Pacemakers, implantable defibrillators and

• A pacemaker treats a slow or blocked heartbeat; an implantable cardioverter-defibrillator (ICD) detects and terminates life-threatening ventricular fast rhythms; cardiac resynchronisation therapy (CRT) re-times a failing ventricle in selected patients with heart failure.
• Each device is named by a short code that states which chambers are paced and sensed and how it responds, for example VVI or DDD.
• The commonest indications are sinus node dysfunction and high-grade atrioventricular block for pacemakers; a prior ventricular fibrillation or hemodynamically significant ventricular tachycardia, or high-risk structural heart disease with reduced ejection fraction, for an ICD; and symptomatic heart failure with a reduced ejection fraction and a wide left bundle branch block for CRT. ^[4]
• After implantation the device needs regular remote checks, battery replacement when due, and special handling during MRI and surgery.
• Decisions rest on current guidelines: the 2021 European Society of Cardiology (ESC) Guidelines on cardiac pacing and CRT, the 2018 bradycardia guideline and the 2017 ventricular arrhythmia guideline issued jointly by the American cardiology college, the American Heart Association and the Heart Rhythm Society, and the 2022 AHA/ACC/Heart Failure Society of America (HFSA) heart failure guideline, as summarised in the Merck Manual and other open sources cited here.

By the end of this page you should be able to:

1. Explain the difference between a pacemaker, an ICD and a resynchronisation device, and match each to its typical patient.
2. Read and explain a three-to-five letter pacemaker code, and justify mode selection (AAI, VVI, VDD, DDD).
3. List the principal indications and the principal non-indications for a permanent pacemaker.
4. State when an ICD is indicated for secondary or primary prevention of sudden death, and what happens during a shock or inappropriate shock.
5. State the CRT indications and the electrocardiogram (ECG) and imaging cut-offs that must be met.
6. Describe routine device-clinic follow-up and remote monitoring, including how often devices are checked.
7. Describe MRI, surgical and everyday precautions for a patient with an implanted device.

An artificial pacemaker is a battery-powered generator connected to one or more leads that deliver electrical stimuli to the heart muscle. It is used mainly to prevent or treat bradycardia and conduction block. An implantable cardioverter-defibrillator also paces the heart but, in addition, senses dangerous ventricular rhythms and can deliver antitachycardia pacing or an electrical shock to terminate them. Cardiac resynchronisation therapy uses a pacemaker system with a lead in the left ventricle, usually via the coronary sinus, to restore coordinated contraction in selected patients with heart failure and ventricular dyssynchrony. A CRT device may be a pacemaker alone (CRT-P) or combined with an ICD (CRT-D).

Modern practice is built around the idea that bradycardia, sudden death and pump failure are three separate problems, and a single device family can address more than one of them when the patient has more than one risk.

A pacemaker has three jobs: it paces (creates a heartbeat when the heart does not beat on time), it senses (detects the patient's own electrical activity), and it decides (inhibit itself when the heart is working, or respond when it is needed). The combination of these three functions is expressed as a code of three to five letters. The first letter states the chamber paced, the second the chamber sensed, the third the response to a sensed beat, the fourth whether the rate increases ("modulates") with exercise, and the fifth whether pacing is multisite. ^[2]

VVI and DDD devices are the types most commonly used, and they give equivalent survival benefits. ^[2] In VVI the ventricle is paced and sensed, and a sensed beat inhibits the pacemaker; this suits permanent atrial fibrillation with a slow ventricular rate. DDD paces and senses both chambers, so it preserves atrioventricular synchrony. Compared with VVI, physiologic systems (AAI, VDD, DDD) appear to reduce the risk of atrial fibrillation and heart failure and to improve quality of life a little. ^[2] Mode switching is an automatic change of mode (for example from DDDR to VVIR during atrial fibrillation) that prevents rapid tracking of atrial flutter or fibrillation. Rate-responsive mode (R) raises the paced rate during exercise by sensing body movement or respiration.

Leadless ventricular pacemakers combine generator and lead into one small cylinder that is anchored inside the right ventricle. They are transvenous devices with no pocket and no transvenous leads, and the currently used types are VVI or VVIR. ^[2] They are an option when upper venous access is difficult, when pocket infection risk is high, or when a single-chamber ventricular device is sufficient. ^[1]

The 2021 ESC Guidelines on cardiac pacing and CRT emphasize minimizing unnecessary ventricular pacing, which is a class IA indication in patients with sinus node dysfunction who receive a DDD-type pacemaker. ^[1] The reason is simple: unnecessary right ventricular pacing can produce left ventricular dyssynchrony and eventually heart failure, so algorithms that avoid pacing when the patient's own rhythm is adequate are preferred.

The indications below are summarized from the Merck Manual Professional Edition (February 2025), which adapts the 2018 ACC/AHA/HRS guideline on bradycardia and cardiac conduction delay, the earlier ACCF/AHA/HRS device-based therapy guidance, and the 2022 AHA/ACC/HFSA heart failure guideline. ^[2]

Permanent pacing is generally for bradycardia or high-grade conduction block that is not expected to resolve. The central problems (as summarised in the source used here) are sinus node dysfunction and acquired high-grade or third-degree AV block. A reversible cause (drug toxicity, acute inferior infarction that may recover, Lyme disease, transient vagal surges) should always be sought and treated first; pacing for transient block is inappropriate. ^[2]

Further indications in the same source include congenital heart disease, fascicular and bundle branch block (for example QRS prolongation greater than 110 ms in Anderson-Fabry disease; LVEF 36-50% with LBBB and QRS 150 ms or more as part of CRT), hypertrophic cardiomyopathy, and hypersensitive carotid sinus syndrome. ^[2]

The ESC working group adds disease-specific refinements: implantation is recommended if AV block does not resolve within less than 5 days after myocardial infarction, and it is a class I indication after transcatheter aortic valve implantation (TAVI) for persistent AV block, new-onset alternating bundle branch block, or pre-existing right bundle branch block with a new conduction disturbance. Early device implantation (CRT-D or CRT-P) can be considered after anterior infarction with acute heart failure. ^[1]

The 2021 ESC algorithm for a patient with bradycardia or conduction disease starts with history and an ECG and extends to polysomnography, genetic testing when progressive conduction disease begins before the age of 50 years, targeted tests (thyroid function, Lyme serology, electrolytes), cardiac imaging when structural disease is suspected, carotid sinus massage (after excluding carotid stenosis) and tilt testing for suspected reflex syncope, and exercise testing when symptoms occur with exertion or chronotropic incompetence is suspected. Ambulatory ECG monitoring is used to match rhythm with symptoms, and an implantable loop recorder is used when symptoms are infrequent (less than monthly) and non-invasive testing is unhelpful. Dual-chamber pacing should be considered for adenosine-sensitive syncope with paroxysmal AV block lasting more than 10 seconds, and for severe, recurrent, unpredictable syncope in patients over 40 years with asystolic pauses over 3 seconds if symptomatic (or over 6 seconds if recorded without symptoms). ^[1]

Class III entries in the table above are the main non-indications: asymptomatic bradycardia, first-degree or second-degree type I AV block, block that is expected to resolve, vasovagal syncope that can be avoided by simple measures, and AV block caused by a substance that can be stopped. In practical terms, pacing is not indicated when the patient is symptom-free and safe without it, because every implant adds long-term risks of infection, thrombosis, tricuspid valve damage and lead failure. An independent exception is the ICD, which may be contraindicated where there is no reasonable expectation of survival with acceptable functional status for one year, or severe, medication-refractory heart failure in a patient who is not a transplant or CRT-D candidate. ^[3]

An ICD treats the terminal rhythms of sudden death. The generator is placed subcutaneously or under the pectoral muscle with transvenous leads to the right ventricle (and sometimes the right atrium); a biventricular ICD also carries a left ventricular epicardial lead via the coronary sinus, making it a CRT-D. Subcutaneous ICDs (S-ICD) and short-term wearable defibrillators exist for selected patients. In addition to shocks, modern (tiered-therapy) devices provide antibradycardia pacing, antitachycardia pacing to terminate responsive tachycardias without a shock, and stored intracardiac electrograms for review. ^[3]

Battery life of an ICD generator is typically 5 to 7 years. ^[3]

Indications separate secondary from primary prevention. ^[3]

Secondary prevention (the device is preferred treatment):

• An episode of ventricular fibrillation or hemodynamically significant ventricular tachycardia not due to a reversible or transient cause such as electrolyte disturbance, an antiarrhythmic effect or acute myocardial infarction.
• Sustained monomorphic VT induced by an electrophysiology study in a patient with syncope of undetermined origin.

Primary prevention in structural heart disease (selected high-risk phenotypes), including:

• Ischemic cardiomyopathy, New York Heart Association (NYHA) class II or III heart failure on optimal medical therapy, with left ventricular ejection fraction (LVEF) 35% or less measured at least 40 days after myocardial infarction and at least 90 days after revascularisation.
• Ischemic cardiomyopathy, NYHA class I, LVEF 30% or less measured at those same intervals.
• Non-ischemic dilated cardiomyopathy, NYHA class II or III, LVEF 35% or less.
• Ischemic cardiomyopathy with nonsustained VT, LVEF 40% or less after infarction, and VT or ventricular fibrillation inducible at electrophysiology study. ^[3]
• Arrhythmogenic right ventricular cardiomyopathy with sustained VT, resuscitated cardiac arrest, or severe ventricular dysfunction; long QT, short QT, Brugada and catecholaminergic polymorphic VT syndromes with documented ventricular arrhythmia on appropriate drug therapy; early repolarisation with sustained VT or cardiac arrest. ^[3]

The ESC position is that CRT-D should be used where the patient has both a resynchronisation indication and an ICD indication. ^[1]

Class IIa/IIb (less strong) entries include hypertrophic cardiomyopathy with high-risk features, cardiac sarcoidosis with impaired but preserved ejection fraction and risk markers, lamin A/C cardiomyopathy, and arrhythmogenic cardiomyopathy without sustained arrhythmia but with risk factors; idiopathic dilated cardiomyopathy with unexplained syncope; and idiopathic dilated cardiomyopathy in class I heart failure with severe dysfunction. Device therapy is generally not useful (class III) for syncope without inducible VT and without structural disease, for incessant VT/VF or for an arrhythmia that can be ablated or that has a reversible cause. ^[3]

Shock delivery and antitachycardia pacing are therapies, not preventions: the abnormal rhythm still happens, and the device rescues the patient from it. Many patients never receive a shock during the life of the device, while some receive multiple shocks (a "storm"). Antitachycardia pacing often aborts ventricular tachycardia painlessly. Because of this, patients prone to frequent episodes may also need an antiarrhythmic drug to reduce shocks and prolong device life. ^[3]

Malfunction takes two forms. An ICD may deliver inappropriate pacing or shocks, triggered by sinus tachycardia, supraventricular tachycardia, atrial fibrillation or flutter, or by noise such as a lead fracture. Or it may fail to deliver therapy when needed, because of lead migration, undersensing, rising pacing threshold from fibrosis, or battery depletion. ^[3]

When a patient reports that the ICD has discharged without symptoms (no syncope, dyspnoea, chest pain or persistent palpitations), arrange device-clinic or electrophysiology follow-up as soon as feasible, because the device can be interrogated to determine whether the therapy was appropriate. If symptoms accompanied the discharge, or multiple shocks were delivered, urgent referral is needed to look for a treatable cause such as myocardial ischaemia or an electrolyte abnormality, or for device malfunction. ^[3]

CRT is a pacing intervention for selected heart failure patients with electrical dyssynchrony. Candidates usually have ischemic or non-ischaemic dilated cardiomyopathy, a prolonged QRS (130 ms or more, particularly typical LBBB), a left ventricular end-diastolic dimension of 55 mm or more and an LVEF of 35% or less in sinus rhythm. ^[4] Dyssynchrony may be atrioventricular (poor timing between atrial and ventricular contraction), interventricular (right and left ventricles out of step), or intraventricular (different segments of the left ventricle contracting at different times). It is suspected from the ECG and confirmed with advanced echocardiography techniques such as tissue Doppler and strain rate. ^[4]

In patients with heart failure (NYHA class II, III or IV) CRT reduces heart failure hospitalisation and all-cause mortality. There is little to no benefit with permanent atrial fibrillation, right bundle branch block, nonspecific intraventricular conduction delay, or only mild QRS prolongation (less than 150 ms). ^[4]

The clearest evidence, quoted from the 2021 ESC Guidelines on cardiac pacing and CRT, is for patients in sinus rhythm with LVEF 35% or less, QRS duration more than 150 ms and LBBB morphology despite optimal medical therapy. With a narrower QRS (130-149 ms) and non-LBBB morphology the recommendation is less strong. CRT rather than right ventricular pacing is recommended for LVEF under 40% with atrial fibrillation and anticipated RV pacing above 20%, or LVEF under 50% when AV junctional ablation is planned. His bundle pacing should be considered when a coronary sinus lead cannot be placed. ^[1] Conduction system pacing (His bundle or left bundle branch area) achieves resynchronisation by recruiting the native conduction system and is increasingly used where biventricular lead placement fails. ^[4]

CRT systems use a right atrial lead, a right ventricular lead and a left ventricular lead, and can be CRT-P or CRT-D. Leads may be placed transvenously or surgically via thoracotomy. ^[4]

The choice between devices follows the clinical problem rather than the device:

This is a summary of the operative pathway; details follow institutional protocol.

Preparation and consent. Assess the indication and review device type, location (left or right infraclavicular pocket) and lead configuration. Confirm the patient's rhythm, anticoagulation status and infection risk. For ICDs and CRT, pre-procedure optimisation of heart failure and antiarrhythmic therapy and device programming matter as much as the procedure itself. Informed consent covers infection, bleeding, pneumothorax, lead dislodgement, cardiac perforation, vascular injury, device malfunction and the need for future reoperation.

Equipment. A suitable operating or catheter-laboratory environment with X-ray, pacing analysis and backup defibrillation. Transvenous leads (plus a pocket) for conventional systems; for CRT, a coronary sinus lead delivery kit. For subcutaneous ICDs, no transvenous leads. A real-time physiologist or technician to test sensing and capture thresholds.

Steps. For a transvenous pacemaker: venous access (commonly cephalic, axillary or subclavian), advancement of leads to the right atrium/right ventricle (and coronary sinus for CRT), fixation, testing of capture, sensing and impedance, creation of a generator pocket, connection, and layered closure. For CRT the left ventricular lead position is chosen to achieve stable capture with good mechanical response. Subcutaneous systems are implanted with tunnelling rather than intravascular leads. Leadless systems are deployed transvenously entirely within the right ventricle.

Aftercare. Haemostasis and wound care, ECG documentation and a chest X-ray for lead position. Before discharge the device is interrogated and programmed to its final settings, remote monitoring is enrolled and activated, and programming of alerts specific to device type is set. Patients are taught wound care, activity restrictions and how to recognise infection or lead problems. The British Heart Rhythm Society and AHA periprocedural guidance stresses that device checks, identification of system type and pre-operative assessment protocols reduce operative risk in patients with devices. ^[6]

Regular follow-up is a lifelong task, not an after-thought. It includes checking battery status, lead integrity, capture and sensing thresholds, heart failure diagnostics, arrhythmia burden and therapy appropriateness, and re-programming as the clinical state changes.

Routine remote monitoring is now the standard of care for patients with these devices. ^[5] Remote monitoring reduces in-person visits and allows earlier detection of actionable events such as arrhythmias and heart failure decompensation. ^[5]

Practical intervals from the 2023 HRS/EHRA/APHRS/LAHRS expert consensus on the remote device clinic are: ^[5]

• Remote interrogations at least every 3-12 months for pacemakers and every 3-6 months for ICDs when the device lacks continuous connectivity. ^[5]
• More frequently, every 1-3 months, as the device approaches battery "elective replacement" indicator. ^[5]
• In systems with continuous connectivity, in-person visits can in suitable low-risk patients be spaced to as much as 24 months apart. ^[5]
• Remote monitoring enrolment within 2 weeks of implant (and ideally before discharge) is beneficial; enrolment within 3 months of implant has been associated with improved survival, most clearly in CRT-D recipients. ^[5]

The older 2015 consensus recommended in-person or remote interrogation every 3 months, and this remains a common schedule in many centres. Real-world practice varies with reimbursement and connectivity, and quarterly scheduled transmissions are the commonest programming in the United States. ^[5]

Each clinic contact should also assess pulse generator longevity. Modern pacemakers with improved circuitry, battery and corticosteroid-eluting leads have longer life, but exchange is ultimately needed. ICD generators last about 5 to 7 years. ^[3] Leadless pacemakers are retrieved or deactivated and replaced when depleted, depending on type.

Reprogramming and battery replacement. Interrogate and reprogram as disease progresses (for example turning on CRT in a patient who develops pacing-induced cardiomyopathy). A pacemaker-dependent patient whose device shows signs of depletion needs prompt elective replacement, and a failing lead may need extraction or abandonment (with the hazard that abandoned leads can be hazardous in an MRI field). ^[2]

Inappropriate ICD shocks. When a patient reports a shock without worrying symptoms, arrange device-clinic follow-up soon, because the device can be interrogated to determine whether the therapy was appropriate; inappropriate therapy is usually a programming or supraventricular-tachycardia problem. Multiple shocks or shocks with symptoms warrant urgent review for ischaemia, electrolyte abnormality or device malfunction. ^[3]

MRI. Older pacemakers are a relative contraindication to MR imaging because of lead heating, torque and induced currents. MRI-conditional systems are designed to withstand MRI when the manufacturer's instructions are followed, including generator and lead combination, programming to an "MRI-safe" mode and, usually, scanning within specified field strengths. In the 2021 ESC Guidelines on cardiac pacing and CRT, MRI could be performed safely following manufacturer's instructions in patients with MRI-conditional pacemakers and leads. ^[1] Even an abandoned lead that was MRI-conditional when connected can be hazardous when exposed to a magnetic field, because it can heat. ^[2] Non-conditional devices occasionally undergo carefully supervised "MRI on the conditions" protocols in experienced centres, but that is a specialist decision.

Surgery and diathermy. Surgical electrocautery generates electromagnetic interference that can inhibit pacing, trigger inappropriate shocks or corrupt sensing. Practical protections are ^[6]:

• Identify the device and its recent follow-up before any procedure involving diathermy.
• Place the pacemaker in an asynchronous mode (DOO, AOO, VOO) or use a magnet to achieve asynchronous pacing; a magnet has no effect on the pacing function of an ICD.
• For ICD patients, tachyarrhythmia detection should be programmed "off" before electrocautery; if a programmer is not available, a magnet over the generator can disable tachytherapy (though it does not change pacing). ^[6]
• Use electrocautery in short bursts, prefer bipolar diathermy where possible, and monitor the patient (arterial line or pulse oximetry, because the ECG may be unreadable during diathermy).
• Keep a magnet available, and be aware that magnet effects and responses vary between manufacturers (some ICDs revert to normal function only after the magnet is removed; some leadless and Biotronik devices do not respond to a magnet). ^[6]
• For procedures below the level of the umbilicus many guidelines consider it reasonable not to deactivate the ICD, provided standard intra-operative precautions are taken.

Everyday life. Cellular telephones should not be placed directly against the generator, but normal use for talking is fine. Walking through metal detectors does not cause malfunction provided the patient does not linger. Security wands should not be held over the device. MRI-conditional systems have no special everyday restrictions. Swimming, driving and strenuous activity need individual advice based on the underlying arrhythmia, with local legal driving rules for ICD patients. Avoid body-building or contact sports that strike the generator pocket; avoid deep scuba diving pressure hazards if unsure. Anticoagulants and antibiotics before dental or invasive procedures follow the device and cardiac indications, not the device alone.

End-of-life. When a patient is dying and an ICD is delivering shocks (for example during refractory ventricular tachycardia), deactivating tachytherapy can be an important and compassionate act; pacing can be left active to support comfort. Families and hospice teams should know how to arrange this.

Complications are those of the procedure and the hardware:

• Infection of pocket or leads, including endocarditis; usually requires complete system removal. ^[2]
• Lead-related: dislodgement, fracture, insulation failure, tricuspid regurgitation from leaflet interference, and eventual extraction difficulty. ^[2]
• Oversensing and undersensing, failure to pace, failure to capture, and pacing at an abnormal rate. ^[2]
• Pacemaker-mediated tachycardia; cross-talk inhibition; pacemaker syndrome from AV dyssynchrony, which is managed by restoring AV synchrony (AAI, VDD or most commonly DDD). ^[2]
• Psychosocial and physical burden: visible pocket, inappropriate shocks, electrical sensitivity, and "phantom shocks". ^[3]

• Pregnancy. Devices are safe in pregnancy and essential when indicated; fetal radiation during implantation should be minimised. Leadless and subcutaneous options reduce fetal exposure slightly. Obtain cardiology and device-team advice for delivery planning and for magnetic or surgical exposure.
• Children and congenital heart disease. Epicardial rather than transvenous leads are often used in small children, and growth may require excess lead length. Pacing thresholds, battery life and venous capacity all differ from adult practice.
• Older adults and frail patients. Frailty, cognitive impairment and limited life expectancy may shift the balance toward pacemaker rather than ICD therapy, and class III ICD indications explicitly exclude patients with no reasonable expectation of acceptable survival.
• Low-resource settings. A basic dual-chamber or single-chamber pacemaker, a simple ICD, and a thoracotomy-placed epicardial CRT lead extend care to regions without full catheter-laboratory and corporate support. Device longevity and lead salvage matter more where replacement is costly. Remote monitoring may be unavailable; scheduled in-person interrogation can be organised in simple, regular clinics.

• A pacemaker prevents bradycardia and block; an ICD rescues from ventricular fibrillation and unstable tachycardia; CRT re-times the failing ventricle.
• The pacemaker code (three to five letters: A/V/D for chamber paced and sensed, I/T/D/O for response, R for rate response) determines how it behaves; VVI and DDD are the commonest modes and give equivalent survival.
• The commonest indications for permanent pacing are sinus node dysfunction and high-grade AV block; always treat reversible causes first. ^[2]
• AV block that does not resolve within less than 5 days after infarction, or persistent AV block after TAVI, is an indication for permanent pacing.
• An ICD is indicated for a resuscitated VF/VT without reversible cause, inducible VT in syncope, and for primary prevention in selected ischemic and non-ischemic cardiomyopathies with LVEF 35% or less (or 30% or less in class I ischemic disease). ^[3]
• CRT benefits symptomatic heart failure with a reduced ejection fraction and QRS 130 ms or more, best with LBBB and QRS over 150 ms; little benefit with right bundle branch block or QRS under 150 ms.
• CRT-D is used when both CRT and ICD indications are present. ^[1]
• ICD generators last 5 to 7 years; pacemaker leads and batteries eventually need replacement or abandoned.
• Routine follow-up is lifelong: remote monitoring is standard, with quarterly (every 3-6 months for ICDs) and less frequent interrogation, rising to every 1-3 months near battery depletion.
• Inappropriate ICD shocks and shocks with symptoms both need timely review: first interrogate, then treat ischaemia, electrolytes or malfunction.
• MRI is safe only with MRI-conditional systems following manufacturer's instructions; abandoned leads remain hazardous.
• Surgery needs prior device identification, asynchronous or off-mode tachytherapy for ICDs, short-burst diathermy and monitoring; magnet responses differ by manufacturer.
• Conduction system pacing, leadless pacemakers and subcutaneous ICDs expand the choices, but each has specific indications and limits.

• Cardiology topics in the library include ECG interpretation, atrial fibrillation and flutter, syncope, valvular heart disease, heart failure, sudden cardiac death, hypertrophic cardiomyopathy, channelopathies, arrhythmogenic cardiomyopathy and bradycardia assessment (search the index; links are added as those topics are verified).