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‫بسم هللا الرحمن الرحيم‬
Cardiovascular System (CVS 227)
ELECTROCARDIOGRAM (ECG)
BASIC PRINICPLES
Dr.Mohammed Sharique Ahmed Quadri
Asst. professor in physiology
Al Maarefa College
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OBJECTIVES
Identify the concept of ECG leads.
 Describe basic components of ECG.
 Relate each components of ECG with the
electrical event in the heart.
 Understand the concept of QRS axis.

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CONDUCTION SYSTEM OF THE HEART
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ELECTROCARDIOGRAM

Electrocardiogram:
Measure of the electrical activity of the heart per unit
time.
 Potential
differences generated by heart’sectrical activity
are conducted to body surface where they can be
recorded by electrodes placed on the skin.

Does NOT measure the flow of blood through the
heart.

Electrocardiograph : device that records the electrical
activity of the heart
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PRINCIPLE OF RECORDING ECG
The body is a good conductor of electricity
Battery
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ELECTROCARDIOGRAPH
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ECG
What is Lead?
 ECG tracing recorded on the paper
by a single electrode.

What is Electrode?
 It is a metallic plate placed on skin which
conducts electricity.

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LEADS ARE LIKE PICTURES OF THE HEART
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LEADS ARE LIKE PICTURES OF THE HEART
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BE CAREFUL !! DON’T PUT WRONG LEADS
DON’T PUT WRONG LEADS
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STANDARD 12-LEAD ELECTROCARDIOGRAM

Six Limb Leads :
Frontal plane currents (right, left, inferior, superior)
 Three Bipolar Limb Leads( standard limb leads)
 named
 Three


as , I, II , III
Unipolar Limb Leads ( augmented limb leads)
named as aVR, aVL, aVF
Six precordial (chest) leads
Horizontal plane (anterior, posterior, right and left
forces)
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BIPOLAR / STANDARD LIMB LEADS
Record the electrical potential difference in the
frontal plane
 In bipolar lead, we record potential difference
between two active electrodes.

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UNIPOLAR (AUGMENTED) LEADS
Record electrical potential in the frontal
plane
 In Unipolar lead, we record potential
difference between active electrode and
other electrode at zero potential.

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EINTHOVEN’S TRIANGLE
The axis of the limb
leads form an
equilateral triangle
around the heart’s
electrical field
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PRECORDIAL (CHEST) LEADS
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HEART IN THREE DIMENSION
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ELECTRODES & WAVES
If current is flowing towards the recording electrode,
we get upward deflection.
If current is flowing away from the recording
electrode, we get downward deflection.
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Standard 12 Lead ECG
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THE ECG PAPER

Horizontally
 One
small box - 0.04 s
 One large box - 0.20 s

Speed of paper is 25mm/sec
1mm
Vertically
 One
large box - 0.5 mV
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BASIC COMPONENTS

Waves

Segments & intervals
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BASIC COMPONENTS
NORMAL IMPULSE CONDUCTION
Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers
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IMPULSE CONDUCTION & THE ECG
Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers
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THE “PQRST”
• P wave - Atrial
Depolarization
• QRS - Ventricular
Depolarization
• T wave - Ventricular
Repolarization
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THE PR INTERVAL
Atrial depolarization
+
delay in AV junction
(AV node/Bundle of His)
(delay allows time for
the atria to contract
before the ventricles
contract)
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SEGMENTS & INTERVALS

PR interval: 0.12 – 0.20 sec

QRS interval : 0.08 – 0.10sec

QT interval : 0.40-0.43sec

ST interval :0.32 - sec
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THE ELCTRICAL AXIS OF THE HEART

Vector is diagrammatic way to show strength &
direction of impulse
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MEAN ELECTRICAL AXIS

Is some total of all
vectors generated by
the action potential of
individual ventricular
myocytes.

we measure the way
the vectors looks as it
travels under each of
various electrode
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Axis
Determination of Mean Electrical
The QRS axis is determined by overlying a circle, in the frontal
plane. By convention, the degrees of the circle are as shown.
The normal QRS axis lies between -30o and +110o.
A QRS axis that falls between -30o
and -90o is abnormal and called left
axis deviation.
-90o
-60o
-120o
-150o
A QRS axis that falls between
+110o and +180o is abnormal and
called right axis deviation.
-30o
180o
o
A QRS axis that falls between +180
and -90o is abnormal and called
Indeterminant (extende rt axis ) .
0o
30o
150o
120o
90o
60o
FORMATION OF HEXAAXIAL SYSTEM
I
III
II
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FORMATION OF HEXAAXIAL SYSTEM
aVR
aVL
aVF
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FORMATION OF HEXAAXIAL SYSTEM
-90
-120
-60
aVR
-150
aVL
-30
180
0I
+150
+30
III+120
+60 II
+90
aVF
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THE HEXA-AXIAL SYSTEM FOR DETERMINING QRS AXIS

When a vector is exactly
horizontal and directed
toward the person’s left
side,the vector is said to
extend in the direction of
0 degrees

From this zero reference
point, the scale of
vectors rotates
clockwise:
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HOW TO CALCULATE THE QRS AXIS

Leads & there
isoelectric
partners
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REFERENCES

Text book of medical physiology by GUYTON &
HALL 11th edition

Text book physiology by GANONG

The ECG made Easy by John R.Hampton sixth
edition
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