A normal electrogram (ECG) is composed of a ' ' wave, a QRS wave (complex), and a T wave. The ' ' wave is a small upward wave that represents electrical excitation or atrial depolarization which leads to contraction of both the atria. QRS wave (complex) begins after a fraction of a second of the wave. It begins as a small downward deflection and continues as a larger upright and triangular wave, ending as a downward wave at its base. It represents ventricular depolarization. The ' ' wave is dome-shaped which represents ventricular repolarization. The potential generated by the recovery of the ventricle from the depolarization state is called the repolarization wave. The end of the ' ' wave marks the end of the systole.

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