Localization of Characteristic Peaks in Cardiac Signal: A Simplified Approach

Fundamentals Of Ecg

An ECG is generated by a nerve impulse stimulus to heart. Contraction and relaxation of cardiac muscle occurs as a result of electrical changes within the myocardial cells, referred to as depolarisation and re-polarisation. Electrodes attached to the skin on the limbs and chest wall can sense this electrical activity and transmit it to an electrocardiograph. The electrocardiograph then converts this information into waveforms, which are recorded on graph paper to produce an electrocardiogram, commonly known as an ECG, [Ambu EKG guide for UK, http://www.ambu.co.uk], as demonstrated in Figure 1.

Figure 1.

The ECG signal

The human heart has four chambers -Two atria and two ventricles. The muscle mass of the atria is relatively small and the electrical changes accompanying the contraction are therefore equally small. Contraction (i.e. depolarisation) of the atria causes the ECG wave called P. Since the ventricular mass is large, there is a large deflection of the ECG when the ventricles contract (i.e. depolarise) and this is called the QRS complex. The T wave of the ECG is caused by the return of this ventricular mass to the resting electrical state called re-polarisation. Characteristics of these waves P, QRS, T for a normal ECG signal are listed below (Witsarut Srisawat, 2013):

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  Amplitude:

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    P-wave — 0.25 mV;

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    R-wave — 1.60 mV;

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    Q-wave — 25% R wave;

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    T-wave — 0.1 to 0.5 mV;

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  Duration:

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    P-R interval: 0.12 to 0.20 s;

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    Q-T interval: 0.35 to 0.44 s;

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    S-T interval: 0.05 to 0.15 s;

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    P-wave interval: 0.11 s;

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    QRS interval: 0.09s.