P wave = activation (depolarization) of atria. PR interval = time interval between onset of atrial depolarization and onset of ventricular depolarization. QRS complex = depolarization of ventricles, consisting of the Q, R, and S waves. QT interval = time interval between onset of ventricular depolarization and end of ventricular repolarization. R-R interval = time interval between 2 QRS complexes. T wave = ventricular repolarization. ST segment plus T wave (ST-T) = ventricular repolarization. U wave = probably after-depolarization (relaxation) of ventricles.