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Cardiological signal processing
Origin of ECG ± The ECG is measured using electrodes attaches to the body surface and
connected to the instrumentation (ECG) amplifier. The output of the ECG will be positive for
the points in the time that cardiac vector points towards the electrode connected to the
positive terminal of the amplifier; otherwise the output will be negative.
The time varying motion of the cardiac vector produces the body surface ECG for one heart
beat, with each heart beat this ECG inscribes a series of deflection that are labelled as
P,Q,R,S,T and U as shown in fig
Pwave ± Represents atrial depolarization.
Q,R,Swave ± Reflects ventricular depolarisation.
Twave ± corresponding to ventricular repolarisation.
Uwave ± undetermined origin.
P,R interval ± From the onset of the Pwave to the onset of the Q,R,S complex reflects the time
required for the conduction of the cardiac impulse from the Sa node to the ventricles.
Q,T interval - Reflects or represent the time duration required for both ventricular
depolarization and repolarisation.
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TP segment - R epresent t e time segment corresponding to t e per iod when the hear t is
completel repolar ised.
For normal recording of EC , which is a plot of voltage / time a cali ration of 10 mm / mv
and paper speed of 25 mm / sec.
The def lection in the EC w/f may also understood using the concept of an electr ical di pole
which is def ined as a shor t distance across the di pole can be represented by vector indicating
the magnitude and duration.
Fig - R epresents the slow moving depolar isation of the atr ial that begin at the SA node gives
r ise to the P wave. The is delayed in the AV node as shown in f ig b, Which gives r ise to the
iso electr ic region af ter the P wave.
As park ing system star ts deliver ing the stimulus to the ventr icular muscle, The Q wave makes
its appearance. A rapid depolar i ation of the ventr icular muscle is responsi ble for the
production of the fast moving vector which produces the R wave as shown in f ig c. The peak
of the R wave is attained when most of the cells are depolar i ed as shown in f ig d. The f inal
phase of the ventr icular depolar i ation occurs as excitation spreads toward the base of the
ventr icles gives r ise to the S wave.
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ECG QRS DETECTION TECHNIQUES
QRS detection is an important and integral part of any sophisticated ECG processing system.
There are different techniques are used for detection of ECG QRS
1) Template techniques
Template cross correlation and subtraction methods requires standard pattern, Then compared
against it for observing the similarity or dissimilarity.
- Template cross correlation method
This method is based on the concept of correlation. Signal to be correlated if their
wave shapes match or similar.
In this method the QRS template is cross correlated with the incoming signal. For
implementing this method we need to define cross correlation function and correlation
coefficient.
Thus the cross correlation function estimate r xy(m) of the sequences x(n) and y(n),
defined by
r xy(m) =
The cross correlation coefficient xy at lag m is defined as
xy(m) =
Here the QRS template thought of as a window that moving over the incoming signal point
by point. During this process the value of the cross correlation coefficient always falls
between +1 and -1.
+1 ± represents perfect match between signal and template.
-1 - represent a perfect mismatch.
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And any value in between is indicative of closeness of the shapes of two waveforms.
Since the objective of this exercise is to estimate if and when the QR S complex occurred, one
is not concerned with the shape or amplitude of the resulting cross correlation function.
- Template subtraction
In this method the QR S template is compared with the incoming signal and difference between them is noted.
Here the segment of the incoming signal is stored and each point of this segment is subtracted
from the corresponding point of the template and difference in values noted. This difference
will be closed to zero when the QR S complex of the incoming EC signal is aligned with the
template. And the number of such zeros is a measure of the QR s duration.
Advantages
1) Involves only the number of sample points that constitutes the template.
2) No multi plication and summation.
Disadvantages
1) The signal to noise ratio of the incoming signal needs to be verylarge.
2) Pre-processing is required before the method can be applied.
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2) Differentiation technique
- Simple high speed QR S width detection algor ithm
Differentiation forms the basis of many QR S detection algor ithm .since it is basically a high
pass f ilter, The der ivative amplif ies the higher frequencies character istics of the QR S
complex while attenuating the lower frequencies of the P and T waves. This pr inci ple is
exploited in the two methods.
The absolute values of the 1st and 2nd der ivative are calculated by
Y0(nT) = | x(nT) - x(nt-2T)|
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Y1(nT) = |x(nT) ± 2x(nT-2T)+x(nT-4T)|
These two data buffers Y0(nT) and Y1(nT) are scaled and then summed.
c = 1.3 Y0(nT) +1.1 Y1(nT)
Once this condition is met for a data point Y2(nT), The next eight points meet or exceed the
threshold, then the segment might be a part of the QRS complex.
Advantage : 1) It produces a pulse which is proportional in width to the complex.
Disadvantage: it is very much sensitive to high frequency noise.
- A high speed QRS detection algorithm.
It recognizes QRS complex based on analysis of slope, amplitude and width. In order to
attenuate noise, the signal is passed through a band pass filter composed of cascaded LPF and
HPF. Subsequent processes are differentiation, Squaring and low averaging of the signal.
The LPF and HPF isolates the predominant QRS energy and the higher frequency associated
with EMG noise and power line interference.
Differentiation is used for fencing the high slopes that normally distinguish the QRS
complexes from other ECG waves.
Squaring is used to perform to make the data positive prior integration.
Moving window integrator is used to maintain time sequence.