT), which uses specialised pacing to coordinate ventricular contraction in selected patients with heart failure and electrical dyssynchrony.

These devices perform different jobs.

Having a pacemaker does not mean a person is at high risk of sudden cardiac death, and having an ICD does not simply mean that the heart is “too slow.”

Understanding the underlying electrical problem is essential.

What controls the heartbeat?

Normally, electrical activation begins in the sinoatrial (SA) node, which acts as the heart’s dominant natural pacemaker.

The electrical impulse then travels through the atria and reaches the atrioventricular (AV) node before passing through the specialised conduction system to activate the ventricles.

This sequence allows the atria and ventricles to contract in a coordinated fashion.

Harrison’s describes the SA node as the predominant pacemaker, with subsidiary pacemakers in the AV node and specialised conduction system capable of generating impulses if normal SA-node activity fails.

Problems can occur at several levels.

The heart may:

• generate impulses too slowly
• fail to generate impulses reliably
• conduct impulses poorly from the atria to the ventricles
• develop excessively rapid ventricular rhythms
• develop electrical dyssynchrony that reduces pumping efficiency

The appropriate device depends on which problem is present.

What is a pacemaker?

A permanent pacemaker is an implanted electronic system that monitors the heart’s electrical activity and provides electrical stimulation when required.

The device generally consists of:

• a pulse generator containing the battery and electronics
• one or more pacing leads, depending on the system
• electrodes that sense cardiac electrical activity and deliver pacing impulses

The pacemaker does not necessarily pace every heartbeat.

Modern devices can sense the patient’s intrinsic rhythm and deliver stimulation when the programmed criteria indicate that pacing is required.

Harrison’s describes modern pacemakers as programmable devices with different pacing and sensing configurations.

Why might someone need a pacemaker?

Pacemakers are primarily used when the heart rate is too slow or the normal conduction system cannot reliably maintain an adequate ventricular rate.

Two major categories are:

Sinus node dysfunction

The SA node may become unable to generate an adequate heart rate.

This can produce:

• persistent or intermittent bradycardia
• pauses
• chronotropic incompetence
• alternating slow and fast rhythms in some patients
• dizziness
• fatigue
• exercise intolerance
• presyncope
• syncope

Harrison’s identifies permanent pacing as the main therapeutic intervention for symptomatic sinus-node dysfunction.

AV conduction disease

The electrical impulse may fail to travel normally from the atria to the ventricles.

Depending on the degree of conduction abnormality, this can produce a slow ventricular rate or pauses.

High-grade AV block can be particularly important because the normal atrial impulses may not reliably reach the ventricles.

Permanent pacing may therefore be required when the underlying conduction abnormality creates an inadequate or unreliable ventricular rhythm.

Does every slow heart rate require a pacemaker?

No.

A low heart rate by itself does not automatically mean that a pacemaker is needed.

Bradycardia can occur for many reasons, including:

• medications
• increased vagal tone
• athletic conditioning
• sleep
• acute illness
• myocardial infarction
• electrolyte abnormalities
• intrinsic conduction-system disease

The important question is whether the bradycardia is clinically significant and whether it is responsible for symptoms or creates an unreliable rhythm.

Harrison’s notes that some forms of sinus bradycardia require no specific treatment or only temporary support, and that potentially reversible causes should be considered before committing a patient to permanent pacing.

What symptoms can a significant slow rhythm cause?

When the heart rate is inadequate for the body’s needs, cardiac output may fall.

Possible symptoms include:

• fatigue
• weakness
• dizziness
• light-headedness
• reduced exercise tolerance
• shortness of breath
• confusion
• near-fainting
• fainting

Some patients have few symptoms despite significant electrical abnormalities.

Others may experience dramatic episodes.

This is why the decision to implant a pacemaker requires assessment of the rhythm and its clinical consequences rather than relying on one isolated pulse measurement.

What is an ICD?

An implantable cardioverter-defibrillator (ICD) is a device designed primarily to protect appropriately selected patients against life-threatening ventricular tachyarrhythmias.

An ICD can:

1. monitor the heart rhythm
2. recognise potentially dangerous ventricular tachycardia or ventricular fibrillation
3. deliver electrical therapy when programmed criteria are met

Depending on the device and rhythm, therapy may include:

• antitachycardia pacing
• cardioversion
• defibrillation shock

Harrison’s describes ventricular tachyarrhythmias occurring in the setting of structural heart disease as an important indication for ICD therapy and notes that ICD implantation can reduce mortality from sudden cardiac death in appropriate candidates.

Why would someone need an ICD?

ICDs are used in selected patients who have a significant risk of sudden cardiac death from ventricular arrhythmias.

This can include patients who have:

• survived cardiac arrest due to ventricular tachycardia or ventricular fibrillation
• sustained ventricular tachycardia in appropriate clinical settings
• certain structural heart diseases associated with substantial arrhythmic risk
• selected cardiomyopathies
• selected patients with significant left ventricular systolic dysfunction despite appropriate treatment

The precise indications depend on the underlying disease and current clinical guidelines.

An ICD is therefore not simply a stronger pacemaker.

Its primary purpose is protection against dangerous ventricular tachyarrhythmias.

Pacemaker vs ICD

The distinction is important.

An ICD may also provide pacing, but its defining feature is its ability to recognise and treat potentially fatal ventricular arrhythmias.

What is cardiac resynchronization therapy?

Cardiac resynchronization therapy (CRT) is a specialised pacing treatment used in selected patients with heart failure and electrical dyssynchrony.

In some patients with heart failure, the ventricles do not contract in a coordinated fashion.

Instead of both ventricles contracting in an appropriately synchronised sequence, electrical conduction abnormalities can cause delayed activation of one ventricle.

CRT attempts to coordinate ventricular activation through pacing.

Harrison’s describes CRT using pacemaker or ICD platforms, typically with a left ventricular pacing lead, in selected patients with heart failure and ventricular mechanical dyssynchrony.

A CRT system can therefore be:

• CRT-P — cardiac resynchronization with pacing
• CRT-D — cardiac resynchronization combined with defibrillator capability

These devices are used for different clinical situations.

How is a pacemaker implanted?

A conventional transvenous pacemaker is generally implanted using a small incision in the upper chest.

The basic procedure involves:

1. Anaesthesia and preparation

The implantation site is cleaned and local anaesthesia is administered, often with sedation depending on the patient and procedure.

2. Venous access

A vein is accessed so that the pacing leads can be introduced into the cardiovascular system.

Harrison’s describes the subclavian–superior vena cava venous system as a common route for transvenous pacemaker implantation in adults.

3. Lead placement

One or more leads are advanced through the venous system into the appropriate cardiac chamber.

Depending on the pacing system, leads may be positioned in:

• the right atrium
• right ventricle
• both chambers
• or other locations using specialised approaches

4. Electrical testing

The leads are tested to ensure that they can:

• sense cardiac electrical activity
• deliver pacing effectively
• maintain acceptable electrical characteristics

5. Generator placement

The pulse generator is placed beneath the skin, usually in a pocket created in the upper chest.

6. Programming

The device is programmed according to the patient’s rhythm problem and the characteristics of the implanted system.

The wound is then closed.

How is an ICD implanted?

The implantation procedure is broadly similar to a pacemaker when a transvenous ICD system is used.

A generator is placed under the skin and one or more leads are positioned within the cardiovascular system.

The difference is the device’s capabilities.

An ICD must be able to:

• sense cardiac rhythm
• distinguish programmed tachyarrhythmias
• deliver antitachycardia pacing where appropriate
• deliver cardioversion or defibrillation when required

The system is programmed according to the patient’s arrhythmic risk and clinical circumstances.

Some patients may instead be candidates for a subcutaneous ICD, which avoids placing a conventional transvenous lead inside the heart.

Harrison’s notes that a completely subcutaneous ICD may be appropriate for patients at risk of arrhythmic sudden death who do not require pacing.

What happens when an ICD detects a dangerous rhythm?

The ICD continuously monitors cardiac electrical activity.

If a rhythm falls within a programmed treatment zone, the device can provide therapy.

Depending on the rhythm and programming, it may first attempt:

Antitachycardia pacing

Small, rapid pacing impulses can sometimes interrupt a ventricular tachycardia without delivering a shock.

Cardioversion

A synchronised electrical shock can be delivered to terminate certain tachyarrhythmias.

Defibrillation

A higher-energy shock can be delivered when a life-threatening rhythm such as ventricular fibrillation is detected.

Patients may therefore experience an ICD shock as a sudden, strong electrical sensation.

An ICD shock does not necessarily mean that the device malfunctioned.

It may mean that the device detected and treated a potentially dangerous rhythm.

However, an unexpected or repeated shock requires prompt communication with the treating team.

What happens if an ICD delivers several shocks?

Multiple shocks over a short period can occur when a patient is experiencing recurrent ventricular arrhythmias.

This can represent a medical emergency.

Patients with an ICD are generally given instructions about what to do after a shock, and these instructions vary according to the circumstances and device programme.

A shock accompanied by:

• chest pain
• severe breathlessness
• fainting
• persistent palpitations
• neurological symptoms
• ongoing instability

requires emergency medical assessment.

Repeated shocks should also be reported promptly even if the patient feels relatively well afterward.

What are the complications of pacemaker implantation?

Pacemaker implantation is generally well established, but complications can occur.

Early complications include:

• bleeding or haematoma
• infection
• pneumothorax
• cardiac perforation
• lead dislodgement
• diaphragmatic or phrenic nerve stimulation
• vascular complications

Harrison’s specifically lists infection, haematoma, pneumothorax, cardiac perforation, phrenic nerve stimulation and lead dislodgement among possible acute complications of transvenous pacemaker implantation.

Long-term complications can include:

• device infection
• lead failure
• generator problems
• inappropriate programming
• interaction between pacing and native cardiac electrical activity
• pacing-induced mechanical dyssynchrony

Harrison’s also describes pacemaker syndrome, which can occur when pacing disrupts appropriate atrioventricular synchrony.

What are the complications of ICD implantation?

ICDs have many of the same implantation-related complications as pacemakers because conventional systems also involve a generator and leads.

Potential complications include:

• bleeding
• infection
• lead dislodgement
• pneumothorax
• cardiac perforation
• inappropriate shocks
• lead malfunction
• generator failure
• psychological distress associated with shocks

There is also the possibility of receiving a shock when the rhythm is not actually a life-threatening ventricular arrhythmia.

Modern programming aims to reduce inappropriate therapy, but no system eliminates the possibility completely.

Can a pacemaker or ICD become infected?

Yes.

Implanted cardiac devices are foreign material and can become infected.

Infection can involve:

• the generator pocket
• the leads
• the bloodstream
• the cardiac structures associated with the device

Harrison’s identifies permanent pacemakers and ICDs as the principal types of cardiovascular implantable electronic devices involved in device-related endocarditis.

Device infection can be serious and may require prolonged treatment and, in some cases, removal of the infected hardware.

Symptoms such as:

• fever
• redness around the device
• swelling
• drainage
• increasing pain at the implant site
• unexplained bloodstream infection

should be evaluated promptly.

How long does a pacemaker or ICD battery last?

The generator contains a battery and electronic circuitry.

Battery longevity varies according to:

• device type
• pacing requirements
• frequency of pacing
• amount of energy delivered
• ICD shocks
• programmed settings
• other device functions

The generator can eventually be replaced without necessarily replacing the entire system.

The leads may remain in place if they continue to function appropriately.

Regular device follow-up allows clinicians to monitor battery status before depletion becomes an emergency.

What is device interrogation?

Device interrogation means checking the implanted device using specialised equipment.

The clinician can evaluate:

• battery status
• lead function
• sensing
• pacing
• stored rhythm events
• ICD therapies
• programmed settings

Modern systems may also support remote monitoring.

This allows some device information to be reviewed without requiring every assessment to occur during an in-person appointment.

Can a person live normally with a pacemaker?

Many people return to normal or near-normal daily activities after recovery.

A pacemaker is designed to support the heart’s electrical function without requiring the patient to consciously operate it.

Patients can generally:

• walk
• exercise according to medical advice
• work
• travel
• sleep normally
• participate in everyday activities

Restrictions immediately after implantation are usually related to wound healing and preventing lead displacement rather than permanent lifestyle limitations.

Individual advice depends on the device and patient’s condition.

Can a person travel with a pacemaker or ICD?

Travel is generally possible for many patients after device implantation.

Patients should carry information identifying:

• the device type
• manufacturer
• implantation date
• treating centre or cardiologist
• relevant device identification information

Airport and security screening procedures should be discussed with the device team, particularly because security systems may interact with implanted electronic devices.

Patients should also ensure that they have access to medical care at their destination if they have significant underlying cardiac disease.

The device itself should not be regarded as a reason to avoid all travel.

Are mobile phones or household electronics dangerous?

Most ordinary household electronics can generally be used safely, but patients should follow the specific manufacturer’s recommendations for their device.

Modern implanted cardiac devices are designed with safeguards against common sources of electromagnetic interference.

A sensible principle is to keep potentially strong electromagnetic sources away from the implanted device unless the manufacturer or treating team provides specific guidance.

If a patient notices unusual symptoms or device alerts around a particular piece of equipment, they should discuss the situation with their device clinic.

Can someone undergo an MRI with a pacemaker or ICD?

This depends on the specific device system.

Some modern pacemakers and ICDs are designed to be MRI-conditional, meaning MRI may be possible under specified conditions.

Other systems may have restrictions.

The device, leads and programming all matter.

Patients should never assume that an implanted cardiac device automatically makes MRI impossible—or automatically makes MRI safe.

The radiology and cardiology teams need to identify the exact device system before an MRI is performed.

What is leadless pacing?

Traditional pacemakers generally use leads connecting the generator to the heart.

Leadless pacemakers are self-contained devices implanted directly within the heart and therefore do not require a conventional transvenous pacing lead.

Harrison’s describes leadless pacemakers as an option for selected patients requiring single-chamber ventricular pacing.

They are not suitable for every pacing indication.

The appropriate device depends on the patient’s rhythm disorder and the pacing requirements.

Pacemaker vs ICD vs CRT

These categories can overlap.

For example, an ICD can also provide pacing, while a CRT-D combines resynchronization and defibrillator functions.

Does an ICD cure the underlying heart disease?

No.

An ICD treats the electrical consequence or risk of dangerous ventricular arrhythmia.

It does not remove the underlying condition that created the risk.

For example, ventricular arrhythmias may occur in patients with:

• ischemic cardiomyopathy
• previous myocardial infarction
• cardiomyopathies
• inherited electrical disorders
• structural heart disease

The underlying condition still requires treatment.

Harrison’s emphasises that the risk of sudden cardiac death is strongly influenced by the underlying cardiac disease responsible for the ventricular arrhythmia.

The ICD is therefore one part of a broader treatment strategy.

Read our guide on Heart Transplant.

Can an ICD be used in heart failure?

In selected patients, yes.

Heart failure can increase the risk of ventricular arrhythmias, but not every patient with heart failure needs an ICD.

The decision depends on factors including:

• left ventricular systolic function
• underlying cause of heart failure
• duration and optimisation of medical therapy
• arrhythmic history
• expected survival and overall clinical condition
• timing after myocardial infarction or revascularisation
• presence of electrical dyssynchrony

Harrison’s stresses that left ventricular function should be evaluated after appropriate medical treatment and, when relevant, after revascularisation before certain primary-prevention ICD decisions are made.

Modern indications should be determined using current guidelines rather than applying the thresholds in an older edition of Harrison’s directly.

What should patients ask before device implantation?

Before receiving a pacemaker or ICD, useful questions include:

1. What electrical problem is the device treating?
2. Why do I need a device rather than medication or observation?
3. Do I need a pacemaker or an ICD?
4. Would CRT be appropriate in my case?
5. How many leads will be implanted?
6. Where will the generator be positioned?
7. What complications are most relevant to me?
8. How often will the device need to be checked?
9. What happens when the battery reaches the end of its service life?
10. Can I travel, exercise and undergo MRI with this specific device?
11. What should I do if the ICD delivers a shock?
12. Which symptoms should make me seek emergency care?

The key point

Pacemakers and ICDs are different forms of cardiac device therapy designed for different electrical problems.

A pacemaker primarily supports patients whose heart rate is too slow or whose conduction system cannot reliably maintain an adequate rhythm. Harrison’s identifies symptomatic bradycardia and unreliable escape rhythms as important considerations for permanent pacing.

An ICD is designed to detect and treat dangerous ventricular tachyarrhythmias and can reduce sudden-death risk in appropriately selected patients.

CRT adds another dimension by coordinating ventricular contraction in selected patients with heart failure and ventricular electrical/mechanical dyssynchrony.

The device is therefore only one part of treatment. The underlying rhythm disorder, structural heart disease, heart failure or other cardiac condition still requires ongoing management and follow-up.

Written and Reviewed By Dr. Munaf Ahmad Ansari

Medical information note: This article is written for information purpose only. Individual treatment decisions should be based on contemporary clinical guidelines and assessment by an appropriately qualified medical team.