Electrocardiogram

ECG is the non-invasive method used to record the electrical activity of the heart over time by attaching electrodes to the surface of the patient’s skin. It is a diagnostic tool which provides information about the state of the heart. It is widely used by physicians to study heart function and to diagnose heart diseases [40]. ECG device records the electrical cardiac signals as characteristic waves which reflect different periods of the heart cycle. Figure 2.6 illustrates a typical ECG beat signal and its various waves.

2.1.3.1 ECG Waves and Interval

The normal ECG beat, known as Normal Sinus Rhythm (NSR), starts with a P-wave followed by a QRS complex and a T wave. The P wave is produced by the spread of

Figure 2.6: A typical ECG waveform (Adapted from [35]).

electric currents during the atrial depolarisation [40]. The QRS consists of the Q, R, and S waves and represents the electrical wave spreads over the ventricles during the ventricular depolarisation. The T wave represents the repolarisation of ventricles. The repolarisation of the atria occurs during ventricular depolarisation and is buried within the QRS complex, so it is not visible on ECG signal [41]. A U wave may be present after the T wave which represents the late repolarisation of Purkinje fibres in ventricles. The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The PR interval represents the time required for the electrical impulse to travel from SA node through the AV node and to enter the ventricles. The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The QT interval reflects the time for both ventricular depolarisation and repolarisation.

Analysing the shape and lengths of the ECG waves (morphologic features of ECG signal) provides crucial information about the cardiac electrophysiology. Any malfunc- tion in heart changes the morphological pattern of ECG signal and results in a change in the normal size and shape of the ECG waves. The medical experts can diagnose heart diseases and malfunctions of heart by checking the change in clinical signatures

Figure 2.7: Standard 12-lead ECG placement (Adapted from [43]).

of heart and comparing them to their nominal range. It worths to mention that other parameters such as age, sex and gene are also taken into account for the actual clinical diagnosis [42].

2.1.3.2 ECG Signal Acquisition

The electrical activity of the heart can be measured from the body surface using surface electrodes. The standard 12-lead ECG is the most common and accepted method for representation of the heart’s electrical activity [35]. The 12-lead ECG system uses ten electrodes to provides 12 different views of the heart. Four electrodes are placed on the limbs (right arm (RA), left arm (LA), right leg (RL), and left leg (LL)), and six electrodes are placed on the patient’s chest. Figure 2.7 depicts the location of these electrodes.

The 12-lead ECG consist of twelve leads: three standard limb leads (Leads I, II, and III), three augmented limb leads (Leads aVR, aVL, and aVF) and six precordial leads (leads V1, V2, V3, V4, V5, and V6). These leads are categorised in bipolar and unipolar. The bipolar leads are measured between two electrodes (positive and negative electrodes) and unipolar leads are measured between one electrode (positive electrode) and a reference point. The standard limb leads are bipolar while the augmented limb leads and the precordial leads are unipolar.

Einthoven [44] introduced the lead positions and connections for the standard (bipo- lar) limb leads. The electrodes are placed on the right arm, left arm and left leg of a person as shown in Figure 2.8. The standard limb leads are defined as:

I =ΦLA−ΦRA

II =ΦLL−ΦRA

III =ΦLL−ΦLA

(2.1)

where the VLA is the potential of the left arm, the VRA is the potential of the right arm, and the VLL is the potential of the left leg.

Goldberger developed the augmented (unipolar) limb lead electrode positions [45]. The position of electrodes in the augmented limb leads is similar to the electrode posi- tions in the standard limb leads. The electrode positions and connections for the three augmented limb leads are given in Figure 2.9. The augmented limb leads (aVL, aVR and aVF) are calculated as the potential difference between one of the electrodes and the average of remaining two electrodes:

Figure 2.8: Standard limbs lead positions of Einthoven (Adapted from [43]). aV R=ΦRA− ΦLA+ΦLL 2 aV L=ΦLA− ΦRA+ΦLL 2 aV F =ΦLL− ΦRA+ΦLA 2 (2.2)

Figure 2.9: Augmented lead positions and connections (Adapted from [43]). R stands for resistors which are usually set to 5 K.

across the chest as shown in Figure 2.10. These leads are defined as the potential differ- ence between one of the electrodes and a common reference point which is commonly called Wilson Central Terminal (WCT). The WCT is obtained by a combination of the three standard limb leads and can be calculated as follows:

ΦW CT =

ΦRA+ΦLA+ΦLL

3 (2.3)

Figure 2.10: Precordial lead positions (Adapted from [43]). R stands for resistors which are normally set to 5 K.

The combination of the six limb leads (the three standard limb leads and the three augmented leads) and the six precordial leads provides a three-dimensional view of the heart as shown in Figure 2.11. The six limb leads provide information about the electrical activity of the heart in the frontal plane, while the six precordial leads provide information about the electrical activity of the heart in the horizontal plane. Additional information on ECG lead systems can be found at [46].

Figure 2.11: Frontal and horizontal planes (Adapted from [46]).