ECG Basics
 
 

The Normal ECG Trace

 
 
 
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On this page:The ECG TraceECG Leads
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Overview

    • Square Sizes

      The standard ECG is recorded at a speed of 25mm/sec and gain of 10mm/mV.
    • 40ms = 1mm = 1 small square
    • 200ms = 5mm = 1 large square
    • 0.1mV = 1mm = 1 small square

The ECG Trace

  • A normal ECG trace includes a P wave, a QRS complex and a T wave.
    • The ECG Trace

    • Look For

    • PQRST Complex

    • P wave - indicative of atrial depolarisation
    • QRS complex - indicative of ventricular depolarisation
    • T wave - indicative of ventricular repolarisation

    • Intervals

    • PR interval - the time between atrial depolarisation and ventricular depolarisation, suggestive of AV conduction time
    • QT interval - the time between ventricular depolarisation and repolarisation

ECG Leads

  • A standard 12-lead ECG includes bipolar limb leads, unipolar limb leads and chest leads.

    • Bipolar Limb Leads

    • Electrical potential difference is measured between two limb electrodes.
  • ElectrodesDirectionComplex
    IRA → LA0Lateral
    IIRA → LL+60Inferior
    IIILA → LL+120Inferior

    • Unipolar (Augmented) Limb Leads

    • Electrical potential difference is measured between one limb electrode and a combination of the other limb electrodes.
  • ElectrodesDirectionComplex
    aVLLA-30Lateral
    aVRRA-150Right atrial
    aVFLL+90Inferior

    • Unipolar Chest Leads

    • Electrical potential difference is measured between the chest electrode and the average of the limb electrodes.
  • Complex
    V1Right atrial
    V2Septal
    V3Anterior
    V4Anterior
    V5Lateral
    V6Lateral
 
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