Electrocardiogram

ECG is a graphical representation of electrical activities of the heart. Many studies investigated the characteristics of cardiac anomalies. The normal rate of heartbeat is composed by ECG waves depended on the mechanical action of the heart.

In general, 12-lead ECG is most frequently used to provide precise and useful information. In the measurement of 12-lead ECG, 10 electrodes are placed and twelve ECG signals are derived from these 10 electrodes. The electrodes positions of 12-lead ECG are described as follows:

(1) V1 - V6 are 6 points on the chest from the right of the breast bone extending sideways towards the left hand border of the rib cage. The location of electrode V3 should be placed on the middle between electrodes V2 and V4.

The locations of electrodes V4, V5 and V6 should be placed at the same horizontal height as V4, as shown in Fig.2- 2.

(2) Right arm lead (RA), left arm lead (LA), right leg lead (RL) and left leg lead (LL) are on the right and left arm below the shoulder and on right and left legs below the groin respectively. RA, LA, RL, and LL are usually used to measure lead-I, II, and III ECG, as shown in Fig.2- 3. Here, lead-I ECG is the measurement from RA to LA. Lead-II ECG is the measurement from RA to

LL, and lead-III ECG is from LA to LL.

Fig.2- 2 Electrode positions for 6-lead ECG 　[26]

Although 12-lead ECG provides more precise and useful information in clinical diagnosis, it is not conveniently and easily used at home. Moreover, the attachment of 10 electrodes on the patient will cause the inconvenience of movement. In this study, we monitored long-term lead-II ECG, and focused on R-Wave detection. In general, the waveform of ECG consists of three principal components: P-wave, QRS complex and T-wave, as shown in Fig.2- 4.

Fig.2- 3 ECG lead positions of limp 　[26]

The waveform of P-wave represents auricular depolarization. The complex QRS corresponds to ventricular depolarization preceding the mechanical effect of contraction, the wave R is of a great amplitude because the mass of the ventricles is higher than that of the auricles. The T wave represents the repolarization of the ventricles. Lastly, the wave U, not constant, translates the repolarization of the network of Purkinje 　[25]. These ECG components are listed as below:

(1) P wave: This deflection is due to the atria depolarization

(3) Q wave: This first negative deflection is caused by the ventricle depolarization.

The first positive deflection (R) follows.

(4) R wave: This first positive deflection caused by the ventricle depolarization.

(5) S wave: This first negative deflection follows the first positive deflection (R) during the period of ventricle depolarization.

(6) T wave: This deflection is due to the ventricle repolarization.

(7) U wave: The deflection succeeds the T wave and leads the next P wave. (A positive wave in usual) The mechanism generating this wave forms is unclear yet. One proposal referred this phenomenon to the slower repolarization of the conducting system among ventricles (Purkinje fibers).

The characteristic of ECG intervals are introduced briefly as follows:

(1) RR interval: RR interval stands for the time interval between two successive R waves. The BPM (beat per minute) can be derived by 60/RR interval (60 divided by RR interval).

(2) PR interval: This is the period of AV conduction mainly. The period consists of some processes as follows: (i) atria depolarization (ii) normal conduction delay in AV node (about 0.07 sec) (iii) the period of depolarization waves passing Bundle of His and its branches until the beginning of ventricle depolarization.

The normal value of PR interval is usually from 0.12sec to 0.2sec.

(3) QRS interval: This period represents the whole time that ventricle depolarization takes. The normal value of QRS interval is usually from 0.04sec to 0.11sec.

(4) QT interval: The period is from the starting point of Q wave to the ending of T wave. It stands for the period of electric power contracting of heart.

(5) ST interval: The period is from the ending of QRS complexes to the starting point of T wave. The point connecting the QRS complexes and ST interval is called junction J.

Fig.2- 4 Typical ECG with actions　[43]

In general, the traces of cardiac anomalies are appeared in one or more parameters related to ECG waveform. These waves are related to the activity of the auricles and the ventricles in the form of activation or recuperation. The contraction of the heart muscles (of the heart) produces a visible wave of depolarization in the layout of the ECG and the return to the state of the rest constitutes a wave of repolarization represented by an iso-electric voltage constituting the line base.

The slowly heart beats are the Bradycardia and the accelerated rhythm corresponds to the Tachycardia. The Ventricular Tachycardia is marked following a scar of a former heart attack. Auricular Fibrillation describes completely chaotic activation in the auricles. In this case, the production of the fluctuations is irregular in the line base whereas complex QRS remains normal.

The frequencies of each wave present some variations according to the rate of heartbeat. The change of the rhythm of the beat is called Arrhythmia. The frequency band of the ECG signal is approximately 60 Hz for a normal subject and can go up to 130 Hz for an abnormal patient. Table 2- 2 showed the actions of membranes of the heart in the Normal case and the associated waves like their temporal and frequencies

characteristics.

Table 2- 2 Heart actions and electrical characteristics of a normal arrhythmia

Heart Actions Auricular depolarization P ^{≤ 0.3 80-120} 10

Depolarization of the ventricle

Repolarization of the ventricles T 0.2 ²⁰⁰ 5