The T wave reflects phase 3 of the action potential, which is terminal repolarisation. It starts epicardially due to the longer action potential duration of endocardial cells, resulting in a positive polarity in leads with a high R. Changes in repolarisation can cause characteristic alterations in the T wave, which are significant in arrhythmogenesis. A special characteristic of T-wave changes is that they often occur only fleetingly (i.e. transiently)!
