The normal ST is isoelectric. Elevation or depression is judged in relation to the PR segment.
Some changes may be normal:
- Limb lead depression or elevation up to 1mm and in left precordial leads of up to 2mm.
- J‐depression: the J point (junction between the QRS and ST segment) is depressed without sustained ST depression i.e. depression with a gentle upward slope.
- Early repolarisation in adolescents: the ST segment is elevated and concave in leads with an upright T wave.
Others are pathological, either:
- a downward slope of the ST followed by a biphasic inverted T or
- a sustained horizontal ST segment depression 0.08 sec or longer
Pathological ST segment changes are commonly associated with T wave changes and occur in:
- Myocardial ischaemia or infarction
- Severe ventricular hypertrophy (ventricular strain pattern)
- Digitalis effect