The ST segment is the gap between the end of the QRS complex and the start of the T wave. It represents early ventricular repolarisation. The start of the ST segment is called the junction, or J point.
The key feature of the ST segment is whether it is isoelectric, elevated or depressed. A normal ST segment may be isoelectric (at the baseline), but it may also have some degree of elevation in healthy individuals. This makes it difficult at times to tell the difference between a normal variant and ischemia / infarction.
Ischemia and infarction are some of the most important causes of ST changes, but other causes also include normal variants, hypertrophy, conduction blocks, digoxin, pericarditis, aneurysm, contusion, pulmonary embolism and hypothermia. Reciprocal ST changes are key features of infarction, which means there is ST elevation and depression in leads that are anatomically opposite one another.