1. Electrocardiogram or ECG is the graphic v record of electrical variations produced by the heart during one heartbeat or cardiac cycle. 

2. ECG is taken with the help of an instrument called electrocardiograph or ECG machine. Electrocardiograph records the action potentials generated by the heart muscles.

3. The electrical activity of heart is represented in the form of a graph plotted with time on X-axis against voltage displacement on Y-axis. 

4. A normal ECG is a graph having series of ridges and furrows. There are waves such as P-wave, QRS complex and Twave. 

5. P-wave is a small upwards wave representing impulse generated by SA node. P-wave is caused by atrial depolarization that results in atrial contraction. 

6. QRS-complex wave begins as a downward deflection, continues as a large upright triangular wave and ends’ as a downward wave. 

7. QRS-complex wave represents spreading of impulse from SA node to AV node, then to bundle of His and Purkinje fibres. It causes ventricular depolarization resulting in ventricular contraction.

8. T-wave is a broad upward wave which represents ventricular repolarization resulting in ventricular relaxation. 

9. Functions of ECG are mainly for diagnosis and also for prognosis. It is useful to detect abnormal functioning of heart as in coronary artery diseases, heart block, angina pectoris, tachycardia, ischemic heart disease, myocardial infarction, cardiac arrest, etc.