1. Introduction
U wave: It is generally not found in most of the ECG waveforms. U wave is a small deflection after T wave. In abnormal conditions, U wave amplitude may get inverted or becomes tall with amplitude of 0.2 mV or more.
RR interval: The duration between one R wave in one heart beat to the neighboring R wave is termed as RR interval. It is the heart rate, which represents the duration of cardiac cycle.
The bandwidth of ECG signal is 0.05 to 100 Hz. In some pathological cases, it may also go up to 120 Hz.
1.2 Multi-channel/Multi-lead ECG
+
+
V1 V4
V5
+
aVR
V6
II III
RA LA
LL
V3 aVL V2
aVF
Figure 1.2: Lead configuration for 12-channel MECG recording. Einthoven’s triangle and Wilson’s central terminal are also shown.
synthesized with the help of limb leads using the following linear relations:
aV R= (Lead I+Lead II)/2 aV L=Lead I−(Lead II)/2 aV F =Lead II−(Lead I)/2 .
Wilson’s central terminal is also taken as reference for the chest leads. For recording of chest leads, there are standardized positions on the chest. For example: electrodes are placed at the fourth intercostal space just to the right and left of the sternum, respectively, to record signals from lead V1 and V2. ECG signal at lead V4 is recorded by placing the electrode at the fifth intercostal space at the midclavicular line. Lead V3 is placed between lead V2 and lead V4. For V5 and V6, electrode positions remain at the same level as in the case of lead V4 but positions are at anterior axillary line and midaxillary line.
1. Introduction
Superior
Posterior
Anterior
Inferior 900 aVF 1200
III 600
II
00 I 1500
aVR
300 aVF
L A T E R A L
L A T E R A L
(a)
V1
V2 V3
V4 V6
V5
(b)
Figure 1.3: (a) Electrical axes for vertical directional views of the heart for bipolar limb leads and augmented
1.2 Multi-channel/Multi-lead ECG
Table 1.1: Orientations of important ECG waveforms in different leads of 12-lead MECG with different views of heart
Leads P Wave QRS-complex T wave Views
I upright upright upright lateral
II upright upright upright inferior
III upright upright upright inferior
aVR negative negative negative/upright right atrium/left ventricle
aVL upright upright upright lateral
aVF upright upright upright inferior
V1 upright/biphasic small R wave/QS upright septum
V2 upright/biphasic small R wave/QS upright septum
V3 upright equiphasic/QRS upright upright anterior
V4 upright upright upright anterior
V5 upright upright upright lateral
V6 upright upright upright lateral
1.2.1 Clinical significance of MECG
The human heart is a three-dimensional organ. Therefore, viewing it from all the directions is impor- tant from diagnostic point of view. The clinical 12-lead ECG system captures electrical activities of heart from different narrow angles in the three-dimensional space. A three-dimensional view of heart with the direction of different leads is shown in Figures 1.3(a) and 1.3(b). Original heart image is taken from the web source (https://www.thinglink.com/scene/715516620675481600). It can be seen that the six limb leads (three bipolar and three augmented) and six chest leads view the heart in vertical (frontal) and transverse (horizontal) planes, respectively. The twelve leads align with different anatomical areas of the heart and are categorized in four groups: inferior, lateral, septal, and anterior.
The inferior leads include lead II, III, and aVF and look at the heart from inferior or diaphragmatic surface. Lateral leads include lead I, aVL, V5, and V6 and view the heart from lateral wall of left ven- tricle. Lead V1 and V2 are called septal leads and depict electrical activities of the heart from septal surface (inter-ventricular septum). Anterior anatomy (sternocostal surface) of the heart is captured by lead V3 and V4. Thus, it can be concluded that the MECG signals provide spatio-temporal variations of various electrical activities of the heart and help in better understanding of its functionalities. Any deviations in these signals from their normal characteristics may provide pathological clues, important for detection of cardiac abnormalities.
1. Introduction
1.2.2 Characteristics of ECG signals in different leads
ECG signal characteristics in different channels vary in normal and pathological cases. Different waveforms in the ECG also vary with different views of the heart (from different angles) in all leads.
A summary of the orientations of different ECG waveforms in normal cases is given in Table 1.1. In normal conditions, P wave is upright in leads I and II and always inverted in lead aVR. In abnormal atrial rhythm conditions like in dextrocardia, P wave may get inverted in lead I. The duration of P wave changes in left atrial enlargement, which is prominently visible in leads I, II, and aVF. Most important ECG waveform, i.e., QRS-complex, is positive in lead I and lead V6 and its polarity reverses in lead aVR and lead V1. The height and width of QRS-complex signifies important pathological characteristics in conduction diseases such as bundle branch block. Changes appear in the form of wide, slurred S wave in leads V5, V6, I and a secondary R wave (R’) in V1 or V2 (i.e., M-shaped rSR’, rsR’, or rsr’ complex). The T wave usually follows the orientation of QRS-complex and hence is inverted in aVR and may get inverted in lead III. It is always upright in leads I, II, and V4 through V6.
Commonly, T wave is inverted in lead V1 and occasionally it is accompanied by similar inversion in lead V2. However, isolated T wave inversion is a pathological condition. An inverted T wave in leads I, II, and V3 through V6 accompanied by ST segment depression is highly suggestive of myocardial ischaemia. A tall T wave is observed in the case of severe myocardial ischemia.