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The P wave is a small wave that represents the depolarization of the atria. During this wave, the muscles of the atria are contracting.
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The QRS complex is a rapid down‐up‐down movement. The upward movement produces a tall peak, indicated by R. The QRS complex represents the depolarization of the ventricles.
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The T wave represents the repolarization of the ventricles. Electrical activity generated by the repolarization of the atria is concealed by the QRS complex.
Figure 1. Different periods of the cardiac cycle: (a) electrocardiogram, (b) heart sounds, (c) valves, (d) pressure, and (e) volume of left ventricle.