The ST segment is one of the most closely analyzed regions of the electrocardiogram (ECG) in clinical practice. Interpreting it correctly requires conceptual understanding, technical precision in measurement, and appropriate clinical integration. In this schematic summary from the ECG library, we review its definition, how to measure deviations, the criteria for ST elevation and depression, and their main causes.

Diagram illustrating the ST segment on the electrocardiogram
ST segment.

Definition of the ST Segment

The ST segment is the interval between the end of the QRS complex (J point) and the beginning of the T wave. Electrophysiologically, it predominantly reflects phase 2 of the ventricular action potential (plateau).

Under normal conditions, the ST segment is isoelectric.

Baseline and Reference Point

Baseline

  • Preferably: PQ junction
  • Alternatively: TP segment

J point

The J point is the junction between the end of the QRS and the beginning of the ST segment.

How to Measure ST-Segment Deviation

The magnitude of ST-segment deviation is determined by the vertical distance between the PQ junction and the J point.

  • 1 mm = 0.1 mV
Diagram showing how to measure ST-segment elevation from the J point
How to measure ST-segment elevation.

ST-Segment Elevation

ST-segment elevation is defined as elevation of the J point above the baseline.

  • ≥ 1 mm in at least two contiguous leads

Exception in V2 and V3 according to sex and age.

Isolated ST elevation is not synonymous with acute coronary occlusion.

ST-Elevation Morphologies

Concave

Convex or straightened

  • High specificity for acute myocardial infarction

Causes of ST-Segment Elevation

Ischemic

Non-ischemic

ST-Segment Depression

  • ≥ 0.5 mm, horizontal or downsloping

Causes of ST-Segment Depression

Ischemic

Non-ischemic

References

  1. Scheffer MK, De Marchi MFN, de Alencar Neto JN, Felicioni SP. Eletrocardiograma de A a Z . São Paulo: Manole, 2024.
  2. Scheffer MK, de Alencar Neto JN, De Marchi MFN, Felicioni SP. Eletrocardiograma na síndrome coronária aguda . São Paulo: Manole, 2025.
  3. Thygesen K, et al. Fourth Universal Definition of Myocardial Infarction . Eur Heart J. 2018;39:119-177.