T waves represent repolarisation of the heart, and undergo temporary or permanent change.
Normal: T waves down in V1, V2, never V3, sometimes in III and aVF (1). The T wave may be normally inverted or upright in III and in V1 (and in V2 in young people, and also in V3 in some Afro-Caribbeans).
The T wave may be lengthened or made taller by electrolyte imbalances, especially potassium. Hypokalaemia flattens the T wave and prolongs the QT interval (the duration from the Q wave to the end of the T wave) to more than 0.4 seconds.
Reference:
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