Basics of Electrocardiography (ECG

Basics of Electrocardiography
Dr.U.K.M.Nazmun Ara
FCPS (Paediatrics), MD (Paediatric Cardiology)
Assistant Professor (Paediatric Cardiology)
NICVD, Dhaka
1

Outline
• Review of the conduction system
• ECG leads and recording
• ECG waveforms and intervals
• Determining the QRS Axis
• Calculating the Heart Rate
• Low voltage ECG
• Bundle Branch Block
• Chamber enlargement
• Hyper & Hypokalemia
• Arrhythmia
2

References
• Moss and Adam’s Heart disease in infants, children and adolescents
• Pediatric cardiology by Myung K. Park
• Basic and bedside electrocardiography by Romulo F. Baltazar
• Leo Schamroth – An introduction to electrocardiography
• Internet
3

The conduction system
4
• The cardiac conduction
system is designed for
electrical impulse creation
and propagation.
• Cardiac cells - spontaneous
depolarization, which creates
the cardiac impulse.
• Sinus node - pacemaker of
the heart.
• AV node - create an escape
rhythm if the sinus node is
diseased.
• Bundle of HIS, bundle
branches, and Purkinje fibers

Basic electrophysiology
The heart consists of three special types of cells with different
electrophysiologic properties.
Muscle cells: specialized for contraction and are present in the atria and
ventricles.
Conducting cells: rapid conduction of the electrical impulse
Pacemaking cells: automaticity and are capable of generating electrical
impulses.
5

Action Potential of a Muscle Cell, Conducting Cell and Pacemaking Cell
6

The Transmembrane Action Potential and
the Surface Electrocardiogram.
7

What is an ECG?
An ECG is the recording (gram) of the electrical activity (electro)
generated by the cells of the heart (cardio) that reaches the body
surface.
Useful in diagnosis of…
• Cardiac Arrhythmias
• Myocardial ischemia and infarction
• Pericarditis
• Chamber hypertrophy
• Electrolyte disturbances
• Drug effects and toxicity
8

Recording an ECG
• ECG graphs: – 1 mm squares – 5 mm squares
• Paper Speed: – 25 mm/sec
• Standard Voltage Calibration: – 10 mm/mV standard
9

ECG Leads
Two types of leads –
• Limb leads
• 3 Standard Limb Leads
• 3 Augmented Limb Leads
• Precordial (chest) leads
• 6 Precordial Leads
12

Placement of electrodes
Ten electrodes are used for a 12-lead ECG
The limb electrodes
• RA - On the right arm, avoiding thick
muscle
• LA – On the left arm this time.
• RL - On the right leg, lateral calf muscle.
• LL - On the left leg this time.
13

LEADS I, II, III
• These are the only bipolar leads. All
together they are called the limb leads or
the EINTHOVEN’S TRIANGLE.
15

LEADS aVR, aVL, aVF
• They are also derived from the limb
electrodes, they measure the electric
potential at one point with respect to a null
point.
• They are the augmented limb leads.
16

Arrangement of Leads on the EKG
19

ECG waveforms and intervals
22

P wave
• Because of its anatomic location, the sinus impulse has to
travel from right atrium to left atrium in a leftward and
downward (inferior) direction.
25

Biatrial enlargement
• Signs of both right and left atrial
enlargement are present.
29

The PR interval
• 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 sinus impulse to reach the ventricles.
• The normal PR interval measures 0.12 to
0.20 seconds in the adult.
30

The normal PR interval varies with age and heart rate. The older the
person and the slower the heart rate, the longer is the PR interval.
31

Prolonged PR interval:
• myocarditis (rheumatic, viral, or diphtheric),
• digitalis or quinidine toxicity,
• congenital heart defects (ECD, ASD , Ebstein's anomaly),
• myocardial dysfunctions,
• hyperkalemia,
• otherwise normal heart with vagal stimulation.
Short PR interval: A short PR interval can be seen when the AV node delay is bypassed
such as in Wolff-Parkinson-White (WPW) preexcitation, Lown-Ganong-Levine
syndrome.
Variable PR intervals: seen in wandering atrial pacemaker and in second-degree AV
block type I (Wenkebach).
32

Effect of Lt and Rt oriented lead
34

ST segment
• The ST segment starts from the J point to the
beginning of the T wave.
• It represents the time when all cells have
just been depolarized and the muscle cells
are in a state of sustained contraction.
37

T wave
• Represents rapid ventricular
repolarization.
• The T wave is highly susceptible to all
kinds of influences both cardiac and
non cardiac. Hence it is variable in
nature. It is usually positive in leads
with tall R waves.
38

QT Interval
• The QT interval is affected by heart rate.
• It becomes longer when the heart rate is
slower and shorter when the heart rate is
faster.
• The QT interval therefore should always be
corrected for heart rate (QTc).
• Prolonged QT interval : is defined as a QTc >
0.44 seconds (440 milliseconds) in men and
>0.46 seconds (460 milliseconds) in women
and children.
39

U Wave
• The U wave, if present, is the last positive deflection in the ECG.
• It is likely due to repolarization of the His-Purkinje system
• The size of the normal U wave is small, measuring approximately one-
tenth of the size of the T wave
• Abnormal U Wave: when they are inverted or when they equal or
exceed the size of the T wave. This occurs in the setting of
hypokalemia.
40

The QRS Axis
• The QRS axis represents the net
overall direction of the heart’s
electrical activity.
• For calculating the actual axis the
lead-axis-degree diagram should be
remembered thoroughly.
41

• By near-consensus, the normal
QRS axis is defined as ranging
from -30° to +90°.
• -30° to -90° is referred to as a left
axis deviation (LAD)
• +90° to +180° is referred to as a
right axis deviation (RAD)
42

• The QRS axis is perpendicular to the lead with an equiphasic QRS
complex in the predetermined quadrant (Look for an isoelectric
complex)
Lead I is perpendicular to lead aVF
Lead II is perpendicular to lead aVL
Lead III is perpendicular to lead aVR
44

Step 1
• The axis is in the lower left quadrant (0 to +90 degrees) because the R
waves are upright in leads I and aVF.
Step 2
• The QRS complex is equiphasic in aVL. Therefore, the QRS axis is +60
degrees, which is perpendicular to aVL.
47

Because aVL is negative, the axis is
adjusted further away from 60, thus
the axis is approximately 70 rather
than 60.
52

Plotting the Amplitude of the QRS Complex using
Two Perpendicular Leads
53

Normal QRS Axis
• Normal ranges of QRS axis vary with age.
• Newborns normally have RAD compared with the adult standard.
• By 3 years of age, the QRS axis approaches the adult mean value
of +50 degrees.
• From birth to 3 mon - +90 to +140
degree
• From 3 mon to 12 or 16 yr of age -
+ 90 to + 120 degree
(Schamroth)
54

Calculating the Heart Rate
• Regular heart rate - The distance between
two QRS complexes in large or small boxes is
used for counting the ventricular rate and the
distance between two P waves for counting
the atrial rate.
• Irregular heart rate: When the heart rate is
irregular (atrial flutter or atrial fibrillation), a
longer interval should be measured to provide
a more precise rate.
3- second time line
6-second time interval is used
55

Using the large boxes
Standard ECG paper speed 25
mm per second or 1,500 mm
per minute.
56

Using the small boxes
This is the most accurate method when the heart rate is regular and fast.
57

Using 3-second time markers
• the 3-second time markers (15 large blocks) are printed at the top
margin of the ECG paper.
• The heart rate is calculated by counting the number of QRS
complexes within 3 seconds and multiplied by 20.
• The first complex is the reference point and is not counted.
58

Using 6-second time markers
• If the heart rate is irregular or very slow, a longer time interval such as
6-second time marker (30 large blocks) or even 12-second time marker
is chosen.
• The heart rate is calculated by counting the number of QRS complexes
within 6 seconds and multiplied by 10. If 12 seconds are used, the
number of complexes is multiplied by 5, to obtain the heart rate per
minute.
59

Quick estimation of heart rate
60

Moss and Adam’s Heart disease in
infants, children and adolescents
Pediatric cardiology - Park
61

The low voltage
The QRS is said to be low voltage when:
• The amplitudes of all the QRS complexes in the limb leads are < 5 mm; or
• The amplitudes of all the QRS complexes in the precordial leads are < 10 mm
62

Right Bundle Branch Block
Complete RBBB
• Wide QRS complexes measuring ≥0.12 second (above the upper limit for age )
• V1: Large terminal R’ waves with rR’ or rsR’ configuration.
• V6 ,leads I and aVL: Wide terminal S waves. Septal q waves are preserved.
• Wide and slurred S in leads II, III, and aVF.
63

Incomplete RBBB
• There is delay rather than complete interruption of the impulse to the
right ventricle.
• ECG: An RSR prime pattern in lead V1 with a normal or slightly
prolonged QRS duration.
65

Left BBB
ECG –
• an abnormally prolonged QRS duration,
• absent normal initial forces (no Q waves in leads aVL and V6), and
• QS or rS in lead V1 and a tall notched R wave in lead V6).
66

Left Ventricular Hypertrophy
• QRS voltage criteria - Voltage criteria involve R and S waves in leads
V1, V6, and aVF and Q-wave amplitude in lead V6.
• Repolarization criteria - T-wave negativity in the lateral precordial
leads and the angle (>100 degrees) between the frontal plane QRS
and T axis.
68

QRS voltage criteria (LVH)
1. Tall R-wave in left sided leads (I, aVL, V4-V6) (greater than the 98th percentile for
age) and
• deep S-wave in right sided leads (III, aVR, V1-V3)
2. Tall R wave in aVF, but in the absence of RVH (increased inferior forces), helpful
in supporting a diagnosis of LVH.
69

3. Left axis deviation is supportive of the diagnosis of LVH, especially in infancy.
4. Abnormally prominent Q waves in leads V5 and V6 .
5. In newborns , small R waves and deep S waves over the right precordium
progressing to tall R waves and small S waves in the left lateral precordium, it suggests
that there is left ventricular dominance.
Normal newborn
LVH
70

Repolarization abnormalities (LVH)
T-wave abnormalities
left ventricular strain pattern consists of inverted T waves in the inferior leads
(II, III, and aVF) and left precordial leads (V5 and V6) sometimes can be
associated with depression of the ST segment.
A wide QRS-T angle of >100 degrees is supportive of the diagnosis of LVH.
71

LVH with Pressure overload
• Left ventricle becomes concentrically hypertrophied
• ECG - tall R waves in V5 and V6 associated with depression of the ST segment and
inversion of the T wave (left ventricular “strain”).
72

LVH with Volume overload
• Left ventricle becomes eccentrically hypertrophied.
• ECG - prominent Q waves, tall R waves, and tall and upright T waves in V5 and
V6.
73

Right Ventricular Hypertrophy
1. Tall R-wave in V1 (greater than the 98th percentile for age)
2. Deep S wave in V6 (>98th percentile) is a very sensitive indicator of
RVH.
3. The R/S ratio - it is abnormally increased in lead V1, or abnormally
decreased in lead V6.
74

4. An RSR prime pattern when the R prime amplitude is large.
5. A qR pattern, especially in conjunction with a tall R wave (typically
>10 mm)
6. Right axis deviation can be used to support other findings suggestive
of RVH.
3 yr old child, qR in the
right precordial leads
75

6. An upright T wave after 7 days of age but before adolescence is a
sensitive indicator of increased right ventricular pressure (between 1
week of age and adolescence it is negative).
8. The neonatal pattern, consisting of tall R waves and small S waves in
the right precordium, progressing to small R waves and deep S waves in
the left lateral precordium, suggests right ventricular dominance in
older children.
76

Biventricular Hypertrophy
1. Tall biphasic complexes in mid-precordial leads: (Katz-Wachtel
phenomenon).
2. Right atrial enlargement combined with LVH
3. Voltage discordance - tall QRS complexes in precordial leads, whereas
low voltages in the limb leads, especially bipolar leads I, II, III.
77

Hyper & Hypokalemia
• Potassium is vital for regulating the normal
electrical activity of the heart.
• Increased extracellular potassium reduces
myocardial excitability, with depression of both
pacemaking and conducting tissues.
• Decreased extracellular potassium causes
myocardial hyperexcitability with the potential to
develop re-entrant arrhythmias.
80

Hyperkalemia
• Serum potassium > 5.5 mEq/L - repolarization abnormalities. Peaked T waves, shorter QT interval.
• Serum potassium > 6.5 mEq/L - progressive paralysis of the atria. P wave widens and flattens, PR
segment lengthens resulting in AV block, P waves eventually disappear. More pronounced peaking
of the T waves occur, QRS complexes widen.
• Serum potassium > 7.0 mEq/L - conduction abnormalities and bradycardia. P waves become
unrecognizable, further widening of the QRS complex occurs, S and T waves merge with a very
short ST segment resulting in a sinusoidal wave. High-grade AV block with slow junctional and
ventricular escape rhythms, any kind of conduction block (bundle branch blocks, fascicular blocks),
sinus bradycardia or slow AF.
• Serum potassium level of > 9.0 mEq/L : slowing of the heart rate, asystole, or ventricular flutter
(causes cardiac arrest).
81

Generalized Peaking of the T Waves is the Hallmark of Hyperkalemia.
82

Severe Hyperkalemia with Widening of the QRS Complexes
84

ECG Findings of Hypokalemia
• Dynamic changes in T-wave morphology (T-wave flattening and
inversion)
• ST-segment depression
• Prominent U waves
• Fusion of the T and U waves
• Prolongation of the QU interval
• P pulmonale pattern is occasionally observed
• Frequent supraventricular and ventricular ectopics, SVT, potential to
develop life-threatening VT.
T wave inversion and prominent U waves
in hypokalaemia
85

Arrhythmia
• An abnormality of the cardiac rhythm is called arrhythmia.
• Arrhythmias may cause sudden death, syncope, heart failure,
dizziness, palpitations or no symptoms at all.
• There are two main types of arrhythmia:
1. Bradyarrhythmia : the heart rate is slow
2. Tachyarrhythmia : the heart rate is fast

Normal resting heart rate in the
paediatric population by age
Age Resting heart rate (bpm)
Birth- 1wk 90-160
1wk- 1yr 100-170
1y- 2y 80-150
3y-7y 70-135
7y-10y 65-130
11y-15y 60-120

Sinus tachycardia refers to impulses that originate from the sinus node
with a rate that exceeds 140 beats per minute (bpm) in children and
160 bpm or more in infants .
ECG Findings
• Sinus P waves are present with a higher rate.
• The morphology of the P wave should be upright in leads I, II, and aVF
and in V3 to V6.

91
represents a sinus arrhythmia which is a normal finding particularly in
pediatrics with acceleration during inspiration and slowing during
expiration with no change in the P-wave axis or morphology.

• An abnormal mechanism of tachycardia, or tachyarrhythmia, results
from an area other than the sinus node elevating the heart rate
above the sinus rate or alternatively from an intrinsically abnormal
sinus node.
• So, tachyarrhythmias occur due to abnormal impulse initiation or
conduction and may be classified in several different ways.

Based on mechanism of initiation and
propagation
There are three basic mechanisms by which tachyarrhythmias start:
1. Reentry
2. Abnormal automaticity
3. Triggered activity

Based on the origin of the arrhythmia
1. Supraventricular tachycardia (SVT): above the bifurcation of the
bundle of His, usually in the atria or atrioventricular (AV) junction.
2. Ventricular tachycardia (VT): below the bifurcation of the
bundle of His.

Characterization of Tachycardias
By examining the width of the QRS complex which depends on the
patient’s age and history of cardiovascular surgery
1. Narrow complex tachyarrythmia
2. Wide complex tachyarrythmia

• A, Abrupt onset of sustained monomorphic ventricular tachycardia
with treadmill exercise in a 15-year-old with palpitations. B, Nearly
sustained, somewhat polymorphic, ventricular tachycardia at peak
exercise in an adolescent female with recurrent syncope.
102

AF - ECG Findings
1. P waves not frequently visible , fibrillatory waves are
present.
2. The R-R intervals are irregularly irregular.
3. The ventricular rate is variable.
4. The QRS complexes are normal unless there is
preexistent bundle branch block, ventricular
aberration, or preexcitation.

ECG Findings of Atrial Flutter
1. Atrial rates – regular, between 300 and 600 beats per
minute.
2. Ventricular rates - variable
3. Very regular and uniform flutter waves with a saw
tooth or picket fence appearance.

WPW Syndrome
• It is a clinical entity characterized by preexcitation of
the ventricles with symptoms of paroxysmal
tachycardia.
• Here, a bypass tract is present, which connects the atrium
directly to the ventricle.
• Incidence - 0.1% to 0.3% among the general
population.
• More prevalent in men than women.

ECG Findings
• Short PR interval
• Delta wave
• QRS complex of longer duration than normal for age
• ST and T wave abnormalities

There are three types of AV block based on the severity of
the conduction abnormality:
• First-degree AV block
• Second-degree AV block
Mobitz type I or AV Wenckebach
Mobitz type II
Advanced or high grade
• Third-degree or complete AV block

• Although the PR interval is prolonged in first-degree AV
block, all P waves are conducted to the ventricles and are
always followed by QRS complexes.
• First-degree AV block therefore is a conduction delay rather
than actual block.
• Once a sinus P wave is not conducted to the ventricles, the
AV block has advanced to second degree.

• Second-degree AV block is defined as the failure of at least one
nonpremature atrial impulse to conduct to the ventricles.
• There are three types of second-degree AV block.
 Mobitz type I also called AV Wenckebach (most common)
 Mobitz type II
 Advanced, also called high-grade second-degree AV block

Type I Second-Degree AV Block
Also called AV Wenckebach. The following features
characterize type I second-degree AV block :
1. Two or more consecutive P waves are conducted.
2. Only single P waves are blocked.
3. There is gradual prolongation of the PR interval before a ventricular
complex is dropped.
4. The PR interval always shortens immediately after the pause.
5. The QRS complexes may be narrow or wide but are typically narrow.

Type II Second-Degree AV Block
Characterized by the following features:
• Two or more consecutive P waves are conducted.
• Only single P waves are blocked.
• All PR intervals measure the same throughout.
• The QRS complexes are usually wide because of the presence of
bundle branch block.
• The R-R intervals are constant and measure the same throughout as
long as the sinus rhythm is stable—that is, the heart rate or P-P
intervals are regular .

Advanced second-degree AV block.
When the AV block is 2:1, 3:1, 4:1, or higher, the AV block
cannot be classified as type I or type II because only a single
P wave is conducted (2:1 block) or two or more consecutive P
waves are blocked (3:1 AV block or higher). These are
examples of advanced second-degree AV block.

Advanced 2:1 Second-Degree AV Block
• It is an example of advanced second-degree AV block.
• In 2:1 block, every other P wave is conducted alternating with every other P wave
that is blocked.
• The QRS complexes may be narrow or wide.

Advanced Second-Degree AV Block: 3:1
and Higher
High-grade or advanced second-degree AV block occurs when there
are two or more successive atrial impulses that are not propagated
through the AV node.
Three consecutive P waves, only one is conducted

• Complete AV block (CAVB), or third-degree AV block, is
the complete failure of sinus atrial impulses to conduct to
the ventricles.
• To diagnose this condition, the atrial rate must be faster
than the ventricular rate .

Basics of Electrocardiography (ECG

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