 Dear students, in this topic we shall discuss the electrocardiogram commonly known as ECG. You know that cardiac impulses are based on depolarization and repolarization of the cardiac muscle fibers. These cardiac impulses cause electrical changes while passing through the heart muscles. The electrical crunch generated in this process spreads to the adjacent tissues. Some of this crunch spreads to all body surface. This spread of the crunch to the body surface forms the basis of recording of the electrocardiogram. Dear students, the electrical potentials generated by the crunch can be recorded by placing electrodes of an instrument which is called electrocardiograph on the skin at certain locations. As a result, the electrical changes in the heart can be recorded and these recordings are known as electrocardiogram. An electrocardiogram is in the form of a graph that reflects the various phases of the heartbeat. It reflects depolarization and repolarization as wave deflections. These wave deflections represent specific events of cardiac cycle. For example, we give different designations of these wave deflections and name of different segments. There are three major wave deflections. The first one is a P wave. The second one is a QRS complex. The third one is called a T wave. We will discuss these three major waves in detail. The P wave is caused by the electrical potentials when atria depolarize. It occurs just prior to the contraction of atria. The P wave appears when the ventricles are in the diastole. Dear students, the interval of time between the start of the P wave to the beginning of the QRS complex is known as the PR interval. This interval indicates the amount of time which is required for the sinoatrial node depolarization to reach the ventricles. Sinoatrial depolarization shows the timing of the PR interval. The QRS complex comprises of three separate waves. This complex begins as a short downward deflection which is called Q wave. Then it continues as a sharp upward spike. This spike is called R wave. Then it ends as a downward deflection. This wave is called S wave. The QRS complex indicates depolarization of ventricles. In this interval, ventricles are in systole. That is, blood is being ejected from the ventricles of the heart. Dear students, the T wave is known as repolarization wave. It is caused by the potentials which are generated when ventricles recover from the state of depolarization and repolarize. Here the interval or duration between the completion of depolarization of ventricles and initiation of its repolarization is known as the ST segment. Before starting the T wave and until the end of the QRS complex, this interval is called ST segment. And it represents that now depolarization is over and repolarization has begun. This segment in a normal ECG must be flat. Dear students, electrocardiogram is an important clinical test for the diseases of the heart. Any heart disease which can disturb the electrical activity of the heart produces characteristic types of changes in the patterns of ECG waves. Therefore, understanding of these wave deflections and their patterns is clinically important.