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ECG Basics
 
 

The Normal ECG Trace

 
 
 
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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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