The normal ECG consists of a P wave, a QRS complex, and a T wave. The P wave, which is the result of action potentials that cause depolarization of the atrial myocardium, signals the onset of atrial contraction. The QRS complex is composed of three individual waves: the Q, R, and S waves. The QRS complex results from ventricular depolarization and signals the onset of ventricular contraction. The T wave represents repolarization of the ventricles and precedes ventricular relaxation. A wave representing repolarization of the atria cannot be seen because it occurs during the QRS complex.
