Implantable Cardiac Defibrillators (Implantable Cardioverter Defibrillators)

Implantable cardiac (cardioverter) defibrillators facts

  • Implantable cardioverter defibrillators (ICDs) can prevent death from cardiac arrest.
  • Cardiac arrest due to life-threatening fast heart rhythms is the most common cause of death in the United States.
  • ICDs are implanted in the chest in a manner similar to that of pacemakers.
  • ICDs deliver electrical pulses or shocks to the heart to convert a life-threatening fast rhythm to a normal rhythm.
  • Certain electrical equipment with strong magnetic fields can interfere with ICDs, but most household appliances in good order are safe.

What are implantable cardiac defibrillators (ICDs)?

An implantable cardioverter defibrillator (ICD) is a small electronic device installed inside the chest to prevent sudden death from cardiac arrest due to life threatening abnormally fast heart rhythms (tachycardias). The ICD is capable of monitoring the heart rhythm. When the heart is beating normally, the device remains inactive. If the heart develops a life-threatening tachycardia, the ICD can attempt pacing to bring the hear rhythm back to normal, or it can deliver an electrical "shock(s)" to the heart to terminate the abnormal rhythm and return the heart rhythm to normal.

How does a normal heart function?

The heart is an organ consisting of four chambers that pump blood. The two upper chambers are called the right and left atria, and the two lower chambers called the right and left ventricles. The right atrium receives venous blood (oxygen-poor blood) from the body and pumps it into the right ventricle. The right ventricle pumps the oxygen-poor blood to the lungs to receive oxygen. The oxygen-rich blood from the lungs then travels to the left atrium and is pumped by the left atrium into the left ventricle. The left ventricle delivers the oxygen-rich blood to the rest of the body. In addition to oxygen, the blood carries other nutrients (glucose, electrolytes, etc.) to the organs.

In order to keep a body healthy, the heart must deliver a sufficient amount of blood to the body. As a pump, the heart is most efficient in delivering blood when functioning within a certain heart rate range. Normally, the heart's natural pacemaker called the SA node (a special tissue located on the right atria wall), keeps the heartbeat (heart rate) in the normal range. Electrical signals generated by the SA node travel along special conduction tissues on the walls of the atria and the ventricles. These electrical signals cause the heart muscles to contract and pump blood in an orderly and efficient manner.

How do abnormal heart rhythms decrease blood delivery by the heart?

Abnormal heart rhythms, either too slow or too fast, decrease the delivery of blood by the heart. Bradycardia is a condition in which the heart rate is too slow. Bradycardias can be due to diseases of the SA node or the conduction tissues of the heart. The slow-beating heart delivers an insufficient amount of blood to the body.

Tachycardia is a condition in which the heart rate is too rapid. When the heart pumps too fast, the ventricles do not have enough time to fill their chambers with blood before the next contraction. Therefore, tachycardias can decrease the amount of blood delivered to he body. One of the effects of decreased blood delivery to the body is low blood pressure.

What is the cause of tachycardias?

Abnormally fast heart rates are called tachycardias. Tachycardias are caused by rapidly firing electrical signals arising from the walls of the atria or the ventricles. These rapidly firing signals override the signals generated by the SA node and cause the heart to beat too fast.

Tachycardias caused by electrical signals from the atria are called atrial tachycardias. Tachycardias caused by electrical signals from the ventricles are called ventricular tachycardias.

What are the symptoms of tachycardias?

Symptoms of tachycardias include:

  1. Palpitations or fluttering sensations in the heart;
  2. Lightheadedness (due to low blood pressure);
  3. Fainting spells or loss of consciousness (due to low blood pressure);
  4. Fatigue and weakness (due to lack of blood supply); and
  5. A flushing sensation.


Heart Disease: Causes of a Heart Attack See Slideshow

What are life-threatening tachycardias?

Two common life-threatening tachycardias are ventricular tachycardia and ventricular fibrillation. Ventricular tachycardia is a rapid regular rhythm caused by electrical signals originating from an area of the ventricle. Ventricular tachycardia can decrease blood delivery by the heart and cause low blood pressure. Ventricular tachycardia can also progress to a more serious heart rhythm called ventricular fibrillation.

Ventricular fibrillation is an irregular rhythm, which is a result of multiple rapid and chaotic electrical signals firing from many different areas in the ventricles. A heart undergoing ventricular fibrillation is in a state of standstill called cardiac arrest. The heart muscles quiver and cease pumping which causes a halt in the delivery of blood to the body. Unless ventricular fibrillation is terminated quickly, irreversible brain damage occurs within minutes of the onset of ventricular fibrillation, leading to death.

Sudden cardiac arrest is the most common cause of death in the United States. The most frequent causes of cardiac arrest in the United States are ventricular tachycardia and ventricular fibrillation.

What are the causes of ventricular tachycardia and ventricular fibrillation?

Ventricular tachycardia and ventricular fibrillation are most commonly caused by heart attacks (acute myocardial infarctions) or scarring of the heart muscle from previous heart attacks. Less common causes of ventricular tachycardia and ventricular fibrillation include severe weakening of the heart muscles (cardiomyopathy), medication toxicity (such as digoxin [Lanoxin] toxicity), medication side effects, and blood electrolyte disturbances (such as a low potassium level). Ironically, some medications used in treating heart rhythm abnormalities can cause ventricular tachycardias.

How can ventricular tachycardia and fibrillation be treated and prevented?

Medications have traditionally been used in preventing ventricular tachycardia and fibrillation. Examples of these medications include amiodarone (Cordarone, Nextrone, Pacerone) and beta-blockers such as atenolol (Tenormin), and propranolol (Inderal). Medicines, however, are not very successful in preventing tachycardias or in terminating tachycardias once they occur.

Once a life threatening tachycardia occurs, the most effective treatment is to administer mild electric shock(s) to the heart to terminate the tachycardia and reset the heart rhythm to normal.

If a patient is in cardiac arrest due to ventricular fibrillation, the treatment is the delivery of a strong electrical shock to the fibrillating heart without delay. Irreversible brain and other organ damages can occur within minutes if the normal heart rhythm is not restored. Most patients can potentially be saved if shocks are delivered quickly to convert the fibrillation to normal rhythm before irreversible brain damages occur.

The electrical shocks (mild and strong) that terminate ventricular tachycardia and fibrillation can be delivered by an external defibrillator (a portable unit with pads that deliver electrical shocks to the heart), or by an implantable cardiac defibrillator (ICD). External defibrillators, however, may not be readily available, and rescuers may not be able to administer effective CPR for long periods before paramedics arrive. Therefore, in patients known to be at risk of developing life-threatening tachycardias, ICDs can be implanted in their chests as a preventive measure to terminate tachycardias and fibrillation and avert cardiac arrest.

Who should receive an implantable cardiac defibrillator?

Patients at risk of developing sudden cardiac arrests due to ventricular tachycardias and fibrillations are candidates for ICDs. ICDs do not prevent the occurrence of life-threatening rhythms, but can quickly terminate them when they occur. Recent clinical trials have identified several groups of patients who should receive ICDs. They are:

  • Patients who have survived cardiac arrest;
  • Patients with ventricular tachycardias that significantly decrease the amount of blood delivered by the heart, resulting in low blood pressure;
  • Patients with significant heart muscle damage from prior a heart attack, and have ventricular tachycardia episodes that are not suppressed by medications; and
  • Patients deemed at high risk for sudden death from cardiac arrest based on a history of heart disease and findings from an echocardiogram and ECG.

How are implantable cardiac defibrillators designed?

An ICD consists of one or more leads (conducting wires insulated with silicone or polyurethane) and a defibrillator unit. The defibrillator unit is a small titanium case containing a microchip computer, a capacitor, and a battery.

The leads carry electrical signals between the heart and the defibrillator unit. One end of a lead is placed on the inner wall of the heart while the other end is attached to the defibrillator unit. The leads help the defibrillator unit monitor the natural heart rhythm. The leads also deliver electrical shock(s) from the defibrillator unit to the heart when tachycardias occur.

The microchip computer runs the defibrillator, monitors the natural heart rhythm, instructs the capacitor to send electrical shock(s) when tachycardias occur, determines the strength of the shock(s) sent, and also keeps a record of the heart rhythms as well as the shock(s) sent by the defibrillator.

ICDs have programmable features that allow the doctor to change the cutoff heart rate for activating the defibrillator. Tachycardias with rates higher than the cutoff heart rate activate the firing of shocks by the defibrillator. The doctor can also adjust the strength (amount of energy delivered) of each shock, and the number of shocks delivered with each tachycardia episode.

Most defibrillators now have built-in pacemakers as well. The newer defibrillators can have very sophisticated pacing devices equipped with the ability to pace both the atrium and the ventricle (dual chamber pacers). Cardioverter defibrillators have rapid pacing capabilities. Rapid pacing can sometimes convert a tachycardia to normal rhythm without administering electric shock(s).

The electric pulses and shocks delivered by the ICDs are of such low energy that they do not harm the patient or family members in physical contact with the patient.

How are implantable cardiac defibrillators implanted?

Implantation of an ICD is similar to that of a permanent pacemaker. The procedure, which lasts 1-2 hours, is considered minor in that it does not involve major heart surgery. Patients are typically sedated during the procedure. A local anesthetic is injected under the skin over the area where the ICD will be placed, usually in the right or left upper chest near the collarbone. The lead is then inserted into a vein located in the upper chest near the collarbone. The tip of the lead is placed on the inner wall of the heart with the visual guidance of x-rays. If there is more than one lead, the process is repeated. The other end of the lead (or leads) is connected to the defibrillator unit, which is then inserted under the skin at the incision site. Because there are no nerve endings inside the blood vessels and the heart, the patient usually does not feel the placement of the lead(s).

Heavy sedation is used during the procedure when the defibrillator is tested for proper functioning. Testing an ICD involves inducing a rapid heart rhythm and allowing the defibrillator to detect the abnormal rhythm and then terminate it with a shock (just as the device would be expected to operate in a real-life tachycardia episode).

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What happens after implantation of an implantable cardiac defibrillator?

While in the hospital, the patient's heart rhythms, pulse, and blood pressures are routinely monitored. The doctor may check or adjust the settings on the defibrillator (done from outside the body). The nurses also periodically examine the incision over the implantation site for bleeding, redness, or other signs of infection. It is normal to feel some pain over the incision for 1-2 weeks. Medications are usually given to alleviate pain.

Patients are typically discharged from the hospital the day after the procedure. Once home, the patient can usually return to most activities. Instructions are given to avoid raising the arm over the shoulder on the side of the ICD implantation for several weeks. This precaution is to avoid dislodging the leads before they become secure inside the veins and the heart. Patients are also asked to avoid contact sports, vigorous exercises, and heavy lifting for several weeks.

In a week, the sutures over the incision are removed in the doctor's office. This is a good opportunity to discuss the following issues with the doctor:

  1. Level of physical activity;
  2. Return to work;
  3. Permission to drive automobiles;
  4. How frequently should the ICD and battery level be checked?
  5. What are the signs of device failure?
  6. When to replace the ICD (most ICD batteries last 3-7 years)?
  7. Precautions regarding interference with the device by outside power sources; and
  8. What to do when tachycardias occur.

Call the doctor if there is bleeding from the incision site, increasing pain over the incision site, fever, heat, swelling, or fluid draining from the incision site. Also call if the arm becomes swollen on the side of the implantation or if there is twitching of chest muscles, persistent hiccups, dizziness, fainting, chest pain, or shortness of breath.

What are the complications of implantable cardiac defibrillator implantation?

Common complications include pain, swelling, and minor bleeding at the implantation site. More serious complications are uncommon and typically occur less than 2% of the time. Serious complications include major bleeding requiring blood transfusions, introduction of air into the space between the lung and chest wall (pneumothorax) requiring tube drainage, perforation of the heart muscle by the leads, activation of an intractably fast heart rhythm, stroke, heart attack, need for emergency heart surgery, and death. Although there are no official guidelines, ICDs should be implanted by or in conjunction with a cardiologist specially trained in clinical cardiac electrophysiology (electrical diseases of the heart).

What happens during a tachycardia episode after implantation of an Implantable Cardiac Defibrillator?

When the heart is beating normally, the ICD remains inactive. When tachycardia occurs, the patient typically experiences the symptoms of a fast heart rate. Since tachycardia can lower blood pressure and cause dizziness or fainting spells, the person should lie down or sit down until the symptoms pass. The ICD will either send a series of pacemaker-like weak electrical signals or one or more low energy shocks to convert the tachycardia to normal rhythm. The patient may not feel the pacemaker-like signals, while the low energy shocks may feel like thumps in the chest. Terminating a tachycardia with low energy shocks is called cardioversion.

If ventricular fibrillation occurs, the patient may suddenly feel faint or lose consciousness due to lack of blood pressure and blood supply to the brain. The ICD, sensing the rapid and irregular rhythm, quickly sends a strong shock to terminate the rhythm. Terminating fibrillation with a strong electric shock is called defibrillation. Successful defibrillation promptly restores consciousness. If unconsciousness lasts longer than 30 seconds, emergency 911 should be called.

The patient, or anyone available to assist, should also call emergency (911) if any of the following occur:

  1. Symptoms of tachycardia persist after feeling the shock(s);
  2. Symptoms of tachycardia persist and the patient feels no shocks (possible ICD malfunction); or
  3. The patient feels a series of shocks in a row (possible ICD malfunction or recurrent attacks of tachycardias). Both conditions need prompt medical attention.


Heart Disease: Causes of a Heart Attack See Slideshow

Living with an implantable cardiac defibrillator

After recovering fully from the ICD implantation, most patients can resume normal activities, including exercise and sex. The doctor should prescribe the type and intensity of the exercise. The doctor also decides when the patient can return to work.

Every patient is given an ICD identification card. The ID card contains information regarding the ICD and instructions in case of an emergency. The card should be carried in the patient's wallet at all times and shown to other doctors and dentists. Occasionally, it will need to be shown to security officers at the airport.

What outside electrical sources are safe?

ICDs are well protected from most household electrical appliances in good condition such as radios, televisions, stereos, microwave ovens, electrical blankets, computers, vacuum cleaners, etc.

What outside electrical sources can interfere with the implantable cardiac defibrillator?

Magnetic resonance imaging (MRI scan) is a diagnostic test for studying the brain, the joints, the spine, the liver, and other organs. The strong magnetic field from the MRI scan can interfere with ICDs. Patients with ICDs should not undergo MRI scanning.

Digital cellular phones can interfere with ICDs. Therefore, the cellular phone should be held on the ear opposite from the side of the ICD. Do not carry the cellular phone in the pocket near the chest.

Theft detector gates in certain stores can generate signals that interfere with the ICD. While it is safe for patients with ICDs to quickly walk through these gates, they should not stand at or near the gates.

Similarly, the metal detector gates at airports can send strong signals that interfere with the ICDs. This problem can be avoided by presenting the ICD ID card to the security officers and walking around the gates. Hand held security wands (such as those used by airport security officers) have magnetic fields that can interfere with the device. Scanning by these wands should be avoided.

Heavy-duty electrical powered equipment, arc welders, a running car engine, and certain electrically powered surgical tools can also cause disturbances with the ICD. Patients should obtain permission from their doctors prior to driving a car or operating equipment which may fall into the above category.

Although a running car should not interfere with an ICD during driving, a patient should not lean over a running engine. Any other concerns and precautions should be discussed with your doctor.

What does the future hold for ICDs?

Implantable defibrillator technology continues to improve. The size of ICDs has decreased dramatically which makes the ICD easier to implant, less visible, and more comfortable. Other advances include not only technology improvements, but clinical trials that may expand the indications and uses for this exciting lifesaving device.

Medically reviewed by Robert J. Bryg, MD; Board Certified Internal Medicine with subspecialty in Cardiovascular Disease


Ganz, L., MD., "General principles of the implantable cardioverter-defibrillator." UpToDate. Updated Jul 6, 2016.