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Overview of Pacemakers and Implantable Cardioverter Defibrillators (ICDs)Descripción General de los Marcapasos y Cardiodesfibrilador Implantable (CDI)

Overview of Pacemakers and Implantable Cardioverter Defibrillators (ICDs)

What is a permanent pacemaker?

A permanent pacemaker, a small device that is implanted under the skin (most often in the shoulder area just under the collarbone), sends electrical signals to start or regulate a slow heartbeat. A permanent pacemaker may be used to make the heart beat if the heart's natural pacemaker (the SA node) is not functioning properly and has developed an abnormally slow heart rate or rhythm, or if the electrical pathways are blocked.

Illustration of a pacemaker device and the location where it is placed in the chest
Click Image to Enlarge

A newer type of pacemaker, called a biventricular pacemaker, is currently used in the treatment of ventricular dyssynchrony (irregular conduction pattern in the lower heart chambers) or heart failure. Sometimes in heart failure, the two ventricles do not pump together in a normal manner. When this happens, less blood is pumped by the heart. A biventricular pacemaker paces both ventricles at the same time, increasing the amount of blood pumped by the heart. This type of treatment is called cardiac resynchronization therapy.

What is an implantable converter defibrillator (ICD)?

An implantable cardioverter defibrillator (ICD) looks very similar to a pacemaker, except that it is slightly larger. It has a generator, one or more leads, and an electrode for each lead. These components work very much like a pacemaker. However, the ICD is designed to deliver two levels of electrical energy: a low energy shock that can convert  a beating heart that is in an abnormal rhythm back to a normal heartbeat, and a high energy shock that is delivered only if the arrhythmia is so severe that the heart is only quivering instead of beating. 

Illustration showing the ICD device and the location when placed in the chest
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An ICD senses when the heart is beating too fast and delivers an electrical shock to convert the fast rhythm to a normal rhythm. Many devices combine a pacemaker and ICD in one unit for people who need both functions. After the shock is delivered, a "back-up" pacing mode is available if needed for a short while.

The ICD has another type of treatment for certain fast rhythms called anti-tachycardia pacing, a fast-pacing impulse sent to correct the rhythm.

What is the reason for getting a pacemaker or an ICD?

Pacemakers are most commonly advised in patients whose heartbeat slows to an unhealthy low rate. ICDs are advised in specific patients who are at risk for potentially fatal ventricular arrhythmias (an abnormal rhythm from the lower heart chambers, which can cause the heart to pump less effectively). There may be other reasons why your doctor advises placement of a pacemaker or ICD.

When the heart's natural pacemaker or electrical circuit malfunctions, the signals sent out may become erratic: either too slow, too fast, or too irregular to stimulate adequate contractions of the heart chambers. When the heartbeat becomes erratic, it is referred to as an arrhythmia.

Arrhythmias can cause problems with contractions of the heart chambers by:

  • Not allowing the chambers to fill with an adequate amount of blood because the electrical signal is causing the heart to pump too fast.

  • Not allowing a sufficient amount of blood to be pumped out to the body because the electrical signal is causing the heart to pump too slowly or too irregularly.

The heart's electrical system

The heart is, in the simplest terms, a pump made up of muscle tissue. The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart.

How does the heart beat?

Illustration of a cross section of the heart and electrical pathways. Inset box shows normal heart rhythm
Click Image to Enlarge

An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node), which is a small mass of specialized tissue located in the right atrium (right upper chamber of the heart). In an adult, the sinus node generates an electrical stimulus regularly (for adults, 60 to 100 times per minute under normal conditions). This electrical stimulus travels down through the conduction pathways (similar to the way electricity flows through power lines from the power plant to your house) and causes the heart's lower chambers to contract and pump out blood. The right and left atria (the two upper chambers of the heart) are stimulated first and contract a short period of time before the right and left ventricles (the two lower chambers of the heart). 

The electrical impulse travels from the sinus node through the atria to the atrioventricular node (also called AV node), where impulses are slowed down for a very short period, then continue down the conduction pathway via the bundle of His into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to the right and left ventricles.

Normally at rest, as the electrical impulse moves through the heart, the heart contracts about 60 to 100 times a minute, depending on a person's age (infants normally have very high heart rates). Each contraction of the ventricles represents one heartbeat. The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract.

Under some abnormal conditions, certain heart tissue is capable of starting a heartbeat, or becoming the pacemaker. An arrhythmia (abnormal heartbeat) occurs when:

  • The heart's natural pacemaker develops an abnormal rate or rhythm

  • The normal conduction pathway is interrupted

  • Another part of the heart takes over as pacemaker

In any of these situations, the body may not receive enough blood because the heart cannot pump out an adequate amount with each beat as a result of the arrhythmia's effects on the heart rate. The effects on the body are often the same, however, whether the heartbeat is too fast, too slow, or too irregular. Some symptoms of arrhythmias include, but are not limited to:

  • Weakness

  • Fatigue

  • Palpitations

  • Low blood pressure

  • Dizziness

  • Fainting

The symptoms of arrhythmias may resemble other medical conditions. Consult your doctor for a diagnosis.

What are the components of a permanent pacemaker/ICD?

A permanent pacemaker or ICD has three main components:

  • A pulse generator which has a sealed lithium battery and an electronic circuitry package. The pulse generator produces the electrical signals that make the heart beat. Most pulse generators also have the capability to receive and respond to signals that are sent by the heart itself.

  • One or more wires (also called leads). Leads are insulated flexible wires that conduct electrical signals to the heart from the pulse generator. The leads also relay signals from the heart to the pulse generator. One end of the lead is attached to the pulse generator and the electrode end of the lead is positioned in the atrium (the upper chamber of the heart) or in the right ventricle (the lower chamber of the heart). In the case of a biventricular pacemaker, leads are placed in both ventricles.

  • Electrodes, which are found on each lead.

Pacemakers can "sense" when the heart's natural rate falls below the rate that has been programmed into the pacemaker's circuitry.

Pacemaker leads may be positioned in the right atrium, right ventricle, or positioned to pace both ventricles, depending on the condition requiring the pacemaker to be inserted. An atrial arrhythmia (an arrhythmia caused by a dysfunction of the sinus node or the development of another atrial pacemaker within the heart tissue that takes over the function of the sinus node) may be treated with an atrial permanent pacemaker whose lead wire is located in the atrium.

When the ventricles are not stimulated normally by the sinus node or another natural atrial pacemaker site, a ventricular pacemaker whose lead wire is located in the ventricle is placed/used. It is possible to have both atrial and ventricular arrhythmias, and there are pacemakers which have lead wires positioned in both the atrium and the ventricle.

An ICD has a lead wire that is positioned in the ventricle, as it is used for treating fast ventricular arrhythmias. Commonly, ICDs will have an atrial lead and ventricular lead.

Pacemakers that pace either the right atrium or the right ventricle are called "single-chamber" pacemakers. Pacemakers that pace both the right atrium and right ventricle of the heart and require two pacing leads are called "dual-chamber" pacemakers. Pacemakers that pace the right atrium and right and left ventricles are called "biventricular" pacemakers.

How is a pacemaker/ICD implanted?

X-ray of a single-chamber implanted pacemaker

Pacemaker/ICD insertion is done in the cardiac catheterization laboratory, or the electrophysiology laboratory. The patient is awake during the procedure, although local anesthesia is given over the incision site, and generally sedation is given to help the patient relax during the procedure. A night or two of hospitalization may be recommended so that the functioning of the implanted device may be observed.

Shown here is a chest X-ray. The large, white space in the middle is the heart. The dark spaces on either side are the lungs. The small object in the upper corner is an implanted pacemaker.

A small incision is made just under the collarbone. The pacemaker/ICD lead(s) is inserted into the heart through a blood vessel which runs under the collarbone. Once the lead is in place, it is tested to make sure it is in the right place and is functional. The lead is then attached to the generator, which is placed just under the skin through the incision made earlier. Once the procedure has been completed, the patient goes through a recovery period of several hours.

There are certain instructions related to having an implanted permanent pacemaker or ICD. For example, after you receive your pacemaker or ICD, you will receive an identification card from the manufacturer that includes information about your specific model of pacemaker and the serial number as well as how the device works. You should carry this card with you at all times so that the information is always available to any health care professional who may have reason to examine and/or treat you.

 
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