Electrocardiogram (ECG)

Last revised by Amanda Er on 19 Mar 2022

Electrocardiography is the process of recording an electrocardiogram (ECG). An ECG is a recording of the heart's electrical activity carried out by measuring the potential difference across different points on the skin surface using electrodes.


This is often carried out as a '12 lead' ECG where 10 physical electrodes are used to produce 12 'leads'. In this circumstance, a lead represents the direction the potential difference is measured and is derived from the physical electrodes placed on the skin. 6 electrodes are placed across the chest (V1 to V6) and 1 electrode is placed on each limb (RA, LA, RL, LL).

Chest (praecordial) lead skin electrode positions:

V1: 4th intercostal space, right parasternal

V2: 4th intercostal space, left parasternal

V3: midway between V2 and V4

V4: 5th intercostal space in the nipple line in men (under the breast in women)

V5: midway between V4 and V6

V6: midaxillary line on the same horizontal line as V4 (may not be 5th intercostal space)

These are used to produce three limb leads (termed I, II and III), three augmented limb leads (aVR, aVL and aVF) and six chest (praecordial) leads (V1 to V6). These leads are assessing the heart's electrical activity from a different direction and can be used to assess different anatomical regions of the heart (and also therefore different coronary artery territories). The leads are often grouped as follows:

  • lateral leads - I, aVL, V5, V6
  • inferior leads - II, III and aVF
  • anterior leads - V3, V4
  • septal leads - V1, V2

Pathologies may affect one of those territories or multiple territories, for example, anteroseptal pathology may affect V1, V2, V3 and V4.


There are numerous indications for carrying out an ECG, including:

  • investigation of chest pain (to assess for myocardial ischemia/infarction)
  • investigation of palpitations/suspected arrhythmia
  • known or suspected electrolyte abnormalities (e.g. hyperkalemia)
  • pre-/peri-/post-operative monitoring
  • as part of the 'gating' procedure used when carrying out ECG-gated radiological procedures such as cardiac CT
  • monitoring of medications (e.g. digoxin) or in overdose 2

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