A human heart consists of four chambers which are right and left, atria and ventricles, responsible for the collection and transport of blood throughout the body. The right side of the heart collects the deoxygenated blood from the systemic veins and pumps it towards the lungs whereas the left side collects the oxygenated blood from the pul- monary veins and pumps it to the rest of the body [44].
Cardiac excitation involves the generation of electrical impulses, referred to as APs, by individual cells due to the electrical current flow across the cell membranes and their conduction to neighbouring cells [45]. The conduction system of the heart con- sists of three groups of specialized cells which are known as the Sinoatrial (SA) Node, Atrioventricular (AV) Node and bundle of HIS. Cells within the SA node, also known as Pacemaker cells, which are located on the right atrium, have the fastest rate of AP generation and drive the rest of the heart at this rate [46]. Thus, electrical activation of the heart starts within the SA node which fires electrical impulses that propagates through the AV Node, bundle of HIS down to the Purkinje Fibers [46].
potentials generated by these specialised group of cells through the conduction system during a cardiac cycle [47]. Simplified anatomy of the heart and the generated AP shapes and durations for specialised cardiac cells are illustrated in Figure 2.1. Here it is depicted that different parts of the cardiac conduction system give rise to APs with different shapes and durations, at different times and different locations. In this figure, the APs generated by each group of cells are colour coded and their contribution to an ECG waveform (bottom right corner) during a cardiac cycle can be clearly seen. For instance, the two pink APs generated by the ventricular muscles contribute towards the generation of the QRS complex and the T wave which are important features of the ECG signal and will be explained in the section below.
SA node Atrial muscle AV node Common bundle Bundle branches Purkinje fibres Ventricular muscles 0 100 200 300 400 500 600 700 Time, msec Atria Ventricles
Figure 2.1: Conduction system of the heart and generation of a ECG signal by the temporal and spatial summation of APs [47].
ECG Waveform Morphology
ECG signals reflect the cardiac activation of the heart measured between any two points on the body surface using electrodes. A standard clinical ECG signal is composed of waves at different frequency bands, each reflecting the electrical activation of different parts of the heart. These waves are known as the P waves, QRS complex (combination of Q, R and S waves), and T waves, representing the atrial depolarization, ventricular depolarization and repolarization, respectively. The segments and intervals between these waves such as ST segment as well as PQ/PR interval, QRS width, QT and RR interval also represent different cardiac events and carry diagnostic information. Figure 2.2 depicts a typical ECG waveform where each wave and interval is presented.
Time, msec -1 -0.5 0 0.5 1 Am p li tu d e, m V P Q R S T P Q R PQ/PR interval QRS width ST segment QT interval RR Interval
Figure 2.2: Time domain features of an ECG signal: P, QRS, and T represent atrial depolarization, ventricular depolarization, and atrial and ventricular repolarization respectively [48]
These waves have extremely low amplitudes ranging from 100 µV to 5 mV and low diagnostic frequency bandwidth between 0.05 to 100 Hz [49]. The standard clinical features of the ECG waves for a healthy adult male in sinus rhythm are presented in Table 2.1 [49] and are dependent on several factors such as age, gender, heart rate, respiration patterns and diseases [49].
Table 2.1: Normal values of a Lead II ECG features of a healthy subject in sinus rhythm at 60 BPM [49].
Feature Normal Value Normal Limit
P width 110 ms 20 ms PQ/PR interval 160 ms 40 ms QRS width 100 ms 20 ms QT interval 400 ms 40 ms ST segment 70 ms 10 ms P amplitude 0.15 mV 0.05 mV QRS height 1.5 mV 0.5 mV ST level 0 mV 0.1 mV T amplitude 0.3 mV 0.2 mV
ECG Recording Techniques and Applications
Standard clinical ECG instrumentation consists of ten surface electrodes placed on the chest, limbs and left leg. This system records 12-lead ECG which is the most commonly used method for diagnostic purposes. In wearable WBAN systems usually 1-,3- and 6-lead ECGs are recorded by employing two and three electrodes respectively, in order to minimize the system complexity and power dissipation [34]. The ECG is a fundamental component in patient monitoring and diagnosis, thus accurate ECG signal acquisition and its precise analysis are of great importance and has been subject to numerous research work [33–41, 50]. Diagnosis of the cardiac health conditions mostly rely on the assessment of ECG data based on the interbeat timing and wave amplitudes. Different types of arrhythmias can be distinguished by the morphological and beat- to-beat interval variations and/or missing beats. These morphological abnormalities sometimes can be fatal and often occur sporadically which require long-term and real- time monitoring [26, 33, 34, 38, 40]. ECG is also employed in cardiac implants such as Pacemakers. The detection of abnormalities is used for triggering stimulations by the