Electrocardiogram
(ECG) / Stress Test / Holter Monitor
An electrocardiogram
(ECG or EKG), is a measurement of the electrical
activity of the heart. 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 can indicate
one or more of several heart-related conditions.
Conditions that are not heart conditions may also
cause changes in the ECG.
To better
understand the ECG, it is helpful to understand
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 periodically (60-100 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.
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.
An arrhythmia occurs when:
- the heart's natural
pacemaker develops an abnormal rate or rhythm
- the normal conduction
pathway is interrupted
- another part of the
heart takes over as pacemaker
Almost
everyone knows what a basic ECG tracing looks
like. But what does it mean?
- The first little 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 electrically stimulated to pump blood
to the ventricles.
- 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 electrically
stimulated to pump out blood.
- The next short flat
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 "T wave".
- The next upward curve
is the T wave. The T wave indicates the recovery
period of the ventricles.
When
your physician studies your ECG, he/she looks
at 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 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 the lead.
Many
conditions can cause changes to the ECG. Because
the ECG is a fast, simple, painless and relatively
inexpensive test, it may be used as a part of
an initial examination to help the physician narrow
the scope of the diagnostic process. ECGs are
also done with routine physical examinations so
that comparisons can be made with previous ECGs
to determine if a hidden or undetected condition
might be causing changes in the ECG. Some conditions
which may cause changes in the ECG pattern may
include, but are not limited to, the following:
- ischemia - decreased
flow of oxygenated blood to an organ due to
obstruction in an artery.
- heart attack - also
called myocardial infarction; damage to the
heart muscle due to insufficient blood supply.
- conduction disorders
- a dysfunction in the heart's electrical
conduction system, which can make the heartbeat
too fast, too slow, or at an uneven rate.
- electrolyte disturbances
- an imbalance in the level of electrolytes,
or chemicals, in the blood, such as potassium,
magnesium, or calcium.
- pericarditis - an
inflammation of the sac (thin covering) that
surrounds the heart.
- valvular heart disease
- one or more of the heart's four valves
becomes defective, or may be congenitally malformed.
- enlarged heart - a
condition of the heart in which it is abnormally
larger than normal; can be caused by various
factors, such as valve disorders, high blood
pressure, congestive heart failure, conduction
disturbances, etc.
- chest trauma - blunt
trauma to the chest, such as a motorist hitting
the steering wheel in an automobile accident.
This
list is presented as an example. It is not intended
to be a comprehensive list of all conditions which
may cause changes in the ECG pattern.
An ECG
may also be done for the following reasons:
- to obtain a baseline
tracing of the heart's function (during
a physical examination). This baseline tracing
may be used later as a comparison with future
ECGs, to see if any changes have occurred.
- as part of a work-up
prior to a procedure such as surgery to make
sure a heart condition does not exist that might
cause complications during or after the procedure
- to check the function
of an implanted pacemaker
- to check the effectiveness
of certain heart medications
- to check the heart's
status after an MI, or after a heart-related
procedure such as a cardiac catheterization,
heart surgery, electrophysiological studies,
etc.
An ECG
is one of the simplest and fastest procedures
used to evaluate the heart. An ECG technician,
nurse, or physician will place 10 separate electrodes
(small plastic patches) at specific locations
on your chest, arms, and legs. Six of the electrodes
will be placed on your chest, and one electrode
will be placed on each arm and leg. The electrodes
are self-sticking and will adhere to the skin.
You will be lying down on a stretcher or bed,
and the leads (wires) will be connected to the
electrodes. You will need to lie very still and
not talk during the ECG procedure, as movement
or talking may interfere with the tracing. The
technician, nurse, or physician will start the
tracing, which will take just a few minutes. You
will not feel anything during the tracing. Once
a clear tracing has been obtained, the leads and
electrodes will be removed, and you will be free
to continue on with your usual activities, unless
directed otherwise by your physician. An ECG can
indicate the presence of arrhythmias (an abnormal
rhythm of the heart), damage to the heart caused
by ischemia (lack of oxygen to the heart muscle)
or myocardial infarction (MI, or heart attack),
a problem with one or more of the heart valves,
or other types of heart conditions.
There
are additional ECG procedures which are more involved
than the basic ECG. These procedures include the
following:
- Exercise ECG, or
stress test
The patient is attached to the ECG machine as
described above. However, rather than lying
down, the patient exercises by walking on a
treadmill or pedaling a stationary bicycle while
the ECG is recorded. This test is done to assess
changes in the ECG during stress such as exercise.
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