Causes of Bidirectional Ventricular Tachycardia
Bidirectional ventricular tachycardia (BdVT) is most commonly caused by catecholaminergic polymorphic ventricular tachycardia (CPVT) and digoxin toxicity, though several other important etiologies must be considered.
Primary Causes
Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT)
CPVT is the classic genetic cause of BdVT, triggered by exertion or emotional stress 1. This condition:
- Develops bidirectional VT specifically with exertion or stress 1
- Results from mutations in ryanodine receptor genes (found in 47% of probands) or calmodulin genes 1
- Carries significant risk of sudden cardiac death if untreated
- Requires aggressive therapy with beta-blockers as first-line treatment 1
Andersen-Tawil Syndrome (LQT7)
This autosomal dominant long QT syndrome variant presents with 2:
- Prolonged QT interval with prominent U waves
- Polymorphic or bidirectional VT
- Facial dysmorphisms
- Hyper-/hypokalemic periodic paralysis
- Extracardiac manifestations that help distinguish it from CPVT
Digoxin Toxicity
Historically, BdVT was considered pathognomonic for digitalis poisoning 3. This remains an important cause, particularly in:
- Elderly patients on chronic digoxin therapy
- Patients with renal dysfunction affecting digoxin clearance
- Overdose situations 3
Secondary/Acquired Causes
Acute Myocardial Ischemia
Although rare and limited to case reports, acute coronary occlusion can directly cause BdVT 4. Key features:
- Look for excessively discordant ST segment changes during the wide complex tachycardia
- Apply Sgarbossa criteria to identify acute ischemia in the setting of wide QRS rhythm 4
- This is an exceptionally rare presentation but critical to recognize
Toxic/Metabolic Causes
- Aconitine poisoning (herbal toxin) 5, 3
- Caffeine poisoning - massive overdose (>6g) can induce BdVT through increased intracellular calcium and delayed afterdepolarizations 6
- Pheochromocytoma - particularly with acute hemorrhagic necrosis causing massive catecholamine release 7
Structural Heart Disease
- Fulminant myocarditis 5
- Cardiac sarcoidosis 5
- Arrhythmogenic right ventricular cardiomyopathy 3
- Myocardial disarray with interstitial fibrosis 8
Mechanistic Framework
The underlying mechanism involves:
- Triggered activity from delayed afterdepolarizations (most common)
- Abnormal automaticity
- Reentry 5
The characteristic beat-to-beat QRS axis alternation results from alternate occurrence of triggered activities within different His-Purkinje fibers 6.
Clinical Pitfalls
Do not assume all BdVT is CPVT or digoxin toxicity. The differential is broader than traditionally taught:
- Check digoxin levels if patient is on the medication
- Obtain detailed family history for channelopathies
- Consider acute ischemia in patients with chest pain or ACS symptoms 4
- Screen for toxic ingestions (caffeine, aconite, herbal supplements)
- Evaluate for pheochromocytoma if accompanied by cardiogenic shock 7
- Look for extracardiac features suggesting Andersen-Tawil syndrome 2
In patients with structurally normal hearts and negative workup, genetic testing for CPVT, Andersen-Tawil syndrome, and related channelopathies is essential 1, 9.