• Usual VT is due to reentry and has rate more than 120/mt. AIVR is due to accelerated automaticity and has rate less than 120/mt.
  • Torsades de pointes is due to macro-reentry.

Etiologies

  • Idiopathic- RVOT, fascicular
  • CAD- acute MI, past MI
  • Dilated cardiomyopathy, hypertrophic cardiomyopathy
  • ARVC
  • Drugs

Diagnosis

  • Step 1- search the whole ECG for P waves
    • AV dissociation
      • Especially likely to be seen in leads with low QRS amplitude and in lead v1 (where P waves tend to be prominent)
      • May be absent if there is retrograde atrial activation
      • Entirely specific, but poorly sensitive
    • VA association
      • May be 1:1 or lower.
  • Step 2- search the whole ECG for fusion or capture beats
    • Fusion or capture beats. The latter is premature.
    • Seen only at low rates of less than 160/mt.
  • Step 3- search the precordial leads for RS pattern
    • No RS pattern in any precordial lead
      • Strongly suggests VT. (With RBBB or LBBB, there will be RS in at least one lead.)
    • RS is present in a precordial lead
      • It can still be VT if time from beginning of R to nadir of S is more than 100 msec.
  • Step 4- study QRS morphology in precordial leads
    • RBBB pattern
      • V1-
        • R (can also occur with RVH or PWMI)
        • R with notched downslope producing two rabbit ears, first being taller (Marriott sign, very specific)
        • QR (can also occur with RBBB + MI)
      • V6-
        • QS
        • rS + left axis deviation
    • LBBB pattern
      • V1-
        • R duration 40 msec or more
        • Notched downslope in S
        • Onset of R to nadir of S 60 msec or more
      • V6-
        • QS
        • qR

Other interesting points

    • RV origin VT occurs in young females while LV origin VT occurs in elderly males with heart disease.
    • TDP is usually nonsustained.
    • Ventricular flutter and fibrillation are due to macro-reentry.
    • Ventricular fibrillation is more difficult to defibrillate than ventricular flutter.