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An ST elevation is considered significant if the vertical distance between the ECG trace and the isoelectric line at a point 0.04 seconds after the J-point is at least 0.1 mV (usually representing 1 mm or 1 small square) in a limb lead or 0.2 mV (2 mm or 2 small squares) in a precordial lead. The isoelectric line is either the PR interval or the TP interval. 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%.
The ST segment corresponds to a period of ventricle systolic depolarization, when the cardiac muscle is contracted. Subsequent relaxation occurs during the diastolic repolarization phase. The normal course of ST segment reflects a certain sequence of muscular layers undergoing repolarization and certain timing of this activity. When the cardiac muscle is damaged or undergoes a pathological process (e.g. inflammation), its contractile and electrical properties change. Usually, this leads to early repolarization, or premature ending of the systole.
The exact topology and distribution of the affected areas depend on the underlying condition. Thus, ST elevation may be present on all or some leads of ECG.
It can be associated with: