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ECG Basics

The QRS Complex



  • 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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