ECG is a graphical representation of heart activities during a cardiac cycle. ECG is Electrocardiograph. Suppose a patient is connected to the machine with three electrical leads that is one to each wrist and to the left ankle and it continuously monitors the heart activity. For a detailed evaluation of the heart's function, multiple leads are attached to the chest region.
Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.
P-wave: It represents the electrical excitation (or depolarization) of the atria, which leads to the contraction of both the atria.
QRS complex: It represents the depolarization of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after
T-wave: It represents the return of the ventricles from excited to the normal state (repolarisation).
The end of the T-wave marks the end of systole. Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heartbeat rate of an individual. Since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence, it is of great clinical significance.