It starts at the J point (junction between the QRS complex and ST segment) and ends at the beginning of the T wave. However, since it is usually difficult to determine exactly where the ST segment ends and the T wave begins, the relationship between the RT segment and T wave should be examined together. The typical ST segment duration is usually around 0.08 sec (80 ms). It should be essentially level with the PR and TP segment.
The ST segment represents the period when the ventricles are depolarized. It is isoelectric.
- The normal ST segment has a slight upward concavity.
- Flat, downsloping, or depressed ST segments may indicate coronary ischemia.
- ST elevation may indicate myocardial infarction. An elevation of >1mm and longer than 80 milliseconds following the J-point. This measure has a false positive rate of 15-20% (which is slightly higher in women than men) and a false negative rate of 20-30%.
- ST depression may be associated with hypokalemia or digitalis toxicity.
In fetal monitoring 
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- Mansano, R. Z.; Beall, M. H.; Ross, M. G. (2007). "Fetal ST segment heart rate analysis in labor: Improvement of intervention criteria using interpolated base deficit". Journal of Maternal-Fetal and Neonatal Medicine 20 (1): 47–52. doi:10.1080/14767050601055279. PMID 17437199.