Is Torsades de Pointes a Type of Ventricular Tachycardia?
Yes, torsades de pointes is definitively classified as a specific type of polymorphic ventricular tachycardia that occurs in the setting of a prolonged QT interval. 1
Classification and Definition
Torsades de pointes is polymorphic ventricular tachycardia (VT) characterized by a waxing and waning QRS amplitude that occurs specifically when the QT interval is prolonged. 1 The 2017 AHA/ACC/HRS guidelines explicitly define it as "polymorphic VT that occurs in the setting of a long QT interval." 1
Key Distinguishing Features from Other VT Types
The classification of torsades as a VT subtype is based on several defining characteristics:
- Rate criteria: It meets the fundamental VT definition of ≥3 consecutive ventricular complexes at a rate >100 bpm, specifically ranging from 160-240 bpm 1, 2
- Morphology: Unlike monomorphic VT (stable single QRS morphology), torsades exhibits changing or multiform QRS morphology from beat to beat, which classifies it as polymorphic VT 1
- Context-specific: The critical distinguishing feature is that it occurs exclusively in the setting of marked QT prolongation (>500 ms), which separates it from other polymorphic VTs 1
Why This Classification Matters Clinically
The classification as a specific VT subtype has critical therapeutic implications because torsades requires fundamentally different management than other ventricular tachycardias. 3, 4
Treatment Divergence from Standard VT
- Contraindicated therapies: Standard class I antiarrhythmic agents used for other VTs will worsen torsades and are strictly contraindicated 3, 5
- First-line treatment: IV magnesium sulfate 2g is the treatment of choice, regardless of serum magnesium level 2, 6, 3
- Rate augmentation: Unlike other VTs where slowing the rate is beneficial, torsades requires heart rate acceleration through pacing or isoproterenol to shorten the QT interval 1, 3, 4
Relationship to Ventricular Fibrillation
While torsades is a type of VT, it has a unique relationship with ventricular fibrillation:
- Self-termination: Unlike ventricular fibrillation, torsades frequently terminates spontaneously with the last 2-3 beats showing slowing 1, 7
- Degeneration risk: However, it can degenerate into ventricular fibrillation and cause sudden cardiac death 1, 7
- Distinct entity: The slower rate (160-240 bpm vs >300 bpm for VF) and visible QRS complexes distinguish it from ventricular fibrillation 7, 8
Clinical Recognition Pitfalls
The characteristic "twisting of the points" morphology may not always be visible, especially in nonsustained episodes or with limited ECG leads, but this does not change its classification as polymorphic VT. 1 The diagnosis should be based on the combination of polymorphic VT occurring with marked QT prolongation, not solely on visualizing the twisting pattern 1.