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.
A newer type of pacemaker, called a biventricular pacemaker, is currently
used in the treatment of ventricular dyschrony (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.
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 persons 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 (ATP). When ATP is
used, a fast pacing impulse is 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 physician 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?
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 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 140 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:
Low blood pressure
The symptoms of arrhythmias may resemble other medical conditions. Consult
your physician 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?
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
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.