6 letters are used to describe deflections from baseline on ECG.
The P wave reflects the atrial depolarization.
QRS complex follows P wave. It corresponds to the depolarization of the right and left ventricle. Depolarization wave spreads from the inner to the outer layers of myocardium.
The T wave represents the repolarization of the ventricles.
There are four theories of U wave appearance: 1) repolarization of the papillary muscles, 2) delayed repolarization of Purkinje fibres, 3) after-potentials from the ventricular wall, 4) prolonged repolarization of mid-myocardial cells.