An electrocardiogram (ECG) is a recording of action potentials produced during each heartbeat and the instrument used to record it is an electrocardiograph. In order to obtain a standard ECG, a patient has to be connected to the machine with three electrodes (one to each wrist and to the left ankle), that continuously monitor the cardiac cycle. Each electrocardiogram is composed of three distinct ECG waves, designated as P, QRS and T.
The P wave represents electrical excitation or the depolarization of the atria, which leads to contraction of the atria and is caused by the activation of the SA node. The second wave called the QRS complex represents ventricular depolarization, which is caused by the impulse of the AV node, the bundle of his and Purkinje fibres. The QRS complex begins with a small downward deflection (Q), continues as a large upward deflection (R) and ends with a small downward deflection (S). The dome-shaped T wave represents ventricular repolarization, which repolarizes the depolarised ventricles.
The PR interval includes the P wave and the PR interval. The PR interval is the time taken by the AV node to conduct the impulse. The ST segment is the time gap between depolarization and depolarization in the ventricles. The QT interval represents the time between activation of electrical activity in the ventricles and their return to the resting state.
