27 CHAPTER 3 ELECTROCARDIOGRAM, PHOTOPLETHYSMOGRAM AND WAVELET TRANSFORM This chapter presents the fundamentals of ECG, PPG and wavelet transform. In the first part of this chapter the basics of ECG signal measurement, P, QRS complex, ST segments and T wave are discussed. Using these morphological changes how the abnormalities in cardiac activity can occur is discussed. The second part of this chapter presents the fundamentals of PPG signal measurement and its characteristic features. The third part of this chapter presents the basics of wavelet transform and the use of wavelet transform in signal processing. At the end of the chapter the overview of the proposed work is presented. 3.1 ELECTROCARDIOGRAM An electrocardiogram (ECG) is an electrical recording of the heart and is used in the diagnosis of heart disease. These impulses are recorded as waves called P-QRS-T deflections. Each cardiac cell is surrounded by and filled with a solution that contains sodium (Na+), potassium (K+), and calcium (Ca++). In its resting condition the interior of the cell membrane is considered negatively charged, with respect to the outside. When an electrical impulse is initiated in the heart, the inside of a cardiac cell rapidly becomes positive in relation to the outside of the cell.
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CHAPTER 3
ELECTROCARDIOGRAM, PHOTOPLETHYSMOGRAM
AND WAVELET TRANSFORM
This chapter presents the fundamentals of ECG, PPG and wavelet
transform. In the first part of this chapter the basics of ECG signal
measurement, P, QRS complex, ST segments and T wave are discussed.
Using these morphological changes how the abnormalities in cardiac activity
can occur is discussed. The second part of this chapter presents the
fundamentals of PPG signal measurement and its characteristic features. The
third part of this chapter presents the basics of wavelet transform and the use
of wavelet transform in signal processing. At the end of the chapter the
overview of the proposed work is presented.
3.1 ELECTROCARDIOGRAM
An electrocardiogram (ECG) is an electrical recording of the heart
and is used in the diagnosis of heart disease. These impulses are recorded as
waves called P-QRS-T deflections. Each cardiac cell is surrounded by and
filled with a solution that contains sodium (Na+), potassium (K+), and
calcium (Ca++). In its resting condition the interior of the cell membrane is
considered negatively charged, with respect to the outside. When an electrical
impulse is initiated in the heart, the inside of a cardiac cell rapidly becomes
positive in relation to the outside of the cell.
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The electrical impulse causes this excited state and this change of
polarity, is called depolarization. Immediately after depolarization, the
stimulated cardiac cell returns to its resting state, which is called
repolarization. The resting state is maintained until the arrival of the next
wave of depolarization. This change in cell potential from negative to positive
and back to negative is called an action potential. That action potential
initiates a cardiac muscle contraction. Figure 3.1 shows that the components
of ECG signal.
The ECG is a measurement of the effect of this depolarization and
repolarization for the entire heart on the skin surface, and is also an indirect
indicator of heart muscle contraction, because the depolarization of the heart
leads to the contraction of the heart muscles (Jin Yinbin et al 1998). Although
the phases of the ECG are due to action potentials traveling through the heart
muscle, the ECG is not simply a recording of an action potential. During each
heartbeat, cells fire action potentials at different times, and the ECG reflects
patterns of that electrical activity.
Figure 3.1 Components of an ECG signal
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3.1.1 ECG Lead Configuration
To record an electrocardiogram, a number of electrodes, 3, 6, 10, 12
or 16 can be affixed to the body of the patient. The electrodes are connected
to the ECG machine by the same number of electrical wires. These electrodes
are called as leads.
There are three types of electrode systems:
Bipolar limb leads (or) standard leads
Augmented unipolar limb leads
Chest leads (or) precordial leads
3.1.2 Bipolar Limb Leads (OR) Standard Leads
By convention, lead I have the positive electrode on the left arm and
the negative electrode on the right arm and therefore measure the potential
difference between the two arms. Figure 3.2 shows the Einthoven’s triangle.
In this and the other two limb leads, an electrode on the right leg serves as a
reference electrode for recording purposes. In the lead II configuration, the
positive electrode is on the left leg and the negative electrode is on the right
arm. Lead III has the positive electrode on the left leg and the negative
electrode on the left arm.
Figure 3.2 Einthoven’s Triangle
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These three bipolar limb leads roughly form an equilateral triangle
(with the heart at the center) that is called Einthoven’s triangle in honor of
Willem Einthoven who developed the electrocardiogram in 1901. Figure 3.3
and 3.4 shows the bipolar lead configuration and the output waveform of it.
Figure 3.3 Bipolar Lead Configurations
Figure 3.4 Output Waveforms for bipolar Lead configuration
Whether the limb leads are attached to the end of the limb (wrists
and ankles) or at the origin of the limb (shoulder or upper thigh) makes no
difference in the recording because the limb can simply be viewed as a long
wire conductor originating from a point on the trunk of the body.
3.1.3 Augmented Unipolar Limb Leads
These are termed unipolar leads because there is a single positive
electrode that is referenced against a combination of the other limb
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electrodes. Figure 3.5 and 3.6 shows the augmented unipolar limb leads. The
positive electrodes for these augmented leads are located on the left arm
(aVL), the right arm (aVR), and the left leg (aVF). In practice, these are the
same electrodes used for leads I, II and III. (The ECG machine does the
actual switching and rearranging of the electrode designations).
Figure 3.5 Augmented lead configuration
Figure 3.6 Output waveforms for Augmented Lead configuration
These leads are unipolar in that they measure the electric potential at
one point with respect to a null point (one which doesn't register any
significant variation in electric potential during contraction of the heart). This
null point is obtained for each lead by adding the potential from the other two
leads. For example, in lead aVR, the electric potential of the right arm is
compared to a null point, which is obtained by adding together the potential of
lead aVL and lead aVF.
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3.1.4 The Chest Leads
In addition to the four limb leads, a 12-lead ECG includes six chest
leads. The chest leads sample the electrical activity over small areas of the
heart. The chest leads look at the heart’s electrical activity in a slightly off-
horizontal plane around the front of the chest. This detects problems that
might not be obvious from the standard limb leads, which measure electricity
in a vertical plane. The chest leads are often called V-leads. Figure 3.7 shows
the chest lead. The electrode over the chest is the positive electrode, while the
limb electrodes are all averaged together to form a general ground electrode.
Figure 3.7 Chest Leads
The precordial (chest) leads start with V1, placed beneath the 4th rib
to the right of the sternum. Lead V2 is opposite to V1 at the left side of the
sternum. V3 is halfway to lead V4, which is placed below rib 5 directly down
from the middle of the clavicle. Lead V5 is straight around the chest from V4,
in line with the front of the armpit. V6 is directly around from V5, straight
down from the middle of the armpit (Leslie Cromwell et al 1997; Khandpur,
2003).
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3.1.5 Normal Sinus Rhythm
Each P wave is followed by a QRS complex. P wave rate is 60-100
beats per minute (bpm). If rate is less than 60bpm it is called sinus
bradycardia. If rate is greater than 100 it is called sinus tachycardia
Figure 3.8 presents the normal ECG signal.
Figure 3.8 Normal ECG Signal
3.1.6 Standard conventions for reading an ECG
The rate of paper (i.e. of recording of the ECG) is 25 mm/s, which
results in:
1 mm = 0.04 sec (or each individual block)
5 mm = 0.2 sec (or between 2 dark vertical lines)
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The voltage recorded from the leads is also standardized on the
paper where 1 mm = 0.1 mV (or between each individual block vertically).
Figure 3.9 shows the recording chart. The standards are:
5 mm = 0.5 mV (or between 2 dark horizontal lines)
10 mm = 1.0 mV (this is how it is usually marked on the
ECG)
Figure 3.9 ECG Recording Chart
3.1.7 Waves and Intervals of ECG
3.1.7.1 P Wave
During normal atrial depolarization, the main electrical vector is
directed from the SA node towards the AV node, and spreads from the right
atrium to the left atrium. This turns into the P wave on the ECG, which is
upright in II, III and a VF (since the general electrical activity is going toward
the positive electrode in those leads) and inverted in a VR (since it is going
away from the positive electrode for that lead). A P wave must be upright in
leads II and a VF and inverted in lead aVR to designate a cardiac rhythm as
Sinus Rhythm (Brendan Phibbs 2005).
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The P wave height and width depends not only on the size of the RA
and LA but also the site of origin of atrial impulse. A normal SA nodal origin
of P wave produce the normal shaped P Waves. Ectopic P waves can have a
wide variation of morphology (fully inverted, partially inverted, slurred,
biphasic, notched, rounded, deformed, etc. The morphology is dictated by the
direction of P wave vector and thus it is quite variable in different leads.
Further it is also determined by the inter atrial and intra atrial conduction
(Ariyarajah et al 2005). A P wave can also be of very low amplitude and it
may be entirely isoelectric, which could actually mean the P waves are as
good as absent. This can happen in all leads or in few leads. Atria get
electrically activated but fail to inscribe a P wave. This is termed as isoelectric
P waves.
3.1.7.1.1 The Importance of Isoelectric P Waves
It can happen, both in sinus rhythm and in ectopic atrial rhythm.
Absent P waves should be differentiated form isoelectric P waves. It is
typically described in focal atrial rhythm arising from the right side of the
inter atrial septum near the perinodal tissue. The atrial tachycardia arising
from this site has isoelectric P waves in most of the leads especially in lead V.
The relationship between P waves and QRS complexes helps
distinguish various cardiac arrhythmias.
• The shape and duration of the P waves may indicate atrial
enlargement. The PR interval is measured from the beginning
of the P wave to the beginning of the QRS complex. It is 120
to 200 millisecond long. On an ECG tracing, this corresponds
to 3 to 5 small boxes.
• A PR interval of over 200 millisecond may indicate a first
degree heart block
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• A short PR interval may indicate a pre-excitation syndrome
via an accessory pathway that leads to early activation of the
ventricles, such as seen in Wolff-Parkinson White syndrome.
• PR segment depression may indicate atrial injury or
pericarditis.
• Variable morphologies of P waves in a single ECG lead are
suggestive of an ectopic pacemaker rhythm such as wandering
pacemaker or multifocal atrial tachycardia. Table 3.1 presents
the abnormalities due to variations in P waves.
Table 3.1 Causes of abnormalities and their characteristic features
S No Characteristic Feature of P wave Causes 1 P wave inversion (other than aVR) a) Ectopic atrial focus
b) AV nodal rhythm 2 High amplitude P wave Atrial Hypertrophy (or)
Atrial Dilation a) Mitral valve disease b) Hypertension c) Cor Pulmonale d) Congenital Heart Disease
3 Wide P wave (over 0.11s) Left Atrial Enlargement 4 Biphasic P wave
(2nd half negative in III or V1) Left Atrial Enlargement
5 M shaped or notched P wave Findings: a) Over 0.04s between peaks b) Taller in I than II
a) M-Mitral: Left Atrial Enlargement
6 Peaked P-wave Findings: 1)Tall and pointed 2)Taller in Lead III than in I
P-Pulmonale: Right Atrial Enlargement
7 P-wave absent a) Sinoatrial node block b) AV nodal rhythm
8 Inverted P-wave Dextrocardia
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3.1.7.2 QRSComplex
The QRS complex is a structure on the ECG that corresponds to the
depolarization of the ventricles. Because the ventricles contain more muscle
mass than the atria, the QRS complex is larger than the P wave. In addition,
since the bundle of his (Purkinje system) also coordinates the depolarization
of the ventricles, the QRS complex tends to look "spiked" rather than rounded
due to the increase in conduction velocity. A normal QRS complex is 0.06 to
0.10 sec (60 to 100 ms) in duration. The duration, amplitude, and morphology
of the QRS complex is useful in diagnosing cardiac arrhythmias, conduction
abnormalities, ventricular hypertrophy, myocardial infarction and other
disease states. Q waves can be normal (physiological) or pathological. Normal
Q waves represent depolarization of the inter-ventricular septum. For this
reason, they are referred to as septal Q waves and can be appreciated in the
lateral leads I, aVL, V5 and V6 (Khandpur 2003).
3.1.8 Diseases Related with Abnormal QRS Complex
3.1.8.1 Tachycardia
Tachycardia typically refers to a heart rate that exceeds the normal
range for a resting heart rate. Ventricular tachycardia (VT or V-tach) is a
potentially life-threatening cardiac arrhythmia that originates in the ventricles.
It is usually an irregular, wide QRS complex with a rate between 120 and 250
beats per minute (Holly L., 2009) Ventricular tachycardia has the potential of
degrading the more serious ventricular fibrillation. Ventricular tachycardia is
a common and often lethal, complication of a myocardial infarction
(heart attack).
3.1.8.2 Ventricular Fibrillation
Ventricular fibrillation occurs in the ventricles (lower chambers) of
the heart; it is always a medical emergency. If left untreated, ventricular
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fibrillation (VF or V-fib) can lead to death within minutes. When a heart goes
into V-fib, effective pumping of the blood stops. V-fib is considered as a form
of cardiac arrest, and an individual suffering from it will not survive unless
cardiopulmonary resuscitation (CPR) and defibrillation are provided
immediately.
3.1.8.3 Bradycardia
A slow rhythm, (less than 60 beats/min), is labeled Bradycardia.
This may be caused by a slowed signal from the sinus node (termed sinus
Bradycardia), a pause in the normal activity of the sinus node (termed sinus
arrest), or by blocking of the electrical impulse on its way from the atria to the
ventricles (termed AV block or heart block).Bradycardia may also be present
in the normally functioning heart of athletes or other well-conditioned
persons.
3.1.8.4 Bundle Branch Block
A bundle branch block refers to a defect of the heart's electrical
conduction system. When a bundle branch becomes injured due to underlying
heart disease, myocardial infarction, or cardiac surgery, it ceases to conduct
electrical impulses appropriately. This results in altered pathways for
ventricular depolarization. Since the electrical impulse can no longer use the
preferred pathway across the bundle branch, it may move instead through
muscle fibers in a way that both slows the electrical movement and changes
the directional propagation of the impulses. As a result, there is a loss of
ventricular synchrony, prolonged ventricular depolarization and
corresponding drop in cardiac output (George A. Perera et al 1941).
3.1.8.5 ST Segment
The ST segment connects the QRS complex and the T wave and has
duration of 0.08 to 0.12 sec (80 to 120 ms). It starts at the J point (junction
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between the QRS complex and ST segment) and ends at the beginning of the
T wave. However, since it is usually difficult to determine exactly where the
ST segment ends and the T wave begins, the relationship between the ST
segment and T wave should be examined together. The typical ST segment
duration is usually around 0.08 sec (80 ms). It should be essentially level with
the PR and TP segment.
3.1.9 Types of ST-Segment
ST-segment elevation was classified into three types according to
the morphology of the ST elevation after the J point on any pericardial
derivation: figure 3.10 shows the three different types of ST-Segment
elevation in ECG signal. Concave type where ST-T segment rises with
downward convexity, straight type where ST-T segment rises obliquely like
an inclined plane and convex type where ST-T segment rises with an upward
convexity (Gettes and Cascio 1991). Figure 3.11 presents various disorders
related to the morphological changes in ST segment.
Figure 3.10 Three different types of ST-Segment elevation in ECG