ELECTROCARDIOGRAM (ECG) Cardiovascular System (CVS 227) BASIC PRINICPLES Dr.Mohammed Sharique Ahmed Quadri Asst. professor in physiology Al Maarefa College بسم الله الرحمن الرحيم 1

 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. 2

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CONDUCTION SYSTEM OF THE HEART 4

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 5

PRINCIPLE OF RECORDING ECG Battery The body is a good conductor of electricity 6

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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. 8

LEADS ARE LIKE PICTURES OF THE HEART 9

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11 DON’T PUT WRONG LEADS

STANDARD 12-LEAD ELECTROCARDIOGRAM  Six Limb Leads : Frontal plane currents (right, left, inferior, superior)  Three Bipolar Limb Leads( standard limb leads)  named as, I, II, III  Three Unipolar Limb Leads ( augmented limb leads)  named as aVR, aVL, aVF  Six precordial (chest) leads Horizontal plane (anterior, posterior, right and left forces) 12

BIPOLAR / STANDARD LIMB LEADS  Record the electrical potential difference in the frontal plane  In bipolar lead, we record potential difference between two active electrodes. 13

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. 14

EINTHOVEN’S TRIANGLE The axis of the limb leads form an equilateral triangle around the heart’s electrical field 15

PRECORDIAL (CHEST) LEADS 16

HEART IN THREE DIMENSION 17

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. 18

Standard 12 Lead ECG 19

THE ECG PAPER  Horizontally  One small box s  One large box s  Vertically  One large box mV 1 mm 20 Speed of paper is 25mm/sec

BASIC COMPONENTS  Waves  Segments & intervals 21

BASIC COMPONENTS

NORMAL IMPULSE CONDUCTION Sinoatrial node AV node Bundle of His Bundle Branches Purkinje fibers 23

IMPULSE CONDUCTION & THE ECG Sinoatrial node AV node Bundle of His Bundle Branches Purkinje fibers 24

THE “PQRST” T wave - Ventricular Repolarization QRS - Ventricular Depolarization P wave - Atrial Depolarization 25

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) 26

SEGMENTS & INTERVALS  PR interval: 0.12 – 0.20 sec  QRS interval : 0.08 – 0.10sec  QT interval : sec  ST interval : sec 27

THE ELCTRICAL AXIS OF THE HEART  Vector is diagrammatic way to show strength & direction of impulse 28

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 29

Determination of Mean Electrical Axis 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 -30 o and +110 o. 0o0o 30 o -30 o 60 o -60 o -90 o -120 o 90 o 120 o 150 o 180 o -150 o A QRS axis that falls between -30 o and -90 o is abnormal and called left axis deviation. A QRS axis that falls between +110 o and +180 o is abnormal and called right axis deviation. A QRS axis that falls between +180 o and -90 o is abnormal and called Indeterminant (extende rt axis ).

FORMATION OF HEXAAXIAL SYSTEM I IIIII 31

aVR aVL aVF FORMATION OF HEXAAXIAL SYSTEM 32

aVR aVL I III II +90 aVF FORMATION OF HEXAAXIAL SYSTEM 33

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: 34

HOW TO CALCULATE THE QRS AXIS  Leads & there isoelectric partners 35

REFERENCES  Text book of medical physiology by GUYTON & HALL 11 th edition  Text book physiology by GANONG  The ECG made Easy by John R.Hampton sixth edition 36