ELECTROCARDIOGRAM (ECG)Cardiovascular System (CVS 227)

BASIC PRINICPLESDr.Mohammed Sharique Ahmed Quadri

Asst. professor in physiologyAl Maarefa College

الرحيم الرحمن الله بسم

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CONDUCTION SYSTEM OF THE HEART

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PRINCIPLE OF RECORDING ECGBattery

The body is a good conductor of electricity

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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

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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.

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LEADS ARE LIKE PICTURES OF THE HEART

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STANDARD 12-LEAD ELECTROCARDIOGRAM Six Limb Leads :

Three Bipolar Limb Leads( standard limb leads) Frontal plane currents (right, left, inferior, superior) named as , I, II , III

Three Unipolar Limb Leads ( augmented limb leads)

named as aVR, aVL, aVF

Six precordial leads Horizontal plane (anterior, posterior, right and

left forces)8

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

Vertically One large box - 0.5 mV

1mm

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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 fibers19

IMPULSE CONDUCTION & THE ECG

Sinoatrial node

AV node

Bundle of His

Bundle Branches

Purkinje fibers

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THE “PQRST”

• T wave - Ventricular Repolarization

• QRS - Ventricular

Depolarization

• P wave - Atrial Depolarization

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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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MEAN ELECTRICAL AXIS OF THE HEART:QRS AXIS Normal range

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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 -30o and +110o.

0o

30o

-30o

60o

-60o-90o

-120o

90o 120o

150o

180o

-150o

A QRS axis that falls between -30o and -90o is abnormal and called left axis deviation.

A QRS axis that falls between +110o and +180o is abnormal and called right axis deviation.

A QRS axis that falls between +180o and -90o is abnormal and called Indeterminant (extende rt axis ) .

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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