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Pager-sized pacemakers rest just under
the collarbone to stimulate hearts that beat
too slowly.
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Each individual heartbeat is
actually a collection of several
muscle movements spurred
into action by electrical impulses.
The first electrical signal comes from
the heart’s own natural pacemaker,
the sinoatrial node, comprised of
electrically active cells and located
in the upper right heart chamber. This
node sends a steady stream of electrical
signals along a pathway through the
heart’s upper chambers. The signals
then travel to the electrical bridge –
the atrioventricular node – between
the upper and lower chambers and,
finally, move to the lower chambers.
A problem at any point in the electrical
pathway can wreak havoc with the
regular beating of a heart. Luckily, an
artificial pacemaker – a small, battery
-operated device – can take over
the role of the heart’s own electrical
system, if necessary.
What is a pacemaker?
Although it weighs just about
an ounce, a pacemaker contains a
powerful battery, electronic circuits
and computer memory that together
generate electronic signals. The signals,
or pacing pulses, are carried along
thin, insulated wires, or leads, to the
heart muscle. The signals cause the
heart muscle to begin the contractions
that cause a heartbeat.
A pacemaker is implanted just below
the collarbone in a procedure that takes
about two hours. It is programmed to
stimulate the heart at a pre-determined
rate, and settings can be adjusted at
any time. Routine evaluation, sometimes
even via telephone, ensures the
pacemaker is working properly and
monitors battery life, which generally
runs from five to ten years.
When is a pacemaker needed?
The most common reason for a pacemaker
is a heartbeat that slows to
an unhealthy rate, or bradycardia. A
pacemaker resets the heart rate to an
appropriate pace, ensuring adequate
blood and oxygen are delivered to the
brain and other parts of the body.
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TAKE NOTE
Full-contact sports can damage a pacemaker.
Magnetic Resonance Imaging (MRI) tests employ powerful magnets to create
images. If you are scheduled for an MRI test, you should make certain your
doctor knows you have a pacemaker.
Electronic security systems, such as those common in airports, can pose
problems if exposure is prolonged. Passing through such systems, even
several times, does not pose a threat. However, patients with pacemakers
should avoid the “wand” used in some security checks.
Microwave ovens, cellular phones and electric blankets do not, as myth
might have it, affect pacemaker functioning.
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Types of pacemakers
Three basic types exist to serve
different purposes:
Single-Chamber Pacemakers – In
a single-chamber pacemaker, only
one wire (pacing lead) is placed into
a chamber of the heart. Sometimes it
is the upper chamber, or atrium.
Other times it is the lower chamber,
or ventricle.
Dual-Chamber Pacemakers – In dualchamber
pacemakers, wires are placed
in two chambers of the heart. One lead
paces the atrium and one paces the
ventricle. This approach more closely
matches the natural pacing of the
heart. This type of pacemaker can
coordinate function between the atria
and ventricles.
Rate-Responsive Pacemakers –
These have sensors that automatically
adjust to changes in a person's physical
activity.
Other devices – Some devices, such
as implantable cardioverter defibrillators
(ICDs), designed primarily for
other purposes, can function as pacemakers
in certain situations.
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In the largest clinical trial of pacemakers conducted in the United States,
researchers found that patients with sick sinus syndrome benefit from dualchamber
pacemakers. They are less likely to develop atrial fibrillation or
mild forms of heart failure compared to people who are treated with singlechamber
pacemakers.
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When are pacemakers used?
Pacemakers may be prescribed for
a number of conditions, including:
Bradycardia – a condition in which
the heart beats too slowly, causing
symptoms such as fatigue, dizziness
or fainting spells. Bradycardia may
be caused by the wear and tear of
age or by conditions such as sick
sinus syndrome (SSS) or heart block.
Atrial fibrillation – a common heart
rhythm disorder in which the upper
chambers of the heart beat rapidly
and chaotically. Sometimes people
with atrial fibrillation can also have
slow rhythms. Medicines used to
control atrial fibrillation may result
in slow rhythms which are treated
by pacemakers.
Heart failure – a condition in which
the heartbeat is not sufficient to supply
a normal volume of blood and oxygen
to the brain and other parts of the
body. A special pacemaker can be
carefully programmed to increase the
force of muscle contractions in the
heart. This is called “biventricular
pacing” or “resynchronization” therapy.
Syncope – a condition best known
as the common faint, is usually not
serious. Some patients faint when
their heart rhythm becomes very
slow. For a small percentage of people
who experience severe and frequent
fainting spells, a pacemaker may
prevent the heart rate from slowing
to the point of fainting.
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