An implantable cardioverter defibrillator (AICD) is a device that is implanted in the chest to monitor for and, if necessary, correct episodes of rapid heartbeat. If the heartbeat gets too fast (ventricular tachycardia), the AICD will stimulate the heart to restore a normal rhythm (anti-tachycardia pacing). In cases where the heartbeat is so rapid that the person may die (ventricular fibrillation), the AICD will also give an electric shock (defibrillation) to “reset” the heartbeat.

An AICD is similar in many respects to an artifical pacemaker, which is another type of device that corrects an abnormal heart rhythm. However, pacemakers are usually chosen to correct a heart rhythm that is too slow (bradycardia), whereas AICDs are used to correct a heart rhythm that is too fast (tachycardia). And there are patients who need both bradycardia pacing and anti-tachycardia pacing. In these patients, an AICD will be used to pace the heart.

The AICD 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 rhythm (antiarrhythmics). Once the AICD is in place, it runs on batteries for about four to seven years, depending on how often an electric shock is discharged. AICD batteries will not run out unexpectedly. Physicians can detect when the battery is running low during a routine office visit.

People with AICDs need to be careful in certain situations. More information on the lifestyle considerations of living with an AICD can be found by clicking on the following link: Living with an Implantable Defibrillator.

What is an implantable cardioverter defibrillator (AICD)?
An implantable cardioverter defibrillator (AICD) is a device that is implanted in the chest to monitor for and, if necessary, correct episodes of rapid heartbeat. The implantable cardioverter defibrillator gets its name from the two functions that it performs. First, the AICD sends small electrical charges to the heart to “reset” it when it goes too fast. This process of converting one rhythm or electrical pattern to another is called cardioversion. Second, the AICD will send stronger charges to “reset” the heart if it begins quivering instead of beating. The act of stopping this potentially fatal quivering of the heart (ventricular fibrillation) is called defibrillation.

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

• Anti-tachycardia pacing. When an AICD 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 are generally not felt by the patient.
• Bradycardia pacing. Like an artificial pacemaker, the AICD 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.

Modern AICDs can be programmed for all of the above functions. The AICD 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.







Who may benefit from an AICD?
A series of landmark studies confirmed that AICDs significantly reduce the risk of sudden cardiac death due to arrhythmias. In fact, some of the studies were ended earlier than scheduled due to the superior benefit shown by the AICD in comparison to drugs used alone.

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

• Previous heart attacks, with weakened functioning of the left ventricle. The performance of the left ventricle is expressed numerically as the left ventricular ejection fraction. It represents the proportion of blood in the heart that is pumped out with each beat. A normal range is between 55 and 75 percent. An ejection fraction below 40 percent has been shown to increase the risk of sudden cardiac death. In heart attack survivors with reduced ejection fractions, it has been found that an AICD plus antiarrhythmic drugs significantly lowers the risk of sudden cardiac death, as compared to antiarrhythmics used alone.
• History of ventricular tachycardia (VT) or ventricular fibrillation (VF). For these patients, AICDs have clearly improved survival compared to antiarrhythmic drugs.
• Coronary artery disease. Patients with coronary artery disease may have an underlying arrhythmia. Studies have shown that, in patients with coronary artery disease who received an AICD, cholesterol reducing drugs may have an anti-arrhythmic effect that can reduce the recurrence of ventricular tachycardia or ventricular fibrillation.
• Cardiac arrest.

Not every patient needs an AICD. AICDs 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 an abnormally slow heartbeat (bradycardia), the preferred treatment is an artificial pacemaker.

How an AICD works
Similar in structure to a pacemaker, the AICD consists of three parts:

• Generator
• Leads
• Electrodes

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 two 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. Thus, 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, as described below.

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. While 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.

Depending on the needs of the patient, the two leads of an AICD can be placed on two of these chambers in a variety of combinations depending on the needs of the patient. Currently, there is great research interest in placing leads in both the right and left ventricles for patients with heart failure. By simultaneously pacing both ventricles, the AICD may restore normal heart synchronization. Although results of this technique appear promising, it is still considered to be experimental.

How invasive is an AICD?
Because surgery is needed to implant the AICD, this is considered an invasive treatment option. However, AICD 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 to dress in a hospital gown for the procedure. The patient’s heart rate and blood pressure 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 AICD 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 AICD, which is inserted into the tissue under the collarbone and above the breast. Once the AICD is implanted, the physician will test it several times by causing the heart to fibrillate, making sure the AICD 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 takes about an hour.

Immediately following the procedure, a chest x-ray will be taken to confirm the proper placement of the wires in the heart. Patients will rest for several hours and their vital signs will be closely monitored. The AICD’s programming may be adjusted using a magnetic wand that is passed over the chest, during which the patient will not feel anything different.

After the initial operation, the physician may induce ventricular fibrillation or ventricular tachycardia prior to the patient’s discharge. This allows the physician to program the AICD for maximum efficiency.

Depending on the patient’s age and overall health, a short stay in the hospital is usually required following AICD insertion. The physician will provide specific instructions regarding 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.

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

• Remove any remaining sutures (stitches) or surgical staples.
• Check the site for any signs of infection.
• Offer further instructions for how to live with an AICD. 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.

Is there any risk of complications with an AICD?
Although the insertion of an AICD requires only minor surgery, it still carries some risks. While complications are rare, patients should report any of the following symptoms immediately:

• Redness, warmth, tenderness or swelling of the incision site, alone or with a fever. Sometimes a hard ridge forms where the incision was closed. This will fade away as the wound heals.
• Drainage of liquid from the incision site, alone or with a fever.
• Increased shortness of breath, prolonged hiccuping or difficulty breathing.
• Fainting, lightheadedness or dizziness.
• Fast or pounding heartbeats (palpitations).
• Chest pain.
• Re-experiencing the same symptoms that they had before surgery.

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
• Death

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

• Advanced age
• Obesity (more than 20 pounds heavier than one’s ideal weight or 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 AICD itself. As with any mechanical device, the AICD or the wires may malfunction. A small percentage of patients report at least one time in which the AICD delivered a shock when no arrhythmia was present. If patients feel a shock from the AICD, they should call their physician. 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, because their AICD may need reprogramming.

Some patients may sustain a cluster of episodes of ventricular tachycardia and/or ventricular fibrillation that results in multiple shocks (discharges) being administered by the AICD. 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. Compared to AICD patients not having such an episode, AICD 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 AICD and consider appropriate treatment.

How long will an AICD last?
Since the battery of the AICD is sealed within the main box of the AICD, that entire box must be replaced when the battery is low. Most modern AICDs use lithium batteries that need to be replaced every four to seven years, depending on how often an electric shock is discharged.

AICD 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. AICDs will continue to function for approximately six months after the ERI is activated, allowing plenty of time to schedule an elective replacement procedure.

Aside from the ERI, changes may occur in the way the AICD 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 AICD.