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ECG Basics

The ST Segment



The ST segment on an ECG trace reflects early ventricular repolarisation, and is classically used to assess for cardiac ischaemia.
  • Look For

  • The section between the end of the QRS complex (J point) and the beginning of the T wave.

ST Elevation

ST elevation is classically associated with ST elevation myocardial infarction (STEMI) and as such is a medical emergency if seen on ECG. However, there are several other causes of ST elevation that it is important to be aware of.
  • Look For

    Elevation of the J point (meeting point of the QRS complex and ST segment)
  • >2mm in V1-V3
  • >1mm in limb leads
  • ST Elevation
  • Causes of ST Elevation

  • ST elevation myocardial infarction
  • Prinzmetal angina (coronary vasospasm)
  • Tako-tsubo cardiomyopathy
  • Pericarditis
  • Left bundle branch block
  • Left ventricular hypertrophy
  • Benign early repolarisation (physiological)
  • Subarachnoid haemorrhage
  • Interpretation

  • Localised ST elevation with reciprocal ST depressionST elevation myocardial infarction, prinzmetal angina, tako-tsubo cardiomyopathy
  • ST elevation discordant with the QRS complexLeft bundle branch block, left ventricular hypertrophy
  • Mild concave ST elevation with tall T waves in at least 1 precordial lead ± notch at end of QRS complex (j-point notching)Benign early repolarisation
  • Widespread ST elevationPericarditis, subarachnoid haemorrhage, tako-tsubo cardiomyopathy
  • Examples

  • Benign early repolarisation with concave ST elevation, tall T waves and J-point notching:
  • ST Elevation
Massive ST elevation (“tombstoning” pattern) associated with large MI:
  • ST Elevation

ST Depression

  • Look For

  • Depression of the J point (meeting point of QRS complex and ST segment) of >1mm. Note whether the ST segment is horisontal, downsloping or upsloping.
  • ST Depression
  • Causes of ST Depression

  • Exercise
  • Cardiac ischaemia without infarction - downsloping ST depression
  • Non-ST elevation MI (NSTEMI)
  • ST elevation MI (STEMI) - reciprocal to ST elevation
  • Hypokalaemia - widespread downsloping ST depression
  • Digoxin - reverse tick ST depression
  • Left ventricular hypertrophy - downsloping
  • Left bundle branch block
  • Example

  • ST depression, T wave flattening and QT prolongation associated with hypokalaemia:
  • ST Depression


ST elevation can be used to localise a cardiac lesion, while ST depression cannot.
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