CARDIAC RESYNCRONIZATION THERAPY (CRT)
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 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 the 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
nod 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.
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 mistiming in the contractive 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
Cardiac Resynchronization Therapy Process
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.