Chapter 2 Fundamental of ECG Signals
2.1 ECG characteristic
Goal of an ECG device is to detect, amplify, and record the electrical changes caused bydepolarization and repolarization of heart muscle. Each heart muscle cell, at rest condition, has a negative charge across its cell membrane and is called polarized. The negative charge can be increased to zero, and the phenomenon of depolarization causes the heart to contract.
Afterwards, the heart muscle cell will be recharged, called repolarization, which makes the heart to expand. A cardiac cycle begins when the sinoatrial node (SA) generates the impulse, which will run through the heart. The conducting system of the heart can be summarized as follows:
1. The impulse generated from SA node will signal the muscle in the atria to beat, resulting in contraction,during which the blood is pushed from atria into ventricles.
2. The impulse propagates to atrioventricular(AV) node and then delays for about 1 millisecond to allow the blood to fill the ventricles.
3. The impulse propagates to the ventricles through the right bundle branch (RBB), the left bundle branch (LBB), and other nerves. Thisresultsin the ventricular contraction, during which the blood is pushed to the body.
4. The muscle cells are recharged (repolarization) and it expands the atrial so that the blood is allowed to return to the heart. Then, the next heartbeat repeats.
A typical cardiac cycle (ECG cycle) is composed of a P wave, a QRS complex, and a T wave. In addition, there are U wave and J wave within an ECG cycle, but their amplitudesare so low that they are often ignored. Different types of wave reflect the different stages of the heartbeats. A detailed description of each wave and its duration are shown in Fig2.1and Table2.1. ECG is the most important tool to diagnose any damage to the heart. Symptoms like arrhythmia and myocardial infarction can be easily detected through ECG. For example, hyperacute T waves indicate that acute myocardial infarction may occur.
PR
Figure 2.1 Typical cardiac cycle.
Table 2.1 Descriptions of waves and durations.
Name Descriptions Duration
P wave Represents theatrial depolarization. Shorter than 0.12 seconds PR interval The interval between the points of the P
wave to Q wave.
Longer than 0.12 seconds and shorter than 0.2 seconds.
QRS complex Represents the ventricular depolarization and atrial repolarization.
Longer than 0.08 seconds and shorter than 0.12 seconds.
ST segment The period during which the ventricles are depolarized.
Longer than 0.08 seconds and shorter than 0.12 seconds T wave The last wave of a normal cardiac cycle,
representing the ventricular repolarization. Shorter than 0.16 seconds RR interval The interval between two R waves. Longer than 0.6 seconds and
shorter than 1.2 seconds While the heartbeats are caused by a series of electrical activities in the heart, we can collect and record these signals by attaching electrodes on the surface of the skin. When measuring ECG, usually more than two electrodes are used and they can be combined into a pair whose output is called a lead. The most common clinically-used one is 12-lead ECG where ten electrodes are used. Each electrode has a specific label (name), including RA, LA, RL, LL, V1, V2, V3, V4, V5, and V6. The placements and labels of electrodes are illustrated inFig2.2 andTable 2.2, respectively.
RA LA
RL LL
(a) (b)
Figure 2.2 Placements of electrodes
Table 2.2 Placements of electrodes
Electrodes’ label Placement
RA On the right arm.
LA On the left arm.
RL On the right leg.
LL On the left leg.
V1 In the space, between rib 4 and rib 5, to the right side of the breastbone.
V2 In the space, between rib 4 and rib 5, to the left side of the breastbone.
V3 In the place between lead V2 and lead V4.
V4 In the space between rib 5 and rib 6, and on an imaginary line extended from the collarbone’s midpoint.
V5 In the place between lead V4 and lead V6.
V6 In the space horizontally even with lead V4 and V5, and on an imaginary line extended from the middle of the armpit.
The twelve leads contain sixprecordial leads (V1~V6) in the horizontal plane, three standard limb leads (I, II, III), and three augmented limb leads (aVR, aVL, aVF) in the frontal plane.The twelve leads can also be divided into two types: bipolar and unipolar. While the former has one positive and one negative pole, the latter has two poles with the negative onemade of signals from many other electrodes. For example, leads I, II, and III are bipolar leads, while others are unipolar leads. Thedefinitions of twelve leads are given as below.
1. Lead aVR: the positive electrode is on the right arm and the negative one is a combination of two electrodes on the left arm and left leg.
( ) / 2
RA LA LL
Lead aVRV V V (2.1)
2. LeadaVL: the positive electrode is on the left arm and the negative one is a combination of two electrodes on the right arm and left leg.
( ) / 2
LA RA LL
Lead aVLV V V (2.2)
3. Lead aVF: the positive electrode is on the leftleg and the negative one is a combination of two electrodes on the left arm and right arm.
( ) / 2
LL LA RA
Lead aVFV V V (2.3)
4. Lead I: the positive electrode is on the leftarm and the negative one is on the right arm.
( LA RA)
Lead I V V (2.4)
5. Lead II: the positive electrode is on the leftleg and the negative one is on the right arm.
( LL RA)
Lead II V V (2.5)
6. Lead III: the positive electrode is on the leftleg and the negative one is on the left arm.
( LL LA)
Lead III V V (2.6)
Of the 12 leads in total, each records the electrical activity of the heart from a different perspective, which also correlates to different anatomical areas of the heart for the purpose of identifying acute coronary ischemia or injury. Two leads that look at neighbouring anatomical areas of the heart are said to be contiguous. The relevance of this is in determining whether an abnormality on the ECG is likely to represent true disease or a spurious finding.
Modern ECG monitors offer multiple filters for signal processing. The most common settings are monitor mode and diagnostic mode. In monitor mode, the low-frequency filter is set at either 0.5Hz or 1Hz and the high-frequency filter is set at 40Hz. This limits artifacts for routine cardiac rhythm monitoring. The high-pass filter helps reduce wandering baseline and the low-pass filter helps reduce 50- or 60-Hz power line noise.