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Cardiac resynchronization relies on electric
leads to correct an arrhythmia commonly
diagnosed in patients experiencing heart
failure. The leads electrically stimulate heart
muscle to synchronized the contractions of
the heart’s two lower chambers, or ventricles.
Only when the lower chambers beat in harmony
can they contract with enough force to
push blood carrying oxygen through the body.
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More than 22 million people
worldwide suffer from congestive
heart failure (CHF),
a potentially debilitating disease.
Until recently, lifestyle changes,
medication and, sometimes, heart surgery
were the only treatment options.
Patients with severe symptoms, however,
received little, if any, relief from
such approaches. To make matters
worse, up to 40 percent of patients
with CHF also have an arrhythmia
that further reduces the heart’s ability
to beat properly.
Cardiac resynchronization therapy
(CRT) is an innovative new therapy
that can relieve CHF symptoms
by improving the coordination of
the heart’s contractions.CRT builds on the technology used
in pacemakers and implantable
cardioverter devices. CRT devices
also can protect the patient from
slow and fast heart rhythms.
Overview of a heart beat
The heart is comprised of four chambers:
two upper atria, and two lower
ventricles. An electrical system
controls the synchronized pumping
action of these chambers.
The normal heartbeat originates in
a section of the right atrium known as
the sinoatrial, or SA node. The electrical
signal from the sinoatrial node
spreads through both atria causing
them to contract and squeeze blood
into the ventricles. The electrical signal
then passes through an electrical
bridge known as the atrioventricular
or AV node. After a split second delay, the signal continues to the
ventricles by way of a specialized
network known as the left and right
bundle branches.
The bundle branches separate to
the left and right ventricles, which
enables the electrical signal to stimulate
both ventricles simultaneously.
This coordinated contraction, or
squeezing, of the ventricles is necessary
for optimal pumping of blood
to the body and lungs.
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ABOUT CONGESTIVE HEART FAILURE
Damaged heart muscle can become so weak that it can no longer pump
effectively, leading to cardiomyopathy and CHF.
Coronary artery disease and heart attacks are the most frequent causes of
CHF, but inherited disorders, viral infections and toxins, such as alcohol, also
can cause heart muscle damage. Symptoms of CHF typically include shortness
of breath, swelling of the feet and legs, abdominal swelling, fatigue,
exercise intolerance, diminished appetite and depression.
Most often, medications aim to control CHF symptoms, such as the build up
of excess fluid that causes leg swelling and makes it difficult to breath.
Medications can reduce fluid retention, strengthen the heart’s squeezing ability
and relax blood vessels, thereby reducing the resistance to blood flow and
easing the heart’s workload.
In addition, lifestyle changes, such as low-salt diets and exercise, can help
control symptoms.
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Uncoordinated contractions
When there is a delay in electrical
signal transmission through the
left bundle branch, this causes left
bundle branch block (LBBB). Because
the electrical signal to the left ventricle
is delayed, the right ventricle begins
to contract a fraction of a second
before the left ventricle, instead
of simultaneously. The result is an
asynchronous, or uncoordinated
contraction of the ventricles and
a mis-timing in the contraction pattern
of the left atrium and ventricle.
Other conduction abnormalities,
such as right bundle branch block
(RBBB), also may contribute to less
efficient contraction of the heart. This
further reduces the pumping ability
of the already weakened heart muscle.
Cardiac Resynchronization
Therapy
The concept behind CRT is quite
simple. Resynchronization restores
the normal coordinated pumping
action of the ventricles by overcoming
the delay in electrical conduction
caused by bundle branch block. This
is accomplished by means of a special
type of cardiac device.
These powerful, “built-in” devices
have enormous potential to improve
the quality of life and probably survival
for patients with heart failure.
The CRT device
Pacemakers are typically used to
prevent symptoms due to an excessively
slow heartbeat. The pacemaker
continuously monitors the heartbeat
and, when necessary, delivers tiny,
imperceptible electrical signals to
stimulate the heartbeat. Most pacemakers
have two electrode wires,
or leads, one in the right atrium
and one in the right ventricle. This
ensures the pacemaker will maintain
the normal coordinated pumping
relationship between the upper and
lower chambers of the heart.
The wires that carry the electrical
signals connect to an electrical pulse
generator placed under the skin in
the upper chest. In addition to the
two leads (right atrium and right
ventricle) used by a common pacemaker,
the CRT device has a third
lead that is positioned in a vein on
the surface of the left ventricle.
This allows the CRT device to simultaneously
stimulate the left and right
ventricles and restore a coordinated,
or “synchronous,” squeezing pattern.
This is sometimes referred to as
“bi-ventricular pacing” because both
ventricles are electrically stimulated
(paced) at the same time. This reduces
the electrical delay and results in
a more coordinated and effective
heart beat.
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WHEN IS CRT THE RIGHT CHOICE?
The ideal candidate for a CRT device is someone with:
| | Moderate to severe CHF symptoms, despite lifestyle changes and medication |
| | A weakened and enlarged heart muscle |
| | A significant electrical delay in the lower pumping chambers (bundle branch block) |
Some CRT candidates also have a high risk of Sudden Cardiac Death. For
these patients, a special CRT device can stop potentially life-threatening rapid
heartbeats by delivering an electrical shock known as defibrillation. This device
incorporates a standard implantable cardioverter defibrillator (ICD) with a CRT
pacemaker creating a “CRTD” device. (The “D” refers to defibrillation.)
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CRT results
The response to CRT can vary greatly
among patients. Clinical studies
involving more than 2000 patients
worldwide demonstrate modest
improvements in exercise tolerance,
CHF severity, and quality of life in
most patients. Improvement may
happen quickly, but sometimes it can
take several months.
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