What is the difference between a defibrillator and a pacemaker?

Difference Between Defibrillators and Pacemakers

Pacemakers treat slow heart rhythms (bradycardia) by delivering small electrical impulses to maintain adequate heart rate, while defibrillators (ICDs) treat life-threatening fast rhythms (ventricular tachycardia/fibrillation) by delivering high-energy shocks to terminate these arrhythmias—though importantly, every ICD also includes full pacemaker capabilities. 1

Primary Function and Purpose

Pacemakers

• Designed to prevent bradycardia by delivering low-energy electrical impulses that stimulate heart muscle contraction when the heart's natural pacemaker fails or conduction is blocked 2
• Maintain an adequate heart rate when the heart's natural pacemaker is too slow or when electrical conduction blocks prevent normal rhythm 2
• Relieve symptoms of bradyarrhythmia including fatigue, syncope, and exercise intolerance 3
• Do not treat or prevent tachyarrhythmias or sudden cardiac death 1

Implantable Cardioverter-Defibrillators (ICDs)

• Prevent sudden cardiac death by detecting and terminating life-threatening ventricular tachycardia or ventricular fibrillation 1
• Deliver high-energy shocks (cardioversion/defibrillation) to terminate dangerous fast rhythms 1
• Every ICD includes both pacing and shock therapies—the device provides full pacemaker function for bradyarrhythmias in addition to shock capability for tachyarrhythmias 1
• Reduce sudden cardiac death risk by 20-40% in appropriate patient populations 4

Device Capabilities

Pacemaker Features

• Single-chamber (atrial or ventricular) or dual-chamber pacing options 1, 5
• Rate-responsive capabilities that adjust pacing rate based on physiologic demand 6
• Programmable lower rate limits and AV intervals 1
• Cannot deliver cardioversion or defibrillation shocks 1

ICD Features

• All modern ICDs incorporate multiple tachycardia detection zones with tiered therapy including antitachycardia pacing, low-energy cardioversion, and high-energy defibrillation 1
• Programmable ventricular demand pacing for bradycardia management 1
• Extensive diagnostics with stored electrograms of rhythms before and after therapy 1
• Antitachycardia pacing terminates 96% of detected ventricular tachyarrhythmias without requiring shocks 1
• Dual-chamber ICDs can distinguish supraventricular from ventricular arrhythmias using atrial sensing 1

Clinical Indications

When to Use a Pacemaker

• Complete heart block with symptomatic bradycardia, heart failure, or requiring drugs that suppress escape rhythms 3
• Type II second-degree AV block (even if asymptomatic) 3
• Sinus node dysfunction with documented symptomatic bradycardia 3
• Bifascicular block with intermittent complete heart block 3
• Post-MI persistent advanced AV block 3

When to Use an ICD

• Survivors of cardiac arrest due to ventricular fibrillation or ventricular tachycardia not due to reversible causes 1
• Sustained ventricular tachycardia with structural heart disease 1
• Patients at high risk for sudden cardiac death who have inducible ventricular arrhythmias at electrophysiologic study 1
• Patients with aborted sudden death without inducible arrhythmias at EP study (when no other therapy can be assessed) 1

Critical Distinctions in Perioperative Management

Pacemaker Considerations

• May require reprogramming to asynchronous mode (AOO/VOO/DOO) to prevent electromagnetic interference from inhibiting pacing in pacemaker-dependent patients 1
• Backup pacing capability should be immediately available 1
• Risk of oversensing leading to inappropriate pacing inhibition in pacemaker-dependent patients 4

ICD Considerations

• Shock therapies must be disabled before procedures using electrocautery to prevent inappropriate shocks from electromagnetic interference 1
• External defibrillation pads/paddles should be placed at least 8 cm from the ICD generator when possible 1
• All antitachyarrhythmic therapies must be restored immediately postoperatively with device interrogation 1
• Emergency external defibrillation equipment must be immediately available in case ventricular arrhythmias occur while ICD therapies are disabled 1

Important Caveats

Device complexity: Modern ICDs are significantly more complex than pacemakers, requiring specialized follow-up by trained cardiac electrophysiologists with 24-hour availability 1

Cost considerations: ICDs cost substantially more than pacemakers, with antitachycardia pacing adding 5-10% to device cost 1

Dual functionality: Because every ICD includes pacemaker capabilities, patients with ICDs receive both bradycardia and tachycardia management, but the reverse is not true—pacemakers cannot treat tachyarrhythmias 1

Magnet response differs: Placing a magnet over a pacemaker typically causes asynchronous pacing, while magnet application to an ICD disables tachycardia detection and shock therapies but does not affect pacing function 1, 7