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

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CONDUCTION SYSTEM OF THE HEART 3

PRINCIPLE OF RECORDING ECG Battery The body is a good conductor of electricity 4

ELECTROCARDIOGRAM  Electrocardiogram: Measure of the electrical activity of the heart per unit time.  Potential differences generated by heart are conducted to body surface where they can be recorded on electrodes on the skin.  Does NOT measure the flow of blood through the heart.  Electrocardiograph : device that records the electrical activity of the heart 5

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ECG  What is Lead?  ECG recorded on the paper after putting the electrode.  What is Electrode?  It is a metallic plate which conducts electricity. 7

LEADS ARE LIKE PICTURES OF THE HEART 8

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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 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) 11

BIPOLAR / STANDARD LIMB LEADS  Record the electrical potential difference in the frontal plane  In bipolar lead, we record potential difference between two active electrodes. 12

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. 13

EINTHOVEN’S TRIANGLE The axis of the limb leads form an equilateral triangle around the heart’s electrical field 14

PRECORDIAL (CHEST) LEADS 15

HEART IN THREE DIMENSION 16

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. 17

Standard 12 Lead ECG 18

THE ECG PAPER  Horizontally  One small box s  One large box s  Vertically  One large box mV 1 mm 19

BASIC COMPONENTS  Waves  Segments & intervals 20

BASIC COMPONENTS

NORMAL IMPULSE CONDUCTION Sinoatrial node AV node Bundle of His Bundle Branches Purkinje fibers 22

IMPULSE CONDUCTION & THE ECG Sinoatrial node AV node Bundle of His Bundle Branches Purkinje fibers 23

THE “PQRST” T wave - Ventricular Repolarization QRS - Ventricular Depolarization P wave - Atrial Depolarization 24

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) 25

SEGMENTS & INTERVALS  PR interval: 0.12 – 0.20 sec  QRS interval : 0.08 – 0.10sec  QT interval : sec  ST interval : sec 26

THE ELCTRICAL AXIS OF THE HEART  Vector is diagrammatic way to show strength & direction of impulse 27

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 28

MEAN ELECTRICAL AXIS OF THE HEART: QRS AXIS  Normal range 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 ).

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 31