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SIGNAL AVERAGED ECG
INTRODUCTION
Signal-averaged electrocardiography
(SAECG) is a special electrocardiographic
technique, in which multiple electric
signals from the heart are averaged to
remove interference and reveal small
variations in the QRS complex, usually the
so-called "late potentials".
These may represent a predisposition
towards potentially dangerous ventricular
tachyarrhythmias
INTRODUCTION
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.
INTRODUCTION
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.
Procedure
A resting electrocardiogram (ECG) is recorded in
the supine position using an ECG machine
equipped with SAECG software; this can be done
by a physician, nurse, or medical technician.
Unlike standard basal ECG recording, which
requires only a few seconds, SAECG recording
requires a few minutes (usually about 7-10
minutes), as the machine must record multiple
subsequent QRS potentials to remove
interference due to skeletal muscle and to obtain
a statistically significant average trace.
For this reason, it is important for the patient to
lie as still as possible during the recording.
Significance
Late potentials are taken to represent delayed and
fragmented depolarisation of the ventricular myocardium,
which may be the substrate for a micro-re-entry
mechanism, implying a higher risk of potentially dangerous
ventricular tachyarrhythmia.
This has been used for the risk stratification of sudden
cardiac death in people who have had a myocardial
infarction, as well as in people with known coronary heart
disease, cardiomyopathies, or unexplained syncope.
Still, the real predictive value of these findings is
questioned. Late potentials may be found in 0-10% of
normal volunteers.
When used as a prognostic factor for the development of
ventricular tachycardia, they have a sensitivity of 72% and
a specificity of 75%, yielding a positive predictive value of
20% and a negative predictive value of 20%.
ADVANTAGE OF SAECG
Filtered ECG that is able to detect low amplitude potentials
filtered out of standard ECGs.
Myocardial scar (infarction, ARVD) creates zones of slow
conduction that appear as low amplitude late potentials on
SAECG. Areas of slow conduction are necessary
components for reentry.
Late potentials from within scar sometimes are not
detected in SAECG
– Bundle branch block delays depolarization ipsilateral to the site
of block. The delayed conduction may conceal late potentials
on SAECG.
– The base of the left ventricle is the last area to depolarize
when bundle branch block is not present. Inferior scar is easier
to detect on SAECG than anterior scar because the inferior
scar boarder zone is activated later than the anterior wall.
Therefore, the late potentials are not concealed by
depolarization in other areas of the ventricle.
Criteria for abnormal SAECG
Root mean squared voltage of the
terminal 40 msecs is less than 20
microvolts. This shows low voltage
potentials
late
in
ventricular
depolarization and reflect depolarization
in slowly conducting scar boarder zones
Total QRS duration greater than 114
msec
Duration of the low amplitude signal that
is less than 40 microvolts is greater than
38 msec