Polymorphic Ventricular Tachycardia and Ventricular Fibrillation¶
Chapter 262 | Part 6: Disorders of the Cardiovascular System
KEY CLINICAL POINTS¶
- Polymorphic VT is characterized by continuously changing QRS morphology, often associated with QT prolongation (e.g., torsades de pointes).
- Acute MI, electrolyte abnormalities, and genetic mutations (e.g., long QT syndrome) are major etiologies.
- Magnesium sulfate (1–2 g IV) and beta-blockers are first-line therapies; catheter ablation and ICDs are indicated for refractory cases.
- Noninvasive ablation (e.g., stereotactic radiation) and genetic testing are emerging treatment options.
- ICDs are recommended for patients with recurrent VT, structural heart disease, or a history of cardiac arrest.
1. DEFINITION & OVERVIEW¶
Polymorphic ventricular tachycardia (VT) is a rapidly changing arrhythmia with continuously shifting QRS complexes, often linked to QT prolongation (e.g., torsades de pointes). Unlike monomorphic VT, polymorphic VT does not necessarily indicate a fixed focus or structural abnormality. It may arise from reentry with variable pathways, spiral waves, or multiple foci. Ventricular fibrillation (VF) is a common complication, particularly in acute MI or congenital arrhythmia syndromes.
Table 262-1 Causes of QT Prolongation and Torsades des Pointes¶
| Category | Causes | Notes |
|---|---|---|
| 1. Congenital Long QT Syndromes | Long QT syndrome type 1 (KCNQ1), type 2 (KCNH2), type 3 (SCN5A) | 80–90% of cases |
| 2. Electrolyte Abnormalities | Hypokalemia, hypomagnesemia, hypocalcemia | Common in acute MI or drug toxicity |
| 3. Drug-Induced QT Prolongation | Class IA (quinidine, disopyramide), Class III (sotalol, amiodarone), Macrolides (erythromycin), Antipsychotics (haloperidol) | Avoid in LQTS patients |
| 4. Cardiac Conditions | Myocardial ischemia, myocarditis, stress cardiomyopathy | Associated with acute MI or structural disease |
| Category | Causes | Notes |
|---|---|---|
| 5. Endocrine Disorders | Hypothyroidism, hyperparathyroidism, pheochromocytoma | Contribute to QT prolongation |
| 6. Intracranial Disorders | Subarachnoid hemorrhage, thalamic hematoma | May trigger arrhythmias |
| 7. Nutritional Disorders | Anorexia nervosa, celiac disease, malnutrition | Affect electrolyte balance |
1.1 Clinical Features¶
Sustained polymorphic VT typically degenerates into VF. Common presentations include syncope, near-syncope, or cardiac arrest. Preceding episodes may include nonsustained VT or premature ventricular contractions (PVCs).
1.2 Differential Diagnosis¶
Differential diagnoses include monomorphic VT, ventricular tachycardia due to structural heart disease, and arrhythmias from electrolyte disturbances or drug toxicity.
2. EPIDEMIOLOGY¶
Polymorphic VT is a common cause of sudden cardiac death, particularly in acute MI. Approximately 10% of patients with acute MI develop VT that degenerates to VF, with highest risk in the first hour post-MI. Congenital long QT syndrome (LQTS) accounts for 30–40% of nonischemic dilated cardiomyopathy cases. Brugada syndrome predominantly affects males, with a 10:1 male-to-female ratio.
2.1 Risk Factors¶
Risk factors include structural heart disease, electrolyte imbalances, QT-prolonging drugs, and genetic predispositions (e.g., LQTS, Brugada syndrome).
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Polymorphic VT arises from abnormal repolarization and reentry mechanisms. Key causes include: - QT prolongation (e.g., LQTS, drug toxicity) - Acute MI or ischemia - Genetic mutations (SCN5A, KCNQ1, KCNH2) - Electrolyte disturbances (hypokalemia, hypomagnesemia) - Structural heart disease (e.g., hypertrophic cardiomyopathy, Brug, dilated cardiomyopathy) Reentry occurs via variable pathways, spiral waves, or multiple foci, often in areas of scar or fibrosis.
3.1 QT Prolongation Mechanisms¶
QT prolongation results from impaired repolarization due to genetic mutations (e.g., LQTS), drug effects (e.g., sotalol, amiodarone), or electrolyte imbalances. Torsades de pointes is a hallmark of severe QT prolongation.
4. CLINICAL FEATURES¶
Patients typically present with syncope, near-syncope, or cardiac arrest. Sustained episodes may degenerate into VF. Preceding episodes often include nonsustained VT or PVCs. In congenital LQTS, episodes are often triggered by bradycardia, exercise, or sudden auditory stimuli.
4.1 Complications¶
Complications include sudden cardiac death, hemodynamic instability, and progression to VF. Recurrent episodes may indicate ongoing MI or structural disease.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include monomorphic VT, ventricular tachycardia due to structural heart disease, and arrhythmias from electrolyte disturbances or drug toxicity. Brugada syndrome and LQTS must be distinguished from other ST-segment elevations (e.g., LV hypertrophy, pericarditis).
5.1 Key Differentiators¶
Brugada syndrome: coved ST elevation with negative T waves in V1–V3; LQTS: prolonged QTc with torsades de pointes; early repolarization syndrome: J-point elevation without arrhythmia.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis requires ECG, electrolyte testing, and history of QT-prolonging drugs. Key findings include: - QTc >440 ms (men) or >460 ms (women) - Torsades de pointes morphology - ST-segment abnormalities (Brugada pattern) - Family history of sudden death ECG and Holter monitoring are essential for detecting intermittent episodes.
6.1 Diagnostic Criteria¶
QTc >440 ms (men) or >460 ms (women) with torsades de pointes morphology. Brugada syndrome is diagnosed by characteristic ECG findings (coved ST elevation in V1–V3) and episodes of syncope or VF.
7. MANAGEMENT & TREATMENT¶
Acute management includes magnesium sulfate (1–2 g IV), beta-blockers, and defibrillation for VF. Long-term strategies involve: - Beta-blockers (nadolol, propranolol) - Avoidance of QT-prolonging drugs - ICD implantation for recurrent VT or structural disease - Catheter ablation for focal VT - Noninvasive ablation (stereotactic radiation) for refractory cases
7.1 Acute Management¶
Magnesium sulfate (1–2 g IV) suppresses recurrent episodes. Isoproterenol infusion (100–120 bpm) may suppress PVCs. Defibrillation is required for VF.
7.2 Long-Term Strategies¶
ICDs are indicated for patients with recurrent VT, LV dysfunction (ejection fraction <35%), or a history of cardiac arrest. Catheter ablation is effective for VT originating from the right ventricular outflow tract.
8. PROGNOSIS & COMPLICATIONS¶
Prognosis depends on underlying cause. Patients with acute MI and polymorphic VT have higher in-hospital mortality, but survivors are not at increased risk for sudden death. Recurrent episodes may indicate ongoing MI or structural disease. Complications include sudden cardiac death, hemodynamic instability, and progression to VF.
8.1 ICD Outcomes¶
ICDs reduce sudden death risk in patients with recurrent VT, LV dysfunction, or a history of cardiac arrest. Annual mortality rates for Brugada syndrome range from 1–5% with ethanol septal ablation.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Avoid QT-prolonging drugs; ICDs are safe. Pediatrics: LQTS may present with syncope during exercise. Elderly: Higher risk of drug interactions and electrolyte disturbances. Genetic testing is critical for familial LQTS or Brugada syndrome.
9.1 Genetic Counseling¶
Genetic testing is recommended for families with LQTS or Brugada syndrome. Mutations in SCN5A, KCNQ1, and KCNH2 are common. Family screening is essential for asymptomatic carriers.
10. KEY POINTS & CLINICAL PEARLS¶
- Polymorphic VT is often linked to QT prolongation (e.g., torsades de pointes) and requires immediate magnesium sulfate. 2. ICDs are indicated for recurrent VT, structural disease, or a history of cardiac arrest. 3. Avoid QT-prolonging drugs in LQTS patients. 4. Noninvasive ablation (e.g., stereotactic radiation) is a promising treatment for refractory cases. 5. Genetic testing is critical for diagnosing congenital arrhythmia syndromes.