Graphic record of summated electrical activity of heart obtained by electrodes placed on body surface.

Electrocardiograph is the instrument to record ECG

Willem Einthoven (1860 -1927) was the first to develop ECG Machine in 1903. He, for the first time, recorded electrical activity of heart from body surface. He named it ‘Elektrokardiogramme’ (EKG) which later on anglicized to ‘Electrocardiogram’ (ECG). He won Noble Prize in 1924

  • Cells at rest are polarized –negative from interior & positive from exterior
  • Action Potential records voltage inside the cell
  • ECG records voltage out side the cells

Cardiac Impulse

Cardiac impulse arises in SA node

Spreads to both atria and AV node

Delay of 0.13 sec in AV node

Goes to right and left bundle branches

Through purkinje fibersdistributed to the whole cardiac muscle

Principles of ECG

Electrical activity is recorded when part of muscle fiber is depolarized/repolarized

No electrical activity is recorded when muscle fiber is fully depolarized/repolarized

Positive electrode

  • present in area of positivity
  • upward deflection will be recorded

Negative electrode

  • present in area of negativity
  • downward deflection will be recorded

Electrocardiograph –ECG Machine

A Galvanometer having an electromagnet and amplifier detects voltage difference.

Amplifier enhances the signal

Electromagnet has two poles positive & Negative

Leads connect the poles of ECG machine with electrodes

Electrodes are placed on body surface


Isoelectric line or segment

A flat line on ECG tracing with out any wave


A flat line on ECG tracing having at least one ECG wave

Calculation of heart rate

60/time in sec between 2 consecutive R waves

300/no of large squares between 2 consecutive R waves

1500/no of small squares between 2 consecutive R waves

6 second strip

ECG Lead

Pair of two electrodes connected to electrocardiograph with wires constitute a LEAD

A lead measures voltage difference between two electrodes

Stylus (needle) of ECG machine moves up and down with changing voltage

Waves are recorded on ECG paper which moves beneath the stylus –ECG recording

Types of Leads:

Limb leads

I, II, III -Bipolar leads

aVR, aVL, aVF –Unipolarleads

Chest leads

V1, V2, V3, V4, V5, V6 –Unipolarleads

Lead Axis is the imaginary straight line connecting the two electrodes is called AXIS of the lead

By convention axis of a lead directs toward +ve electrode

A lead records potential difference along its axis (between two electrodes)

Bipolar limb leads

+ve electrode is placed on one limb and –ve on the other

Measure electrical potential difference between +ve and –ve electrodes

Bipolar limb leads

Lead I

+ve electrode on left arm

-ve on right arm

Lead II

+ve electrode on left foot

-ve on right arm

Lead III

+ve electrode on left foot

-ve on left arm

Einthoven’s Triangle

An equilateral Triangle formed by the Axes of three Bipolar limb leads with heart in its centre

Einthoven’s law

Electrical potentials of Bipolar limb leadsI and III equal that of lead II

I + III = II

Unipolarlimb leads

Also called Augmented limb leads

+ve electrode is placed on one limb and the other two limbs jointly act as negative electrode

+ve electrode is called ‘exploring electrode’

-ve electrode is called ‘indifferent electrode’

Voltage of indifferent electrode is kept at almost zero

Actual voltage in the area of +ve electrode is measured

Augmentation enhances the voltage by about 50%

Lead aVF

+ve electrode on left foot

Both upper arms act as indifferent electrode

Lead aVL

+ve electrode on lest arm

Right arm and left foot act as indifferent electrode

Lead aVR

+ve electrode on right arm

Left arm and left foot act as indifferent electrode

Chest (Precardial) leads


+ve electrode is placed on chest

All the three limbs jointly act as negative (indifferent) electrode

V1 to V6

Frontal plane leads

Limb leads: Record electrical activity in frontal plane (superior/inferior & right/left)

Horizontal plane leads

Precardial leads: Record electrical activity in horizontal plane (anterior/posterior & right/left)

Cardiac Vector

Any entity having direction and magnitude can be represented by a Vector

Electrical currents generated in the heart have both direction and magnitude –so can be represented by vector

Moving wave of depolarization or repolarization can be represented by a VECTOR

Vector is represented by an arrow

By convention arrow head points towards +ve area

Length of the arrow represents voltage

Instantaneous Mean Vector

Many vectors summate/cancel each other and the net resultant vector is called InstantaneousMean Vector

Instantaneous Mean Vector is recorded on ECG according to its relation with a particular lead axis

Septal vector

Left to right & generally downwards

Base to apex vector

Right to left and downwards

Apex to base vector

Left to right and upwards

Mean QRS Vector

Represents mean electrical axis of ventricles

Normal direction is +59°

Parallel with lead II axis

Direction is determined by Hexagonal reference system

Determining limb lead potentials from mean QRS vector

Drawing limb lead axes

Drawing of mean QRS vector

Drawing perpendiculars on lead axes

Drawing projected vectors

Bipolar limb leads

Can be determined by knowing net Potential & Polarity in any two bipolar limb leads

Lead I and III are used preferably

Subtraction of smaller deflection from the larger one to get net potential

Net potential is plotted along axes of lead I and III


A perpendicular to any lead axis that passes through centre of the heart divides the lead into two hemifields

One hemifieldis +ve (towards +ve electrode) and the other is -ve (towards –ve electrode)

Deviation of Mean QRS Vector

Left axis deviation

From -30o to -90o

Right axis deviation

From +110oto +180o

Extreme right axis deviation

Between 180oand -90o

Conditions affecting Mean QRS Vector

1. Ventricular hypertrophy

Deviates towards hypertrophied ventricle

2. Bundle branch block

Deviates towards the block

3. Myocardial infarction

Deviates away from infarction

4. Ventricular ectopic pace maker

Depends upon locationof the pace maker

T Wave

At start of repolarization myocardium is fully contracted

High intraventricular pressure reduces blood to endocardium, so it repolarizes low (last)

Epicardium near Apex of the heart repolarizes first

T wave vector remains base to apex

T wave polarity is similar to QRS complex

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