An AICD (automatic implantable cardioverter-defibrillator) is not a pacemaker, but the two devices are closely related. An AICD, more commonly called an ICD, is designed to detect and stop dangerously fast heart rhythms by delivering a high-energy shock. A pacemaker, by contrast, sends small electrical pulses to speed up a heart that beats too slowly. The key distinction: a pacemaker keeps your heart from going too slow, while an ICD keeps it from going too fast in a way that could kill you.
That said, the line between the two has blurred significantly. Most new ICDs can act as both a pacemaker and a defibrillator, which is one reason people often confuse them.
What Each Device Actually Does
A standard pacemaker treats bradycardia, meaning your heart rate drops too low. The two most common reasons people get pacemakers are AV block (a delay or blockage in the heart’s electrical signals between its upper and lower chambers) and sick sinus syndrome, where the heart’s natural pacemaker fires too slowly. The device continuously monitors your heart rate and delivers tiny electrical pulses, measured in millionths of a joule, to prompt a normal beat when needed. You don’t feel these pulses.
An ICD monitors for the opposite problem: life-threatening fast rhythms like ventricular tachycardia and ventricular fibrillation. These are the rhythms responsible for sudden cardiac death. When the device detects one, it can deliver a shock of up to 80 joules to reset the heart. That’s millions of times more energy than a pacing pulse, and patients who receive a shock typically describe it as a sudden, sharp kick in the chest.
Why Modern ICDs Blur the Line
Most ICDs implanted today include full pacemaker capabilities built in. This means a single device can speed up a slow heart, try to override a dangerous fast rhythm with carefully timed pacing pulses, and deliver a high-energy shock as a last resort. It’s essentially a pacemaker with a defibrillator on top.
The pacing-first approach matters because ICDs have a feature called anti-tachycardia pacing (ATP). When the device detects ventricular tachycardia, it first tries to interrupt the abnormal circuit by sending a rapid burst of small pacing pulses, timed to arrive at precisely 85 to 88 percent of the interval between the dangerous beats. If ATP works, the arrhythmia stops painlessly, and you may not even notice it happened. Only if pacing fails does the device escalate to a full shock.
One exception to this overlap is the subcutaneous ICD (S-ICD), a newer design that sits just under the skin rather than threading wires into the heart. The S-ICD can deliver shocks but cannot provide ongoing pacing for a slow heart rate or use anti-tachycardia pacing. European cardiology guidelines recommend it only for patients who don’t need bradycardia support or ATP, and doctors must consider whether a patient might develop a pacing need in the future before choosing this option.
Who Gets Which Device
The conditions that lead to each device are quite different. Pacemakers go to people whose hearts beat too slowly, causing symptoms like dizziness, fainting, or fatigue. The device fills in the gaps when your heart’s electrical system isn’t firing reliably.
ICDs go to people at risk for sudden cardiac death. Current guidelines from the American College of Cardiology and American Heart Association rate ICD implantation as appropriate for patients with an ejection fraction (a measure of how well the heart pumps) of 35 percent or below, whether caused by a prior heart attack or another form of heart muscle disease. ICDs are also recommended for people with hypertrophic cardiomyopathy or certain congenital heart conditions. Some patients receive an ICD after surviving a cardiac arrest or a documented episode of dangerous ventricular arrhythmia.
Because most ICDs include pacemaker functions, a person who needs both a defibrillator and pacing support can get a single device rather than two separate implants.
Physical Differences in the Hardware
The devices look similar from the outside: both are small metal cases implanted under the skin near the collarbone, connected to the heart by thin wires called leads. But the internal engineering is substantially different.
Pacemaker leads are relatively simple, containing one or two conductor wires in a coaxial or coradial design. Modern pacemaker leads can be as thin as 4.1 French (about 1.4 millimeters) in diameter. ICD leads are thicker and more complex because they need to carry both low-voltage pacing signals and high-voltage defibrillation shocks. They use a multilumen design with separate internal channels for each conductor, plus one or two shock coils wrapped around the lead body. This extra hardware makes ICD leads bulkier and stiffer than pacemaker leads.
The generator (the metal case containing the battery and computer) is also larger in an ICD because it houses capacitors capable of storing and releasing the energy needed for a defibrillation shock.
Battery Life and Long-Term Maintenance
Both devices run on internal batteries that cannot be recharged, so the generator must be surgically replaced when the battery runs low. Pacemakers generally last 10 to 15 years, depending on how frequently they need to pace.
ICD battery life has improved considerably. A real-world analysis of a prospective defibrillator database found median ICD battery longevity of 10.8 years overall. Single-chamber ICDs lasted about 11 years on average, while dual-chamber ICDs (which do more pacing) lasted around 8.5 years. Factors that shorten battery life include frequent shock delivery, heavy use of pacing functions, and certain programming settings.
When a battery replacement is needed, the procedure involves opening the pocket under the skin, disconnecting the old generator, and attaching a new one to the existing leads. The leads themselves often last longer than the generator, though they can sometimes fracture or develop insulation problems over the years.
What the Experience Feels Like
Living with a pacemaker is largely invisible. The electrical pulses are far too small to feel, and most people forget the device is there between checkups. Activity restrictions are minimal after the initial healing period.
Living with an ICD involves the same baseline experience, since the pacing functions work identically. The difference is the possibility of receiving a shock. Most ICD patients go months or years without a shock, especially since ATP successfully terminates many fast rhythms before a shock becomes necessary. When a shock does fire, it’s sudden and unmistakable. Some people describe it as being kicked in the chest; others say it feels like a strong jolt. The shock lasts a fraction of a second. While startling and sometimes painful, it means the device just prevented a potentially fatal arrhythmia.
Both devices require periodic follow-up visits, typically every 6 to 12 months, though many modern devices can transmit data wirelessly to your doctor’s office from home. These remote checks reduce the number of in-person visits needed.