ECG

BY:Kiran R (1RV10EE026)Sunil Fernandes (1RV10EE52)Suraj K ( 1RV10EE053)

Introduction The electrocardiogram (ECG) is a time-varying

signal reflecting the ionic current flow which causes the cardiac fibers to contract and subsequently relax. The surface ECG is obtained by recording the potential difference between two electrodes placed on the surface of the skin. A single normal cycle of the ECG represents the successive atrial depolarisation/repolarisation and ventricular depolarisation/repolarisation which occurs with every heart beat.

Simply put, the ECG (EKG) is a device that measures and records the electrical activity of the heart from electrodes placed on the skin in specific locations

Basic Working The ECG is nothing but the recording of the hearts electrical

activity. The deviations in the normal electrical patterns indicate various cardiac disorders. Cardiac cells, in the normal state are electrically polarized. Their inner sides are negatively charged relative to their outer sides.

These cardiac cells can lose their normal negativity in a process called depolarization, which is the fundamental electrical activity of the heart.

This depolarization is propagated from cell to cell, producing a wave of depolarization that can be transmitted across the entire heart. This wave of depolarization produces a flow of electric current and it can be detected by keeping the electrodes on the surface of the body.

Once the depolarization is complete, the cardiac cells are able to restore their normal polarity by a process called re-polarization.

• The earlier method of ECG signal analysis was based on time domain approach

• But this is not always sufficient to study all the features of ECG signals. So, the frequency domain analysis is made use of

• To accomplish this, FFT (Fast Fourier Transform) technique is applied.

Measuring ECG ECG commonly measured via 12

specifically placed leads

Typical ECG A typical ECG period consists of P,Q,R,S,T

and U waves

P,Q,R,S,T Theory•ECG is composed of 5 waves - P, Q, R, S and T. This signal could be measured by electrodes from human body in typical engagement. •Signals from these electrodes are brought to simple electrical circuits with amplifiers and analogue – digital converters.

ECG Waves P wave: the

sequential activation (depolarization) of the right and left atria

QRS comples: right and left ventricular depolarization

T wave: ventricular repolarization

U wave: origin not clear, probably ”afterdepolarizations” in the ventrices

ECG Example

Elements of the ECG:

• P wave

• Depolarization of both atria;

• Relationship between P and QRS helps distinguish various cardiac arrhythmias

• Shape and duration of P may indicate atrial enlargement

QRS complex:

• Represents ventricular depolarization

• Larger than P wave because of greater muscle mass of ventricles

• Normal duration = 0.08-0.12 seconds

• Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc.

• Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI

PR interval

• From onset of P wave to onset of QRS

• Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)

• Represents atria to ventricular conduction time (through His bundle)

• Prolonged PR interval may indicate a 1st degree heart block

ST segment:

• Connects the QRS complex and T wave• Duration of 0.08-0.12 sec (80-120 ms)

T wave:

• Represents repolarization or recovery of ventricles• Interval from beginning of QRS to apex of T is referred to as the absolute refractory period

QT Interval:• Measured from beginning of QRS to the end of the T wave• Normal QT is usually about 0.40 sec• QT interval varies based on heart rate

Need for using DSP A number of emerging medical applications not only

electrocardiography (ECG), but also digital stethoscope, and pulse oximeters require DSP processing performance at very low power.

The main problem of digitalized signal is interference with other noisy signals like power supply network 50 Hz frequency and breathing muscle artefacts.

These noisy elements have to be removed before the signal is used for next data processing like heart rate frequency detection.

Digital filters and signal processing should be designed very effective for real-time applications in embedded devices.

Steps Involved Signal acquisition : ECG signal for digital signal

processing and heart rate calculation is acquired by a measurement card of known sampling frequency. Analogue signal pre-processing is done by a simple amplifier circuit designated for ECG signal measurement.

Raw signal acquired by measuring card with simple ECG amplifier circuit.

Digital signal processing with digital filters: In this part there is described noise elements filtering and baseline wander elimination with digital filters. The main noise elements are power supply network 50 Hz frequency and breathing muscle movements.

Filtered Waveforms

Signal after network 50 Hz and baseline wander filtering..

Energy signal after R-peaks filtering.

Heart rate detection algorithms: Two Types – 1) Statistical Computing

2) Differential ComputingThese algorithms compute heart rate frequency from the signal energy.

Autocorrelation method can be used because the ECG signal is quasi-periodical. Matlab provides a very simple way of using autocorrelation method in signal processing which is very useful for this purpose.

The second algorithm aims at detecting heart rate as a difference between R waves in ECG. These waves are filtrated by band pass filters firstly and then the signal energy is computed. The wave’s peaks are detected by peak detector or signal thresholding.

[1] Autocorrelation of energy signal

Autocorrelation function of signal energy

[2] Thresholding of energy signal

Which method to adopt? From the Figures we can see that the differential

computing methods yield better results than statistical computing method in a way that they adapt well to real-time processing simulation.

The main problem of autocorrelation function algorithm is that the quality of signal is under par and fluctuates for fast signal changes.

More about Digital Signal Processors Texas Instruments [TI] is a pioneer producer of

DSPs. The TMS320C5515 is the most widely used DSP

in Electro-Cardiograms owing to its good performance and low power consumption.

The TMS320C5515 Digital signal processor (DSP) board

DSP Software Architecture

Front end Architecture

• Ischemic Heart Disease

• AV node blocks Detected on ECG

• Arrhythmias Detected on ECG

• Fibrillation Detected on ECG

Diseases generally diagnosed using ECG

• www.ecglibrary.com/

• library.med.utah.edu/ecg/

• dsp.ti.com