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.
Other related procedures that may be used to assess the heart include
exercise electrocardiogram (ECG), Holter monitor, signal-averaged 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 cardiac CT scan. Please see these
procedures for additional information.
heart is, in the simplest terms, a pump made up of muscle tissue. The
heart's pumping action is regulated by an electrical conduction system
that coordinates the contraction of the various chambers of the heart.
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 node (also called AV node), where impulses are slowed
down for a very short period, then continue 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 the
right and left 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 ECG from the
normal tracing may indicate one or more of several heart-related
Almost everyone knows what a basic ECG tracing looks like. But what does
first short upward notch of the ECG tracing is called the "P wave." The
P wave indicates that the atria (the two upper chambers of the heart)
are contracting to pump out blood.
The next part of the tracing is a short downward section connected to a
tall upward section. This next part is called the "QRS complex." This
part indicates that the ventricles (the two lower chambers of the heart)
are contracting to pump out blood.
The next short upward segment is called the "ST segment." The ST segment
indicates the amount of time from the end of the contraction of the
ventricles to the beginning of the rest period before the ventricles
begin to contract for the next beat.
The next upward curve is called the "T wave." The T wave indicates the
resting period of the ventricles.
When the physician views an ECG, he or she studies the size and length
of each part of the ECG. Variations in size and length of the different
parts of the tracing may be significant. The tracing for each lead of a
12-lead ECG will look different, but will have the same basic components
as described above. Each lead of the 12-lead ECG is "looking" at a
specific part of the heart, so variations in a lead may indicate a
problem with the part of the heart associated with a particular lead.
Some reasons for your doctor to request an ECG 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 identify irregular heartbeats
To determine the status of the heart prior to procedures such as
surgery and/or after treatment for conditions such as a heart
attack (myocardial infarction, or MI), endocarditis
(inflammation or infection of one or more of the heart valves),
or after procedures such as heart surgery or cardiac
To assess the function of an implanted pacemaker
To determine the effectiveness of certain heart medications
To obtain a baseline tracing of the heart's function during a
physical examination that may be used as a comparison with
future ECGs, to determine if any changes have occurred
There may be other reasons for your doctor to recommend an ECG.
An 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 on your specific medical condition.
Be sure to discuss any concerns with your doctor 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
Anatomical considerations, such as the size of the chest and the
location of the heart within the chest
Movement during the procedure
Exercise or smoking prior to the procedure
Electrolyte abnormalities, such as too much or too little
potassium, magnesium, and/or calcium in the blood.
Your doctor 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 doctor of all medications (prescribed and
over-the-counter) and herbal supplements that you are taking.
Notify your doctor if you have a pacemaker.
The area(s) where the electrodes are to be placed may be shaved.
Based on your medical condition, your doctor may request other
An 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
Generally, an 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 ECG will be started. It will take only a short time for the
tracing to be completed.
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 doctor instructs you differently.
Generally, there is no special care following an ECG.
Notify your doctor if you develop any signs or symptoms you had prior to
the test (for example, chest pain, shortness of breath, dizziness, or
Your doctor may give you additional or alternate instructions after the
procedure, depending on your particular situation.
The content provided here is for informational purposes only, and was
not designed to diagnose or treat a health problem or disease, or
replace the professional medical advice you receive from your physician.
Please consult your physician with any questions or concerns you may
have regarding your condition.
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procedure and related health conditions. We hope you find these sites
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For more information, call 1.866.CALL.MLH.