Overview
of Pacemakers
An implanted
pacemaker is a small device that is implanted
under the skin and sends electrical signals to
start or regulate a slow heartbeat. An implanted
pacemaker may be used to stimulate the heartbeat
if the heart's natural pacemaker (the sinoatrial,
or SA, node) is not functioning properly, has
developed an abnormally slow heart rate or
rhythm, or if the electrical pathways are blocked.
Rhythm
problems are common in teens and young adults
with congenital heart disease due to changes in
scar tissue and other results from prior surgeries
and procedures.
Children's
pacemakers may be placed under the skin in one
of several locations. Young children (infants,
toddler, preschool, and young school-aged children)
often have the pacemaker generator placed in the
abdomen, since the fatty tissue found there can
help protect the generator from normal everyday
childhood activities such as playing. As a child
gets older (nearing adolescence), the generator
is often placed in the shoulder area, just under
the collarbone.
An implantable
cardioverter defibrillator (ICD) is a small device,
similar to a pacemaker, that is implanted under
the skin, often in the shoulder area, just under
the collarbone. An ICD senses the rate of the
heartbeat. When the heart rate exceeds a rate
programmed into the device, the ICD delivers a
small, electrical shock to the heart to slow the
heart rate. Many newer ICDs can also function
as a pacemaker by delivering an electrical signal
to regulate a heart rate that is too slow or pace
out of a rapid rhythm.
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).
ICDs
may be recommended for persons with significant
ventricular rhythm problems which could pose a
risk for sudden death. These rhythm problems are
often associated with worsening heart failure.
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.
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 (60-190 times per minute,
depending on the age of the child and his/her
activity level). 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,"
just like the sinus node. An arrhythmia may occur
when:
- the heart's natural
pacemaker (the sinus node) 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.
A permanent
pacemaker has two components, including the following:
- 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).
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, 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.
- A ventricular arrhythmia
(an arrhythmia caused by a dysfunction of the
sinus node, an interruption in the conduction
pathways, or the development of another pacemaker
within the heart tissue that takes over the
function of the sinus node) may be treated with
a ventricular pacemaker whose lead wire is located
in the ventricle.
- 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.
There may be one lead wire for each chamber,
or one lead wire may be capable of sensing and
pacing both chambers.
- 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 done in the hospital, either as a
short-stay surgical procedure, or in the cardiac
catheterization or electrophysiology laboratory.
The child is given medication to help him/her
relax during the procedure.
In older
children and teenagers who receive a transvenous
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. This procedure is usually
performed in the catheterization laboratory.
The picture above 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.
In younger
children, the pacemaker may be placed into the
abdomen through a small incision. A second incision
is made in the chest to visualize the heart. The
lead(s) are guided to the heart, then placed on
the heart's surface. This procedure is usually
performed in the operating room. Once the procedure
has been completed, the child goes through a recovery
period of several hours and often is allowed to
go home the day after the procedure.
After
receiving a pacemaker or ICD, you will receive
an identification card from the manufacturer that
includes information about your child's 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 your child. A medical identification
bracelet or necklace can also be worn by your
child to alert others about the pacemaker or ICD
in case of emergency.
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