An ICD, or implantable cardioverter defibrillator, is a small device placed under the skin that continuously monitors your heartbeat and delivers an electrical shock if it detects a life-threatening rhythm. It’s designed to prevent sudden cardiac death in people whose hearts are at risk of beating dangerously fast or chaotically. The device is roughly the size of a matchbox and sits just below the collarbone, connected to the heart by thin wires called leads.
Worth noting: “ICD” also stands for the International Classification of Diseases, a coding system the World Health Organization maintains so hospitals and countries worldwide can record and compare health data in a standardized way. If you’re looking for that, the current version is ICD-11. The rest of this article covers the heart device.
How an ICD Works
An ICD does three things. First, it senses your heart’s electrical activity around the clock, reading each heartbeat to determine whether the rhythm is normal. Second, if it detects a dangerously fast rhythm (ventricular tachycardia or ventricular fibrillation), it classifies what’s happening using built-in algorithms that evaluate heart rate and the number of abnormal beats in a row. Third, it responds with the appropriate level of treatment.
For a fast but organized rhythm, the device often tries a gentler approach first: it sends a rapid series of small electrical pulses to interrupt the abnormal pattern and nudge the heart back to normal. This is called antitachycardia pacing, and most people don’t feel it. If that doesn’t work, or if the rhythm is chaotic and immediately dangerous, the ICD delivers a high-energy shock to reset the heart. People who’ve experienced a shock typically describe it as a sudden jolt or kick in the chest. It’s startling but brief.
Most modern ICDs also include a built-in pacemaker. If your heart beats too slowly after a shock, or if you have an underlying slow-rhythm problem, the device can deliver steady, low-energy pulses to keep your heart rate from dropping too low.
How an ICD Differs From a Pacemaker
A standard pacemaker treats slow heart rhythms. It sends small electrical signals to keep the heart from beating too slowly, but it can’t stop a dangerously fast rhythm or deliver a life-saving shock. An ICD handles both directions: it can pace a slow heart and shock a fast one. Think of a pacemaker as a gentle accelerator and an ICD as an accelerator plus an emergency brake. If your doctor is primarily concerned about sudden cardiac arrest from a fast, chaotic rhythm, an ICD is the device that addresses that risk.
Who Gets an ICD
ICDs are implanted for two broad reasons. The first, called secondary prevention, is straightforward: you’ve already survived a cardiac arrest or a documented episode of dangerous ventricular arrhythmia. The device is placed to protect you if it happens again.
The second reason, primary prevention, applies to people who haven’t had an event yet but are statistically at high risk. The strongest guidelines recommend an ICD for people whose heart pumps 35% or less of its blood with each beat (a normal heart pumps about 55% to 70%). This reduced pumping ability can result from a prior heart attack or from a condition called nonischemic cardiomyopathy, where the heart muscle weakens for other reasons. Landmark clinical trials showed clear survival benefits for these patients compared to medication alone.
People who’ve had a heart attack and whose pumping function drops below 30% are also candidates, even without symptoms, as long as at least 40 days have passed since the heart attack. The waiting period matters because the heart sometimes recovers pumping strength on its own during that window. On the other end, ICDs are generally not recommended for people with very advanced heart failure and a life expectancy under one year, since the device wouldn’t meaningfully extend survival.
The Implantation Procedure
Getting an ICD is a surgical procedure, but it’s not open-heart surgery. It typically takes one to three hours and is done under sedation or general anesthesia. The surgeon makes a small incision just below the collarbone, usually on the left side, and creates a pocket in the tissue to hold the pulse generator. Thin, flexible leads are threaded through a vein into the heart and connected to the generator. The surgical team then tests the device to confirm it can sense the heart’s rhythm and deliver therapy correctly.
A newer option, the subcutaneous ICD, places the lead just under the skin along the breastbone rather than threading it into the heart through a vein. The generator sits along the side of the rib cage. This approach avoids the risks associated with leads inside the heart and blood vessels, though it can’t provide the gentle antitachycardia pacing that traditional ICDs offer.
Most people go home the same day or the next morning. You’ll likely be told to limit arm movement on the implant side for several weeks while the leads settle into position and the incision heals.
Battery Life and Replacement
ICD batteries are not rechargeable, and the device will eventually need to be replaced. Modern ICDs generally last five to seven years or longer, depending on the manufacturer and how often the device delivers therapy. Some newer battery technologies maintain stable power output through roughly 90% of their total capacity, which extends usable life. One recent analysis found that certain devices had a 99% replacement-free survival rate at six years.
Your cardiology team monitors battery status at regular checkups, often remotely through a home transmitter that sends data to your doctor’s office while you sleep. When the battery reaches a predetermined low point, the device signals that it’s time for an elective replacement. The replacement procedure is simpler than the original implantation: the surgeon opens the existing pocket, disconnects the old generator, connects a new one to the existing leads, and closes the incision. It’s typically an outpatient procedure.
Risks and Complications
Like any implanted device, ICDs carry risks. Short-term complications include infection at the implant site, bleeding, and lead dislodgement, where a wire shifts out of its correct position and needs to be repositioned. These issues are relatively uncommon in experienced centers.
The longer-term concern is lead failure. Over a decade, roughly 40% of ICD leads develop some form of malfunction, which can range from minor insulation wear to fractures that cause the device to misread the heart’s signals. Lead problems can trigger inappropriate shocks, where the device delivers a jolt even though the heart rhythm is actually fine. Inappropriate shocks are one of the most distressing experiences ICD patients report, and they’re a key reason for regular device monitoring.
Living With an ICD
Most people with ICDs return to normal daily life, including work, exercise, and travel. The adjustments are real but manageable, and they mostly involve keeping certain devices and magnets away from the area over your ICD.
Cell phones are generally safe but should be held to the ear on the opposite side of your implant and kept at least 6 inches from the device. That means no storing your phone in a breast pocket on the implant side. Headphones, both earbuds and over-ear styles, contain magnets that can interfere with the ICD if held too close, so the same 6-inch rule applies. Don’t drape headphones around your neck so they rest on your chest.
At airport security, walk through metal detectors at a normal pace without lingering. If a handheld wand is used, let the security officer know you have an implanted heart device and ask them to keep the wand moving rather than holding it over your chest. MRI scans were historically off-limits for ICD patients, though many newer devices are designed to be MRI-compatible under specific conditions. Your cardiology team can confirm whether your particular device qualifies.
Strong electromagnetic fields from industrial welding equipment, large motors, and high-power radio transmitters can interfere with ICD function. For CB or ham radios, the general guidance scales with power output: keep low-wattage radios at least 6 inches away, mid-range radios at least 12 inches, and higher-power transmitters at least 2 feet from the device.
The psychological side of living with an ICD matters too. Some people feel reassured knowing the device is there. Others develop anxiety about receiving a shock, particularly after their first one. If the worry starts affecting your sleep, activity level, or mood, it’s worth bringing up with your care team, since targeted counseling and support groups for ICD patients are widely available.