-ECG refers to graphical recording of electrical activities of heart-electrocardiogram.

-The electrical activity occurs prior to mechanical component of cardiac cycle.

-Electrocardiography is the technique and electrocardiograph is the instrument used to record electrical activities.


• Uses of ECG :

1) To record heart rate ( when heart rate is very high)

2) To record rhythm of heart rate

3) Abnormal condition in the heart ( ectopic phase makers )

4) Cardiac ischemia

5) Heart attack

6)Coronary artery diseases

7) Hypertrophy of ventricles.


Principle of ECG:

– It is the summated electrical activity of heart. It is recorded from the surface of body by using electrodes which are then connected to a amplifier (ECG machine) and recorded on the paper.

-The graph paper has small squares on the x-axis it shows the time in mili seconds and in y-axis it shows amplitude in mili volts .

On x-axis 1mm=0.04sec

On y-axis 1mm=0.1mv.


•ECG leads :


There are 2 types of leads jn ECG

1)Bipolar leads – both are active electrodes

2)Unipolar leads – 1 electrode is indifferent / inactive and 2 nd electrode is active.

-These leads are made by placing series of electrode on the surface of body.

-These electrodes are connected to ECG machine.

-There are total 12 leads including bipolar and unipolar leads.


*Bipolar leads : 

In bipolar leads there are 3 limb leads : lead 1, lead 2 , lead 3.


Einthoven’s principle :

-Here bipolar leads are used for the first time by Einthoven.

-He developed lead 1, lead 2 and lead 3.

-The electrodes are connected to right arm, left arm, and left foot.

-The electrodes are connected which makes a triangle heart is assumed as the centre of triangle.


Lead 1 :

It is between left arm and right arm

Left arm is positive

Right arm is negative.


Lead 2 :

It is between right arm and left foot

Left foot is positive

Right arm is negative.

Lead 3 : It is between left arm and left foot.

Left foot is positive

Left arm is negative.


*Unipolar leads :

In unipolar leads there are three unipolar augmented limb leads and unipolar chest leads.


Augmented limb leads :

These are aVL , aVR , and aVF.

They are recorded by the same leads but recordings are amplified.

The indifferent electrode is connected through any 2 limb and the active electrode is connected to the respective limb.


In aVR – active electrode is connected to right arm.

In aVF – active electrode is connected to left foot.

In aVL – active electrode is connected to left arm.


Chest leads :

They are unipolar leads also known as precardial chest leads.

There are 6 chest leads beginning from V1 to V6.

Here indifferent electrode is connected through all the 3 limb leads.

V1 – It is located in the right 4 th intercostal space just on the right border of  sternum.

V2 – lt is located in the left 4 th intercostal space just on the left border of sternum.

V3 – It is located in between V2 and V4.

V4 – It Is located in the left 5 th intercostal space at the midclavicular line.

V5 – It is located in the left 5 th intercostal space in the anterior axillary line.

V6 – lt is located in the left 5 th intercostal space in the mid axillary line.


• Waves of ECG :

There are S waves on ECG.

They are Q, R, S, T.

Sometimes 6 th wave is seen on ECG.

Q, R and S waves together form QRS complex.

P wave :

It is a first positive wave in ECG.

I represents atrial depolarization.

The duration is 0.1 sec .

The amplitude is 0.12 mV.

Significance – P wave is absent in atrial fibrillation , SA node block.

P wave is increased in amplitude in hypokalemia.


QRS complex :

It has both positive and negative waves

Q wave is initial , small, negative.

R wave is large and positive.

S wave is negative wave following R wave .

In QRS complex Q represents depolarization of base of ventricles.

R wave is produced due to depolarization of ventricular muscle and apex of heart .

S wave represents depolarization of base of ventricles at the AV ring.

Duration of QRS is 0.1 sec .

Amplitude in Q wave is 0.1 mV, R wave is 1 mV, S wave is 0.4 mV.

Significance – QRS complex is prolonged in hyperkalemia QRS complex us prolonged also in bundle branch block.


U wave : 

It is not usually seen when it is seen it is a positive wave.

It indicates bradycardia, thyrotoxicosis, hypercalcemia, and hypokalemia.


• Intervals of ECG : 


(1) P-R interval :

It is the duration of time between beginning of P wave to the beginning of QRS complex.

Normal duration is about 0.18 sec.

P-R interval represents arterial depolarization and conduction through AV node.

P-R interval shorten when heart rate increases.

P-R interval increases in decrease in heart rate , first degree heart block and in AV nodal delay.


(2) Q-T interval :

It is the time between onset of Q wave to the end of T wave.

Normal duration is 0.4 sec.

It indicates depolarization and repolarization.

Q-T interval is prolonged in myocardial infarction, hypothyroidism , and hypocalcemia .

Q-T interval is shortened in hypocalcemia.


(3) S-T segment : 

It represents ventricular repolarization .

It is a time between end of S wave and the onset of T wave.

Duration is 0.08sec.

S-T segment is shortened in hypercalcemia.

S-T segment is prolonged in hypocalcemia.

S-T segment is depressed in acute myocardial infarction.


(4) R-R interval :

It is the duration between 2 successive R waves.

R-R interval is used to count heart rate when it increases more than 100 per minute .

R-R interval signifies duration of cardiac cycle .

Normal R-R interval is 0.8 sec.

When R-R interval decreases by 0.5 sec , cardiac cycle significantly reduced and heart rate tremendously increased.



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