Ventricular Tachycardia

 

• Definitions and pathophysiology

  Ventricular tachycardia

  • Tachycardia arising from a ventricular ectopic focus.
  • Often occurs in a structurally abnormal heart. Injury to the ventricular myocardium can leave strands of functioning tissue among damaged tissue, creating the possibility of a re-entrant loop, which is the commonest trigger for VT.
  • 'Sustained' when it lasts >30 seconds.
  • VT may or may not be accompanied by a cardiac output.

  Ventricular fibrillation

  • Irregular, chaotic ventricular activity without effective cardiac output, and without identifiable P wave, QRS, or T waves on ECG.

• DDx: Broad complex tachycardia

  • Ventricular tachycardia (80%).
  • Supraventricular rhythm (20%) with abnormal conduction through ventricles ('aberrancy'), due to conduction defect (RBBB/LBBB), accessory pathway (WPW), or electrolyte imbalance (↑K+, Na+ channel blockade).

• ECG findings

  • HR is usually 150-250.
  • Spread of depolarisation through ventricular myocardium is slower than through conduction pathways, creating a broad QRS (>0.12 seconds).
  • The QRS complex can be monomorphic (commoner) or polymorphic.
  • Signs of AV dissociation: P waves and QRS at different rates, atrial capture beats (normal QRS appears amidst a run of VT), fusion beats (P wave fused with a broad QRS).

• Management

  In cardiac arrest:

  • VF and pulseless VT are shockable rhythms.

  If there is a pulse, VT is treated with:

  • Synchronised DC shocks (up to 3) if unstable – SBP <90, syncope, cardiac ischaemia, HF – followed by amiodarone 300 mg IV over 20 mins, further shock, then amiodarone 900 mg over 24h.
  • Straight to amiodarone IV if stable, according to guidelines, though a recent RCT suggests procainamide is superior.

• Torsades de pointes

  Definition

  • Polymorphic VT with a characteristic pattern of QRS complexes of increasing and decreasing magnitude.
  • A complication of congenital or acquired long QT syndrome (LQTS).

  Signs and symptoms

  • Can present with sudden ↓BP, dizziness and syncope.
  • Usually self-resolves quickly but can progress to sustained VT or VF.

  Management

  • Magnesium sulphate IV.