SAECG

1. SIGNAL AVERAGED ECG
RAMAKRISHNA JANAPATI
NIZAMS INSTITUTE OF MEDICAL
SCIENCES

2. • The signal-averaged electrocardiogram (SAECG)
is a computerized technique for detecting subtle
abnormalities in the surface electrocardiogram
(ECG) that are not visible to the naked eye.
• The SAECG is derived by computing the
arithmetic mean of multiple ECG complexes

3. High Resolution Electrocardiography
 A high-resolution ECG detects very low amplitude signals
in the ventricles called 'Late Potentials' in patients with
abnormal Hearts.
 A standard electrocardiogram cannot detect these
signals. These signals are embedded in the ECG but
ordinarily obscured by skeletal muscle activity and other
extraneous sources of "noise" encountered in recording a
standard ECG.
 The presence of late potentials is widely accepted to
have prognostic significance in patients after AMI

4. SAECG
 The ECG is a graphical representation of the electrical
potentials generated by the heart
 Based on the resolution of the digital recording of analog
ECG signals, the instruments & techniques may be
categorized into 2 types:
♦ 1) Low-resolution (or standard) ECG, and
♦ 2) High-Resolution ECG (HRECG)
 A standard 12-Lead ECG is a typical example of a widely
used low-resolution instrument that records 10 sec of
cardiac data

5. A SAECG is a typical example of a High-
Resolution ECG
SAECG records ventricular ECG signals of very
low magnitudes called 'Ventricular Late
Potentials' (VLP) by averaging a number of
signals (QRS)
The presence of VLPs is indicative of risk for
subsequent occurrence of arrhythmic events,
mainly VT

6. SAECG / Indications
Vulnerability to VT
risk stratification post-MI
Unexplained syncope
Post-operative patients

7. • The goal of these SA ECG techniques is to detect
occult derangements of ventricular activation, or
late potentials, present during sinus rhythm that
appear to be a hallmark for sustained ventricular
arrhythmias
• The ‘‘noise’’ in orthogonal ECGs ranges from 8 to
10 Mv and is generated primarily by skeletal
muscle activity

8. • The temporal and spectral features of ECGs that
identify patients with VT are masked by this level
of noise
• The purpose of signal averaging is to improve the
signal-to noise ratio to facilitate the detection of
low-amplitude bioelectric potentials
• Signals may be averaged by temporal or spatial
techniques

11. SAECG
The high-pass filtering used to record late potentials
in relation time is called time domain analysis
because the filter output corresponds in time to the
input signal.
Because late potentials are high-frequency signals,
Fourier transform can be applied to extract high-
frequency content from the signal-averaged ECG,
called frequency domain analysis.

12. Temporal technique
• The following requirements must be met for
temporal averaging to work effectively
• First, the signal of interest must be repetitive
and invariable.
• Time varying signals, such as ectopic or
premature complexes, are eliminated before
averaging by comparing incoming signals
against a previously established template

13. • Second, the signal of interest must be time-locked
to a fixed point, such as the peak of the QRS
complex, that is easily detectable and serves as a
timing reference for the averaging algorithm.
• Third, the signal of interest and the noise must be
independent and remain independent during
averaging.
• Current systems reduce noise to < 1.0 micV.

14. • Most signal processing systems use time-domain
analysis to detect late potentials in the terminal
QRS complex.
• Detection of these microvolt waveforms, which
are continuous with the QRS complex, requires
high-gain amplification and appropriate digital
filtering to reject low frequencies associated with
the plateau and repolarization phases of the
action potential, ST segment and T wave.

15. • Orthogonal, bipolar XYZ ECG leads are recorded,
averaged, filtered and combined into a vector magnitude
called the filtered QRS complex.
• Analysis of the filtered QRS complex typically includes
1) the filtered QRS duration
2) the root-mean-square voltage of the terminal 40 ms of
the filtered QRS
3) the duration that the filtered QRS complex remains ,40
mV.
• Values of these measurements are dependent on the
high pass corner frequency.
• Filter frequencies of 25 to 100 Hz have been
investigated; most recent systems use a 40-Hz high-pass
filter.

16. • Bipolar X, Y, and Z leads are used to record
approximately 250 ECG cycles.
• The SAECG literally averages multiple QRS
complexes that are then digitalized and
filtered and further processed with spectral
analysis to eliminate noise

17. An HRECG instrument consists of 4 key components: 1) Amplifiers, 2) Bandpass filters,
3) Analog/Digital converter, and 4) SAECG Processor. The SAECG Processor may in turn
be functionally divided into the following components: a) Signal Averager, b) Bidirectional
Bandpass Filter, c) Filtered Vector Magnitude, & d) SAECG Quantifier. In addition, the
instrument includes 7 ECG leads. These leads are bipolar, orthogonal electrodes comprising
X+, X-, Y+, Y-, Z+, Z-, & ground placed in a particular fashion on the body surface.
These electrodes are usually referred to as XYZ leads.

18. SAECG/ Frequency Analysis
A sequence generated by sampling a time-domain
signal like the ECG can be represented in the frequency
domain by taking the fast Fourier transform.
Spectral analysis considers the QRS complex (or P-
wave) to be composed of multiple simple waveforms,
typically sinusoids.
Spectral analysis thus decomposes the QRS complex (or
P-wave) into these constituent signals for analysis
The Fourier transform is a complete description of the
ECG and contains information that may not be seen in
the output of a particular fixed-band filter.

19. Frequency analysis offers potential advantages
for identification and characterization of signals
that differentiate patients with from those
without sustained ventricular tachycardia.
Most studies have calculated the fast Fourier
transform to estimate scalar-lead spectra of the
terminal QRS and ST segment of signal-averaged
Frank X, Y, Z or uncorrected orthogonal leads.
The results have often been expressed as
indexes of the relative contributions of specific
frequencies that comprise these ECG segments.

20. Key issues that affect the spectra of ECG signals are
being investigated. For example, the frequency content
of ECG signals is spatially variable and thus lead
dependent.
Indexes derived from spectra of uncorrected leads
may not be comparable to end points or approaches
developed using corrected leads.
Analysis of multiple segments (spectro temporal
mapping) may allow better separation between noise
and late potentials
SAECG/ Frequency Analysis

21. SAECG/Noise
 Noise be measured in the averaged signal over an interval
of at least 40 msec in the ST or TP segment with a four-pole
Butterworth filter.
 With this approach, noise should be < 1 μV with a 25 Hz
high-pass cutoff or < 0.7 μV with a 40 Hz high-pass cutoff
as measured by the root mean square method from a
vector magnitude of the X, Y, and Z leads.
 The segment for noise level analysis should be determined
automatically
 The inherent noise level of the recording should be low so
that adequate noise reduction can be achieved by
averaging 50-300 beats.
 Averaging a greater number of beats to obtain adequate
noise reduction indicates that baseline noise is excessive
for optimum recording.

22. • Averaging multiple QRS complexes (or P-
waves) "reinforces" the consistent ventricular
or atrial components while diminishing the
inconsistent noise components, thus
improving the signal-to-noise ratio

23. Successive QRS complexes meeting a predetermined coefficient of similarity are aligned
(within the window indicated by dashed lines) on an ongoing basis. Their rolling arithmetic
mean is computed until a predetermined noise threshold is achieved (shown as the typical
criterion of the standard deviation of the TP segment <1 V). The QRS complex is then filtered
and analyzed for late potentials, defined when the filtered QRS duration is >114 ms, root-
mean-square (RMS) voltage in the terminal 40 ms is <20 mV, and low amplitude signal (LAS)
duration (the terminal signal duration from 40 mV to isopotential) is >38 ms.

24. Late Potentials
 Ventricular late potentials in patients with cardiac
abnormalities, especially CAD or following an acute MI ,
are associated with an increased risk of VT and SCD.
 Proponents of SAECG claim that it can obviate the need
for invasive techniques commonly used to identify high-
risk patients for interventions that treat or prevent
ventricular tachyarrhythmia and sudden death.

25. SAECG
 The current data on SAECG show relatively consistent high
negative predictive values, poor positive predictive values,
and variable sensitivity and specificity when the technique
is used on pts with Cardiomyopathy or following a MI
 The available evidence also indicates that combining SAECG
with other tests of cardiac function is superior to using any
single test for risk.
 The utility of SAECG alone as an indicator of risk remains to
be proven.

26. SAECG combined with other standard tests of
risk has been demonstrated to have clinical
utility in patients following an acute MI.
Other patient populations have not been
conclusively shown to benefit from its use

27. SAECG: Normal (left) and abnormal (right) results are shown from a patient with
prior MI and VT.
Bottom panels: Shaded blue areas at the end of each tracing represent voltage
content of last 40 ms of the filtered QRS integral. The small shaded area in the
abnormal study denotes prolonged, slow conduction and suggests the potential for
reentrant ventricular arrhythmias.

28. Late Potentials
One of the constituents of reentrant ventricular
arrhythmias in patients with prior myocardial damage
is slow conduction.
Direct cardiac mapping techniques can record
myocardial activation from damaged areas that occurs
after the end of the surface ECG QRS complex during
sinus rhythm.
These delayed signals have very low amplitude that
cannot be discerned on routine ECG and correspond to
the delayed and fragmented conduction in the
ventricles recorded with direct mapping techniques

29. SAECG
 Signal averaging has been applied clinically most often to
detect such late ventricular potentials of 1 to 25 μV
 Criteria for late potentials are
(1) filtered QRS complex duration (QRSD) >114 –120 ms,
(2) < 20 μV of root-mean-square (RMS) signal amplitude in
the last 40 ms of the filtered QRS complex, and
(3) the terminal filtered QRS complex remains below 40 μV
(low amplitude signal-LAS) for longer than 38 ms

30. Time-Domain Analysis: Results of most studies have been based on analysis of a vector
magnitude of the filtered leads, √x2+y2 +z2, called the filtered QRS complex.
The end of the filtered QRS complex is defined as the midpoint of a 5 msec segment in
which mean voltage exceeds the mean noise level plus 3 times the standard deviation
of the noise sample.
The end point and onset of the filtered QRS complex should be verified visually, and
the system should allow manual adjustment of the automatically determined
end points.

33. Late Potentials
These late potentials have been recorded in 70 –
90% of patients with spontaneous sustained and
inducible VT after myocardial infarction,
in only 0 to 6 % of normal volunteers, and
in 7 to 15 % of patients after myocardial infarction
who do not have VT

34. Late Potentials
Late potentials can be detected as early as 3 h after the
onset of chest pain, increase in prevalence in the first
week after MI, and disappear in some patients after 1
year.
If not present initially, late potentials usually do not
appear later.

Early use of thrombolytic agents may reduce the
prevalence of late potentials after coronary occlusion
Patients with BBB or paced ventricular rhythms have
wide QRS complexes already, rendering the technique
less useful in these cases.

35. Late Potentials
Late potentials also have been recorded in patients
with VT not related to ischemia, such as dilated
cardiomyopathies.
Successful surgical resection of the VT can eliminate
late potentials but is not necessary to cause
tachycardia suppression.
The presence of a late potential is a sensitive, but
not specific, marker of arrhythmic risk and thus its
prognostic use is limited

36. Late Potentials
In specific situations, LPs can be helpful;
EX….for a patient with a prior IWMI
(normally the last portion of the heart to be
activated) who has no late potential has a
very low likelihood of having VT episodes.

37. • The SAECG can be useful to risk stratify the
following populations for sudden cardiac death
Post-myocardial infarction patients
Structural heart disease patients with
depressed heart function
Arrhythmogenic right ventricular dysplasia
(ARVD)
Brugada Syndrome

38. SAECG
 Some data suggest that frequency domain analysis
provides useful information not available in the time
domain analysis.
 Signal averaging has been applied to the P wave to
determine risk for developing atrial fibrillation as well as
maintenance of sinus rhythm after cardioversion.
 Studies suggest that a prolonged SAECG P-wave, equivalent
to "atrial late potentials", may identify patients at risk for
atrial fibrillation
 The overall use of the technique remains limited at
present.

44. • SAECG is an high resolution ECG technique
which can be used to risk stratify VT patients
especially post MI patients