Localization of Characteristic Peaks in Cardiac Signal: A Simplified Approach

Localization of Characteristic Peaks in Cardiac Signal: A Simplified Approach

Subash Khanal (Center for Medical Electronics and Computing, M.S. Ramaiah Institute of Technology, Bangalore, India) and N. Sriraam (Center for Medical Electronics and Computing, M.S. Ramaiah Institute of Technology, Bangalore, India)
Copyright: © 2015 |Pages: 14
DOI: 10.4018/IJBCE.2015010102
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Abstract

Cardiovascular system study using Electrocardiogram (ECG) signals have evolved tremendously in the research domain of medical electronics and signal processing community. The extraction of characteristic points of an ECG signal helps in detection of any irregularities if present. This paper attempts to provide the basic understanding of ECG signal, pre-processing and extraction of fiducial points P, Q, R, S, T and their respective amplitude. Continuous Wavelet Transform (CWT) has been used for the localization of R peaks. Finally, other peaks are localized in time plane, using inter-beat interval dependent search windows. The proposed method was tested using ECG signal data obtained from MIT Physionet-ATM database and Signals derived from medical instrumentation lab of M.S. Ramaiah Institute of Technology. The simulation results were quite promising in terms of accurate localization of characteristic peaks, their respective amplitude in time plane and other features like R-R intervals, QRS width, S-T segment length, etc. The simplified procedure proposed in this study shows clear indication for arrhythmias detection which can be extended for automated ECG signal classification by appropriate selection of pattern classifiers.
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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):

  • Amplitude:

    • o

      P-wave — 0.25 mV;

    • o

      R-wave — 1.60 mV;

    • o

      Q-wave — 25% R wave;

    • o

      T-wave — 0.1 to 0.5 mV;

  • Duration:

    • o

      P-R interval: 0.12 to 0.20 s;

    • o

      Q-T interval: 0.35 to 0.44 s;

    • o

      S-T interval: 0.05 to 0.15 s;

    • o

      P-wave interval: 0.11 s;

    • o

      QRS interval: 0.09s.

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