Describe the principles behind the ECG

The ECG is a graphical representation of the electrical activity of the heart, as measured by the sum of electrical vectors at the patients skin.


An ECG consists of:

  • Electrodes
    Disposable, sticky components which act as conductors due to a silver/silver chloride coating. To reduce electrode impedance, skin should be:
    • Hairless
    • Dry
    • Clean
  • Cables
    Shielded to prevent currents being induced and electrocuting the patient.
  • Processor
  • Monitor

ECG Leads

ECG leads are created by taking the potential difference between two electrodes, which varies by 0.5-2mV through the cardiac cycle as myocardium depolarises. ECG leads are divided into:

  • Limb leads
    Potential difference between limb electrodes:
    • I: RA to LA
    • II: RA to LL
    • III: LA to LL
  • Augmented leads
    Potential difference between the average of the limb leads (called the indifferent electrode) and each individual limb lead.
    • Augmented leads are of much lower voltage and must be amplified
    • Three augmented leads exist (one for each limb electrode)
  • Precordial leads
    Potential difference between the indifferent electrode and one of the six additional electrodes placed on the chest wall.

The relationship between electrodes and leads is described with Einthoven's Triangle:


  • As the myocardial membrane potential changes across the cardiac cycle, a potential difference can be measured at the skin.
    • A depolarisation wave traveling towards the positive electrode (or a repolarisation wave traveling away) will cause an upward deflection in the ECG
  • These potential differences are very small, and therefore need to be:
    • Distinguished from background interference
      Several techniques exist:
      • Common mode rejection
        Identical electrical activity occurring in multiple electrodes is likely due to interference rather than cardiac activity, and is removed from the measured signal.
        • A ground electrode is typically used for this purpose
      • ECG modes
        ECGs can be set to varying levels of sensitivity.
        • Diagnostic mode
          Responds to higher range of frequencies, but is at greater risk of interference.
        • Monitor mode
          ECG responds to a lower range of frequencies, reducing interference but also resolution. This is common on 3-lead ECG.
      • High input impedance
        Minimises signal loss.
    • Amplified
      Frequencies in the desired signal range are amplified.

Sources of Error

  • Improve signal detection
    • Good adherence
    • Optimal skin contact
      Ensure dry and hairless.
  • Minimise external electrostatic forces
    • Earthed
    • Diathermy
    • Shivering


  • ECG electrodes can act as an exit electrode for surgical diathermy


  1. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.
  2. CICM February/April 2016
Last updated 2021-08-23

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