cardiological signal processing

8/6/2019 Cardiological Signal Processing

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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.



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.



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.



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.



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)|



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.

cardiological signal processing

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