DEAR MAYO CLINIC: I've seen emergency defibrillators in public places, but I've also heard that some people need to wear them or have them implanted for heart rhythm problems. Can you explain the differences? 

ANSWER: Pacemakers and defibrillators are devices that help manage heart rhythm problems, but they work differently. 

A pacemaker helps the heart when the rhythm is too slow or irregular. A defibrillator is a specialized pacemaker designed to treat life-threatening heart rhythms that are dangerously fast or chaotic. It acts by shocking the heart out of a dangerous rhythm. 

Sometimes people unexpectedly have an irregular heartbeat that can cause sudden cardiac death or unconsciousness. During this emergency situation, the heart stops or quivers uselessly and is unable to pump in a coordinated way.   

That's when an automated external defibrillator (AED) can be lifesaving. These are the portable devices you see in airports, schools, gyms and other public places. An AED analyzes a person's heart rhythm during sudden cardiac arrest. If it detects a dangerous rhythm, it delivers an electrical shock to help the heart return to a normal rhythm.  

While people who take CPR courses learn how to use an AED, the devices also provide simple step-by-step instructions, including written, picture and voice prompts, so that a bystander can use one in an emergency.  

Defibrillators also can be worn or implanted. In general, they are used for people who are at higher risk of dangerous heart rhythms, including: 

• Those with heart conditions, such as a weakened heart muscle (also called cardiomyopathy), low pumping function or a history of heart attack. 
• Those born with electrical abnormalities of the heart, called channelopathies, or other inherited heart conditions. These can sometimes affect younger people, including athletes, and may cause fainting, sudden cardiac arrest or sudden cardiac death.  

A wearable defibrillator looks like a vest and is worn under clothing. It continuously monitors the heart rhythm and can deliver a shock if it detects a life-threatening rhythm. It's usually worn temporarily, often while a care team is deciding whether a person needs a permanent, implanted device or while the heart is recovering.  

An implantable cardioverter-defibrillator (ICD) is a permanent device placed inside the body. It monitors the heart rhythm and can deliver treatment if a dangerous rhythm occurs. Some ICDs are placed under the skin in the chest, with one or more wires, called leads, connected to the heart.  

Recovery after ICD placement usually takes about four to six weeks. During that time, patients may need to limit certain arm movements, especially on the side where the device was placed, and watch for signs of infection or increasing pain around the incision.  

After recovery, most people return to their usual daily activities. However, they should avoid strong magnetic fields and certain activities, such as arc welding, unless their care team says it's safe. 

Most patients aren't aware of their defibrillator unless it delivers treatment. If a person is unconscious when a shock occurs, they usually don't feel it. If they're awake, the shock may feel like a sudden jolt or kick in the chest. It can be startling, but it's meant to save a life. In some cases, people may not feel the shock at all, and the event is only found later during a device check. 

Anyone who feels a shock from their defibrillator, or is notified by their healthcare team that a shock occurred, should contact their care team for further evaluation. The device may need to be checked, and medications or other parts of the treatment plan may need to be adjusted. 

There's another type of device therapy called cardiac resynchronization therapy (CRT), which may include features of a pacemaker or defibrillator. It's used for some people with moderate to severe heart failure whose lower heart chambers don’t beat in a coordinated way. 

When the heart’s pumping chambers are out of sync, the heart may not pump blood as efficiently. CRT helps coordinate the timing of the heartbeat. 

As with implanted pacemakers or defibrillators, CRT implantation requires a minor surgical procedure to place a device in the chest, and the recovery and post-procedure care are similar. There are two main types: 

• Cardiac resynchronization therapy with a pacemaker (CRT-P). This device sends electrical signals to help the lower chambers of the heart beat together. 
• Cardiac resynchronization therapy with a defibrillator (CRT-D). This device combines resynchronization pacing with defibrillator protection. It may be recommended for people with heart failure who also are at risk for sudden cardiac death. 

Defibrillators, whether they're used in public places, worn temporarily or implanted permanently, are designed to protect people from dangerous heart rhythms. They can be lifesaving, helping people live more safely and confidently. 

Divya Korpu, M.B.B.S., Cardiology, Mayo Clinic Health System, Eau Claire, Wisconsin 

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