Signal-Averaged
Electrocardiogram
(Signal-Averaged
ECG, Signal-Averaged EKG, SAECG)
What
is a signal-averaged electrocardiogram?
An electrocardiogram
(ECG or EKG) is one of the simplest and fastest
procedures used to evaluate the heart. Electrodes
(small, plastic patches) are placed at certain
locations on the chest, arms, and legs. When the
electrodes are connected to an ECG machine by
lead wires, the electrical activity of the heart
is measured, interpreted, and printed out for
the physician's information and further interpretation.
A signal-averaged
electrocardiogram is a more detailed type of ECG.
During this procedure, multiple ECG tracings are
obtained over a period of approximately 20 minutes
in order to capture abnormal heartbeats which
may occur only intermittently. A computer captures
all the electrical signals from the heart and
averages them to provide the physician more detail
regarding how the heart’s electrical conduction
system is working.
Signal-averaged
ECG is one of several procedures used to assess
the potential for dysrhythmias/arrhythmias (irregular
heart rhythms) in certain medical situations.
Other
related procedures that may be used to assess
the heart include resting electrocardiogram (ECG),
Holter monitor, exercise electrocardiogram (ECG),
cardiac catheterization, chest x-ray, computed
tomography (CT scan) of the chest, echocardiography,
electrophysiological studies, magnetic resonance
imaging (MRI) of the heart, myocardial perfusion
scans, radionuclide angiography, and ultrafast
CT scan. Please see these procedures for additional
information.
The
heart's electrical conduction system:
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 regularly
at 60 to 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.
This
electrical activity of the heart is measured by
an electrocardiogram. By placing electrodes at
specific locations on the body (chest, arms, and
legs), a graphic representation, or tracing, of
the electrical activity can be obtained. Changes
in an EKG from the normal tracing can indicate
one or more of several heart-related conditions.
Reasons
your physician may request a signal-averaged ECG
may include, but are not limited to, the following:
- to determine the cause
of chest pain
- to evaluate other signs
and symptoms which may be heart-related, such
as fatigue, shortness of breath, dizziness,
or fainting
- to help identify irregular
heartbeats
- for further evaluation
of dysrhythmias/arrhythmias noted on resting
ECG
There
may be other reasons for your physician to recommend
an exercise ECG.
A signal-averaged
ECG is a quick, noninvasive method of assessing
the heart’s function. Risks associated with
ECG are minimal and rare.
Prolonged
application of the adhesive electrode patches
may cause tissue breakdown or skin irritation
at the application site.
There
may be other risks depending upon your specific
medical condition. Be sure to discuss any concerns
with your physician prior to the procedure.
Certain
factors or conditions may interfere with or affect
the results of the test. These include, but are
not limited to, the following:
- obesity, pregnancy,
or ascites (accumulation of fluid in the abdomen)
- anatomical considerations
such as the size of the chest and the location
of the heart within the chest
- movement during the
procedure
- exercise, intake of
high-carbohydrate meal, and/or smoking prior
to the procedure
- certain medications
- electrolyte abnormalities,
such as too much or too little potassium, magnesium,
and/or calcium in the blood
- Your physician or the
technician will explain the procedure to you
and offer you the opportunity to ask any questions
that you might have about the procedure.
- Generally, fasting
is not required before the test.
- Notify your physician
of all medications (prescribed and over-the-counter)
and herbal supplements that you are taking.
- Notify your physician
if you have a pacemaker.
- The area(s) where the
electrodes are to be placed may be shaved.
- Based upon your medical
condition, your physician may request other
specific preparation.
An exercise
ECG may be performed on an outpatient basis or
as part of your stay in a hospital. Procedures
may vary depending on your condition and your
physician’s practices.
Generally,
an exercise ECG follows this process:
- You will be asked to
remove any jewelry or other objects that may
interfere with the procedure.
- You will be asked to
remove clothing from the waist up. The technician
will ensure your privacy by covering you with
a sheet or gown and exposing only the necessary
skin.
- You will lie flat on
a table or bed for the procedure. It will be
important for you to lie still and not talk
during the procedure, so as not to interfere
with the tracing.
- If your chest, arms,
or legs are very hairy, the technician may shave
small patches of hair, as needed, so that the
electrodes will stick closely to the skin.
- Electrodes will be
attached to your chest, arms, and legs.
- The lead wires will
be attached to the skin electrodes.
- Once the leads are
attached, the technician may key in identifying
information about you into the machine's computer.
- The EKG will be started.
Generally, it will take about 20 minutes to
obtain the necessary ECG information.
- Once the tracing is
completed, the technician will disconnect the
leads and remove the skin electrodes.
You
should be able to resume your normal diet and
activities, unless your physician instructs you
differently.
Generally,
there is no special care following a signal-averaged
ECG.
Notify
your physician if you develop any signs or symptoms
you had prior to the test (e.g., chest pain, shortness
of breath, dizziness, or fainting).
Your
physician may give you additional or alternate
instructions after the procedure, depending on
your particular situation.
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