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ECG Basics

The QRS Complex

February 15th, 2021


The QRS complex indicates ventricular depolarisation.
  • Look For

  • Q wave - an initial negative deflection
  • R wave - a positive deflection
  • S wave - a negative deflection after the R wave

Prolonged QRS Interval

The duration of the QRS interval is useful for determining the origin of an abnormal rhythm, particularly in the setting of tachycardia.
    • Normal Duration

    • 80 - 120ms (2-3mm)
  • Interpretation

  • Narrow QRS complexes (<120ms) - indicative of a supraventricular rhythm from the SA node, atria or AV node.
  • Wide QRS complexes - indicative of a ventricular rhythm, or aberrant conduction of a supraventricular rhythm.
  • Causes of Wide QRS Complexes

  • Ventricular Origin

  • Premature ventricular complexes
  • Ventricular tachycardia
  • Pacemaker rhythm
  • Aberrant Conduction

  • Bundle branch block
  • Accessory pathway, e.g. Wolff-Parkinson-White
  • Hyperkalaemia
  • Sodium channel blocker toxicity - tricyclic antidepressants, type I antiarrhythmics, local anaesthetics
  • Example

  • Wide QRS complexes in the setting of left bundle branch block.
  • Prolonged QRS Interval

Pathological Q Waves

While T wave and ST changes revert post myocardial infarction, Q waves are permanent and thus their presence may indicate previous infarction.
  • Look For

    A negative deflection that is either broad or deep:
  • Broad: >40ms (1mm) in duration
  • Deep: >0.2mV (2mm) or >1/3 of R wave size
  • Pathological Q Waves
Non-pathological Q waves may occur in I, III, aVL, V5 and V5.
  • Causes of Pathological Q Waves

  • Myocardial infarction
  • Hypertrophic cardiomyopathy (inferior / anterior leads)
  • Dilated cardiomyopathy (leads V1 to V4)

Poor R Wave Progression

Poor R wave progression is classically suggestive of anterior MI, though may occur in certain other conditions.
  • Look For

  • Gradual change in the direction of the QRS complex from negative to positive across the chest leads, generally with peak R wave height in V4 that tapers off over V5 and V6.
  • Normal R wave progression:
  • Normal R wave progression
  • Poor R wave progression: 
  • Poor R wave progression 
  • Causes of Poor R Wave Progression

  • Normal variant
  • Anterior myocardial infarction
  • Left ventricular hypertrophy
  • Right ventricular hypertrophy
  • Incorrect chest lead placement
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