An implantable cardioverter defibrillator (ICD) is a device that is implanted in the chest to constantly monitor and correct abnormal heart rhythms (arrhythmias). The devices were developed originally to correct heart rhythms that are too fast, but recent technological advances have increased the pool of possible patients who may benefit from an ICD. 

ICDs are mainly used to treat two forms of abnormal heart rhythms, both of which occur in the ventricles, or lower pumping chambers of the heart. If the ventricles begin to beat too quickly (ventricular tachycardia), the device may emit low-energy electrical pulses that allow the heart to regain its normal rhythm. If the tachycardia progresses to a very rapid, life-threatening rhythm that causes the ventricles to quiver rather than beat (ventricular fibrillation), the device may deliver a relatively stronger jolt to reset the heart rate (defibrillation).

In recent years, ICDs have also been developed that can treat additional heart rhythms. For example, if the heart rhythm becomes too slow (bradycardia), the ICD may function like a traditional pacemaker, emitting pulses that will increase the heart rate. ICDs can also pace the heart in the event of a mildly elevated heart rate. The ICDs response to any situation depends on how it has been programmed. Finally, modern ICDs can record abnormal heart rates and keep a record of any activity. This record can be viewed by the physician to help diagnose underlying heart problems.

Most ICDs are implanted in patients who have suffered from sustained ventricular tachycardia, ventricular fibrillation and/or sudden cardiac death, which occurs when the heart stops beating. If constant pacing is needed, the physician will likely implant a traditional pacemaker. ICDs are not designed to provide constant pacing and doing so will cause their batteries to wear down faster. In turn, the device will have to replaced earlier.

An appropriate ICD is chosen and programmed by the physician according to the patient's needs. The device is implanted into the chest of the patient during a minor surgical procedure (not open-heart surgery). A short stay in the hospital is usually required, and some patients may need to take medications that help the heart maintain a normal heart rhythm (antiarrhythmics). Once the ICD is in place, it runs on batteries for about four to seven years, depending on how often an electric shock is discharged. ICD batteries will not run out unexpectedly. During a routine office visit, physicians can detect when the battery is running low.

An implantable cardioverter defibrillator (ICD) is a device that is implanted in the chest to monitor and, if necessary, correct episodes of an abnormal heart rhythm (arrhythmia). The implantable cardioverter defibrillator gets its name from the two functions that it performs.

In the event of an abnormally fast heart rhythm in the ventricles (ventricular tachycardia), the ICD may emit low-energy shocks to convert the heart rhythm to a more normal rate. This is called cardioversion. If the ventricular tachycardia progresses to a very rapid, life-threatening heart rhythm in which the ventricles quiver instead of beat (ventricular fibrillation), the ICD may emit a relatively larger jolt of electricity to “reset” the heart rate. This is called defibrillation. Ventricular fibrillation usually occurs in the presence of underlying heart disease, such as coronary artery disease, and is a leading cause of sudden cardiac death.

Although the main functions of the ICD are cardioversion and defibrillation, it can also be programmed to do the following:

• Tachycardia pacing. When an ICD senses a fast but rhythmic heartbeat (tachycardia), it releases a series of precisely timed low-intensity electrical pulses that gently interrupt the heart and allow it to return to a slower pace. Whereas both cardioversion and defibrillation involve shocks that may feel like a sudden kick in the chest, these low-intensity stimuli from an ICD are generally not felt by the patient.

• Bradycardia pacing. Like a pacemaker, the ICD can sense an abnormally slow heartbeat (bradycardia) and send small electrical signals to pace the heart until it restores and maintains a normal heart rate. ICDs are not used for patients who need constant bradycardia pacing. Rather, they are used for patients who have multiple heart rhythm problems that include bradycardia.

The ICD also records heart activity and can transmit this information to the physician during a routine check, allowing the physician to better diagnose and monitor the underlying conditions causing the patient’s arrhythmia. These checks can be performed in a physician’s office or even over the telephone (transtelephonic monitoring).

A series of landmark studies confirmed that ICDs significantly reduce the risk of sudden cardiac death due to abnormal heart rhythms (arrhythmias). In fact, some of the studies were ended earlier than scheduled due to the superior benefit shown by the ICD in comparison to drugs used alone.

Based on the results of these studies, an ICD may be recommended for patients who have experienced any of the following:

Not every patient needs an ICD. ICDs are generally not necessary when an abnormal rhythm (arrhythmia):

Is an isolated occurrence with no underlying disease
Is caused by an electrolyte imbalance or drug overdose
Occurred within the first 48 hours after a heart attack

For patients having only an abnormally slow heartbeat (bradycardia), the preferred treatment is a pacemaker.

Similar in structure to a pacemaker, the ICD consists of three parts:

The generator is a small box, usually about 2 inches wide and approximately 3 ounces in weight. Some generators are even smaller, measuring 1 inch in diameter and weighing about half an ounce. They are battery-powered and most use lithium batteries that need to be replaced every four to seven years. The generator is responsible for generating the electric shock.

Attached to the generator are leads, or wires, generally made of platinum with an insulating coating of either silicone or polyurethane. The leads carry the electric shock from the generator.

At the tip of each lead is a tiny device called an electrode that delivers the necessary electrical shock to the heart. The electric shock is created by the generator, carried by the leads and delivered by the electrodes to the heart. The decision of where to put the leads depends on the needs of the patient.

The heart has four chambers. The two lower chambers, or ventricles, perform most of the pumping action. The right ventricle pumps oxygen-poor blood that has returned from the body into the lungs to get more oxygen. The left ventricle takes that oxygen-rich blood and pumps it back out to the body. Though they are pumping the blood to different areas, the ventricles need to be synchronized (pumping at the same time) for blood to circulate properly.

The two upper chambers of the heart are called the atria (or atrium if referring to only one). They also pump blood, but with less force than the ventricles. Blood returning from the body to the heart enters the right atrium, which pumps that blood into the right ventricle and out to the lungs where it picks up a new supply of oxygen. Similarly, the left atrium pumps oxygen-rich blood returning from the lungs into the left ventricle, which in turn pumps the oxygen-rich blood through the aorta and out to the rest of the body. In order for the heart to function properly, the atria must not only be synchronized with each other, but with the ventricles as well.

The lead(s) of an ICD can be placed on one or two of the heart chambers in a variety of combinations depending on the needs of the patient. There are three types of ICDs:

• Single chamber ICD. A lead is attached in the right ventricle. If necessary, a low-energy shock can be delivered to the ventricle to help it contract normally.
  
  
• Dual chamber ICD. In this type of device, one lead is attached in the right atrium and one is attached to the right ventricle. If needed, a low-energy shock is delivered first to the atrium and then to the ventricle. This action helps the heart beast in a normal sequence and rhythm.
  
  
• Biventricular ICD. The leads from the generator are attached in the right atrium, right ventricle and left ventricle. By using this placement, the stimuli helps the heart beat in a more balanced way. This device is used specifically for individuals with heart failure.

Because surgery is needed to implant the ICD, this is considered an invasive treatment option. However, ICD insertion is considered minor surgery and can be performed in either an operating room or an electrophysiology laboratory. Patients may be asked to stop taking certain medications (e.g., anticoagulants) for several days prior to the surgery. They will also be asked to sign a consent form and dress in a hospital gown for the procedure. The patient’s heart rate and blood pressure and other vital signs will be monitored during the implantation.

This insertion site will be cleaned, shaved and numbed with the injection of a local anesthetic. A small cut (incision) is made in the chest wall just below the collarbone. Another incision is made in the vein just under the collarbone. The wires of the ICD are passed through the vein and attached to the inner surface of the heart. The other ends of the wires are connected to the main box of the ICD (containing the generator), which is inserted into the tissue under the collarbone and above the breast.

Once the ICD is implanted, the physician will test it several times by causing the heart to fibrillate, making sure the ICD responds properly. Because the patient is anesthetized, the patient will not feel this test. The incision is then closed by sutures (stitches), staples or surgical glue. The entire procedure may take several hours to complete.

On rare occasions, the ICD may need to be placed outside of a patient's heart. In these cases, the lead is not guided though a vein into the heart but sewn directly into the patient's heart. This type of procedure is performed in open heart surgery.

Following the procedure, a chest x-ray and electrocardiogram (EKG) will be performed to confirm the proper placement of the wires in the heart. The ICD is tested and the device is programmed based on the patient's needs. Prior to the patient’s discharge, the physician may again induce ventricular fibrillation or ventricular tachycardia. This allows the physician to program the ICD for maximum efficiency. Not all hospitals and centers perform this sort of testing.

Depending on the patient’s age and overall health, a short stay in the hospital is usually required following ICD insertion. The physician will provide specific instructions regarding wound care and the patient’s appropriate activity level immediately following the procedure. In general, patients may be instructed not to bathe or shower for at least five days after the procedure. They should also avoid contact sports, heavy lifting or vigorous exercise for several weeks, in order to avoid dislodging the wires.

In addition, the patient will receive important information from the physician about the type of ICD device that has been implanted. Most patients also will receive a card from the ICD manufacturer with information about their specific advice. It is important that patients keep this information with them at all times in case of the need for medical attention.

A follow-up visit at the physician’s office is usually scheduled for two weeks after the surgery. At this visit, patients can expect the physician to do the following:

If there are no complications, complete recovery from the procedure will take about four weeks. During that time, the wires will firmly take hold where they were placed in the heart.

Although the insertion of an ICD usually requires only minor surgery, it still carries some risks. While complications are rare, patients should report any of the following symptoms immediately:

Serious complications from the surgery occur in less than 1 percent of cases. These include:

• Severe bruising or bleeding
  
• Formation of a blood clot
  
• Torn blood vessel
  
• Punctured lung or heart muscle
  
• Stroke
  
• Heart attack
  
• Introduction of air into the space between the lung and chest wall
  
• Inappropriate shock. When a shock is delivered in absence of arrhythmias due to device malfunction or oversensing.
  
• Death

The risk of having one of these complications is increased if people have certain characteristics, such as:

• Advanced age
  
• Obesity (body mass index 30 or greater)
  
• Severe lung disease (often due to smoking)
  
• Use of various medications
  
• Severely decreased heart function

Rarely, there may also be some complications with the ICD itself. As with any mechanical device, the ICD or the wires may malfunction. A small percentage of patients report at least one time in which the ICD delivered a shock when no arrhythmia was present. If patients feel a shock from the ICD, they should call their physicians. If other symptoms occur, such as dizziness, clamminess, palpitations, angina (chest pain), loss of consciousness or blackouts, patients are advised to call an ambulance and go to the emergency room. These symptoms indicate that their ICD may need reprogramming.

Some patients may sustain a cluster of episodes of ventricular tachycardia and/or ventricular fibrillation that results in multiple shocks (discharges) by the ICD. This is known as an “electrical storm.” An electrical storm is when at least three shocks and/or anti-tachycardia pacing signals are delivered within 24 hours. ICD patients surviving an electrical storm have an increased risk of death, particularly within the first three months following the event. What precisely causes electrical storms is not known, but patients experiencing them tend to have ventricular tachycardia and an ejection fraction below 30 percent. They are also less likely to have undergone revascularization (e.g., coronary bypass). Patients who feel multiple discharges in a short time period should contact their physician, who will review the data recorded by the ICD and consider appropriate treatment.

Because the battery of the ICD is sealed within the main box of the ICD, that entire box must be replaced when the battery is low. Most modern ICDs use lithium batteries that need to be replaced every four to seven years, depending on how often an electric shock is discharged.

ICD batteries will not run out unexpectedly. When a battery is running low, the elective replacement indicator (ERI) is activated. Physicians can detect this activation during a routine office visit or during a telephone check (transtelephonic monitoring). ICDs will continue to function for approximately six months after the ERI is activated, allowing ample time to schedule an elective replacement procedure.

Aside from the ERI, changes may occur in the way the ICD operates that will indicate to a physician that the battery is beginning to run low. Patients may or may not be able to feel these changes in function. Regular communication with one’s physician is an important part of a successful experience with an ICD.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about living with an implantable cardioverter defibrillator (ICD):

1. What heart condition do I have that can be helped with an ICD?
   
   
2. How will the ICD correct my condition?
   
   
3. How quickly do I need to have the procedure?
   
   
4. What type of ICD will I receive?
   
   
5. What can I expect in terms of recovery from the procedure?
   
   
6. How will you know if my ICD is functioning correctly?
   
   
7. What factors can affect the function of my ICD?
   
   
8. What are my restrictions with an ICD?
   
   
9. What symptoms indicated a medical emergency?
   
   
10. How will my ICD batteries be checked?
    
    
11. What will be done when my batteries run low?
    
    
12. Will I need medications in addition to the ICD?
    
    
13. What is the difference between an ICD and a pacemaker?
    
    
14. How does an ICD affect pregnancy and childbirth?