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Tachyarrhythmias
 
 

Overview

Ventricular tachycardia (VT) is an imminently life-threatening arrhythmia that must be rapidly diagnosed and treated. Supraventricular tachycardia is a more benign arrhythmia that may also present as a wide complex tachycardia when combined with aberrant conduction; this can be difficult to distinguish from VT.
This section includes a list of causes of wide complex tachycardia, as well as a guide to key findings used to distinguish between causes.
  • Causes of Wide Complex Tachycardia

  • Regular

  • Monomorphic ventricular tachycardia
  • Supraventricular tachycardia with aberrant conduction (bundle branch block)
  • Supraventricular tachycardia with preexcitation (accessory pathway)
  • Pacemaker-mediated tachycardia - atrial tracking or endless loop tachycardia
  • Irregular

  • Polymorphic ventricular tachycardia
  • Atrial fibrillation with aberrant conduction (bundle branch block)
  • Atrial fibrillation with preexcitation (accessory pathway)
  • Other

  • Artefact - due to CPR, shivering, shaking or other movements

Distinguishing Between VT and SVT with Aberrancy

Several ECG features can be used to distinguish between ventricular tachycardia and supraventricular tachycardia with aberrant conduction.
  • Factors favouring VT over SVT with Aberrancy

  • Patient History

  • Structural heart disease (95% of wide complex tachycardias in patients with heart disease will be VT)
  • Normal baseline ECG - no bundle branch block or preexcitation
  • ECG Findings

  • Extreme axis deviation (-90 to 180°)
  • Atypical LBBB or RBBB morphology
  • QRS duration >140 with RBBB pattern or >160 with LBBB pattern
  • AV dissociation - visible, regular P waves (Almost 100% specific for VT)
  • Positive or negative concordance - precordial leads all positive or all negative
  • RS interval >100ms in precordial leads - from onset of R wave to deepest point of S wave
  • R wave peak time ≥50ms in lead II - from isoelectric line to peak of R wave
  • Onset and Offset

  • Wide complex tachycardia initiated by a ventricular complex
  • Fusion beats: a hybrid between a normal QRS complex and a ventricular ectopic
  • Capture beats: a normal, narrow QRS complex produced during due to a conducted atrial beat

Extreme Axis Deviation

Extreme axis deviation, also known as northwest axis, is highly suggestive of ventricular tachycardia.
  • Look For

  • Negative lead I
  • Negative lead aVF
  • Extreme Axis Deviation

Atypical LBBB

  • Typical LBBB

  • Wide QRS ≥120ms
  • Broad R wave in lateral leads (I, aVL, V5, V6)
  • Absent Q waves in I, V5 and V6
  • Atypical LBBB
  • Atypical Findings

  • Initial R wave >30ms
  • Josephson’s sign - notching of the S wave
  • RS interval >100ms in precordial leads
  • Q waves in V6
  • Atypical LBBB

Atypical RBBB

  • Typical RBBB

  • Wide QRS ≥120ms (complete BBB) or 110-120ms (incomplete BBB)
  • Added R wave (R’) in right precordial leads - RSR’ pattern in leads V1 or V2 (R’ taller than R) (Due to delayed conduction to right ventricle)
  • Slurred S wave in lateral leads - S wave duration > R wave duration (or >50ms) in leads I, V5 and V6
  • Atypical RBBB
  • Atypical Findings

  • Monophasic R wave
  • R wave taller than R’ (taller left rabbit ear)
  • Q wave in V1
  • QS waves in V6
  • R:S ratio <1 in V6 (S wave deeper than R wave is tall)
  • Atypical RBBB
  • Atypical RBBB
  • Atypical RBBB

AV Dissociation

The presence of P waves that are regular are not associated with QRS complexes is highly suggestive of VT.
  • AV Dissociation
     

Concordance

Concordance, and particularly negative concordance, is highly suggestive of VT.
  • Features of Concordance

  • Positive concordance - all precordial leads are positive
  • Negative concordance - all precordial leads are negative
  • Positive concordance:
  • Positive concordance
  • Negative concordance:
  • Negative concordance

RS Interval in Precordial Leads

The RS interveal refers to the time from the onset of the R wave to the deepest point of the S wave in the precordial leads.
An RS interval >100ms in the precordial leads is suggestive of VT.
  • RS Interval in Precordial Leads
     

R Wave Peak Time in Lead II

Look for the time from the start of the depolarisation from the isoelectric line to the first deflection in the complex.
An R wave peak time ≥50ms in lead II is suggestive of VT.
  • R Wave Peak Time in Lead II
     

Fusion & Capture Beats

If present in the context of a wide complex tachycardia, fusion and capture beats are suggestive of VT.
Fusion beats are hybrid complexes that occur when a ventricular beat and a supraventricular beat coincide.
A capture beat occurs when a sinus impulse is normally conducted down the AV node and 'captured' by the ventricle, resulting in a normal QRS complex.
  • Fusion & Capture Beats
     
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