Table 3: Phases of the electrocardiogram (ECG).

WaveElectrical activity

P waveAtrial depolarization
PR intervalTime between the onset of depolarization in the atria and the onset of depolarization in the ventricles
QRS complexVentricular depolarization
ST segmentPlateau phase of ventricular depolarization
T waveVentricular repolarization
QT intervalVentricular depolarization and repolarization
U waveA normal component of the surface ECG represents the delayed repolarization of the Purkinje network, seen at the same time as early after depolarization in patients with a prolonged QT interval and TdP

TdP: torsades de pointes, a malignant form of ventricular arrhythmia, is polymorphic.