ECG (electro cardio graph):

A permanent record of the electric events occurring during a cardiac cycle, made on a graph paper in the form of a wave is called electrocardiogram.

• The instrument used to plot this is called electrocardiograph. ECG plays an important role in heart related problems.
• Normally during each beat, human heart shows 5 waves:

(1) P, R & T are above the base line of electrocardiogram, so called positive waves. These are sharp.

(2) Q & S are below the base line of electrocardiogram, so called negative waves. These are dull.

(3) P wave represents the excitation of atria (depolarization) which causes contraction of both atria.

(4) QRS wave represents contraction of ventricles (depolarization).

(5) And T wave represents the repolarization (relaxation) of ventricles.

(6)Depolarization means contraction.

(7)Repolarization means relaxation.