Overview
of Pacemakers
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 heartbeat 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 new
type of pacemaker, called a biventricular pacemaker,
is currently used in the treatment of heart failure.
Sometimes in heart failure, the two ventricles
(lower heart chambers) 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.
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 an electrical shock to the
heart when the heart rate becomes dangerously
fast, or “fibrillates.”
An ICD
senses when the heart is beating too fast and
delivers an electrical shock to convert the fast
rhythm to a normal rhythm. Some devices combine
a pacemaker and ICD in one unit for persons who
need both functions.
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. After the shock is delivered,
a “back-up” pacing mode is used if
needed for a short while.
When
the heart's natural pacemaker has a dysfunction,
the signals it sends 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 (an abnormal rhythm of the
heart, which can cause the heart to pump less
effectively).
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 slow or too irregularly.
Click Image to Enlarge
The
heart is, in the simplest terms, a pump made up
of muscle tissue. Like all pumps, the heart requires
a source of energy in order to function. The heart's
pumping action comes from an intrinsic electrical
conduction system.
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. The sinus node generates an electrical
stimulus periodically (60-100 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 (AV)
node, where impulses are slowed down for a very
short period, then continues 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
both ventricles.
Normally,
as the electrical impulse moves through the heart,
the heart contracts about 60 to 100 times a minute.
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 conditions, almost all heart tissue is capable
of starting a heartbeat, or becoming the pacemaker.
An arrhythmia 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 physician for a diagnosis.
A permanent
pacemaker has three 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.
Many pulse generators also have the capability
to receive and respond to signals that are sent
by the heart itself.
- one or two wires (also
called leads). Leads are insulated flexible
wires that conduct electrical signals to the
heart from the pulse generator. The leads may
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 ventricle (the
lower chamber of the heart).
- electrodes on each
lead
Older
pacemakers sent out electrical signals at a constant
rate, regardless of the heart's own rate.
Pacemaker technology is now much more advanced.
Today, 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 atrium or ventricle
or both, 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 atrial node, 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 primarily for fast ventricular arrhythmias.
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.
Pacemaker/ICD
insertion is usually an outpatient procedure,
done in the cardiac catheterization laboratory,
or the electrophysiology laboratory. The patient
is awake during the procedure, although sedation
may be given to help the patient relax during
the procedure.
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 and often is allowed to go home
the day of the procedure.
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. You
should carry this card with you at all times so
that the information is always available to any
healthcare professional who may have reason to
examine and/or treat you.
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